RealClimate logo


Technical Note: Sorry for any recent performance issues. We are working on it.

What we can learn from studying the last millennium (or so)

Filed under: — mike @ 15 May 2010 - (Español)

With all of the emphasis that is often placed on hemispheric or global mean temperature trends during the past millennium, and the context they provide for interpreting modern warming trends, one thing is often lost in the discussion: space matters as much as time. Indeed, it is likely that the regional patterns of past climate changes, rather than simple hemispheric or global mean temperature trends, will best inform our understanding of the dynamical mechanisms involved. Since much of the uncertainty in future projections relates to regional climate change impacts, it makes particular sense to focus on those changes in the past that involve regional changes and the underlying mechanisms behind them.

For instance, melting of the cryosphere (and consequent rises in sea level), subtle shifts in drought and rainfall patterns, and extreme events, are all regional effects that could be important threats to ecosystems and our environment. Such changes are often associated with phenomena like ENSO or the North Atlantic Oscillation. Yet there remain large uncertainties about how such mechanisms will respond to anthropogenic climate change.

There are a number of potential ways forward to improve our understanding. A first step is to look directly at the time-series of specific systems (like the ENSO index or the ocean temperatures in the North Atlantic) and try to extend them as far back as possible using proxy data. This gives more information on what the natural variations in these phenomena look like, and thus a better idea of how big a forced response would need to be before it could be reliably detected. Secondly, we can look to see if there is a relationship between various natural drivers of climate change (volcanic eruptions, solar variability or orbital forcing say) and any characteristics of these phenomena – amplitude, frequency or duration. Do volcanic eruptions appear to affect El Niño for instance?

For phenomena that need annual or decadal resolution data to be resolved, the last millennium or so is an obvious (and only) time period to be looking at for it is only for that period that there is sufficient paleo-data coverage of high enough temporal resolution. Other periods – such as the mid-Holocene 6000 years ago – are also useful, but the results are more long-term in nature (there is also a discussion of the value of different periods for reducing future projection uncertainty in this recent paper).

There are a number of different approaches to looking at reconstructions in recent centuries – some rely on high density regional networks (as seen in this recent paper by Guiot et al concerning European temperature trends for which they mostly used pollen data) and some rely on wider networks of more diverse proxies which aim to capture longer-range correlations to specific phenomena (such as the recent Mann et al (2009) paper).

When this is done, people usually find that while it was relatively cool in global mean temperatures from the 1400s to the 1800s known as the “Little Ice Age” and relatively mild in the 900s to 1300s interval ( sometimes termed the “Medieval Warm Period”). But the spatial reconstructions reveal, however, why such global terms can be quite misleading, and why alternative phrases such as the “Medieval Climate Anomaly” are being increasingly favored by the community. This latter terminology recognizes that while the interval displayed significant climate anomalies, they varied greatly, even in sign, from region to region. Many of the more profound climate anomalies, moreover, involve variables other than temperature, such as drought, rainfall, and atmospheric circulation. Though the medieval period is seen to be modestly warmer globally in comparison with the later centuries of the Little Ice Age (the peak global mean warmth is likely comparable to mid, but not late, 20th century warmth), some key regions appear to have in fact been colder, while other regions appear to have been warmer. Southern Greenland, for example, appears within uncertainties to have been as warm as today. However, much of the tropical Pacific was unusually cold, suggestive of the La Niña phase of the ENSO phenomenon (a similar conclusion was reached by Trouet et al (2009)). Thus even though some locations may have been as warm or warmer than today, the hemispheric mean appears not to have been.

Why does this matter? It matters because there are plenty of factors that can affect the overall mean temperature (solar variability, volcanoes, greenhouse gases, internal variability etc.) and so it is hard, given the uncertainties in the solar or volcanic reconstructions to precisely attribute the paleo changes in the global or hemispheric mean to these factors. But if we can look at more complex fingerprints of the changes, it might be possible to be more quantitative in those attributions since the spatial fingerprints of the different factors are easier to distinguish. Furthermore, if we can clearly tie the regional patterns to the different forcings, we might be able to use that information to inform regional projections under future conditions.

Thus we can basically say that the warmer conditions of the Medieval era were tied to higher solar output and few volcanic eruptions and the cooler conditions of the Little Ice Age resulted from lower solar output and more frequent volcanic eruptions. But these drivers appear to have had an equally important, though more subtle, influence on regional temperature patterns through their impact on climate phenomena such as ENSO and the North Atlantic Oscillation. The modest increase in solar output during Medieval times appears to have favored the tendency for the positive phase of the NAO, associated with a more northerly jet stream over the North Atlantic. This brought relatively greater warmth in winter to the North Atlantic and Eurasia. A tendency toward the opposite negative NAO phase helps to explain the enhanced winter cooling over a large part of Eurasia during the later Little Ice Age period.

There is some model support for these patterns (see also instance Shindell et al, 2001) when the models include interactive ozone photochemistry to produce this dynamical response to solar forcing, but it is not captured in a simulation of the NCAR CSM coupled model which lacks those processes. Neither model simulation reproduces the apparent La Niña pattern seen in the Medieval temperature reconstructions:

Figure 1: Spatial pattern of mean temperature difference between the MCA and LIA periods (defined at the intervals AD 950-1250 CE and 1400-1700 CE respectively) compared with simulations of two different climate models forced with estimated differences in natural (volcanic and solar) radiative forcing between the two periods (Mann et al, 2009).

Other model simulations, however, using a climate model that exhibits a particular tropical Pacific mechanism, do reproduce such a response. In such models, the tropical Pacific counter-intuitively tends to the cold La Niña phase during periods of increased heating, such as provided by the increase in solar output and low volcanism of the Medieval era. If this response holds for the future, something that is still vigorously debated, it could imply a more La Niña-like response in the future. Most of the state-of-the-art climate models, e.g. those used in the IPCC Fourth Assessment, by contrast, suggest the opposite–a more El Niño-like future climate. The credibility of the models with regard to this phenomenon is not high, however, and lots more work is going to be needed (both on paleo-reconstructions and model improvements) before we can be confident in the future projections of changes in ENSO-like dynamics and mean state.


690 Responses to “What we can learn from studying the last millennium (or so)”

  1. 101
    Dappledwater says:

    73 – Frank Giger. You seem to post here regularly, yet have never heard of the intensification of the hydrological cycle?. Plenty of free papers on the topic available via Google Scholar. Your example seems in line with expectations.

    A very recent paper by Paul J. Durack and Susan E. Wijffels here:

    http://journals.ametsoc.org/doi/pdf/10.1175/2010JCLI3377.1

  2. 102

    Ike Solem 78,

    I regressed Hadley dT on ln CO2 and the PDO for 1880-2007. PDO did indeed affect global temperatures, but it only accounted for about 4% of the variance. CO2 accounted for 76%.

  3. 103

    Ike,

    1st message was eaten. PDO accounts for 4% of dT variance 1880-2007. CO2 accounts for 76%.

  4. 104
    Naindj says:

    Philip Machanick, 80, and Richard Hendricks, 67

    I believe you are talking about indicators of “climate change”, not indicators of “AGW”. If such indicators had ever existed, we wouldn’t need the IPCC to try to sort it out…

  5. 105
    SecularAnimist says:

    Frank Giger, I understand you to be saying that when the science gives robust predictions that AGW will increase certain types of phenomena, then when those phenomena occur, anyone who suggests that they are in fact being caused by AGW just as the science predicted, has no credibility.

    Have I got that right?

  6. 106
    Completely Fed Up says:

    “anyone who suggests that they are in fact being caused by AGW just as the science predicted, has no credibility.”

    I think that Frank is worse than that: if someone suggests that their severity is caused by AGW, Frank thinks they have no credibility.

    Or it at least seems that’s what he’s saying.

  7. 107
    Ike Solem says:

    Here’s the latest from the Heartland Institute conference, on the Little Ice Age and the current warming – apparently, it’s not the “cooling PDO cycle”, says there expert, it’s the “cooling solar cycle” –

    “Observations of the sun show that as for the increase in temperature, carbon doioxide is ‘not guilty,’” Abdussamatov wrote, “and as for what lies ahead in the coming decades, it is not catastrophic warming, but a global, and very prolonged temperature drop.”

    …As historical support for his theory, Abdussamatov cited the observations in 1893 by the English astronomer Walter Maunder, who came to the conclusion that from 1645 to 1715 sunspots had been generally absent. That period coincided with the middle and coldest part of the Little Ice Age.

    Abdussamatov also observed “the most significant solar event in the 20th century was the extraordinarily high level and the prolonged (virtually over the entire century) increase in the energy radiated by the sun,” resulting in the global warming that today climate alarmists believe is a man-made phenomenon.

    Well, that’s the Heartland Conference for you – ignoring the carbon cycle, refusing to admit that infrared-absorbing gases actually warm the atmosphere, insisting that solar explanations or “ocean cycle” explanations account for all global warming, and assuring everyone that a “major cooling event” is only a decade away. How do they keep a straight face?

    @BPL: “PDO accounts for 4% of dT variance 1880-2007. CO2 accounts for 76%.”

    Reference? Supporting information? Mechanistic explanation of the onset of a “PDO event” along the lines of the onset of an ENSO event, either a La Nina (negative phase) or an El Nino (positive phase)? With ENSO, physical oceanographers have come up with reasonable physical explanations for the onset and collapse of El Nino events – but the other putative multidecadal ocean modes like the PDO and the AMO? Statistical data juggling with a priori assumptions might give you those ‘variance’ numbers but they shouldn’t carry much weight in the absence of valid mechanical explanations – for the ENSO case, here’s an excellent set of lecture slides:

    http://www.ldeo.columbia.edu/edu/dees/V1003/lectures/ENSO/ENSO.2100.pdf

    P.S. here’s the most recent global sea surface temp anomaly maps:
    Atlantic Ocean – if you look back a few weeks, you’ll see that the negative cool pools in the mid Atlantic have warmed to neutral, while conditions in the Atlantic Warm Pool are about +2C, and the northwest Atlantic is also at +2C.

    Pacific Ocean – Warm anomalies persist all across the equator, the southern Pacific, and the northwestern Pacific – with cool anomalies in the western Pacific. (A cool anomaly in the western Pacific! Must be the PDO! Someone see how many salmon they caught this year!). El Nino indicators have fallen to neutral – but what will next year bring? Any predictions for ENSO phases a decade from now? How about “PDO phases”?

  8. 108

    “Any predictions on when Black-bellied Whistling Ducks will make it to Canada?”

    Based on the information given, within the decade seems reasonable. . .

    April NCDC numbers are out–warmest April in the instrumental record. Warmest Jan-April, too.

    “It’s just weather”–but it also suggests that 2008 was “just weather” too, not some Easterbrookian “inflection point.” And it’s rather interesting that it continues so warm during such a quiet solar cycle.

  9. 109
    Frank Giger says:

    SA, take a look at post #93 for the paper that actually agrees with me. Another quote:

    “We will conclude that the recent drought was quite typical of historical droughts,
    that winter drought in the Southeast is weakly linked to a cold tropical Pacific Ocean,
    that summer drought is caused largely by internal atmospheric variability and, therefore,
    that there is limited predictability of extended droughts. We will also show that earlier
    centuries had droughts as severe as the recent one but which extended for as long as a
    decade or more. It will also be shown that the recent drought is unlikely to have been
    influenced by anthropogenic climate change
    but that the latter will lead to increased
    precipitation but also increased evapotranspiration with the potential for reduced soil
    moisture and river flow that would place further stress on regional water resources.”

    Phenomena are just that – phenomena. The worst sort of cherry picking can happen if cite every weather event as a demonstration of global warming. It’s not that unusual to have an 18-24 month drought in the SE. Heck, I’ve seen a couple of them (and worse than the last one); to point to this one as the smoking gun lacks credibility. The other option is to claim every weather event as proof of global warming. Equally silly.

    The volcano on Iceland was NOT caused by global warming. That doesn’t mean that melting glaciers don’t effect plate shifting (heck, the Great Lakes are still on the rebound from the last Ice Age ending) and volcanism. But it doesn’t suddenly negate tectonics as the prime mover.

    The analogy is having a friend that can’t even skate suddenly taking up hockey and making the fairly safe prediction that he will pick up an injury in the next six months. And then when one sees a cast on his arm five months later instantly berating him for foolishly taking to the ice, only to find he broke it falling down the stairs leading to rink. The system is sufficiently chaotic to rule out climate models as good predictors of weather – even when they take on long enough durations to imply a trend.

    A better use of projections is my front yard. I’ve been toying with the idea of planting a pair of apple trees, always an iffy proposition in the South. On any given ten year stretch, one can expect it to reach freezing long enough for them to produce apples of any real merit five of them (in my location). However, this is not really realistic if the models hold true; in the future I might only see three years of sufficient freezing out of ten. But I like apple trees, so I’ll probably plant them anyway – and just lower my expectations of how well they’ll bloom on any given year.

    That’s the insideous nature of climate change I’m referring to. It’s more subtle than “it rained/didn’t rain/there was a tornado/there wasn’t a tornado, therefore global warming has arrived” type pronouncements.

  10. 110

    Kevin McKinney @ 108:

    “It’s just weather”–but it also suggests that 2008 was “just weather” too, not some Easterbrookian “inflection point.” And it’s rather interesting that it continues so warm during such a quiet solar cycle.

    It’ll be more interesting when we break the ’98 HadCRUT record during such a quiet solar cycle. Because the solar cycle around ’98 weren’t at all quiet …

    And for anyone who wants to look at the progression of SC23 to SC24, here it is — http://users.telenet.be/j.janssens/Spotless/Spotlessallcycles.png

  11. 111
    Ron Broberg says:

    John N-G — 16 May 201: Is there any evidence that the MCA departed farther from some suitable multi-millenial mean than other carefully-chosen 500-year intervals?

    I sort of agree with Benson @50 but disagree that it is just a noisy wobble. To my eyes it seems to me that the Holocene is in a long term cooling trend. That would be in-line with the most interglacials. A quick warming up to a peak and then a slide back into the glacial period. Here is Holocene graph of GISP2 data from Greenland. The gray bar is a 2 s.d. range.
    http://rhinohide.cx/co2/img/GISP2-10000.jpg

    Last 2000 years GISP2 with Box 2009 appended.
    http://rhinohide.cx/co2/img/GISP2-2000-Box-2009.jpg
    http://journals.ametsoc.org/doi/abs/10.1175/2009JCLI2816.1

    I’ve been told that I’ve used an older GISP2 data set, so the indicated just years may be off just by a bit.

  12. 112
    Ray Ladbury says:

    Frank Giger,
    I agree 100% that attributing any particular event to “climate” is simply silly. Climate after all, has to do with trends. However, there is a big difference between attribution of a single event to climate change and citing a particular event as typical of those expected to happen more frequently as climate change progresses. I think that is perfectly valid. More valid still would be looking at relative frequencies of particular types of events and how they change.

    And unlike the denialists, we need to be scrupulous about pointing out where there is active research and disagreement–e.g. the likely effects on hurricanes–as opposed to areas where there is broad agreement–e.g. increasing drought in the US Southwest.

  13. 113
    Completely Fed Up says:

    “We will also show that earlier centuries had droughts as severe as the recent one but which extended for as long as a decade or more. It will also be shown that the recent drought is unlikely to have been influenced by anthropogenic climate change ”

    Frank:

    “If a one-in-a-hundred event starts turning up one-in-ten times, then you KNOW that something has changed. Even though your one-in-a-hundred event is still not unprecedented (it happened before, roughly once every hundred) and it’s nowhere near a record (it would be less severe than a one-in-a-thousand event).”

    To show that AGW is unlikely to have influenced, you’d have to show that the warming that is the result of AGW would have had no effect on the drought.

    Did the paper do that?

  14. 114
    Rod B says:

    Thomas (88), you in essence say Frank G is incorrect because you have to convince people of AGW, and throwing a bunch of speculative regional or geophysical changes, whether supported by tight science or not, might be effective, so long as you are careful to not get so specific that even mathophobes can pin you down and show it as unscientific and unsupported. To wit: “…it is important to make the case that the frequency of damging events is increasing because of AGW, while trying to avoid the journalistic/rhetorical trap of attributing any single occurrence to AGW.”

  15. 115
    Completely Fed Up says:

    “Reference? Supporting information? Mechanistic explanation of the onset of a “PDO event” along the lines of the onset of an ENSO event…”

    By correlating the PDO effects against the temperature record.

    Where a 0.04 proportion of effect of PDO will reduce the variations of the temperature graph around a level mean.

    It’s called “maths”.

  16. 116
    Rod B says:

    CFU says,
    “…Can you say it would have been as bad if we weren’t warming?

    No.”

    I’ll say it would have been as bad. Prove me wrong.

  17. 117
    flxible says:

    FCH “… we’ve had an exceptionally wet and not-much-sun past 8 months here in Central Texas …”

    And the same up in the PNW, which is why I find the relations to PDO and ENSO in this post interesting, I hopehopehope we’re not seeing a trend developing.

  18. 118
    Completely Fed Up says:

    “I’ll say it would have been as bad. Prove me wrong.”

    Easy:

    You get droughts when it’s hotter and AGW is causing warmer temperatures.

  19. 119
    John E. Pearson says:

    The National Academy of Science of The USA just sent me an e-mail with a link to this page:

    http://americasclimatechoices.org/?utm_medium=etmail&utm_source=National%20Academies%20Press&utm_campaign=NAP+mail+new+05.18.10&utm_content=Downloader&utm_term=

    announcing: “At a public briefing to discuss the reports, Ralph J. Cicerone, president of the National Academy of Sciences, will deliver opening remarks, and members of the panels that wrote the reports will discuss their recommendations and take questions. The briefing starts at 10 a.m. EDT Wednesday, May 19, in the Lecture Room of the National Academy of Sciences building at 2100 C Street, NW, Washington, DC. Those unable to attend the event can watch the live webcast at The National Academies website. “

  20. 120
    Frank Giger says:

    But not everywhere. That’s why scientists do regional probabilities.

    My point isn’t to say climate change isn’t happening or that people aren’t having an effect on it. My point is that we should be very careful in describing any one weather or short term (18 month) event as definative proof.

    When we do that we open up the contrary claim when the situation reverses.

    What if the next 18 months have the South slightly above the climate norm? If the drought is trumpeted as the smoking gun, the rain puts it down. We then have to admit that the drought wasn’t due to climate change or worse, completely switch sides and say the rain is.

    Far more credibility is gained if a more intellectually honest approach is taken. As the climate goes into flux, we should see more variance along the normal bands; rather than drought-normal-normal-normal-wet-normal-normal (“normal” being the expected climate within the average) we will start seeing drought-normal-drought-wet-wet-drought-normal. Swings to the edges of the band are actually worse than a straight trend line, btw.

    What the popular narrative in the Southeast missed was that there would be a bounce back from the drought, and when it lifted chances are it would swing to abnormally wet. Indeed, that’s what we saw last year, and particularly in the fall. The rains returned, giving a very good first harvest but then continued, ruining a fair number for the second one as fields were too bogged with mud for combines and they didn’t get taken up in time to be too late. Heck, even hay bailing suffered.

    The smart farmers are already picking up on this. Hay barn size and location is getting a lot of thought. They’re getting bigger and placed on higher ground for better drainage. Ponds for cattle are being increased and how drainage works for the whole of the area is getting a relook – with up and down years for rain taken into account.

    My point once more is that we should use caution in using specific weather events as “proof” of climate change.

  21. 121

    IS 107,

    My linear regression is enough to show that the idea that the PDO causes it all and CO2 is insignificant is wrong. I really don’t have to get more sophisticated than that.

  22. 122

    Off-topic, but I just finished writing an expression calculator, a little GUI utility for Windows. Email me if you want a copy of the setup program for the Beta version.

  23. 123
    Hank Roberts says:

    Please, kids, cite some kind of source for your claims.

    Here’s the _next_ 20 years, slides and audio.
    Invaluable information, reliable source:
    http://sackler.nasmediaonline.org/2007/ile/jeremy_jackson/jeremy_jackson.html

  24. 124
    CM says:

    CFU #113: ““If a one-in-a-hundred event starts turning up one-in-ten times…”

    What about a drought that stands out against the background of the 20th century but is unremarkable by the standards of the previous millennium? (Which kind of links the discussion to the topic of this post.)

    CFU #113: “Did the paper do that?”

    Why don’t you read it and see? (I did, but I won’t spoil it for you.)

    CFU #118: “You get droughts when it’s hotter…”

    … or when it’s raining less. So it gets a little more complicated when AGW affects precipitation and evaporation.

  25. 125
    Rod B says:

    Ray Ladbury (112) says, “….citing a particular event as typical of those expected to happen more frequently as climate change progresses” is perfectly valid.

    Might be valid but certainly is neither helpful nor scientific. The only value in doing that is to imply and get the recipient to believe that particular events are the direct result of climate change while being able to deny that is what you said.

  26. 126
    Ray Ladbury says:

    Rod,
    Note that I was very careful to state that one must speak of a trend when discussing climate. However, providing examples to indicate which way the trend is going could be quite helpful pedagogically and is certainly NOT unscientific. Would you consider it inconsistent to state that a particular drought is consistent with an increasing trend of droughts?

  27. 127
    Doug Bostrom says:

    One hundred years from now the vastly reduced rejectionist community will still be dismissing the latest storm surges washing over the corroded rebar emerging from the shallow embayments where New Orleans or Dhaka used to stand as simply isolated events.

    Forget any single drought, any one precipitation event, any particular tropical cyclone. Any isolated event can be singled out and dismissed as confirmatory data supporting the notion that our climate is changing.

    On the other hand, collections of excursions in locations significantly geographically isolated from one another seem sufficient to conclude there’s a controlling signal behind the noise of events.

    Collapsing volume of ice in the Arctic concurrent with statistical aberrations in record highs versus record lows in CONUS concurrent with more frequent extreme precipitation variances concurrent with accelerating ice loss at the margins of Greenland concurrent with a cooling stratosphere concurrent with statistical shrinkage of the global glacial ice inventory concurrent with thermal ocean expansion and yet again concurrent with perturbed tundra and permafrost are a set of statistical threads comprising a drapery covering a monumental accidental work of humankind. More threads abound; the warp and woof of this fabric is pretty tightly woven.

    Why look at a single fiber of a single thread?

  28. 128
    Completely Fed Up says:

    “Might be valid but certainly is neither helpful nor scientific.”

    Really?

    “If you keep doing that, you’ll make it bleed”.

    “If you don’t wear your helmet, you could fall off and hurt yourself again”.

    Both seem to be helpful and are citing an event that is typical as a result of the action.

    And how is it not scientific?

    The chance of a molecule in a gas having more than a certain kinetic energy can be related to the boltzmann equation. Typical energies can too. And how these figures change when you change a factor (add a new gas, let some out, warm or cool the gas) can also be expressed scientifically.

    You state a rebuttal with merely the fact that you said “it’s not” as its only argument.

    Do better than this.

  29. 129
    Completely Fed Up says:

    “What about a drought that stands out against the background of the 20th century but is unremarkable by the standards of the previous millennium?”

    By the standards of the century-level event of the previous millenium? Then the event is not remarkable unless you’ve seen another change that should affect the probabilities to become less likely an event.

    By the standard of “the worst that had happened in the previous thousand years”, then you’re talking a century-return-event becoming as likely as a millenial-return-event. This is significant. If it weren’t, then why aren’t they both century-return-events?

  30. 130
    Completely Fed Up says:

    “CFU #118: “You get droughts when it’s hotter…”

    … or when it’s raining less. So it gets a little more complicated when AGW affects precipitation and evaporation.”

    But it can rain less whether it is AGW caused or not. Therefore it is not a variant in the proposed query.

    Proposal: RodB: “It would have been no worse if AGW hadn’t happened”.

    Response: CFU: “Warmer weather makes it worse and AGW is making weather warmer”.

    Rain doesn’t come into it because rain doesn’t make drought not happen in warm weather.

  31. 131
    Completely Fed Up says:

    “My point is that we should be very careful in describing any one weather or short term (18 month) event as definative proof.”

    STRAWMAN!!!

    Nobody is doing that.

    Only you so you can say it shouldn’t be done.

    What IS being done is saying “You see that warm dry horrible thing that happened? Well climate change will cause more such dry warm weather that makes that horrible thing happen happen more”.

    Just because not every shark kills and eats a human doesn’t mean that shark infested waters are safe. Nor does citing a shark attack not make that shark infested water no longer shark infested. Getting rid of the sharks does that.

  32. 132
    David B. Benson says:

    Ron Broberg (111) — Over the Holocene I see an upswing until around 8000 ypb as in
    http://www.globalwarmingart.com/wiki/File:Holocene_Temperature_Variations_Rev_png
    but also for GISP2
    http://www.ncdc.noaa.gov/paleo/pubs/alley2000/alley2000.html

    Millennial averages of GISP2 temperatures are interesting: Up to peak, down a bit, back up to nearly the same peak and then a dramatic plunge for the past 2000 years, the coldest two millennia of the entire Holocene. This last roughly agrees with the orbital forcing of precession, placing the thermal equator furthest to the south around 2000 years ago.

  33. 133
  34. 134
    mike roddy says:

    John, #41, I agree that the Black Death is not likely to have been a major contributor to global temperature drops due to the lack of correspondence with the proxy records.

    One could make a case for the depopulation of Amazonia contributing to the Little Ice Age based on afforestation, as you pointed out, but there may be a flaw here: massive land clearing for agriculture may not have occurred in Amazonia as it did in the Mayan region. The mounds and urban grids that have been discovered in South America tend to be in fertile floodplains more than interior forests.

    The other issue is that Amazon forests do not sequester a lot of carbon on a per acre basis- the figure is around 300 Mt/hectare, far less than what temperate forests hold. And since Amazon inhabitants and their descendants appear to have have practiced slash and burn, the carbon consequences of this are far less than those of industrial logging, which kills competing species and results in far more soil degradation.

  35. 135
    Frank Giger says:

    Oh, CFU, if you’re thinking heat = drought you obviously haven’t spent much time in warmer climes.

    Some of our wettest years have been some of our hottest. You’ve got to walk in rain that’s seventy five degrees to really appreciate it.

    [edit] Pointing to specific weather events and declaring they would be worse or more severe if climate change wasn’t happening is just flat wrong.

    [edit-lets watch the snark]

  36. 136
    Ron Broberg says:

    re Benson 132

    The graph that I found most intriguing is this one:
    http://www.globalwarmingart.com/wiki/File:Ice_Age_Temperature_Rev_png

    A friend of mine states that if AGW prolongs the Holocene interglacial it will be mankind’s greates achievement. He has a point.

    On the other hand, I note that the previous interglacials seemed to have peaked at a slightly higher temp and collapsed back into a glacier state faster than this interglacial. Can slightly higher temps set up conditions which trigger the collapse back into the glacier state? I honestly don’t think anyone knows this answer with any useful degree of certainty.

  37. 137
    Aaron Lewis says:

    Re 123 Hank,
    I use the Feynman method – I walk around screaming and waving my arms until I have THE correct answer. For example, in 2002, my answer was that the Arctic Sea Ice would be substantially seasonal in 2012. All the respectable climate guys said I was crazy, but that was sort of obvious.

    The point is: that still looks like a better number than I could have gotten from the peer reviewed literature at THAT time. It may even be as good a number as you can find in current literature.

    By the by, the answer that I got in 1999, was that fisheries were in deep trouble. Where was the peer reviewed literature? Peer reviewed literature follows problems, it does not lead on them.

  38. 138
    Doug Bostrom says:

    Ron Broberg says: 18 May 2010 at 6:39 PM

    A friend of mine states that if AGW prolongs the Holocene interglacial it will be mankind’s greates achievement. He has a point.

    Hopefully our descendants 50,000 years from now who would most likely actually be affected by the next stade will remember to thank us, even as the intervening generations ask themselves, “WTF was with my ancestors, that they were such happy-go-lucky destructive sociopaths?”

  39. 139
    David B. Benson says:

    Ron Broberg (132) — Archer & Ganopowski “Moveable Trigger” (2005) [Available from David Archer's publications page.] suggests that the next possible stade (massive ice sheets) is about 50,000 years in the future providing we don’t add much more CO2. It is quite a fine point as to whether or not AGW kept the globe from plunging into a stade starting about 2000 years ago. Careful reading of A&G suggests not, as does Crucifix & Rougier (2009). Bill Ruddiman seems to have some papers forthcoming showing the opposite; I’m looking forward to reading those contributions.

  40. 140
    Edward Greisch says:

    Extreme rain event: From local TV news this morning: A Mercer County farmer planted corn this spring. It washed away. He planted again. It washed away again and his fields are under water now. If it dries out soon enough, he will plant a third time. Eventually, he will run out of money to buy seed.
    I never heard of having to plant the same field 3 times to get 1 crop before. This is the third year in a row of too much rain here. This has been the corn belt for more than a century, but now it seems to be a belt for something else.

    GW means the wind keeps on shifting, so the rain keeps on moving. I think the “keeps on moving” is the answer to “AGW caused which was it? flood or drought?” AGW causes both, and the “where” keeps moving. The wet climate can only stay in one place if the global temperature remains constant. If warming is continuing, obviously the rainy climate is not going to stay in any one place for a long time. Bands of wet climate are moving past you. So bands of dry climate are also moving past you. That is how agriculture can be driven to collapse: The farmer has to have a constant climate so he can decide whether to plant rice or cactus. A changing climate creates chaos for the farmer.

  41. 141
    Pekka Kostamo says:

    Re 136: What strikes me about the graphs of ice age temperature records is that they do show such very steep rates of change, both warming and cooling.

    The patterns are not similar to the forcing patterns that result from orbital changes, though the overall causation has been established.

    IMHO this proves beyond reasonable doubt that there exist strong tipping points in the feedback loops, both directions of change. Because of the AGW, the natural tipping point(s) towards cooling will not be passed this time, and there will not be another ice age for a very, very long future (like many millions of years).

    It is really the “anthropocene” era that now begins. Must be very interesting times for the geologists – wittnessing and directly recording such a remarkable transition.

    It is another matter that there probably are still other and different tipping points within the warm new regime our world is entering. Those points we do not yet reliably know, unfortunately.

    OTOH, the discussion of ice ages is mostly a diversion to draw away attention from time scales relevant to current policy wars. Who cares? 100 years into the future is already too long for most people – that being an optimistic view of a family’s lifespan.

  42. 142
    Hank Roberts says:

    Lawrence, don’t forget methane oxidizes to CO2 — I don’t know how fast that happens in water, but probably as fast as it does in air — and the CO2 is what definitely is increasing the ocean pH.

    I don’t know how much difference a change in methane bubbling out will make, but it will definitely _end_up_ contributing to the change in pH of the ocean via CO2.

    And that pH change is utterly predictable and going to reach a troubling point well before the temperature change is going to worry people.

    Google “Jeremy Jackson” “rise of slime” — that coastal real estate may be unattractive due to toxic algae producing nasty stuff in the air long before the sea level rise matters much. It’s already happening many places.

    So I wasn’t saying not to worry; just noting a detail in how it’s happening.

  43. 143
    naught101 says:

    It would be great to see a post of RC about the Interacademy Council IPCC review. http://reviewipcc.interacademycouncil.net/comments.html

    My points were:
    * The IPCC needs to report more frequently. Interim reports, or even annual updates would be very useful.
    * More focus on possible tipping points. Especially estimates of sea-level rise from glacial melt, and estimates of non-linear responses to warming.
    * More transparency with the process – especially which representatives are making which changes to the finial release.
    * Stop being so conservative. Offer an your analysis, and be prepared to defend it when it gets attacked by the fossil fuel lobby and governments.
    * Work with science communicators. Create a lay-person’s version of the report.

    It’d be great to see RC’s take on the whole thing. I know the review probably isn’t really necessary, but while it’s happening it’s a chance to improve on what we already have. What improvements to the IPCC would RC contributors like to see?

  44. 144
    Lawrence Coleman says:

    Hank: My handle on chemistry is not quite adequate to know whether CH4 oxidises to CO2 and then to carbonic acid in seawater and to what effect NaCl have in the equation. or whether it goes straight from CH4 to carbonic acid? I thought there would have to be some lowering of the ph to the CH4. What is unclear also is whether the arctic ocean currents would divert the relatively more acidic water south bypassing the main krill/planckton breeding grounds since the CH4 sediment release is concentrated in a few localised areas?. This to me is a big issue as a compromised krill /planckton breeding ability will have massive effects right through the oceanic food chain. I’ll read your ‘rise of slime right now…
    Side note: the recent volcanic eruptions from iceland’s unpronouncible volcano may well have been caused or triggered by a decreasing ice sheet thickness causing the bedrock and land underneath to actually rise upwards due to less weight from the ice above. This is happening in greenland right now…currently at 1 inch/yr and accelerating. What do you think?

  45. 145

    ScienceDaily: Land under Greenland ice sheet rising at accelerated rates, indicating accelerating ice loss: http://bit.ly/Landice

  46. 146
    Completely Fed Up says:

    “Peer reviewed literature follows problems, it does not lead on them.”

    Uh, the problem of AGW was written in a paper before people had seen it in measurements.

    I think you’re waving your arms around and screaming again…

  47. 147
    Completely Fed Up says:

    “Oh, CFU, if you’re thinking heat = drought you obviously haven’t spent much time in warmer climes.”

    Oh dear, Frank. Your memory is slipping.

    We’re not talking about heat=drought. We’re talking about a drought that was happening getting worse because it’s warmer.

    Are you REALLY saying that droughts get less severe if you cool the area??? Interesting effect…

  48. 148
    Completely Fed Up says:

    “Pointing to specific weather events and declaring they would be worse or more severe if climate change wasn’t happening is just flat wrong.”

    Here’s how it works Frank.

    There’s a drought.

    This means not enough rain to replenish near surface water.

    Now if you make it WARMER, there’s more evaporation, yes?

    Now that makes it even drier, doesn’t it? I mean, less water in the ground is a drought, yes?

    So a drought happening is made worse by warmer weather.

    AGW causes warmer weather.

    Therefore AGW makes a drought worse.

  49. 149
    Completely Fed Up says:

    I guess Frank is OK with drugs in sports, because you can’t point to someone winning having performance enhancing drugs and say that the drugs did that.

    So too RodB and CM.

    All fine with drugs in sports because you can’t point to an event that depends even partially on the effects of a change and say that the change made the event more evident.

  50. 150
    Gilles says:

    LC 144 : NaCl has absolutely no influence on oxidizing. Actually if oceans were in steady state, the CO2 equilibrium concentration depends only on the total amount of CO2 in the atmosphere+Oceans, and the CH4 would only matter following its global released amount – whether it is oxidised in water or in the air is immaterial. But it’s more probably a complicated problem including diffusion throughout the volume. But oxidizing CH4 requires.. oxygen, and I doubt that oxygen concentration is very high below the surface, since it must be regenerated by photosynthesis and solar photons are rather rapidly absorbed. So probably most CH4 is released into the atmosphere before being oxidized in water.


Switch to our mobile site