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How much of the recent CO2 increase is due to human activities?

Filed under: — group @ 7 June 2005 - (Français)

Contributed by Corinne Le Quéré, University of East Anglia.

This question keeps coming back, although we know the answer very well: all of the recent CO2 increase in the atmosphere is due to human activities, in spite of the fact that both the oceans and the land biosphere respond to global warming. There is a lot of evidence to support this statement which has been explained in a previous posting here and in a letter in Physics Today . However, the most convincing arguments for scientists (based on isotopes and oxygen decreases in the atmosphere) may be hard to understand for the general public because they require a high level of scientific knowledge. I present simpler evidence of the same statement based on ocean observations, and I explain how we know that not only part of the atmospheric CO2 increase is due to human activities, but all of it.

On time-scales of ~100 years, there are only two reservoirs that can naturally exchange large quantities of CO2 with the atmosphere: the oceans and the land biosphere (forests and soils). The mass of carbon (carbon is the “C” in CO2) must be conserved. If the atmospheric CO2 increase was caused, even in part, by carbon emitted from the oceans or the land, we would measure a carbon decrease in these two reservoirs.

Number of observations of carbon decreasing in the global oceans: zero.

Number of observations of carbon increasing in the global oceans: more than 20 published studies using 6 independent methods.
The methods are:
(1) direct observations of the partial pressure of CO2 at the ocean surface (Takahashi et al. 2002),
(2) observations of the spatial distribution of atmospheric CO2 which show how much carbon goes in and out of the different oceanic regions (Bousquet et al. 2000),
(3) observations of carbon, oxygen, nutrients and CFCs combined to remove the mean imprint of biological processes (Sabine et al. 2004),
(4) observations of carbon and alkalinity for two time-periods combined with an estimate of water age based on CFCs (McNeil et al. 2002), and the simultaneous observations of atmospheric CO2 increase and the decrease in (5) oxygen (Keeling et al. 1996), and (6) carbon 13 (Ciais et al. 1995) in the atmosphere.

The principle of the last two methods is that both fossil fuel burning and biospheric respiration consume oxygen and reduce carbon 13 as they produce CO2, but the exchange of CO2 with the oceans has only a small impact on atmospheric oxygen and carbon 13. The measure of atmospheric CO2 increase together with oxygen or carbon 13 decrease gives the distribution between the different reservoirs.

All the estimates show that the carbon content of the oceans is increasing by ~ 2±1 PgC every year (current burning of fossil fuel is ~7 PgC per year). One method is able to go back in time and shows that the carbon content of the oceans has increased by 118±19 PgC in the last 200 years. There is some uncertainty about the exact amount that the oceans have taken up, but not about the direction of the change. The oceans cannot be a source of carbon to the atmosphere, because we observe them to be a sink of carbon from the atmosphere.

What about the land biosphere? We know that deforestation has contributed to the increase in atmospheric CO2. Yet because carbon needs to be conserved, observations of the carbon increase in the atmosphere and the oceans combined with estimates of fossil fuel burning tell us that deforestation has been largely compensated by enhanced growth by the land biosphere. For example, during 1980 to 1999, fossil fuel burning was 117±5 PgC, and the carbon increase in the atmosphere and the oceans were 65±1 and 37±8 PgC, respectively. Thus that leaves 15±9 PgC that has been taken up by the land. This 15±9 PgC includes deforestation (and other land-use changes) which reduced the land biosphere by 24±12 PgC, and an additional land uptake of 39±18 PgC in response to elevated CO2 and climate changes (Sabine et al. 2004). Here also there is some uncertainty about the exact amount, but there is no uncertainty that the land biosphere has taken up a quantity of CO2 that is roughly equivalent to the deforestation.

Why are the ocean and land taking up carbon, when we know that warming of the oceans reduces the solubility of CO2 and warming of the land accelerates bacterial degradation of the soils? The answer is that warming is not the only process that influences the oceans and land biosphere. The dominant process in the oceans is the response to increasing atmospheric CO2 itself. If the oceans had not warmed, they might have taken up even more carbon, although we cannot say for sure because warming may have other impacts, for example on marine biota. On land, bacterial degradation of the soils may have increased in response to warming, but for the moment this effect is smaller than the land response to other processes (for example fertilization by CO2 and nitrogen, changes in precipitation, etc).

Is this consistent with what we know of the glaciations? Yes. During glaciations, the balance of processes was very different. Cooling and other climate changes occurred first. The response of the oceans and land biosphere to climate caused the atmospheric CO2 to decrease, which caused more cooling (more on the feedbacks between temperature and CO2 can be found here). During glaciations, there were no external changes in atmospheric CO2 and the oceans and land biosphere responded primarily to climate change. In the last 200 years, there have been large changes in atmospheric CO2 as a result of human activities, and the oceans and land biosphere respond primarily to rising CO2.

In summary, we know that the rise in atmospheric CO2 is entirely caused by fossil fuel burning and deforestation because many independent observations show that the carbon content has also increased in both the oceans and the land biosphere (after deforestation). If the oceans or land had contributed to the rise in atmospheric CO2, they would hold less carbon. Their response to warming may be real, but it is less than their response to increasing CO2 and other climate changes for the moment.

More on the carbon budget can be found in the last IPCC report here, which includes budgets and uncertainties for different time periods and additional numbers for the small contribution of volcanoes and other geological reservoirs.

Bousquet et al. (2000), Regional changes of CO2 fluxes over land and oceans since 1980, Science, Vol 290, 1342-1346.
Ciais et al. (1995), A Large Northern Hemisphere Terrestrial CO2 Sink Indicated by the 13C/12C Ratio of atmospheric CO2, Science, Vol 269, pp. 1098-1102.
Keeling, Piper and Heimann (1996), Global and hemispheric CO2 sinks deduced from changes in atmospheric O2 concentration, Nature, Vol 381, 218-221.
McNeil et al. (2003), Anthropogenic CO2 uptake by the ocean based on the global chlorofluorocarbon data set, Science, Vol 299, 235-239.
Takahashi et al. (2002), Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects, Deep Sea Research, Vol 49, 1601-1622.

42 Responses to “How much of the recent CO2 increase is due to human activities?”

  1. 1
    sustainablog says:

    Global Climate Change and Human Activity
    From RealClimate, a “simpler” explanation on how we know “that not only part of the atmospheric CO2 increase is due to human activities, but all of it.”

  2. 2
    Hank Roberts says:

    Does “deforestation” count the loss of topsoil, or just the biomass of trees harvested? And how about the Great Plains and other grasslands also devegetated by agricultural practices?

    Most “topsoil” is living material and fecal pellets, with a small amount of mineral. I’ve seen how a decade after a fire or logging, landslides start, slumping of topsoil that had been knit together by the tree roots til they rotted out. And I’ve seen what remains after a very hot forest fire, enough to burn the topsoil — it’s gravel, and soot.

    Considering the many feet of topsoil that have disappeared down the rivers since the sodbusters reached the Great Plains — not to mention the Mediterranean and Asian areas reduced to mineral soil by sheep — I wonder if the trees removed in temperate area deforestation count for anywhere near as much living material as the soil they grew in and held in place.

    Soil info like this old article from

    “Mass extinction” is the term used by John Jaenike, University of Rochester ecologist, who is concerned that fungi also may be vanishing from the United States.

    The findings (or rather, the no-longer-findings) of Eef Arnolds–a fungal ecologist at the Agricultural University of the Netherlands, are described by Jeremy Cherfas on page 1458 of the Dec. 6, 1991 issue of _Science_. The scale of loss is shown by comparing surveys carried out in the Netherlands between 1912 and 1954, when an average of 71 species of fungus was found per foray, with the period between 1973 and 1982, when a matched series
    of 15 surveys could turn up only 38 species per foray.

    Both occurrence and size of mushrooms have plummeted. It took 50 times as many chanterelles to make up a kilogram in 1975 as it did in 1958. Arnolds rules out overpicking and forest management practices, because both edible and inedible mushrooms have declined and all types of mature forests show similar drops. …

    The loss of a gourmet item of human food is not nearly so consequential as the loss of the network of fungal filaments that live in close symbiotic association with trees…

  3. 3
    Joel Kuni says:

    Science magazine published an article in 1998 with data that suggests the North American continent is a carbon sink. (See Prevailing winds, which blow in an easterly direction over the continent, contain more CO2 just after they blow off the Pacific Ocean than they do after they exit the continent over the Atlantic.

    One of the theories that might explain this effect is the significant RE-forestation of the continent that has been occuring for more than a century.

    Your article says “We know that deforestation has contributed to the increase in atmospheric CO2.” This suggests a way to reduce CO2 levels. We should strive for a world in which the economic activities of people in all countries will resemble the economic activities of North American peoples.

    Rich people re-forest their land. Historically, becoming rich has required a capitalist, minimal-government system in which fossil fuels are burned in enormous quantities to industrialize and expand the economy. It may seem counter-intuititive, but the net result is apparently a decrease in the CO2 contributed by that economy to the atmosphere.

    It is the planet’s backward, stunted anti-capitalist agrarian economies that are apparently to blame for the atmosphere’s net increase in CO2. Their government leaders should be ashamed of themselves for the global warming they are causing.

    [Response: Except that….. the Fan et al (1998) study was pretty conclusively rebutted by Schmiel et al (2000), and is no longer considered to be valid. The general point that re-forestation is a carbon sink is a good one, but no conceivable amount of reforestation is going to be able to soak up all industrial emissions. – gavin]

    [And just to make the point one more time: The greater fraction of the CO2 in the atmosphere today is from fossil fuel burning (by rich countries), not deforestation -eric]

  4. 4
    Joel Kuni says:

    Thank you for your response to my comment (#3)! The Schmiel article you referenced includes these statements in its conclusion:

    “Inventory data also suggest a sink of the order of 0.3 Pg of carbon per year.”

    “If the total sink is about 0.3 Pg of carbon per year, and the CO2/climate sink is about 0.1 Pg of carbon per year, other processes such as regrowth on abandoned agricultural and harvested forest lands must cause a sink of about 0.2 Pg of carbon per year.”

    These statements seem to support my point. They confirm that North America is a carbon sink, due in part to re-forestation. Is there something else in the article that leads you to conclude otherwise?

  5. 5
    Thor Olson says:

    Is it possible to distinguish the CO2 contributed by the thawing of Arctic permafrost from what is attributed to the burning of fossil fuels and deforestation?

    This may seem like splitting hairs since I am aware that the age of the plant material & trapped methane in the permafrost often far older than the halflife of Carbon 14. I am led to believe that the melting of the permafrost may amplify the human related contributions like compounding interest. If this is a large effect then I would expect to see a continued rise in atmospheric C02 concentrations even after a flattening or mild reduction of human emissions.

  6. 6
    Steve Latham says:

    Before a ton of people respond to Joel’s post (#3) with sociological references describing how the powerful countries prevent the weak nations from escaping their economically backward conditions, I wanted to point out that I’ve never seen in any mainstream media reports that humans aren’t responsible for the rise in atmospheric CO2. On the “GW-is-scarier-than-you-imagine” side, I think I’ve seen mention of concerns that melting permafrost will lead to large releases of CO2 to the atmosphere as unfrozen organic stuff decomposes (+’ve feedback for GW). On the “controlling-emissions-is-silly” side, I know I’ve read arguments in the media that we’re only increasing the amount of CO2 (a natural compound, afterall) insignificantly in any given year.

    It’s very interesting to me that people so unworried about a few percentage points in terms of CO2 concentration are often associated with people who claim that everything is just right here on earth and that if our orbit were a bit closer to or further from the sun (& cetera) life couldn’t exist. I’m not sure if collecting examples of such seemingly at-odds viewpoints and studying the cognitive psychology involved could help people See It To Believe It. Right now, especially as regards ideas related to Global Warming, many people (on both sides) are very thoroughly entrenched in a Must Believe It To See It mindset. Hopefully articles written for laypersons like this one on the rather uncontroversial source of CO2 increase will help. Good luck with stuff about which it is easier for folks to fool themselves.

  7. 7
    dave says:

    Thanks for this review of anthropogenic contributions to atmospheric carbon levels. I had some observations to add about the current state of the terrestrial biosphere carbon sink:

    (1) A few years back, there was much discussion of the terrestrial “missing carbon sink”. Some papers at that time (e.g. A large carbon sink in the woody biomass of Northern forests) determined that the “missing” sink was mostly in the Northern Hemisphere forests taking up about .7 gigatons C/year. But there is a problem. Here’s a quote from Stephen Pacala of Princeton (from USA Today in 2001):

    Past land use released CO2 to the atmosphere, some of which is now being taken back. Because the sink is affected by land use, it is going to decrease in the future as ecosystems complete their recovery…. When the growing forests mature, the amount of carbon they can store will reach its limit, Pacala says. He expects the sink to go away in the next 50 to 100 years.

    His estimate (50 to 100 years) may be optimistic. I don’t think the data is in on this one (see the next point below).

    (2) CO2 fertilization claims are discredited. For example, this abstract by Pacala states that

    We analyze tree growth data from Wisconsin forest inventories completed in 1968, 1983, 1996 and 2002. These show that the rate of forest tree growth decreased steadily over the period, in contrast to the increases predicted by CO2 fertilization models. Measured growth rate changed an average of -0.27% y-1 (95% confidence range: -0.05% to -0.49% y-1), whereas the prediction for CO2 fertilization is 0.16% y-1 (corresponding to a � of 0.36).

    Slower growth means less CO2 uptake on short time scales (they mature more slowly, or never reach full maturity).

    (3) Deforestation proceeds apace in the Amazon and other tropical rainforests. For example, this article Brazil Fells Massive Amazon Timber Fraud Ring states that

    The Ministry of Environment said that deforestation of the Amazon has increased 39 percent since 2002. The agency said the total deforested area covers 26,130 square kilometers.

    Amazon deforestation is taking out about 10k/square miles a year. Now, that’s a land use change. See also this Reuters report here. Obviously, the rainforest regions become a one-time CO2 source when they no longer exist.

    (4) What about the Arctic terrestrial regions? (cf. Thor, #5) Research is just now being carried out to find out what’s going on up there. The recent result Arctic Lakes Vanishing as Planet Warms tells us that the permafrost is melting (these lakes were held up by the permafrost beneath them – they just disappear into the water table below). There’s a whole lot of carbon up there stored in the peat and other frozen soils. If this long-term sink (over the Holocene) becomes a source, we’ve all got less time than we thought for curbing CO2 levels in the atmosphere. A lot of this becomes disolved inorganic carbon (DIC) carried by rivers into the Arctic ocean.

    Sorry to go on so long, but these are important issues re: the rate of accumlation of CO2 in the atmosphere.

    [Response: With respect to the news item about Arctic lakes, the actual scientific article is in Science, here -eric]

  8. 8
    Joel Shore says:

    In regards to #4, you seem to be misinterpretting the quote from that paper. The paper is on carbon sinks and so it only discusses sinks and not the net amount adding to the atmosphere when one considers BOTH sources and sinks.

    In particular, while North America may have sinks totalling a few tenths of a Pg of carbon, the total world emissions of carbon from fossil fuels are something like 5.5 Pg, of which North America accounts for something like 25% or more…i.e., ~1.4 Pg.

    So, while your political biases may want it to be scientifically correct that the “backward, stunted anti-capitalist agrarian economies” are to blame and that capitalism is the savior, the science disagrees with you. [And, it is not surprising that this is the case, given that we have had agrarian economies for millenia with at most only a modest increase in CO2 concentrations in the atmosphere.]

  9. 9
    Ned Ford says:

    I want to echo Joel, #7, in reminding people to start with the total carbon mass of fossil fuel emissions. If you know that the total mass of fossil fuel emissions is roughly double the total annual atmospheric accumulation it’s a little easier to realize that all the other possible explanations are besides the point, even if there is a little source here and a little sink there.

    The comments about rich versus poor give me an opportunity to replay a letter I wrote to a newspaper yesterday – a scientist from a corporation with a very strong climate policy mentioned that affluence increases per capita emissions. I replied that this is not true, because U.S. per capita emissions peaked in 1982, and world per capita emissions peaked in 1979 ( Simplistically, this means that population is the main driver of emissions growth.

    But affluence also reduces population growth. So I remind everyone that the reduction in emissions per capita to date is due to improvements in energy efficiency, and that if we want to find a fast and painless way to increase the rate, we should take a more serious look at the one set of trends that are working in our favor.

    The potential for efficiency remains enormous, and given the likely improvements in technology and changes in societal norms over the next century which it will take us to do the right thing, we are likely to be able to cut fossil fuel use further than most people imagine possible, even if renewables don’t become commercially competitive (which wind is already, and solar is in certain situations).

  10. 10
    Joel Kuni says:

    The Fan paper noted that North American fossil emissions of carbon were 1.6 Pg/yr in 1990. It then presented evidence supporting an estimated carbon uptake of 1.7 +/- 0.5 Pg/yr, which exceeds the emissions. This would imply that the rest of the world owes the U.S. a debt of gratitude for locking away more carbon than it emits.

    The Schimel paper used an entirely different modeling technique, concluding that “the estimates from the VEMAP models are an order of magnitude less than the high atmospherically based estimates of Fan et al.” This implies that the U.S. emits more carbon than the continental sink absorbs.

    Schimel et al. conclude by stating “Inventory and model results are in conflict with high estimates from atmospheric inverse estimates [of Fan, et al.]. The next steps in the quantification of the North American carbon sink will require additional observations.”

    They do not conclude that they are right and Fan is wrong. They conclude that more observations are necessary.

    I am eager to review more scientific evidence that would resolve the discrepancies between these models. Until I see such evidence, I am unwilling to dismiss either one of them or to conclude that the North American sink only totals a few tenths of a Pg of carbon. Doing so would require a leap of religious faith. I don’t do religion.

    With regard to the assertion that agrarian economies have contributed very little to atmospheric CO2, did you happen to read the cover story of the March 2005 Scientific American?

  11. 11
    Pascal says:

    thank’s for your paper

    Some sceptics I know argue that we don’t know ,in the considered period ,the amount of excess CO2 from natural sources as volcanoes , permafrost ,…
    To make the CO2 balance it’s important to know this point.
    Have you an estimation of the natural fluxes and is there an anomaly on that period?
    Even if this paper concerns the sink and not the sources the same sceptics can say that we must know all the fluxes to make a correct CO2 balance.

  12. 12
    Steve Bloom says:

    Re #10: Before everyone trips over themselves congratulating Americans for leading the way out of the impending climate crisis, it would be useful to contemplate the overall U.S. carbon footprint. So many of the goods we consume are produced elsewhere (e.g., Chinese goods sold by Walmart, the numerous polluting industries that were forced to relocate from the LA basin to Mexico in consequence of the push for improved air quality on this side of the border, and the fact that the much of the Brazilian rain forest massacre is driven by U.S. demand for lumber, soybeans and beef) that it’s misleading in the extreme to only consider the direct U.S. carbon balance.

  13. 13
    Terry says:

    To make this thread more persuasive, you should quote some skeptics who think that the rise in CO2 is not attributable to human activity. Best would be some quotes from World Climate Report or SEPP. If you can’t find any there, you should at least cite to Cato or AEI or some more popular source with a disclaimer that skeptics working as climate scientists do not believe this.

  14. 14
    Joel Shore says:

    In regards to #10, I think some problems with the Fan paper, besides the fact that its conclusions are at odds with the Schimel work, is that:

    (1) I don’t think there are any known mechanisms that would explain having nearly so large terrestrial sink in N. America.

    (2) It would be difficult to explain why this sink exists in North America and not elsewhere and to make it consistent with the known facts regarding the buildup of CO2 in the atmosphere from the fossil fuel emissions.

    As for that cover story in Scientific American, my wording that “we have had agrarian economies for millenia with at most only a modest increase in CO2 concentrations in the atmosphere” was chosen carefully to not contradict the hypothesis presented in that article (which is, at any rate, still quite speculative).

  15. 15
    dave says:

    Re: the US terrestrial carbon sink

    Looking at the abstract by Pacala et. al. (Science, Vol 292, Issue 5525, 2316-2320 , 22 June 2001), they found the carbon sink “in the coterminous United States [to be] between 0.30 and 0.58 petagrams of carbon per year” for the 1980-89 period. The US is responsible for 25% of the world emissions, where, as Corrine states, “during 1980 to 1999, fossil fuel burning was [responsible for] 117±5 PgC. You can do the math. (Try a Google search for “Pacala carbon sink” to get the abstract text, I could not get a working link to post here.)

    Also, look at Projecting the future of the U.S. carbon sink by G. C. Hurtt, S. W. Pacala et. al. The conclusion is:

    This study builds on previous studies that have anticipated the decline in global carbon sinks generally and that of the U.S. forest sink in particular. The projections here include all lands (both forested and nonforested) and highlight the uncertainties associated with the long-term effectiveness of fire suppression. We find that without dramatic increases in the area of forests, without substantially positive changes in land-use practices, without large net positive effects of CO2 or climate change in the future, or without some other new significant carbon storage mechanism, the U.S. carbon sink itself will decrease substantially over the 21st century.

    So, the US carbon sink is relatively small and projected to decline over the 21st century. And the paper I cited in part 1 of #7 states that “The results, which cover the years 1981-1999, reveal a picture of biomass carbon gains in Eurasian boreal and North American temperate forests and losses in some Canadian boreal forests”. Also, 70% of these gains (above 30N) were in Eurasia.

    Therefore, 1) North America does not contribute CO2 to the atmosphere and 2) Pigs can fly.

    [Response: An important point to clarify here is that the reason the sink will diminish is that whatever carbon does sink exist in the US is from forest regrowth, following the 16th – early 20th century clearcutting of New England. Once the forests are regrown, then the sink stops working. Of course, we can always cut down the forests again, and if we can find a way to do that without emitting any CO2, then we would have created a new sink. Fortunately for the forests, things don’t work that way, so we won’t probably won’t be clearcutting forests in the name of stopping global warming! — eric]

  16. 16
    Dan Allan says:

    Regarding #10:


    You seem to accept that (a) the planet is warming, (b) this is bad, (c) it is due to increased atmospheric carbon, and (d) this increase is due to humans burning fossil fuels. All of these are huge concessions to us eco-terrorist crazies who have no idea what we’re talking about. Now you seem to have shifted the argument. But your point, as far as I can see, is contradictory:

    (a) there is increased atmospheric carbon due to industrialized economies burning fossil fuels – granted,as far as I can tell, in your statements.

    (b) agrarian economies are to blame for global warming, because they have deforested the land more than industrialized countries (an unproven assertion, but we’ll let it pass) and so the earth is not able to absorb the increased atmospheric carbon that industrialized countries are pumping out.

    Presumably, if we follow your argument that further wealth / industrialization will descrease atmospheric carbon, we should also believe that if we were to return to an agrarian economy, this would accelerate global warming. A pretty strange notion.

  17. 17
    Eli Rabett says:

    The arguments about the role of northern boreal forests also apply to Siberia which is a useful thing to keep in mind when looking at carbon balance.

  18. 18
    dave says:

    Re: Eric’s comment #15

    Forest regrowth, yes. Deforestation in the US was particularly intense until some measures started to be taken early last century. When Europeans first got here, supposedly you could walk from the Canadian border up in New England to the Gulf of Mexico and never leave the forest. Aside from clearing land for farming, American forests supplied Europe with wood. Here’s the publisher’s comment for Douglas MacCleery’s American Forests: A History of Resiliency & Recovery at Powells book store.

    MacCleery recounts how settlers removed much of the American forest for agriculture and commerce during the 19th century. At the beginning of the 20th century, though, demographic changes and an emerging conservation movement helped reduce wildfire and encourage reforestation. Today there is more forestland in the U.S. than there was 75 years ago.

    I had mentioned deforestation in the Amazon in #7 but we should not neglect to mention current forest loss in the US (and Canada) due to suburban sprawl (Exurbanization!) and logging, especially clearcuts. For example, American Forests reports that “work in the Puget Sound, Atlanta, and Chesapeake Bay regions has revealed a stark fact: the heavy tree canopy in all these areas has declined by more than one-third in just twenty-five years”. In addition, previously roadless areas will no longer be protected. This ususally leads to new logging. And rules are changing concerning wildfires. Of course, this is a big subject.

    Many factors will contribute to the projected shrinkage of the North American carbon sink.

  19. 19
    Michael Jankowski says:


    Pacala found in the Science article you mentioned was that the US emits 2-4 times as much C as can be taken-up by US terrestrial components each year . If you “do the math,” it comes out more like 3-5 times, but still close.

    It should be noted that his work was land-based, and land-based studies reportedly tend to have lesser sink values than air-based ones.

  20. 20
    Hank Roberts says:

    Dave wrote:
    > Once the forests are regrown, then the sink stops working.

    Not so, I think, except for the Amazon and the like. That statement ignores the biomass tied up in soils, which in North America at least may take as long to recover as they took to form after the last ice age.

    Personal example from experience: I’m doing restoration on a N. Ca. Coast Range site that — per the Forest Service hydrologist — lost a foot of topsoil in the past century and how has about 2/3 of an inch remaining. (Look at the lichen, which takes a century to reach full size — note how the size decreases down to the band of bare rock just above ground level. That’s one quick check.)

    Loss due to logging, then fire, then sheep grazing in the late 1800s til the sheep started to starve (grin), then lying fallow til trees recovered, then logging, then fire. With each change more soil disappeared.

    The Forest Service folks said my site looks a lot like areas nearby that lost that last fraction of an inch of topsoil, and its associated seed bank, and now are gravel and talus slopes on which nothing grows. The roots rot out from between the rocks, the mycorrhizal web breaks down unsupported, water washes through the crevices, and you lose not only the topsoil but the soil that built up in between the broken rock on the mountainside and held the slow moving landslides on slopes steeper than their angle of repose.

    Ten years after a fire, I can go around my site, take an eight foot half inch steel rebar, pound it hard to get it through the top six inches of rock-and-gravel-and-grass-roots-and-manzanita-roots, and then push it by hand down through the underlying talus slope from which the deeper tree roots have rotted out. It’s amazing to feel it going clunk-clunk-clunk as the rebar pushes down through a talus slope that had been more or less inflated by the infiltration of roots and mycorrhiza and soil growing into the voids there. All the deep roots die after a really bad fire, or after logging.

    Think of “topsoil” as a souffle — living material and gravel. Slam the oven door and it falls.

    Take the area of the forest, times a foot of soil — lose that, lose all the biomass in it. Trees will still grow. The Forest Service (Department of Agriculture) still tends to bulldoze an area flat, pile up everything that will burn, burn it, and plant Ponderosa Pine out here. But they’re starting to take steps to hold the powdery red dust left by that approach, because they see it gets washed away by the next winter’s rain leaving their little P-Pines standing in gravel. Sure, the P-Pines grow faster with no competition. But the Forest Service catches deserved hell from the Salmonid restoration people who are dealing with the runoff from “farming” on steep dry hot slopes. And they’re now leaving brush spread around to turbulate the sheet flow of wind and water across the slopes and catch the bits of leaf and twig and seed that otherwise get washed away with the dust.

    We know from the Amazon that most of the biomass there is in the living trees and forbs atop poor mineral soil. But it’s flat there, at least. Any loose mineral material doesn’t go all that far before some living thing can incorporate it. And they haven’t had a recent glaciation to leave the area covered with fine rock flour to work with.

    But look at, say, Wisconsin, with many meters of loess blown there as the glaciers retreated, then populated as plants took over the area, for ten thousand years. Deep, deep soils. Same for much of the plains states. And those soils are now broken up and to a great extent washed away after civilization and its improvements arrived. Even the European earthworms are damaging the forest there, converting every bit of leaf litter and duff into nice, soluble worm castings — that wash away.

    In temperate forests a LOT of the biomass is not in the standing trees — remember, there is far more life in a “dead” tree than in a “living” tree. It’s only when it starts to rot that the inner core becomes once again living material fixing more carbon, instead of a dead structural center around which the thin living cambium is wrapped.

    I’m an amateur at this. I can’t tell you anything more specific and I don’t know how much of this is available and well known. But I’m sure that a first regrowth of forest to “mature old growth” 100 years or more old is going to happen on whatever little bit of topsoil remains — but those trees have to die to build more topsoil, over and over, for a MUCH longer period of time before you have anything approximating the amount of stored carbon that would have been there before the first loggers arrived.

    Uh, /rant off/ ….

    [Response: If I may summarize your point, the question is how the rate of net biomass accumulation changes as an ecosystem matures. Certainly this is a good question, and there is a lot left to learn about soils and how they store carbon. However, even if net accumulation of biomass continues for a very long time (and produces very deep soils), the rate of accumulation is going to be steady at best — it is certainly not going to increase as the forest matures. And of course when the accumulated biomass reaches what it was when Europeans arrived in North America, well, at best you’re back to where you started. You still won’t have taken more CO2 out of the atmosphere than was originally put in. -eric]

  21. 21
    Joel Kuni says:


    Yes, I speculate that if we were to return to an agrarian economy this might accelerate global warming.

    By “agrarian economy” I mean one that operates without fossil fuels.

    Imagine six billion people trying to feed themselves and keep themselves warm without burning fossil fuels or using fertilizers derived from or produced using those fuels. Vast amounts of forest land would have to be converted to additional farmland and to pastures in order to feed all the beasts of burden who pull the plows, etc. I imagine the remaining forests, if there are any, would quickly be destroyed for firewood.

    Would the destruction and burning of the world’s forests increase global warming? I imagine it might.

    Increasing industrialization and wealth, by contrast, will probably lead to improvements in technologies that replace fossil fuels, thus reversing global warming in the long run (assuming that goal is desired). Isn’t the technology for producing nuclear power already advanced enough that it could theoretically provide all of mankind’s energy needs?

    By the way, I have not been persuaded that global warming is bad. I recognize there are costs and drawbacks that will result from significant global warming, but there are benefits as well (e.g., fewer deaths from hypothermia in the high latitudes, improved agricultural output, improved climates in various regions, the flooding of IRS headquarters in Washington, D.C. etc.)

    Whether the costs outweigh the benefits is not a scientific question. It is a value judgement. Some people will enjoy global warming and others won’t. (It amuses me that the largest group of people currently enjoying the benefits of global warming are climatologists and employees of environmentalist organizations, since their incomes depend upon it.)

    [Response: Whether anything is “good” or “bad” is of course a value judgement. But there are legitimate scientific questions that can be (and many have been) addressed, that may determine (given a particular value system) whether climate change is “good” or “bad”. For example, a ski resort has a legitimate interest in knowing whether there will still be snow in 10, 20, 100 years in their area of operation. Clearly, if the answer happens to be “no”, then for that particular business, climate change is “bad”. There will undoubetdly be others (e.g. lift-assisted mountain bike tour operators who currently can only use the ski resort in summer) for whom it is “good”. The science of climate impacts is therefore entirely legitimate, as science. Or to put this another way, in many cases the question of costs vs. benefits is a scientific question. If you want to learn more, see –eric ]

  22. 22
    Dano says:

    Re #16:

    It’s been shown that agriculture actually cools temperatures, by deforestation changing the albedo.


    Rule of thumb is ~ 500 years/inch topsoil. That’s a lot of carbon.



  23. 23
    Bill Perkins says:

    This is a fascinating website that I only discovered today. Obviously I’m climatology ignoramous, but I want to thank the professionals for providing this educational posting and the many probing commentators that help flesh out the issues .

    I will search the site further and I find it mezmerizing.

    But a basic question I have not seen addressed in this article or its referenced predecessor article herein regarding the CO2 concentration increases (“from 280 to nearly 380 ppm” in the last 150 years) is what the impact is on global warming. Obviously, this 35% increase in CO2 doesn’t mean the temperature goes up 35%. I am aware there are other greenhouse gases that also have an impact. What is the correlation and what is prognossis if this trend continues?

    Also, all of this seems to be a good argument for expanding nuclear energy production as an alternative to fossil fuels for generating electricity. Is that a legitimate alternative, albeit with its own hazards, with respect to greenhouse gas production?

    Thanks for your patience.

  24. 24
    Pascal says:

    I’m not a scientific of Realclimate but some ideas to answer.
    The radiative forcing of CO2 is a heat flux in W/m2.
    The delta of this flux between 100 and 1000 ppm of CO2 can be approximated by : delta F = 5.3* log(C/C0)
    Where C is the actual (for example) CO2 concentration equal 365 ppm
    and C0 = 280 ppm .
    So the delta F = 1.4 W/m2.
    The others GES as CH4, CFC , NOx ,O3,…together have a delta F of approximately 0.4/0.6 W/m2.
    So the total increasing of delta F is about 2.0 W/m2.
    This delta flux increases the temperature and consequently increases the water vapour concentration in the atmosphere.
    This vapour is also a GES and increases the 2.0 W/m2 with a multiplication factor of 1.6 /2.0.
    The delta flux becomes around 4.0 W/m2.
    The resulting delta temperature is ,in application of Wien law,

    delta T = 0.1885 delta F = 0.75 °K (for a black body)

    In fact the delta T is below this value because thermal inertia of oceans ( 0.85W/m2). So the delta T is around 0.6°C.

    I hope some corrections of this calculations from Realclimate guys.

  25. 25
    Lynn Vincentnathan says:

    In response to arguments that rich U.S. helps reduce CO2 – even if it were true, it would be misleading. We live in a global economy. Check where all your products are made. The buyers/consumers are ultimately responsible for the CO2 & other pollutants generated in their manufacture, even if they were manufactured elsewhere. And how did we Americans come to possess our great land with great carbon sinks, & get so rich in the first place?? That’s several whole other issues.

    I’m reminded of the contrarian argument that the CO2 humans are emitting is all aborbed, & it’s the natural CO2 sources that are causing the increase in the atmosphere. Sort of like creative accounting.

    [Response: Amazingly enough, one hears this argument all the time (though I’ve never seen it actually written down, presumably because it would be too easy a target). The isotopic data disprove this idea definitively. See our earlier post on this, and the excellent letter in Physics Today.- eric]

  26. 26
    Michael Jankowski says:

    “I’m reminded of the contrarian argument that the CO2 humans are emitting is all aborbed, & it’s the natural CO2 sources that are causing the increase in the atmosphere”

    I’ve never seen/heard that argument before.

    The only argument I’ve seen along those lines is the one that claims that the vast majority of the CO2 humans are emitting is sequestered in soils, oceans, etc, or used by plants, but that “naturally” warmer temps result in the release of sequestered CO2. At least, that’s along the explanations I’ve seen for why historical changes in CO2 levels often seem to lag corresponding temperature changes.

    [Response: That’s essentially the same argument. In any case both arguments are wrong. -eric]

  27. 27
    Robert says:

    Regarding the isotopic data mentioned above, where in the carbon cycle is
    carbon-14 created (and how)? I googled, but couldn’t find the answer.

    Thanks in advance.

  28. 28
    paul gosling says:

    Re 22#

    I am not sure how they come to this conclusion. For most of the time corn is green. It is only yellow for a few weeks of the year. Then the land is ploughed and the dark soil is exposed.

  29. 29
    Benjamin Harrison says:

    Re: #27

    The isotopic data evaluated above is derived from the fractionation of the naturally occuring stable isotopes of Carbon, 12 and 13. Carbon 14 is not mentioned, though it can be used in a somewhat similar manner. 14C is generated through the reaction of 14N with a neutron in the upper atmosphere generated through the collision of O2 and N2 molecules with cosmic rays (generally high energy H and He nuclei). Changing atmospheric concentrations of 14C are notably driven by many of the same processes, but with the added source of cosmogenic production.

  30. 30
    Dan Allan says:


    A couple of points re your post #21:

    First, there is room for disussion regarding whether a modest amount of global warming is bad, but I don’t believe there can be much debate that a large amount of global warming would be horrible – there is no easy work-around for the relocation of a third of the world’s major cities due to sea-level rise.

    Affluence probably does correlate to an interest in finding alternative energy sources. But that is because governments in wealthy countries have imposed high taxes on gasoline, or forced environmental regulations on industry – such as those demanded by Kyoto. So presumably you support these taxes and regulations? Because I think it would be kind of hypocritcal to say yes, we should be allowed to burn unlimited fossil fuels now in order to generate the affluence needed to produce a cleaner environment, and then oppose the laws passed by affluent countries that will yield a cleaner environment.

    Regarding nuclear power – first uranium is a finite resource. second, there is no easy way to get a nuclear plant to power a car. somehow, you need to convert the energy into a battery or fuel cell, and there a variety of serious inefficiencies you encounter, depending on what technology you choose. third, it is necessary to site nuclear plants near population centers because the energy is gradually lost over miles of high tension lines. Finding many, many sites (and we’re talking at least in the many hundreds) near population centers, with a water source for cooling, is next to impossible. So, even if you are willing to accept the disposal issues, melt-down risk, eyesore factor, and thermal pollution to the water (and honestly I am not opposed to nuclear power – there are no ideal energy sources out there), it is only a very partial solution.

    – dan

  31. 31
    Hans Erren says:

    re: #26

    Rorsch A, Courtney RS, Thoenes D, 2005, Global Warming and the Accumulation of Carbon Dioxide in the Atmosphere, Energy & Environment, Vol. 16, No. 1. (January 2005), pp. 101-126.

    Not that I support their conclusions….

  32. 32
    Henry Molvar says:

    Re: Response to #25
    Dear Eric:
    You respond to Ms. Vincentnathan’s second paragraph (a straw man?) but not the first, which is amenable to scientific inquiry. It is the main point of her post,and by far the more interesting, with an illusrative example added. Is there a problem with doing a little outside the box research?

    [Response: See below.]

  33. 33
    Timothy says:

    #32 Henry: The points raised in paragraph 1 of #25 are indeed interesting and raise many important issues about how carbon quotas would be distributed and paid for/traded in a future world where carbon release was restricted.

    The reason Eric did not respond to them [I assume] is simply that this is a site for climate science and I think that the points raised in the 1st paragraph are more political in nature.

    [Response:That’s right. But there is another reason too. All of us at RealClimate have other things to do (like our real jobs!). We feel neither obliged to write, nor interested in writing, a response to each and every comment we get. This is regardless of whether we agree with it, disagree with it, think it appropriate or not. We simply don’t have the time. Earlier on in the site’s history, we might well have deleted a lot of the comments above for being completely off topic. However, we realized a) it took too much time and b) writers felt they were being singled out arbitrarily. In general our tendency is to respond to comments that are either in serious need of correction or clarification, or which we consider particularly cogent. Most other stuff gets left alone. And in no case can one attribute the lack of a response to any particular opinion about what was written. -eric]

  34. 34

    The combination of natural and anthropogenic CO2 fluxes clearly adds complexity to attempts to sort out the issue in few words, written or spoken.

    Given the economic tenor of many news stories , an analogy to inflation may be useful in clarifying the idea of slow but steady radiative bracket creep, as the CO2 forcing can be outlined in terms of its effect on the radiative balance, which reduces to watts/M2 and their rate of change .

    Am I correct in calculating that the present rate of growth in the anthropenic forcing from CO2 from fossil fuel consumption is currently somewaht greater than a milliwatt/ year, or about 4 microwatts per day ?

  35. 35
    Rick Collins says:

    What proportion of the CO2 produced on earth is due to the fact that there are more humans respirating today than 1,000 years ago?

    [Response: Zero. None. Nada. ]

  36. 36
    Henry Molvar says:

    #33 and response: Thank you Timothy and Eric. I understand and agree, somewhat red-faced!

  37. 37

    Global Warming
    On the issue of global warming, I am always on the lookout for simple explanations from the scientific community that can be used to communicate to hoi polloi why global warming is occurring primarily because of human activity (especially emissions…

  38. 38
    Luke Silburn says:

    Its interesting to see the discussion of the various carbon sinks across the planet and the fluxes between them. Has any work been done on estimating the sum total of carbon there is in the overall planetary system?

    It occurs to me that this is a useful framing question for the issue, so its something that has probably been done. Assuming this is so, what kind of quantities are we talking about; how does it break down across the lithosphere, biosphere, hydrosphere, atmosphere; and what kind of confidence do we have in these numbers? If it hasn’t been done, what is it that makes it not worth doing?


  39. 39
    Eli Rabett says:

    Luke, google There are many good discussions and lots of power point presentations.

  40. 40
    John says:

    Is it possible to separate the effect of fossil fuel burning from a release of oceanic carbon on atmospheric carbon 13?

  41. 41
    Eli Rabett says:

    Sorry Luke, that should have been google the term carbon cycle. I inserted what turned out to be a non printing set of characters around the carbon cycle to set it off. My mistake.

  42. 42
    CharlieT says:

    Re #28 and #22
    If the movement from dark forests to lighter coloured crops caused a cooling up to 1900 then this would surely all have been reversed since the 1960’s,in temperate climes? Modern whole-crop harvested cereals have been recorded as producing more biomass than willow plantations.
    (To do this they will have intercepted as much light.)
    -UK wheat nitrogen fertilser use increased from about 80Kg/Ha to 190Kg/Ha in the period 1965 to 1990, increasing both the leaf area and darkness of the canopies.
    Is there a rural heat-sea to go with the urban heat-islands?
    Is it only a negligible effect?

    [Response: You are correct, recent temperate reforestation has partly reversed the cooling due to land use since the 19th Century. The overall effect is still of cooling since there has still been net deforestation even in the mid-latitudes, and much more clearly in the tropics. – gavin]