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  1. So if there was more anthropogenic CO2 in the atmosphere by 1800 than we previously thought, and we have used 350Gt more of the ‘trillion-tonne budget’ than we thought, does that mean climate sensitivity is lower than expected?

    Comment by Fred Zimmerman — 15 Apr 2011 @ 10:00 AM

  2. May I offer you some cleaner versions of the illustrations shown here? I redrew them and you can grab them from this location if you wish to use them:

    thank you,

    Comment by jg — 15 Apr 2011 @ 10:08 AM

  3. 1) Do you have a quantitative estimate of the increased CH4 concentrations due to early anthropocene emissions?

    2) A 9 ppm response from reduced solubility seems large… that would imply ~1 degree C temperature response to the 24 ppm of CO2 increase? (Denman et al., IPCC 2007 estimate a response of 6.9 to 10.2 ppm/degree C)



    Comment by M — 15 Apr 2011 @ 10:31 AM

  4. Very nice summary. What I did find missing was the clearing of forests, not just for agriculture, but for building. This trend has reversed in most industrial nations, but continues in the poorest regions.
    My question for those here is two-fold.
    1. What is the difference in CO2 uptake between the removed forests and the planted crops?

    [Response: The sink would depend on the broad climatic regime, the natural disturbance regime (i.e. rate and sizes of disturbances), and species/physiological diversity (which governs the growth rate dynamics in response to disturbance). The pool however, would be greatly different of course.--Jim]

    2. What was the albedo effect from deforestation?

    [Response: Very good and important question. Will depend on latitude and altitude (generally greater effect moving north and up), leaf type (generally greater effect from conifers), and the albedo of the planted crops, which in turn depends on the cropping methods used.--Jim]

    These answers may help with Fred’s question earlier.

    Comment by Dan H. — 15 Apr 2011 @ 10:33 AM

  5. This is great! This is one of the most interesting and provocative ideas around and thanks to Professor Ruddiman for taking the time for a thoughtful and fascinating post!

    Comment by Andy — 15 Apr 2011 @ 10:44 AM

  6. @4 / Dan H.: global urban land extent is now estimated ~3% ( so not sure how significant we would find clearing of forests for buildings.

    Comment by Fred Zimmerman — 15 Apr 2011 @ 10:56 AM

  7. This seems an especially useful example of real science at work:

    1) Hypotheses, based on early research, with (properly)-mixed reception, i.e., serious people took the ideas seriously (look at blurbs on Plows, Plagues and Petroleum), but new hypotheses that drastically change worldviews require work to get beyond “interesting, not yet proven.” (Of course, Bill has multiple hypotheses).

    2) Counter-arguments, actually carried out in peer-reviewed literature. (yeh!)

    3) Modification & refinement of hypotheses.

    4) Other scientists get stirred to reexamine existing data and see if it bears on the problem.
    This issue is particularly interesting because it is highly interdisciplinary, and the relevant papers are scattered all over the literature.

    5) Other scientists are stirred to do new research and investigations.

    6) As evidence accumulates, hypotheses may still be unresolved, may be refuted … or (as in this case, I think), the evidence is piling up that some old assumptions are likely wrong.

    7) At this point, the larger “missing” factors seem to be identified, with research focused not on the structural possibilities, but on uncertainty bounds.

    8) The argument may not be over, but I think that the evidence has built, and many more scientists are involved.
    I speculate that the result may be a major contribution to our knowledge of the Earth’s recent history.

    9) I think this is an example of real science that is especially instructive:
    a) The hypotheses are simple enough for us lay people to understand. Many aren’t.
    b) I guess that the whole progress from start to wider acceptance is 10-15 years, wit ha small enough number of key papers to easily watch.
    c) And it’s happening *now*, so it’s not already long-established textbook stuff.

    Hence, I hope it will become a good teaching example.

    Comment by John Mashey — 15 Apr 2011 @ 11:35 AM

  8. My attempts to respond to comments 1-7 (with thanks to #5 and to John Mashey in #7):

    Re comment 1: Although our results mean more anthropogenic CO2, they also mean less natural CO2, so there is no net overall change. The Wisconsin group (John Kutzbach and Steve Vavrus) and I have used the mainstream climate sensitivity to convert our estimated early anthropogenic GHG anomalies to temperature (2.5C per CO2 doubling in earlier papers, 2.75-3C more recently).

    Re comment 2: Thanks. I had pointed out the problem to the editors, and they have now fixed it.

    Re comment 3: As I noted, Dorian Fuller estimated that the amount of irrigated land was ~35% of modern by 1000 AD. Using the modern area irrigated and best available estimates of its contribution to the current CH4 concentration, he estimates that at least 80% of the observed ~100 ppb CH4 rise ~100 ppb from 5000 to 1000 years ago came from rice irrigation. The livestock spread he mapped will obviously yield quite a lot more CH4.
    Our estimate of the 9-ppm solubility effect was based on a full GHG anomaly of 40 ppm for CO2 and 310 ppb for CH4 (which adds a non-trivial forcing). If you use the 24-ppm number for direct CO2 emissions instead, you would get something like a 7-ppm solubility effect. The Wisc group and I plan to run a ‘seeding’ experiment with CO2 lower by 24 pp and CH4 by 310 ppb (or slightly less) in an A/OGCM with biochemical feedback to see what CO2 feedbacks we get from solubility and Southern ocean processes.

    Re comment 4, first question: The average difference in standing carbon pools between forests and either pastures or cropland is generally in the range of 13-18 Gt per million square kilometers. That includes related changes in carbon in the upper soil layers. The numbers are higher in tropical rain forest and lower in subtropical dry forests and boreal forests.
    Second question: the Wisconsin group (including post-doc Feng He) is running an experiment on the biophysical effects of Kaplan’s clearance pattern. Previous experiments have shown that boreal forest clearance causes a large albedo-driven winter cooling, but almost no one was living in boreal regions until a few centuries ago, so we expect little or no cooling from that for most of the last 7000 years. Nearly everyone lived in the tropics, subtropics, or mid-latitudes, where biophysical effects would likely have been smaller.

    Re comment #6: Mark Williams, in “Deforesting the Earth”, says that forest clearance for agriculture was 10x larger than that for any other purpose: home building, ship building, creating charcoal for furnaces, etc.

    Comment by Bill Ruddiman — 15 Apr 2011 @ 12:40 PM

  9. They noted that this spread of livestock would also have made a major contribution to anthropogenic methane emissions and atmospheric concentrations but did not attempt to estimate the amount… – the article

    I mentioned this aspect here a couple of years ago. In the United States, the introduction of cattle was accompanied by a significant kill off of wild ruminants.

    The forests they cut down were different from the ones we see today.

    Amos Parker, traveling west of Nacogdoches in 1834, had this to say of the longleaf pinelands:

    “Immediately after leaving the town we came into pine woods again; to all appearance, the same we had already passed over — rolling, sandy soil; the trees straight and tall, but standing so far apart that a carriage might go almost anywhere among them. The grass grew beneath them, and we could see a great distance as we passed along.”

    Comment by JCH — 15 Apr 2011 @ 12:51 PM

  10. Fred,
    I was more referring to using the lumber in building homes, ships, etc. Research has shown an increase in forested area in the U.S. over the past century or so.

    Comment by Dan H. — 15 Apr 2011 @ 12:51 PM

  11. @6:
    Building does not depend exclusively on urbanization: rural people need shelter too; therefore deforestation for building is in large part proportional to population in tree-covered areas, plus the (small) use for non-residential building (ceremonial, government, etc).
    Besides other significant sources of deforestation for human use come from the use of fuelwood and charcoal for heating and cooking, and the construction of boats and ships (in preindustrial Europe, from pre-Roman to Early Modern times, entire forests disappeared as trees were cut for ship building). Note that burning emits carbon directly, while large portions of wood cut for building and shipbuilding end sunken on land and sea.

    Comment by Hector M. — 15 Apr 2011 @ 1:27 PM

  12. > increase in forested area

    Area differs from volume. Look again at that historical description. Compare it to contemporary descriptions:“dog+hair”+forest

    Comment by Hank Roberts — 15 Apr 2011 @ 5:47 PM

  13. More recent work on historical land use in New England, historian William Cronon’s Changes in the Land.

    Comment by Mark A. York — 15 Apr 2011 @ 7:24 PM

  14. Excellent article. I’ve followed this hypothesis with interest for nearly a decade now.

    “This emerging view bears on a current discussion over whether or not to designate an ‘Anthropocene’ interval (a time of major human influence on Earth’s system) and, if so, when to place its beginning…”

    “Forest clearance for cultivation and pastureland is the largest transformation of Earth’s surface that has occurred to date. If well over half of this key transformation occurred prior to the industrial era, then an argument can be made for placing the start of the anthropocene at an earlier time. One possible solution would be to designate two stages: an ‘early anthropocene’ (a time of slow but growing and cumulatively large transformations that began ~7000 years ago for CO2 and ~5000 years ago for CH4) and a ‘late anthropocene’ to mark the many accelerating changes of the industrial era.”

    The early Holocene is being squeezed out. Perhaps we are moving inexorably to viewing the Holocene as the Anthropocene. The time in which our species rose from having a regional to a global impact. By 11,500 years ago our species was already having a dramatic influenece on megafauna (outside Africa), and so influencing the ecological balance.

    Comment by Bruce Tabor — 15 Apr 2011 @ 8:49 PM

  15. The article attributes decreased per capita land clearing to increased population pressure. I was thinking that the inadvertent selection effects applied to crop seeds (presumably farmers use the larger seeds for planting) should have resulted in slowly increasing yields of agricultural species. So less acreage could produce more agricultural commodity. It probably wasn’t just the pressure from increasing population that forced the decreasing per capita land clearing, but most likely a lot smaller crop area was needed to feed a human.

    Comment by Thomas — 15 Apr 2011 @ 10:29 PM

  16. Dan H @ 10, Hank Roberts @ 12 Re changes in forest acres and forest carbon:

    US total Forest and woodland
    Area (10^6 ha)
    1700 = 432
    1990 = 262
    Total carbon (PgC)
    1700= 57.6
    1990 = 25.6
    Houghton, R.A. and J.L. Hackler, Changes in terrestrial carbon storage in the United States. 1: The roles of agriculture and forestry. Global Ecology and Biogeography, 2000. 9:125–144.

    Comment by Rick Brown — 16 Apr 2011 @ 12:27 AM

  17. Thomas, never forget technology. Primitive not-very-wonderful ploughing gives way to better ploughs – thereby leading to better yield for cropping as against seeding by simple scattering. Better yield per area planted feeds more people.

    The fact that ploughing and other cultivation exposes soil carbon to oxidation isn’t a problem – except to your descendants.

    Comment by adelady — 16 Apr 2011 @ 2:23 AM

  18. “Besides other significant sources of deforestation for human use come from the use of fuelwood and charcoal for heating and cooking, and the construction of boats and ships (in preindustrial Europe, from pre-Roman to Early Modern times, entire forests disappeared as trees were cut for ship building).” – Hector m.@11

    Do you have any specific, well-docuemnted examples of entire forests disappearing from these causes in preindustrial Europe? Charcoal, and timber for ships and boats, would have come from managed woods, which were valuable properties, carefully protected and monopolised by their owners. Higher-quality fuelwood too would have been a managed crop, while the lower orders would have gathered fallen branches and offcuts. Population growth would have exercised pressure on the extent of forested areas, but almost entirely through replacing forests by farmland.

    Comment by Nick Gotts — 16 Apr 2011 @ 5:52 AM

  19. I wonder whether this research is taking into account evidence presented in books like “1491″ that human populations in the New World before Columbus were greater than that of Europe. Some anthropolists provide evidence of substantial alteration of the Amazon during that period. Colonizatin by Europeans brought diseases that reduced Native American populations by on the order of 90%. There is some evidence that major increases in game and forests ocurred in the 1600-1700s throughout North America in response to declines in Native American populations.

    Comment by BillD — 16 Apr 2011 @ 7:56 AM

  20. My attempts to respond to comments 1-7 (with thanks to #5 and to John Mashey in #7):

    Re comment 1: Although our results mean more anthropogenic CO2, they also mean less natural CO2, so there is no net overall change. The Wisconsin group (John Kutzbach and Steve Vavrus) and I have used the mainstream climate sensitivity to convert our estimated early anthropogenic GHG anomalies to temperature (2.5C per CO2 doubling in earlier papers, 2.75-3C more recently).

    Re comment 2: Thanks. I had pointed out the problem to the editors, and they have now fixed it.

    Re comment 3: As I noted, Dorian Fuller estimated that the amount of irrigated land was ~35% of modern by 1000 AD. Using the modern area irrigated and best available estimates of its contribution to the current CH4 concentration, he estimates that at least 80% of the observed ~100 ppb CH4 rise ~100 ppb from 5000 to 1000 years ago came from rice irrigation. The livestock spread he mapped will obviously yield quite a lot more CH4.
    Our estimate of the 9-ppm solubility effect was based on a full GHG anomaly of 40 ppm for CO2 and 310 ppb for CH4 (which adds a non-trivial forcing). If you use the 24-ppm number for direct CO2 emissions instead, you would get something like a 7-ppm solubility effect. The Wisc group and I plan to run a ‘seeding’ experiment with CO2 lower by 24 pp and CH4 by 310 ppb (or slightly less) in an A/OGCM with biochemical feedback to see what CO2 feedbacks we get from solubility and Southern ocean processes.

    Re comment 4, first question: The average difference in standing carbon pools between forests and either pastures or cropland is generally in the range of 13-18 Gt per million square kilometers. That includes related changes in carbon in the upper soil layers. The numbers are higher in tropical rain forest and lower in subtropical dry forests and boreal forests.
    Second question: the Wisconsin group (including post-doc Feng He) is running an experiment on the biophysical effects of Kaplan’s clearance pattern. Previous experiments have shown that boreal forest clearance causes a large albedo-driven winter cooling, but almost no one was living in boreal regions until a few centuries ago, so we expect little or no cooling from that for most of the last 7000 years. Nearly everyone lived in the tropics, subtropics, or mid-latitudes, where biophysical effects would likely have been smaller.

    Re comment #6: Mark Williams, in “Deforesting the Earth”, says that forest clearance for agriculture was 10x larger than that for any other purpose: home building, ship building, creating charcoal for furnaces, etc.

    I thank everyone so far for the very civil tone of this discussion. Here are responses to comments 9-19:

    To comment #9: The most likely reason for ‘clean-looking’ forests was heavy burning of understory vegetation by early Americans. Three papers in the special issue (those by Richard Nevle, Jed Kaplan, and Julia Pongratz, and their respective colleagues) examine the issue of whether the reforestation (and C sequestration) that occurred after disease swept away 85-90% of early Americans had a significant effect on CO2 levels. The Nevle and Kaplan papers concluded yes; Pongratz concluded no.

    To comment #11: See my response in #8 to comment #6. A widely cited book says agriculture was by far the major source of deforestation. Wood use for other purposes was locally intensive but not regionally extensive.

    To comment #12: I wasn’t sure what point Hank Roberts was making about the ‘dog-hair’ forests. Anyway, as for his comment about the area deforested being decoupled from the volume, see my reply in #8 that the generally accepted range of carbon emissions per area deforested is 13-18 GT per million square kilometers. This conversion number is an index of the volume/area relationship that is widely used by ecologists and land-use modelers, and it puts the volume/area relationship in a narrow range.

    To comment #13: About early American burning, see my response above to comment #9.

    To comment #14: Thanks. I agree that the Holocene is disappearing, but maybe that is just as well. To me, it was always just another in a long series of interglacials and not deserving of special designation. Humans are the main (only?) reason it is different.

    To comment #15: Boserup and others assume that people use the ‘easiest’ technology available to get their food (with the least labor). Hunter-gatherers in most regions could eat well and nutritiously on a few hours of gathering a week and an occasional mammal kill. Boserup showed that farming the same plot of land every year was much more labor-intensive (weeding, keeping livestock as a source of manure, providing them with fodder for the winter, picking insects, protecting crops against wild animals, etc), so in this view people would not have chosen to do this unless there was pressure to do so.

    To comment #17: In the industrial era, mechanical plows opened up vast areas of prairie and steppe. But plow improvements in previous times were different. They were used mainly to keep repeatedly cultivated plots weed-free and did not increase per-capita land use. But they did increase labor.

    To comment #18: Jed Kaplan and colleagues have pointed to historical evidence of large-scale deforestation in Europe well before the industrial era. The forests disappear as a (non-linear) function of population density, at a time when most people were farmers. Consistent with major deforestation being caused by farming.

    To comment #19: See my response to comment #9 above.

    Comment by Bill Ruddiman — 16 Apr 2011 @ 8:42 AM

  21. Jim,

    Your answer to Dan H on albedo in #4 was a bit vague. Can you quantify it just a bit, at least going so far as to say whether or not a particular scenario represents an increase or decrease in albedo, i.e. more or less reflection, without bothering with how much?

    I’d also point out that season is probably relevant as well, first because in cropland the albedo will change more with the seasons (darker in the spring before growth begins, then more foliage, then changing dramatically as either crops are harvested, or plants die off). Second, the relative importance of the albedo in different seasons would vary (due to the change in the angle of incidence of sunlight, and length of the day).

    I’m just trying to get a handle on which direction, overall, albedo changed (i.e. exaggerating or negating some CO2 increases), and a broad guess at a factor (1/8? 1/4? 1/2?). I understand that the answer is a research paper all to itself… just a heavily qualified educated guess is fine.

    Comment by Sphaerica (Bob) — 16 Apr 2011 @ 9:55 AM

  22. #18, 20–I offer this diffidently, as the research I’ve done on the topic is rather casual.

    But I believe it is historically correct that management attempts (beginning by the end of the 17th century) were driven in part by the status of specific oak species as ‘strategic materials’ for naval warfare–Colbert, the French Intendant, is said to have planted forests for the purpose. And English shipbuilders were increasingly forced to import North American oak, though they considered it less desireable.

    To illustrate, Nelson’s “Victory”–the flagship at Trafalgar, and incidentally still a commissioned warship, though it’s now aground, not afloat–required more than 3,000 oak trees for its construction, as well as smaller numbers of other species.

    The picture I get is one of management attempts that probably helped, but which were being developed in response to impending or breaking supply crises.

    That doesn’t give much info on how shipbuilding compared with land clearance, of course; given that the overwhelming demand was specifically for oak, I’d think that shipbuilding’s impact must have been limited geographically–what percentage of European forest was pure stands of oak?

    Two cents. . .

    Comment by kevin mckinney — 16 Apr 2011 @ 10:21 AM

  23. @18:

    “Do you have any specific, well-docuemnted examples of entire forests disappearing from these causes in preindustrial Europe?”

    The construction of the Spanish Armada.

    It is anecdotal on my part, granted: I lived in Spain for many years, and the people still speak wistfully of the great forests of Iberia before that little venture…

    But quite supported in the literature.

    Comment by Jaime Frontero — 16 Apr 2011 @ 10:25 AM

  24. Like Dan H (#4) I was surprised that the summary, at least, makes no mention of albedo. The Earth Radiation Budget Experiment (ERBE) showed significant seasonal difference in albedo and even a trend.
    If the earth’s albedo was to change from 0.31 to 0.30 this would lead (assuming the Stefan–Boltzmann 4th power law) to a 2.3 C increase in temperature which in turn would have a significant impact on CO2 exchange processes.

    Comment by Ron Manley — 16 Apr 2011 @ 10:56 AM

  25. I guess it has been taken into account, but it seems to me that the warmer and drier conditions of the Holocene would in themselves lead to more natural forest and grass fires, so increasing CO2 output. This natural feedback greatly outweighs any effects of human use of fire.

    Comment by David Horton — 16 Apr 2011 @ 5:11 PM

  26. re: #25
    “This natural feedback greatly outweighs any effects of human use of fire.”

    That is a strong assertion that you have numbers for the two effects.

    Can you cite peer-reviewed research that backs that (with credible) numbers?

    Comment by John Mashey — 16 Apr 2011 @ 6:43 PM

  27. David Horton @25 — Holocene both warmer and wetter than during LGM, as expected from the physics. Here is a LGM vegetation type world map:

    Comment by David B. Benson — 16 Apr 2011 @ 6:47 PM

  28. Trouet, V., A. H. Taylor, E. R. Wahl, C. N. Skinner, and S. L. Stephens (2010), Fire-climate interactions in the American West since 1400 CE, Geophys. Res. Lett., 37, L04702, doi:10.1029/


    Despite a strong anthropogenic fingerprint on 20th Century wildland fire activity in the American West, climate remains a main driver. A better understanding of the spatiotemporal variability in fire-climate interactions is therefore crucial for fire management. Here, we present annually resolved, tree-ring based fire records for four regions in the American West that extend back to 1400 CE. In all regions, years with high fire activity were characterized by widespread yet regionally distinct summer droughts. Overall fire activity was high in late Medieval times, when much of the American West was affected by mega-droughts. A distinct decline in fire activity in the late 16th Century corresponds with anomalously low temperatures during the Little Ice Age and a decline in Native American fire use. The high spatiotemporal resolution of our fire record discloses a time-frequency dependent climatic influence on wildfire regimes in the American West that needs to be accounted for in fire models.

    Comment by Rick Brown — 16 Apr 2011 @ 7:40 PM

  29. re #26 John I was responding to “The most likely reason for ‘clean-looking’ forests was heavy burning of understory vegetation by early Americans”. I know this is the popular view (in Australia as well) but it is presented without any backing of numbers, “credible” or otherwise. I have written extensively on this topic in Australia over several decades – there is no evidence here that Aboriginal burning significantly changed vegetation patterns or affected animal species. You are asking me to prove a negative. But it does seem to me that the change from Pleistocene climates to Holocene ones almost certainly must have affected natural fire regimes.

    Comment by David Horton — 16 Apr 2011 @ 8:46 PM

  30. re #27 Thanks David. Yes, I know the Holocene was a lot better than the LGM. But it in turn was worse than the previous few thousand years (when in Australia, for example, inland lakes were full that have been essentially dry ever since). I suppose all I am saying is that a change in vegetation patterns in the Holocene in response to changes in climate from late Pleistocene will in turn have affected natural fire regimes which n turn will have affected ghg production even without postulated human actions.

    Comment by David Horton — 16 Apr 2011 @ 8:50 PM

  31. re #27 Thanks David. Yes, I know the Holocene was a lot better than the LGM. But it in turn was worse than the previous few thousand years (when in Australia, for example, inland lakes were full that have been essentially dry ever since). I suppose all I am saying is that a change in vegetation patterns in the Holocene in response to changes in climate from late Pleistocene will in turn have affected natural fire regimes which in turn will have affected ghg production even without postulated human actions.

    Comment by David Horton — 16 Apr 2011 @ 9:05 PM

  32. This information is very timely because I had been thinking along these lines for the last few weeks. (I’m a retired art historian, but have spent most of my time since retirement studying climate change. I had wondered about the slow temperature descent over the last 8000 years and why it differed from temperature descents following the four previous optimums, which were rapid. I had wondered about an anthropogenic contribution related to burning and clearing accompanying the spread of civilizations and whether there enough humans and carbon-creating activities to affect the climate significantly, that is, enough to slow the descent indicated by the rhythm of the Milankovitch cycle. I was wondering whom I could ask (after I paid my taxes), so this information is most gratifying. Can anyone tell me whether the properties Milankovitch identified as causing major temperature changes over the last 800,000 years or so are presently in a relationship that should reduce global temperature (were it not for offsetting conditions, such as anthropogenic CO2 creation?

    Comment by John Ward — 16 Apr 2011 @ 10:26 PM

  33. re: #29
    With all due respect, Australia is not the world, and I would suggest that aboriginal burning might not be the same as slash-and-burn to clear forests for agriculture.
    I live in California, which fears hot/dry fire seasons as much as you do in Oz. (and Rick, thanks for the paper, but its not the same thing).

    Once again, you wrote:
    “This natural feedback greatly outweighs any effects of human use of fire.”

    I am unconvinced of this general assertion.
    The point is that wildfires may increase if climate becomes hotter&drier, but after a forest burns, it usually grows back, which is not what happens when humans clear forests and keep them clear.

    A large part of Nevle,Bird, Ruddiman and Dull (2011, to appear in The Holocene) deals with biomass burning in the American neotropical area, which of course is different from arid/semi-arid areas, and has a lot more biomass.

    At the very least, the issue is being argued in the journals. Marlon, et al (2008) argue one way, these folks think human land use matters more.
    So, maybe hold the argument until that paper appears, and hope that Bill will return to explain that one also.

    Comment by John Mashey — 17 Apr 2011 @ 1:32 AM

  34. A couple of questions and points.

    Can someone explain the low point in GHG and its decline more or less matching the Holocene Climatic Optimum?

    There is this statement:

    “One possibility currently being investigated by Kaplan and colleagues is larger early per-capita burning by farmers (and those cultures still in the hunter-gatherer stage)”.

    What burning do hunters and gathers do? Do we really mean slash and burn which would make more sense and probably leave behind scant evidence?

    Much of the evidence of more extensive land clearing seems to come from the last 2,000 years and this could understandably account for GHG rise during that time. But what about the 3,000-5,000 before that? I am dubious there was that much outside of the Mediterranean and China before the last 2,000 years. During the last 2,000, of course, we can point to extensive agriculture in North, South, and Middle America.

    Comment by Jim Cross — 17 Apr 2011 @ 9:17 AM

  35. Catching up on the last day of comments:

    Several comments (#4, 21, and 24) seek answers to the possible albedo (‘biophysical’) effects of early deforestation. Although not my expertise, I suggest Bala et al. ( They show that complete deforestation of boreal forests would cause major winter cooling, because the dark green spruce/fir canopy gives way to bright reflective snow-covered ground. Clearing mid-latitude forests (with far fewer evergreens) has a much smaller winter cooling effect. Clearing tropical forests produces two counteracting effects: a small warming from losing the moderating effects of water vapor recycled by trees in evapotranspiration, versus a small cooling effect from extra clouds.
    So — how do these results apply to pre-industrial clearance? The answer obviously depends on the pattern of early clearance you trust. Jed Kaplan’s historically grounded earlier clearance? Or the previous model simulations that suggested little pre-industrial clearance? Given my stated preference for Jed’s reconstruction, what does it imply about the effects of pre-industrial clearance on global climate?
    For most of the last 7000 years, there was no significant clearance of boreal forests, because the far north was a poor place to farm. Some clearance occurred in the eastern Baltics and western Russia after 1400 when western Europe ran out of wood and started to look to other areas for more lumber (including the east coast of North America). But it is hard to argue for major human effects on albedo in boreal regions.
    Pre-industrial forest clearance in mid-latitude Europe and China may have caused some winter cooling in areas with persistent snow cover, but not during the snow-free summers. Most pre-industrial forest clearance occurred in the tropics, but probably with relatively little net effect on regional/global temperature. As noted in my post #8, the Wisconsin group is working with Kaplan’s group to model the climatic effects.

    As for comments about fire (#25-30): Experts are divided about the relative importance of natural versus anthropogenic fire. But there are a few cases where the answer is clear. After the Maori people arrived in New Zealand in 1280, an estimated population of ~100,000 burned vast parts of the islands during the next century or two (McWethy et al., 2009. The Holocene 19: 883-897. Doi: 10.1177/0959683609336563). They did that to let the Sun in to boost the growth of understory bracken with very nutritious roots. The numbers work out to almost 100 hectares burned per person (compared to typical per-capita forest clearance for agriculture in the range of 1-10 ha).

    Comment by Bill Ruddiman — 17 Apr 2011 @ 10:24 AM

  36. What about fossil fuels?

    Is the use of coal in both Europe and China for thousands of years, collected from beaches, the surface or simple mines, now considered negligible compared to deforestation?

    Also peat? E.g. the creation of the Norfolk Broads in England due to mass use of peat as a fuel. Also negligible in comparison?

    Comment by Gregory Norris — 18 Apr 2011 @ 5:05 AM

  37. Thanks for a great update on the latest research in this area. Do you plan to update Plows, Plagues and Petroleum?

    Comment by Danny Yee — 18 Apr 2011 @ 8:45 AM

  38. John,
    The Milankovitch cycles are such that every 4th cycle has low eccentricity.

    Our current position is shown here:

    Based on Milankovitch theory, the next ice age is a long way off.

    Comment by Dan H. — 18 Apr 2011 @ 10:43 AM

  39. This is old information to most commentors but for those who are new to this idea, Professor Ruddiman wrote a great book that fully explains his hypothesis called Plows, Plagues, and Petroleum: How Humans Took Control of Climate (New in Paper) (Princeton Science Library) published 2010

    Comment by Tony Noerpel — 18 Apr 2011 @ 11:03 AM

  40. Bill and RealClimate:

    Thanks for the posting. I’m pleased that we’re looking at the actual causes of pre-industrial climate variation and taking a very close look at human impacts. I’m sure the closer we look, the more we’ll find.

    I recently read (Hale: Lords of the Sea) that forests around Athens were depleted for shipbuilding purposes circa 400 BC.

    I’m intrigued by the possibility that the little ice age was caused by population declines in the Americas.

    Comment by jimster — 18 Apr 2011 @ 6:50 PM

  41. What am I missing? The article says “14.2 GtC per ppm” … but atmospheric CO2 went
    up 2 ppm in 2009 with emissions of 29 GtCO2 (~8 GtC).

    Comment by Geoff Russell — 19 Apr 2011 @ 2:49 AM

  42. My morning look at the last day of comments (36>41):

    RE comment #36: I have made crude estimates of these fuel sources. For example, what would have been the effect if some estimated % of the population in China used coal in their stoves for heat and cooking over the last 2000 years? For coal in China and peat and coal in northern Europe, I got emissions estimates of a few GtC each, much smaller than agricultural clearance. These guess-timates should be redone by people who are more qualified.

    RE comments #37 and #39: Princeton Press thought enough of Plows, Plagues… to re-release it in their Princeton Library Series and ask if I wanted to write a new afterword. I did and it updated things to 2009-2010. Some of the new stuff is in my post here.
    I have begun writing an intro-level textbook (e-book) focused on the natural-versus-anthropogenic debate. Its scope and tone would be much like the points John Mashey made in #7: an example of a real scientific debate in action, and one that is understandable. Of course, the burden will be on me to be honest.

    Re comment #38: My post here did not address a corollary of the early anthropogenic hypothesis: whether (small) ice sheets would now be growing in the far northern hemisphere, had it not been for GHG’s from early farming. The orbital data alone suggest we are close to the point of glacial inception, but not at the current or pre-industrial GHG’s. A special-issue paper by Vavrus et al suggests the lower values without early farming would be enough to start glacial inception, but another special-issue paper by Vettoretti-Peltier suggests they would not.

    Re comment 40: See the special-issue papers by Pongratz et al and Nevel et al. for two different views on this proposed link.

    Re comment 41: The current situation is different, because it is dominated by short-term C exchanges that are far from long-term balance. But if we stopped C emissions today and then came back 1000 years in the future, ~85% (some say 90%) of our industrial-era C emissions would be in the ocean.

    Comment by Bill Ruddiman — 19 Apr 2011 @ 7:52 AM

  43. Stocker et al have a paper on this topic in BioGeoSciences ( )


    “A wide range of land use scenarios has been applied in
    the BernCC coupled climate-carbon cycle model to simulate
    the potential human impact on carbon emissions and atmospheric
    CO2. Carbon emissions arising from preindustrial
    anthropogenic land cover changes have not been sufficient
    to explain the timing and amplitude of the reconstructed increase
    in atmospheric CO2 over the Holocene.”


    “The model results suggest that human activities
    may have contributed a few ppm to the 20 ppm CO2
    rise between 7 and 2 kyr BP, implying that natural mechanisms

    I’d be curious to hear prof Ruddiman’s take on this paper, as they seem to arrive a opposite conclusions.

    Comment by Bart Verheggen — 19 Apr 2011 @ 10:10 AM

  44. Like Bill, I too thank everyone for staying on topic and civil, and asking good questions and making good comments. As John has noted, this is how scientific discussions should proceed, and there’s no inherent reason they cannot occur in the blog world as well.

    Bob, Bill addressed your question well, and if I was vague it was because I was just mentioning general considerations. I agree with him that the albedo effects of clearing conifer forests, whose time-integrated albedo is as low or lower than any other forest type (typically, very dark foliage with relatively long lifetimes), and which tend to dominate in areas with winter snow cover, is typically much more significant than for other forest/biome types. But if pre-history human densities were low in such areas, which makes sense, then, as Bill explained, clearing rates would also have been low and thus the global effect due to albedo changes, minimal. Your statements in your second paragraph are right, and was at least partially what I was referring to when I mentioned the importance of the cropping system, as that determines the seasonal course of both albedo and evapotransipiration.

    Bill also mentioned the latter. This can be very important indeed, at least locally to regionally, perhaps even larger. Forest clearing drastically changes the seasonal course of surface and subsurface water flow, and the corresponding evapotranspiration rates have an enormous, and often critical, effect on the local energy budget. If the clearing was as great as has been estimated in some of these studies, this could have had a widespread effect on the surface temperature, depending on other critical variables, particularly cloud formation rate, height, and lifetime.

    This is a topic with a good literature base, much current interest, and worthy of another post. If time permits, which it does not right now, we’ll do that. Or I can at least try to pull together some relevant references and post them.

    Comment by Jim — 19 Apr 2011 @ 2:26 PM

  45. I was pleased to read this post, because I’ve long been uncomfortable with the seeming assumption that human contribution to GHGs was negligible prior to the industrial revolution.
    Something I’ve wondered about is how much impact one of the practices of early hunters may have had: burning large areas of grass land in order to chase large herds of animals over cliffs, etc. – a practice that appears to reach back at least 12,000 years, and perhaps started much earlier. The CO2 release itself might have been relatively insignificant, but how did it shift the balance of grasslands and forests?

    [Response: Excellent observation, and relevant in many locations now as well. Bill will have some thoughts here, but a couple of points from my corner. First, burning by native people was practiced in many places not under a slash and burn regime, but at low intensities (i.e. surface fires). This had several possible purposes related to ease of sight and movement through a forest, as for example in relation to hunting and warfare, in addition to the regenerative effects on e.g., stump sprouting shrubs and oaks (a response to top kill), and/or the increased light and T at ground surface that would favor, e.g. bulb sprouting in the Alliums (onions) and related groups. Your second point--type conversion--is critical in arid and semi-arid areas, including now. For example, a large part of the Sierra Nevada, the Cascades, the Coast Ranges, and some Intermountain mountain ranges have a tenuous, unstable dynamic between shrub- and forest lands, the veg type turning rapidly--and for long periods--based on fire severity and extent, which in turn has both vegetation condition, and climatic, drivers. This is a serious concern relative to the long term forest persistence and/or stability in these areas.--Jim]

    Comment by Geno Canto del Halcon — 19 Apr 2011 @ 2:54 PM

  46. Re#45: Lighting fires is still the main way of clearing land and keeping it clear for livestock … see Lauk and Erb … 2.2 gigatonnes dry matter in Sub-Saharan Africa.

    A farmer burning to plant a crop lights a small fire because they can’t plant a huge area, but those running cattle can and do burn much bigger areas. You don’t need huge modern populations to light huge fires. So per capita burning rates would rise quickly as you travel back in time.

    Comment by Geoff Russell — 20 Apr 2011 @ 12:09 AM

  47. #45 and response

    I think you are really stretching it with the idea that hunters and gatherers are setting fires and clearing land. It makes no sense for hunter-gatherers to destroy the grasslands which is the primary source of food for large game animals they are hunting and a key part of hunting and gathering is gathering of plants which would also be destroyed by fire. Is there anywhere today where hunters and gatherers setting large fires?

    The notion that native people need to burn for ease of sight and movement is laughably ethnocentric. Native people can hunt and move in jungles without burning them down.

    I really am missing the imperative to move beyond a slash and burn explanation. Certainly in tropical areas of Southeast Asia, MesoAmerica, and South America probably the earliest agriculture was slash and burn. Much of this I believe would leave little evidence of its magnitude or extent simply because with a diminishing population the areas would grow back over in jungle and with increasing population the form of agriculture would have to be replaced with cultivated crops which would mask the earlier presence of slash and burn.

    [Response: You might want to do some reading. The evidence for widespread native burning is utterly incontrovertible, including for the uses mentioned. And we're discussing surface fires here. They don't "destroy" plants, they simply kill the aerial parts near the ground. promoting a suite of positive processes, including fertilization, increased light levels, sprouting from underground meristems, and pathogen and insect mortality. Fire is a regenerative, and in many cases, essential process. Furthermore, it wasn't just natives involved--there's lightning as well. As for evidence: charcoal--Jim]

    Comment by Jim Cross — 20 Apr 2011 @ 5:31 AM

  48. This morning’s responses (to comments 43-47):

    Re comment #43: The paper by Beni Stocker et al. is the most recent in a series of previous land-use papers referred to in my initial post but not named. Their first two simulations use variations on the HYDE model with the built-in assumption of small, constant per-capita land use for the last 7000 years. The point of my post here was to summarize historical and archeological evidence that this assumption is not valid.

    Their last two simulations assume higher early land use, but go toward the opposite extreme by assuming no (or virtually no) deforestation during the last 1000 years, which is clearly implausible. They also show no obvious clearance in the very heavily populated Aztec and Inka empires.

    In contrast, Jed Kaplan’s simulation uses a historically-based relationship between forest clearance and population density in Europe. One of the authors on this paper is Kees Klein Goldewijk, the person who created the HYDE model with the assumption of small constant per-capita clearance (used by Stocker for the first two simulations). Kees’ presence on Kaplan’s paper is an indication of a new ‘emerging view on early land use”.

    Re comment #44: Nothing to add, except support for Jim’s idea of a future post on the albedo effects of clearing.

    Re comments #45-47: These contrasting opinions echo my point in comment #35 that experts simply don’t agree about natural versus anthropogenic burning. In addition to intentional burning, some scientists have argued that “escaped fires” were common and would have burned large areas during dry seasons (Charles Kay at Utah State for one).

    As for reasons for hunter-gatherers to burn (#47): Fires don’t “destroy grasslands”. They allow sun in so that berries can grow, which feed humans and attract animals and fowl that humans can hunt, and they fertilize grasses for the next season’s growth, which attracts grazers. Here on the U.S. east coast, those grazers included not just deer, but also buffalo, a common place name from upstate NY southward at least as far as Virginia.

    Also #47: Charcoal leaves reasonably permanent evidence of long-ago deforestation and reforestation. The paper by Nevle et al in the special issue provides a good summary of what was occurring in central and South America before (and after) Columbus.

    Comment by Bill Ruddiman — 20 Apr 2011 @ 8:12 AM

  49. Thanks for your reply, Bill.

    I understand that you disagree with Stocker’s assumption regarding historical per-capita land use. However, they also seem to argue that a relatively low biogenic carbon flux is inferred from the isotopic composition of icecore CO2. Doesn’t that provide a limit to the amount of biogenic carbon that early human societies could have emitted via their landuse?

    Comment by Bart Verheggen — 20 Apr 2011 @ 9:18 AM

  50. I am having a hard time finding the Nevle article but from what I can gather the proof you are talking about is from about the last 1-2 thousand years. I wouldn’t dispute that, What I am questioning is the 3-5 thousand years before that when GHG started rise.

    Patterns of land use were much different in the last 2 thousand years. The extensive Mississippian based on maize cultivation arose during this time. This was real agriculture, not hunting and gathering, and not slash and burn either, although probably an extensive amount of burning was involved in much of the manner you suggest.

    But this evidence with which I largely agree doesn’t account for the GHG rise in the early period so it might be that natural processes in addition to human activity is at work.

    Comment by Jim Cross — 21 Apr 2011 @ 7:01 AM

  51. Wow!

    This discussion that provides an example of science in action that is courteous, informative, and wise. Thanks to the patience that provides responses and updates. I don’t know why my mind (only partially educated in science) is so stubborn, but I finally have a better idea about some of the background, such as the Milankovich cycle, thanks to you all.

    I find this discussion broadening; it’s easy to visualize/imagine what is being talked about and see how the various influences develop and how partial information can be accumulated and tested. I do wonder if when natural cycles are disturbed traces of influence of what would happen absent all our interference persist; of course angles, orbits, and distances and are still there, so perhaps I don’t know what I’m trying to say. Once it is changed, how powerful is the influence that might return it to “normal”? (Setting aside that perhaps existential “normal” doesn’t really exist; what we have is a concatenation of effects.)

    My primary involvement has been communication, and I am always hoping for ways to talk about this that reach outside the very small circle of people who know enough and those who can believe six impossible things before breakfast. The stubborn persistence of bad thinking is perfectly illustrated by a comment on Mooney’s latest (science of anti-science over at MotherJones):

    “the quantum mumbo-jumbo is non-elegant and has yet to yield fruit”

    One doesn’t know if to laugh or to cry.

    Comment by Susan Anderson — 21 Apr 2011 @ 8:54 AM

  52. ps. you may assume that I really carefully all relevant comments and look at links, despite my inability to grasp some of the science and keep all the detail in mind.

    But real reason for this return is to mention:

    1. In my childhood (50s) the neighboring area was burned over annually, and it was not long ago that regular prophylactic fires were common, including forests. I know this is not the period being discussed, but may illustrate how people think and manage.

    2. If you have ever hiked through various forms of scrub, you can easily imagine that people would clear it for simple convenience, not to say survival. Without all mod cons various effects such as pest control would also apply.

    Comment by Susan Anderson — 21 Apr 2011 @ 9:05 AM

  53. Just two comments to respond to this morning (#49 and #50).

    Re comment #49: Paragraphs 12-16 in my original post dealt with this problem (not easy to explain!). The d13CO2 index complied by Elsig et al (Tom Stocker’s group) does indeed indicate a small net release of terrestrial carbon over the last 7000 years. However that index is the sum of many separate inputs and outputs, including natural C emissions from areas with weakening monsoons, C storage due to increased CO2 fertilization as CO2 rose by 22 ppm, and C removed from the atmosphere and stored in peat. My opinion is that the Elsig et al. estimate of storage in peat is far too low, and a more realistic (much larger) estimate would require a large offsetting input from other terrestrial emissions to yield that small net d13CO2 release. I argue that anthropogenic emissions are the most likely source. As an imperfect analogy, think of seeing an iceberg and trying to estimate its total volume from the part sticking up above the water line. Allowing for a more realistic estimate of larger peat burial is a bit like looking underwater to see how big the iceberg really is.

    Re comment #50: The Nevle article is not online yet, but you could read Dull et al. DOI: 10.1080/00045608.2010.502432 for a sense of the debate. However, by well-respected estimates, only 7 million of 55-60 million early Americans lived in North America, so the main American contribution would be from Mexico and South America, where agriculture developed and spread earlier. And of course an even larger contribution from the several hundred million people in China, India, and Europe. Because experts disagree on fire, I don’t mean to push this early burning idea too hard, but simply to point it out as a possible explanation for the early CO2 rise (in addition to higher per-capita cultivation). Jed Kaplan is now working on a model of earlier burning based on modern patterns: see Thonicke et al ( People burn extensively at low population density, less when the density passes a threshold, and hardly at all when the density becomes very high. That would help boost early CO2 emissions.

    Comment by Bill Ruddiman — 21 Apr 2011 @ 9:12 AM

  54. However, by well-respected estimates, only 7 million of 55-60 million early Americans lived in North America, so the main American contribution would be from Mexico and South America…

    I was always taught that Mexico is a part of North America. Do climate scientists use a different definition?

    Comment by Greg Simpson — 21 Apr 2011 @ 11:54 AM

  55. RE: #47, Jim Cross -I believe if you look into this, instead of making assumptions, you’ll find that there is substantial evidence that such practices occurred – and may have been a significant factor in the disappearance of megafauna in both the Old world, and the new. Current human activities are a great example of how, despite scientific evidence that our activities may not be in our best interest, we do them anyway. Paleolithic people didn’t have scientist to tell them that their practices were unwise, either. We are talking about highly mobile cultures that likely failed to perceive the ultimate negative outcome of what they were doing.

    Comment by Geno Canto del Halcon — 21 Apr 2011 @ 2:43 PM

  56. If you have increasing farming + increasing CO2, you can also imaging cause and effect the other way around. Humanity profited from the better weather and or favourable CO2 levels for crops, and farmed more. Or you can have another common cause pushing both. Some sources put farming earlier – starting 8,500 BC. So there is plenty of scope to fit an effect to a developement in farming be it old Testiment or 20th Century.

    Comment by Sean — 21 Apr 2011 @ 5:32 PM

  57. #55 There is no “substantial evidence that such practices occurred”

    [Response: Yes, there most certainly is, in abundance--Jim]

    nor that they were “a significant factor in the disappearance of megafauna” – I know this is a popular belief but it ain’t necessarily so.

    To make it clear – I think it highly likely that land-clearing and other early agricultural practices may have had an impact on ghg levels in the atmosphere prior to the industrial revolution, although I also think that this is such a small factor in the context of burning coal and oil as to be of academic rather than any other interest. It is quite clear that the big take-off in ghg production corresponds to the industrial revolution, whatever small steady climb there was before that.

    However I think that prior to agriculture the effects of hunter-gatherers on ghg would have been effectively zero. I pointed out above that changes in climate leading to changes in vegetation lead in turn to natural changes in fire regimes. That is, the base line for assessing hunter-gatherer activities isn’t zero, and that being so I think you would be extremely hard pressed to demonstrate that hunter-gatherer use of fire (even if you accepted that this was significant, which I don’t) had an effect over and above whatever changes in natural fire regimes were occurring.

    [Response: Well, I have direct and strong evidence that burning by Native Americans in Yosemite kept forest carbon content below 50% of what it now is, which likely held ~ true through a very large part of mid elevation forests in California, not to mention large areas in other western states. People are missing Geno's point, which had to do with the forests that never fully develop, from a biomass standpoint, due to frequent fire, whether there was initially a veg type conversion or not. You don't have to burn forests down to affect terrestrial carbon stores--Jim]

    Comment by David Horton — 21 Apr 2011 @ 5:49 PM

  58. I know Australia isn’t the world, but it is a place often used as the poster child for hunter-gatherers using fire to extensively modify the environment and cause extinctions. The most recent publication, featuring a large number of leading Australian paleobotanists, is as follows:

    Late Quaternary fire regimes of Australasia Quaternary Science Reviews xxx (2010) 1-19
    S.D. Mooney, S.P. Harrison, P.J. Bartlein, A.-L. Daniau, J. Stevenson, K.C. Brownlie, S. Buckman, M. Cupper, J. Luly, M. Black, E. Colhoun, D. D’Costa, J. Dodson, S. Haberle, G.S. Hope, P. Kershaw, C. Kenyon, M. McKenzie, N. Williams

    Extract from the Abstract
    “…. On orbital time scales, fire in Australasia predominantly reflects climate, with colder periods characterized by less and warmer intervals by more biomass burning… There is no distinct change in fire regime corresponding to the arrival of humans in Australia at 50 - 10 ka and no correlation between archaeological evidence of increased human activity during the past 40 ka and the history of biomass burning…”

    [Response: We're not talking about orbital time scales David, and one needs to look at the full body of literature to get the picture, not an extract from one abstract. Nor are we discussing megafaunal extinctions--Jim]

    Comment by David Horton — 21 Apr 2011 @ 8:36 PM

  59. “one needs to look at the full body of literature to get the picture” agreed, and there is literature over a number of years (including my own) which supports what the above paper (in itself a review of literature and 28 different charcoal records) is saying. The problem is that we need to get back to a null hypothesis of no effect by hunter-gatherers, and then, if you can demonstrate a particular effect in a particular time and place then good luck to you. The problem for a number of years now is that the reverse process has been followed, in which the null hypothesis is taken to be “of course hunter-gatherers modified the environment by using fire, prove that they didn’t” and I was concerned that the discussion here was following that path.

    [Response: Truth by re-framing the null hypothesis? And whatever some may think, there is in fact a much larger literature showing that burning by natives was extensive, which is why it's the consensus viewpoint. But all of this is frankly, off track. The point relative to this discussion is not whether natives burned or not, but rather how much area burned, regardless of cause.--Jim]

    Comment by David Horton — 21 Apr 2011 @ 9:29 PM

  60. I think there may be huge confusion here about what hunters and gatherers do.

    Hunters and gathers are nomadic, They move from place to place frequently in search of games and new plant foods. Quote from Wikipedia:

    “Hunter-gatherer societies tend to be relatively mobile, given their reliance upon the ability of a given natural environment to provide sufficient resources in order to sustain their population and the variable availability of these resources owing to local climatic and seasonal conditions. Individual band societies tend to be small in number (10-30 individuals), but these may gather together seasonally to temporarily form a larger group (100 or more) when resources are abundant.”

    The California Indians mentioned above were not hunters and gatherers. They were primarily sedentary and resided in villages, often quite large in size, and lived off acorns, salmon, and other abundant resources. Quote from a California government site:

    “Like everywhere else, in California, villages were fiercely independent and governed internally, The abundant food supply allowed for the establishment of villages of up to 1000 individuals, including craft speci-alists (spam?) who produced specific objects and goods for a living.”

    This is not a description of hunters and gatherers. Of course, like many native peoples around the world, they did hunt and gather but people even in the modern United States hunt and gather today, but that does not make us a hunter gatherer society.

    There would be little reason for true hunters and gatherers to clear land because they were not likely to be in the same location again for another year and they would have to clear the land again by that time. On the primarily sedentary population would have great incentive to clear land.

    Regarding the extinction of the mega fauna, the reasons for it are a considerable topic of debate but the extinction boundary is more like 10,000 years ago and occurred around the time of the Younger Dryas. There is the comet theory related to this that has since been proven to be unlikely. However, the Younger Dryas, whatever its cause, may be reason enough to explain the extinction, but if we want to introduce the human element, it might have been caused by true hunters and gatherers hunting, not setting the forests on fire.

    Comment by Jim Cross — 22 Apr 2011 @ 5:49 AM

  61. We may really have more of terminology problem here than a real disagreement. And maybe an under appreciation of the diverse ways in which native peoples lived.

    We have true hunters and gatherers that are mobile and in small groups.

    When these groups went they came into areas of relatively abundant plant and wildlife, they would be become more sedentary and live in larger groups.

    In tropical jungle areas, they might adopt a slash and burn approach moving perhaps once a year or so as the tropical land exposed by fire would lose fertility.

    In any case, to support large population densities, they would have to adopt agricultural practices wherever this was possible.

    Burning and use of fire would be factor in all of the above except for the true hunter gatherers.

    Early inhabitants of the New World probably were hunter gatherers with some settling and becoming sedentary where conditions permitted. Later in the tropical areas there was slash and burn. After that came true agriculture which became prominent in the Mississippian Culture, the Mayan, the Andean cultures, and possibly areas of the Amazon. But this was in the last 2,000 primarily where it became large scale.

    Comment by Jim Cross — 22 Apr 2011 @ 7:02 AM

  62. Responses to posts 51-61:

    Re comments 51 & 52: Thanks for your interest. As for your questions in paragraph 2: The original case for the early anthropogenic hypothesis rested on evidence that CO2 and methane trends in previous interglacials headed downward and were natural in origin, so the upward gas trends late in this interglacial are not likely to be natural. The new evidence for large land use millennia ago points to the same conclusion. As for getting back to ‘normal’: If we stopped emitting methane, the anthropogenic part of the amount now in the atmosphere would be gone in a decade. But CO2 stays a lot longer (see David Archer’s book featured on this site).

    Response to post 54: Valid point, although the area from the highlands of Mexico down into the lowlands of Guatemala is often lumped into “MesoAmerica”. Is ‘meso’ different from ‘central’?

    55-61: The (interesting) discussion here has veered off farther into burning, and the varying opinions posted again show that scientists disagree. My comment about early fire was not about the megafaunal extinction or (mainly) about hunter-gatherers. Its focus was the time of early farming when CO2 levels were rising fast (6000-3000 years ago). By this time, global population was in the high tens of millions, on its way to reaching 200-300 million by 2000 years ago. So — there were lots of farmers who were not hemmed in too tightly by neighbors and were thus free to burn. [Also -- thanks to David Horton for alerting me to a different view about 'fire sticks' inAustralia.]

    Comment by Bill Ruddiman — 22 Apr 2011 @ 7:58 AM

  63. Jim Cross,

    If a society lives by hunting and gathering, it’s a hunter-gatherer society, even if sedentary. Note that your wikipedia article only says h-gs tend to be mobile: it’s not part of the definition.

    Comment by Nick Gotts — 22 Apr 2011 @ 7:59 AM

  64. Jim “The point relative to this discussion is not whether natives burned or not, but rather how much area burned, regardless of cause”. I thought the point was the contribution, if any, of human activity to ghg production prior to the industrial revolution. I don’t want to prolong this too much, but the point is important. There tends to be an assumption that in the absence of human activity there is an absence of fire. For the more sophisticated, who realise that there is a natural fire regime (the fires sparked by lightning for example), the assumption is that human burning, even by hunter-gatherers, greatly increased the amount of fire. This is the proposition that I (thanks Bill) and a few others have been arguing against. And Jim I didn’t change the null hypothesis. The original one was that humans had little if any effect on environment in Australia. This null hypothesis was reframed by the early proponents of fire stick farming. From that point on every example of fire activity in the past, every ethnographic record, every environmental change, every apparent “fire adaptation” of plants, was taken as “proof” of Aboriginal use of fire. My view is that while Aborigines obviously used fire, probably caused bush fires on occasion, this fire activity took place within the context of, not as an addition to, the natural fire regime.

    Comment by David Horton — 22 Apr 2011 @ 3:12 PM

  65. RE: David Horton: What are the differences between hunter-gatherers and modern man?

    Comment by Hellen — 22 Apr 2011 @ 3:48 PM

  66. Hellen, I don’t understand the question?

    Comment by David Horton — 22 Apr 2011 @ 4:48 PM

  67. Hellen @65 — All modern peoples are the direct decendants of hunter-gathers and all are considered to be the same species, Home sapiens.

    Comment by David B. Benson — 22 Apr 2011 @ 5:34 PM

  68. Hellen. The difference is that =some= hunter gatherers had a much easier and more pleasant life, obviously not compared to ‘modern man’ in advanced economies, but certainly compared to practically every peasant agricultural existence I know of. Indigenous people living along the mouth of the River Murray spent the warm months along the beaches. Gathering lunch by standing in warm sand wiggling your toes and picking up shellfish in places that are now major holiday destinations sounds pretty good to me. As summer came to a close, they’d move inland up the river, plenty of roots to dig, massive – unbelievably huge – freshwater fish to catch. I doubt these people had much impact on such an abundant landscape, apart from the occasional campfire becoming wildfire.

    Whereas the activities of agricultural societies throughout Africa, Europe and the Middle East 5000+ years ago would have been noticeable. Ploughing alone exposes soil carbon to oxidation – and then there is stubble burning – another dislocation of the biological carbon cycle.

    Comment by adelady — 22 Apr 2011 @ 5:55 PM

  69. Other recent support for The Ruddiman Hypothesis;

    Chinese Cave Speaks of a Fickle Sun Bringing Down Ancient Dynasties
    Richard A. Kerr

    A 1.2-meter-long chunk of stalagmite from a cave in northern China recorded the waning of Asian monsoon rains that helped bring down the Tang dynasty in 907 C.E., researchers report on page 940 of this week’s issue of Science. A possible culprit, they conclude: a temporary weakening of the sun, which also seems to have contributed to the collapse of Maya civilization in Mesoamerica and the advance of glaciers in the Alps.

    Full story at

    Climatic Fluctuations in Last 2,500 Years Linked to Social Upheavals

    And from Polytechnique Federale de Lausanne
    New model of man’s role in climate change
    January 24, 2011

    The Roman Conquest, the Black Death and the discovery of America — by modifying the nature of the forests — have had a significant impact on the environment. These are the findings of Swiss scientists who have researched our long history of emitting carbon into the environment.

    The Columbian Encounter and the Little Ice Age: Abrupt Land Use Change, Fire, and Greenhouse Forcing – Annals of the Association of American Geographers

    When I was researching net primary production (NPP) numbers for plugging into Biochar/Biofuel systems climate potential, looking through Dr. Bill Ruddiman’s work at UVA on legacy CO2 and the agricultural revolution, It’s support of Johannes Lehmann’s previous work at Cornell of a potential 10 GtC, and the added perspective of palioclimatic effects of soil carbon loss, brought together many loose threads for me.

    Dr. Dull’s recent work brings even more support, related even closer to practices of Terra Preta soils in the Amazon. The BC, charcoal & pollen evidence is hard to ignore

    I’m glad this work by Dr. Dull is getting attention. Together with Dr. William Woods and citing Bill Ruddiman’s work, the pieces of anthropogenic climate change fall into place.

    The implications are really important. Dull, et al, argue that the re-growth of Neotropical forests following the Columbian encounter led to terrestrial biospheric carbon sequestration on the order of 2 to 5 GtC, thereby contributing to the well-documented decrease in atmospheric C recorded in Antarctic ice cores from about 1500 through 1750. While the paper does not extend to the medieval maximum, from charcoal in lake bed studies it documents increased biomass burning and deforestation during agricultural and population expansion in the Neotropics from 2500 to 500 years BP, which would correspond with atmospheric carbon loading and global warming 1100 to 650 years BP.

    Dr.Dull gives us hard numbers for what Charles Mann has tried to get across to us in “1491″, that we don’t give mankind near enough credit for creating our biosphere. Just as Michael Pollan’s “Botany of Desire” showed us how plants have manipulated us to spread them around the globe, the message of man’s mutuality with nature is more than seeping into the data everywhere.

    Since we have filled the air , filling the seas to full, Soil is the Only Beneficial place left.
    Carbon to the Soil, the only ubiquitous and economic place to put it.

    Thanks for your efforts.

    Erich J. Knight
    Chairman; Markets and Business Committee
    2010 US Biochar Conference, at Iowa State University

    Recent NATURE STUDY;
    Sustainable biochar to mitigate global climate change

    Comment by Erich J. Knight — 22 Apr 2011 @ 9:41 PM

  70. What is the cutting down of palm oil trees for biofuel doing to the climate?

    Comment by J. hirsch — 23 Apr 2011 @ 2:21 PM

  71. Responses to comments 63-70:

    The discussion has wandered off into issues not raised in the original post, so one more brief response here to recent comments about fire. The clearest evidence that early agricultural humans used fire extensively are their first arrivals on islands: New Zealand (already mentioned), Madagascar, and Pacific islands with drier natural climates and lower elevations (see, for example, Rollet and Diamond, Nature 431: 443-446). I find these cases convincing enough to make me think that the use of fire could have been extensive in many other regions that lack this kind of ‘controlled-experiment’ evidence.

    Comment by Bill Ruddiman — 24 Apr 2011 @ 5:14 PM

  72. “Most previous modeling simulations relied on the simplifying assumption that per-capita clearance and cultivation remained small and nearly constant during the late Holocene, but historical and archeological data now reveal much larger earlier per-capita land use than used in these models”

    Once again, the models got it wrong.

    [Response: Once again, the contrarian bypasses his frontal cortex when someone uses a word that triggers a knee-jerk reaction. Clue: the land use changes are an input into the models. - gavin]

    Comment by Charles — 25 Apr 2011 @ 11:59 AM

  73. [Response: Once again, the contrarian bypasses his frontal cortex when someone uses a word that triggers a knee-jerk reaction. Clue: the land use changes are an input into the models. - gavin]
    Well, they admit the models relied on a ‘simplifying assumption’. That makes them wrong in my book (or at least fairly useless).
    Oh, by the way Gavin, accusing me of bypassing my frontal cortex is a rather nasty thing to say. Is this the kind of “knee-jerk reaction” you make when you hear a dissenting voice?

    [Response: Oh please. If I had a dime for every time someone like you sees the word 'model' used and instantly says 'see the models are wrong', I would certainly have enough to retire and pay some else to rebut this kind of nonsense. It is not an 'admission' that models use simplifying assumptions - it is the whole point. As George Box said "All models are wrong. Some may be useful", but if you want to play games "Oh my! A scientist said a model was wrong!", please do it elsewhere. - gavin]

    Comment by Charles — 25 Apr 2011 @ 1:29 PM

  74. #73–

    Charles, perhaps you missed this in school, but the use of ‘simplifying assumptions’ is a common and useful tactic in all kinds of investigations. And yes, those assumptions do get questioned.


    Comment by Kevin McKinney — 25 Apr 2011 @ 3:31 PM

  75. Charles @73 At least Gavin implied that you have a frontal cortex. Pretty generous on his part, I think.

    Comment by Rick Brown — 25 Apr 2011 @ 4:46 PM

  76. Charles #73 “..fairly useless in my book…” Oh please! As though you own a book.

    Comment by One Anonymous Bloke — 25 Apr 2011 @ 5:56 PM

  77. @74
    [edit--nope, sorry, you've been responded to twice and it's perfectly clear that you're here to argue, not to learn. Not going to let this discussion get dragged off topic any further. Accuse me of censorship, or whatever else makes you feel good, if you like--Jim]

    Comment by Charles — 25 Apr 2011 @ 6:09 PM

  78. Has anyone looked into the different carbon uptake by crops compared to the forest they are replacing? Conversely, what is the increase in CO2 output generated by 7 billion people?

    [Response: Yes, lots of work on the former, and for typical agronomic crops on a per unit area basis, there is no comparison. These are largely herbaceous (i.e. non-woody) in nature, so the carbon cycles rapidly rather than accumulating, although how much accumulates can be strongly influenced by the cropping system used, as in e.g. no-till systems. Fruit and nut crops are better. And then there are all the related carbon costs of agriculture (esp. fuel, fertilizer). As for the latter, the effect of human biomass on carbon is a wash--all human respiration being balanced by some (ultimate) photosynthesis somewhere.--Jim]

    Comment by Dan H. — 25 Apr 2011 @ 7:37 PM

  79. Jim,
    Your first response was fine, but then you seemed to negate it with your second. If human respiration is balanced somewhere, why would combustion not also be balanced somewhere? By that reasoning, all additional atmospheric carbon dioxide would ultimately be balanced.

    [Response: No. It is related to the source of the carbon. CO2 from respiration comes from food (photosynthesis) which drew CO2 from the current atmosphere - thus it is carbon neutral. CO2 from the combustion of biomass (as long as the biomass is regrown) is also neutral. However, CO2 from fossil fuel combustion is adding carbon to the atmosphere that was drawn from very ancient sources, and so that is an addition to the active carbon pool. - gavin]

    Comment by Dan H. — 26 Apr 2011 @ 5:55 AM

  80. #71

    Sorry for my contributions to wandering off topic.

    Still much of the evidence you continue to cite of extensive use of fire is coming from the last 2,000 years. New Zealand, for example, was settled only 700 years ago. Even Madagascar settlement dates back only 2,000 years.

    Is it mostly speculation that there was extensive use of fire by a large enough number of human populations spread over a large enough area to account the increasing GHG levels 5,000 years ago? Or, do you have evidence that I missed regarding the period prior to the last 2,000 years? Or, do you account for it simply by the spread of agriculture through Europe and Asia?

    Comment by Jim Cross — 26 Apr 2011 @ 6:34 AM

  81. Jim #80.

    You’re right about some settlement and migration. otoh, agriculture was well under way in many regions of the Middle East, Egypt, Europe 5000+ years ago. And Australia was settled ~60,000 years ago. Most of those settlements were, unsurprisingly, in the same favourable coastal regions as modern populations – but some stayed in the drier more difficult areas. Fire could be useful for hunting in those regions.

    I think we get a bit blinded by our own wildly inappropriate land and energy uses to make good judgments about these historical activities. Firstly, early agriculture (and some current practice) was pretty destructive and inefficient. When we look at how much soil carbon can be exposed and oxidised by ploughing, let alone the dreadful multiple ploughing used in some areas/ times, and add in stubble burning, we can see that these activities can have a large effect. Not the same huge instantaneous effect we can create with our large populations and modern equipment but a steady cumulative effect from persistent, repeated, unrelenting disruption of the biological carbon cycle.

    Think of it as the difference between large volcanic eruptions (us) and the constant production of gases from small geo-active regions (them).

    Comment by adelady — 26 Apr 2011 @ 7:48 AM

  82. #78-79, and inlines–

    A little at cross-purposes, I think.

    DanH is asking about combustion *of biomass*, not fossil fuels as Gavin’s response seems to presume.

    If I read Jim’s inline correctly, the answer to Dan’s second comment is that the change from forest to cropping creates a CO2 pulse, since the accumulated biomass C is rapidly dumped into the atmosphere when burned. The annual crops do not accumulate a lot of biomass, so the carbon that they store is much lower. (Jim’s caveats about type of crop apply, of course.)

    [Response: Dan's questions were not perfectly clear;his use of the word "additional"
    implies adding new carbon to the pool, i.e. fossil fuel C, and thus Gavin's (correct) answer. My answer to the 2nd question was just pointing out that human respiration (and the respiration of all organisms on the planet for that matter) adds nothing to the active C cycle--it is all simply the return of atmospheric carbon originally fixed by plants, to the atmosphere. However, I wasn't quite clear enough there, because when you account for all of the fossil fuels used in modern agricultural production (fuels for plowing, planting, pesticide production and application, harvesting, and product shipping, plus fertilizer production), it's no longer carbon neutral. Early ag would have also not have been carbon neutral, but primarily for a different reason: carbon release from vegetation and soils due to clearing/burning. It's also important to mention that various forms of low intensity ag, like organic practices, conservation or no-till, dispersed or "personal scale" ag/hort, etc., all greatly affect the final C ledger. Agriculture has a very big and important effect on current GHG production --Jim]

    Presumably that pulse would not be an ‘ongoing’ source of C, but Dr. Ruddiman’s theses is that it’s been taking place over millennia.

    You could turn Dan’s argument around, I suppose, and ask the question, well, what about the biomass represented by 7 billion humans? But the math doesn’t work.

    As a “back of the back of the envelope” pseudo-calculation, when I was looking at the forest mortality question a few days back, I looked at research on the emerald ash borer, which is killing North American ash trees right now. I found out that the population of ash trees in the US is something like twice the global human population. It’s probably fair to assume that most of those trees mass a good deal more than any human. Of course, that’s just one woodland species, by no means the most numerous, and just one country, which though large still comprises only a few percent of the world’s land area.

    Conclusion: forest biomass has to be several orders of magnitude greater than human biomass.

    (Just for fun, and as a WAG–”wild-ass guess”–I’d venture 5-6 orders of magnitude (minus 1, plus 3) without looking any of my ‘sub-guesses’ up. But I bet Jim has much tighter estimates not far from his fingertips.)

    [Response: I've not a clue on the total carbon in humans, but the biomass of all animal life on the planet is not even close to that of plants--Jim]

    Comment by Kevin McKinney — 26 Apr 2011 @ 8:42 AM

  83. Thanks for elaborating, Jim.

    C in humans shouldn’t be hard: Wikipedia has us at 18% C with a mean body mass of 70 kg, so 7 billion of us would be 12.6 kg x 7 billion, or about 88 billion kg, or 88 million metric tonnes. (Just about what Kuwait emitted in 2007, and they were ranked #47.)

    Comment by Kevin McKinney — 26 Apr 2011 @ 2:40 PM

  84. Of course, I made that same tired mistake of comparing C (humans) to CO2 (Kuwait’s 2007 emissions.)

    My bad, but it really doesn’t affect the point.

    [Response: Right, it doesn't. And I also should clarify what I said in my comment above about early ag not being carbon neutral: this would be true only when cleared land remains cleared, i.e. in continuous agricultural production, or repeatedly burned, etc.--Jim]

    Comment by Kevin McKinney — 26 Apr 2011 @ 2:49 PM

  85. (Selective) responses to comments 72-84:

    Re comment #72: Forcefully (!) answered by Gavin (#72, 73), who I can only imagine must be up to his keester (probably higher) with ‘gotcha’ comments like this about models; by Kevin (#74); and by Jim (#77), who rightly cut ‘Charles’ off from any more of his irrelevant diversions.

    Re comment 79: Again, Gavin answered it.

    Re comment 80: In the last 2000 years that encompass these island examples, it is clear that these early farmers used fire extensively immediately upon arriving. But people had been using fire for hundreds of thousands of years in Africa, at least tens of thousands of years in Eurasia, and for more than 10,000 years in the Americas. And in all those places they had been farming for 10,000 years, or close to it. Is it really conceivable that when farmers arrived on these islands they were suddenly seized by a brand-new (previously unprecedented) urge to burn much of their new landscapes?

    Re Jim’s reply to comment 84: There is a half-disguised subtlety about the effects of shifting cultivation on carbon emissions. When people clear a new plot of forest in order to farm, there is an obvious and immediate effect on carbon emissions and atmospheric CO2 concentrations because crops and pastures have much less standing carbon stored than forests.
    The half-disguised part is the footprint that farmers leave behind in plots previously cleared but now abandoned. From a cup-half-full perspective, the trees growing back on these abandoned plots restore carbon. But the more relevant cup-half-empty perspective notes that those all those previously occupied plots take decades to restore the equivalent carbon of a “mature” forest”. So: it is the cup-half-empty view that matters for carbon emissions and atmospheric CO2. Even though people move around, and forests grow back, they leave a trailing C footprint that is tied to population (non-linearly, as my original post showed).
    Some years ago at a meeting, I had this argument with Paul Crutzen, and he disagreed with me. At breakfast the next morning, he came right up to me and said:”You are right!” I suppose this is blatant name-dropping, but it is one of my fondest career memories.

    Comment by Bill Ruddiman — 26 Apr 2011 @ 5:09 PM

  86. Now that this very interesting thread is getting in to carbon cycle and agriculture more generally, I hope I can ask a (couple of) question(s) not too much off topic:
    Is there a good place to read about the effects of modern conventional (“industrialised”) agriculture on carbon in the soils?
    Of course the original clearing of forests has released carbon. But how big is the effect of “degrading” the carbon content of the soils today?
    I suspect it could be temperature dependent. Has anybody been quantifying what effects global warming will have?

    Comment by MS — 27 Apr 2011 @ 1:38 AM

  87. 1) I grew up on a farm established ~1850 in Western PA. I have sketches from that time showing house, barn and pasture in front of the barn, from the view of the woods on the property opposite. When the farm was sold for development, the pasture was left clear, but of course the cows had departed a few years earlier anyway. (Hence, that form of tree suppression stopped.)

    We visited about 20 years later, and at least to look, one would never know there had been a pasture, because the same dense woods had regrown. [That regrowth speed would not be typical everywhere, of course.]

    2) While much of the land had been cleared long ago, there were still a few acres of woods, and sometime trees at the edge needed to be cleared back. How did we do it? Steel axes, chainsaws, and if not cutting for firewood, use tractor to drag wood away to a wood/brushpile to be burned later.

    Here’s a Penn State paper “trees of Western Pennsylvania about the history, including possible effects of native Americans and then the realities of tree clearing.
    ‘To these settler, “the trees were the enemy”‘

    fire usage of natives in California seems to have been different.

    3) Clearing trees for farmland is *hard* work, even with chainsaws and tractors.
    With only steel axes and horses/oxen to drag the logs (the way the land was originally cleared) it’s rather more work.

    Fire was a whole lot easier, especially thousands of years ago.

    Comment by John Mashey — 27 Apr 2011 @ 11:28 AM

  88. Has anybody factored in the use of wet rice cultivation, which began about 5000 years ago.

    Comment by D. Price — 27 Apr 2011 @ 5:22 PM

  89. re: 88 I’m sure Bill will say more when he next looks in, but of course:

    1) That’s part of Ruddiman, Kutzbach, Vavrus (2011).

    2) And earlier (among others) is Early rice farming and anomalous methane trends; Ruddiman, Zhou, Wu, Yu (2008).

    That’s another example of archeological research being brought to bear

    Comment by John Mashey — 27 Apr 2011 @ 10:04 PM

  90. Regarding comment 86 by MS;

    To me, in the long run, the final arbiter / accountancy / measure of sustainability will be
    soil carbon content. Once this royal road is constructed, traffic cops ( Carbon Board ) in place, the truth of land-management and Biochar systems will be self-evident.

    A dream I’ve had for years is to base the coming carbon economy firmly on the foundation of top soils. My read of the agronomic history of civilization shows that the Kayopo Amazon Indians and the Egyptians were the only ones to maintain fertility for the long haul, millennium scales. Egypt has now forsaken their geologic advantage by building the Aswan dam, and are stuck, with the rest of us, in the soil C mining, NPK rat race to the bottom. The meta-analysis of Syn-N and soil Carbon content show our dilemma, from University of Illinois & ISU;

    The Ag Soil Carbon standard is in final review by the AMS branch at USDA. Both Congressional Ag Committees have asked for expansion of Soil Carbon Standard to ISO status.
    Read over the work so far;

    Comment by Erich J. Knight — 28 Apr 2011 @ 2:11 AM

  91. (Selective) responses to comments 86-90:

    Re comment 86: Out of my expertise. Jim?

    Re comment 87: based on personal observations here in Virginia, regrowth depends on ‘what from’. Clear-cutting of hardwoods leaves stumps and roots that regenerate lots of forest carbon in a couple of decades. These clear-cuts would be like ancient woodlots cut on a rotating basis.
    Fully cleared pastures and croplands that are abandoned take longer, and reoccupation probably depends on the kinds of pioneering trees. Here in VA, cedar and locust trees come in, but not right away. I suspect from John’s story that maples and others with their helicopter seeds may be quicker.

    Re comment 88: You should look more closely at my original post (paragraphs 7-9). Dorian Fuller and colleagues have a paper about to come online in the Holocene special issue that shows a very large and early expansion of rice irrigation across Asia. Their estimates suggest that CH4 emissions from rice irrigation can explain much (~70%?) of the observed CH4 increase between 5000 and 1000 years ago. They also map the expansion of livestock across Asia and Africa, most of which has occurred since 5000 years ago. Livestock emissions are though to be larger than those from rice irrigation today, and likely were in the past.
    Earlier, I had been looking for archeological data on the spread of rice irrigation and ended up publishing with Chinese colleagues a 2008 paper in Quaternary Science Reviews (doi: 10.1016/j.quascirev.2008.03.007). An effort by Li and colleagues had also been underway to determine the spread of rice across southern Asia. Their paper was published in Quaternary International (doi: 10:1016/jquaint.2008.02.009). In my opinion, the recent compilation by Fuller and colleagues, which was very carefully vetted for unambiguous archeobotanical evidence of domesticated irrigated rice, is now the definitive effort.

    Comment by Bill Ruddiman — 29 Apr 2011 @ 8:54 AM

  92. #91 comment on 88

    Bill, this is the sort of thing I have been asking about.

    Wouldn’t this expansion of rice been preceded by burning and CO2 rises?

    I don’t really have any concrete evidence of this but it seems that the first step in any new area would be slash and burn which would then be followed by permanent agriculture where crops, climate, and geography make that is possible. The first would lead to CO2 rise and the second more to CH4.

    Comment by Jim Cross — 29 Apr 2011 @ 6:39 PM

  93. MS (86)

    Lots of work on that topic. Send me ( your email address and I’ll send you some stuff.

    Comment by Jim — 29 Apr 2011 @ 7:28 PM

  94. What would have been the effect on atmospheric CO2 of early metalworking?
    I suggest that if we could estimate the quantity of ores mined at various times and in various places we could estimate the fuel required to work the metals and the amount of CO2 produced from metals smelted from carbonates.

    The production of metals, pottery, glass, lime, leather, soap and many other items requires land and fuel – and all of these processes add to atmospheric levels of CO2. I believe that reasonable estimates could be made for the emissions of CO2 due to early industries and the amount of land used. I suggest that the clearing of trees for better access to the land for surface mining may also be relevant to these studies.

    Comment by Patrick Lockerby — 30 Apr 2011 @ 8:36 PM

  95. Repeated use of the products of material technologies has the larger C foot print,axes & plows etc.

    Moira Wilson of the University of Manchester has developed a ceramic dating technique to draw an exact time line. At an accuracy of one year, which sounds perfect to my interest in Terra Preta soil development year over year. we could see the speed at which the system built on itself once initiated.
    Archaeological dating by re-firing ancient pots – , And provide great data to compare with the spectroscopic survey of NASA’s Space Archaeology; $364K Terra Preta Program;

    Our farming for over 10,000 years has been responsible for 2/3rds of our excess greenhouse gases. This soil carbon, converted to carbon dioxide, methane and nitrous oxide began a slow stable warming that now accelerates with burning of fossil fuel. The unintended consequence has been the flowering of our civilization. Our science has now realized the consequences and developed a more encompassing wisdom.

    Carbon to the Soil, the only ubiquitous and economic place to put it.

    Comment by Erich J. Knight — 1 May 2011 @ 10:41 PM

  96. population ?

    Comment by john byatt — 3 May 2011 @ 4:50 AM

  97. re: #96
    Population? There might be some resemblance, but I’d guess a better fit would be the difference of (measured CO2 – natural CO2 without human influence), and of course we don’t know that for sure, although Ruddiman, Kutzbach, Vavrus (2011) and Kutzbach, Vavrus, Ruddiman, Phillippon-Berthier certainly bear on that, i.e. from which Bill’s 2nd sketch in the original post come.

    But back to the CO2 record, interesting all by itself.
    Akin to the graph mentioned in #96, here’s a 1,900-year Law Dome CO2 record extracted from:
    The graph is truncated @ 1900AD, no to “hide the incline” but to avoid having 1900- squeeze the vertical scale so much.

    This plots that data, basically a subset of the one in #96. Since I’m not good at eyeballing comparing diagonals to see rates of change, I graphed 25,51, and 75-year linear regressions (Excel SLOPEs)to get a simplistic sort of first derivative and ignoring uncertainties in both date and CO2 values. That gives this graph.

    One can summarize this as:
    1) 1AD-1000AD rate of change only rarely hit +/- 0.05ppm/year over 25,50, or 75-year time-scales.

    2) By 1300AD, one starts to see much stronger gyrations at all those scales.

    3) The ~1600AD event is unusual.

    4) By 1750AD Industrial revolution has overpowered everything else.

    Comment by John Mashey — 3 May 2011 @ 3:50 PM

  98. Selective responses to comments 92-96:

    Re comment 92: Yes, CO2 emissions from clearance would have preceded CH4 emissions from rice irrigation, but probably only by years (decades?). Sediment data in the areas of China most favorable for irrigated rice show a decrease in elm pollen, a kind of tree likely to have been growing in wet lowland areas later converted to rice farming.

    Re comment 94: As noted earlier, all of those pre-industrial manufacturing activities are thought to have had a much smaller overall effect than clearance for farming, at least until very recent centuries, and then only locally.

    Re comment 96: You didn’t take the time to make your point. Anyway: yes, population played a role in the CO2 rise since 1850, but that’s also the time when unprecedented C emissions from fossil fuels add to the previous ones that were mainly from deforestation. The whole point of my original post was to summarize new evidence that those pre-industrial GHG emissions were not linked linearly to population, as has been widely assumed

    Those following this discussion might want to check out a recent paper by Mitchell et al in JGR (doi: 10.1029/2010JG001441). They measured decadal-century scale CH4 variations over the last 1000 years in the WAIS (West Antarctic Ice Sheet) ice core, and found variations very close to those measured earlier at the Law Dome site, thus validating both records. They found weak to negligible correlations of CH4 with reconstructed temperature and precipitation, but a suggestive correlation of CH4 minima with anthropogenic episodes: Genghis Kahn’s early-1200’s assault on China and its agricultural infrastructure (less rice irrigation), the mid-1500’s spread of European diseases in the Americas (reduced biomass burning), and the early-1600’s civil unrest and population losses in China. The analysis by Mitchell et al also shows that invoking an anthropogenic explanation for those CH4 dips would require a level of anthro-driven CH4 emissions (the baseline level before those events occurred) that lies at or above the higher end of previous anthropogenic estimates (mine included).

    Comment by Bill Ruddiman — 3 May 2011 @ 3:59 PM

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