How much of the atmospheric CO2 is anthropogenic?

The answer according to a paper just published in the American Geophysical Union journal, Geophysical Research Letters is that about half of anthropogenic CO2 emissions remain in the atmosphere and about half are taken up by natural carbon sinks…

New data show that the balance between the airborne and the absorbed fraction of carbon dioxide has stayed approximately constant since 1850, despite emissions of carbon dioxide having risen from about 2 billion tons a year in 1850 to 35 billion tons a year now.

This suggests that terrestrial ecosystems and the oceans have a much greater capacity to absorb CO2 than had been previously expected.

The results run contrary to a significant body of recent research which expects that the capacity of terrestrial ecosystems and the oceans to absorb CO2 should start to diminish as CO2 emissions increase, letting greenhouse gas levels skyrocket. Dr Wolfgang Knorr at the University of Bristol found that in fact the trend in the airborne fraction since 1850 has only been 0.7 ± 1.4% per decade, which is essentially zero.

The strength of the new study, published online in Geophysical Research Letters, is that it rests solely on measurements and statistical data, including historical records extracted from Antarctic ice, and does not rely on computations with complex climate models.

This work is extremely important for climate change policy, because emission targets to be negotiated at the United Nations Climate Change Conference in Copenhagen early next month have been based on projections that have a carbon free sink of already factored in. Some researchers have cautioned against this approach, pointing at evidence that suggests the sink has already started to decrease.


Watts Up With That?

The research conducted by Dr Wolfgang Knorr of the University of Bristol shows that since 1850 approximately 54% of anthropogenic CO2 emissions are absorbed by natural carbon sinks (i.e. plants, oceans) irrespective of the total volume of emissions…

Figure 1. The annual increase in atmospheric CO2 (as determined from ice cores, thin dotted lines, and direct measurements, thin black line) has remained constantly proportional to the annual amount of CO2 released by human activities (thick black line). The proportion is about 46% (thick dotted line). (Figure source: Knorr, 2009)

The point is that no matter how much CO2 humans emit, from 8 tons to 8 gigatons, 44% of it is taken up by natural carbon sinks. Mankind accounts for about 6 gT’s of atmospheric carbon (primarily CO2) each year. The natural variability of Earth’s carbon cycle is 6 to 7 times as large as current anthropogenic CO2 emissions.

Mankind accounts for 5-6 Gt of CO2 emissions per year. Natural sources account for 190-225 Gt per year. The natural variability of 35 Gt is 6 to 7 times as large as the total anthropogenic emissions.

In other words, there is no such thing as a natural balance between carbon sources and sinks. Most geoscientists already knew that was the case, because there is no such thing as a natural balance of anything. If there was such a thing, the Earth’s atmosphere would have long ago run out of CO2; and we would be on a pathway to running out again in 25 million years…

H/T to Bill Illis for gathering these paleo-climate data into one spreadsheet.

Despite all of the CO2 emitted into the atmosphere by humans, a linear regression would predict that we are on a course to run out of CO2.

If I subtract 56% of the annual anthropogenic emissions (the airborne fraction) from the ice core/instrumental record, CO2 would still have climbed to ~350 ppmv due to the warm-up from the Little Ice Age…

Another way of approaching this is to take the CO2 concentration from 1750 (ice core data) and add the cumulative anthropogenic emissions to it. The funny thing is that up until about 1960, atmospheric CO2 levels were lower than the cumulative anthropogenic emissions and that if I subtract the cumulative emissions from the atmospheric concentrations, I get a curve that basically tracks the temperature changes…

The blue dots and green curve are the cumulative annual difference between anthropogenic emissions and CO2 levels recorded in ice cores and at Mauna Loa. This essentially subtracts the net annual anthropogenic component of CO2.

So if mankind never discovered how to burn things, atmospheric CO2 would have risen to 330 to 350 ppmv from 277 ppmv in 1750 instead of the current 385 ppmv. Plant stomata data clearly show that natural warming and cooling episodes over the last 10,000 years have routinely caused atmospheric CO2 levels to fluctuate between 270 and 360 ppmv. Plants “breathe” CO2 through microscopic epidermal pores called stomata. The density of plant stomata varies inversely with the atmospheric partial pressure of CO2. Several recent studies of plant stomata from living, herbarium and fossil samples of plant tissue have shown that atmospheric CO2 fluctuations comparable to that seen in the industrial era have been fairly common throughout the Holocene and Recent times.

Plant stomata measurements reveal large variations in atmospheric CO2 concentrations over the last 2,000 years that are not apparent in ice core data (Kouwenberg, 2004)…

Kouwenberg (2004) Figure 5.4: Reconstruction of paleo-atmospheric CO2 levels when stomatal frequency of fossil needles is converted to CO2 mixing ratios using the relation between CO2 and TSDL as quantified in the training set. Black line represents a 3 point running average based on 3–5 needles per depth. Grey area indicates the RMSE in the calibration. White diamonds are data measured in the Taylor Dome ice core (Indermühle et al., 1999); white squares CO2 measurements from the Law Dome ice-core (Etheridge et al., 1996). Inset: Training set of TSDL response of Tsuga heterophylla needles from the Pacific Northwest region to CO2 changes over the past century (Chapter 4).

Century-scale fluctuations in atmospheric CO2 concentrations have also been demonstrated in the early Holocene (Wagner et al., 1999)…

(Wagner et al., 1999)Fig. 1. (A) Mean SI values (±1 ) for B. pendula and B. pubescens from the early Holocene part of the Borchert section (Netherlands; 52.23°N, 7.00°E) and reconstructed CO2 concentrations. The scale of the section is in centimeters. Three lithological (Lith.) units can be recognized (18): a basal gyttja (=), succeeded by Drepanocladus peat (//), which is subsequently overlain by Sphagnum peat ( ). Six conventional 14C dates (in years before the present) are available (indicated by circled numbers): 1, 10,070 ± 90; 2, 9930 ± 45; 3, 9685 ± 90; 4, 9770 ± 90; 5, 9730 ± 50; and 6, 9380 ± 80. Summary pollen diagram includes arboreal pollen (white area) with Pinus ( ) and with Betula ( ) and nonarboreal pollen with Gramineae ( ) and with Cyperaceae, upland herbs, and Ericales ( ). Regional climatic phases after (18): YD, Younger Dryas; Fr., Friesland phase; Ra., Rammelbeek phase; and LP, Late Preboreal. For analytical method, see (13). Quantification of CO2 concentrations according to the rate of historical CO2 responsiveness of European tree birches (Fig. 2). P indicates the reconstructed position of the Preboreal Oscillation.

So… If mankind had never burned any coal, oil or natural gas, atmospheric CO2 levels would only be 30 to 50 ppmv lower than the current ~385 ppmv.  And… There’s no evidence that anthropogenic CO2 emissions have caused any warming in the last decade…

The Earth has not warmed over the last 11 years.

Or in the last thousand years…

The long term temperature trend over the last 2,000 years is flat and the warm-up out of the Little Ice Age began 260 years before atmospheric CO2 levels began to rise.


12 Responses to “How much of the atmospheric CO2 is anthropogenic?”

  1. SYSTÉM v rulete Says:

    I cannot believe this is true!

  2. Vyhrát v rulete Says:

    Sometimes it’s really that simple, isn’t it? I feel a little stupid for not thinking of this myself/earlier, though.

  3. Pengar Internet Says:

    Great idea, but will this work over the long run?

  4. ferdiegb Says:

    “The natural variability of Earth’s carbon cycle is 6 to 7 times as large as current anthropogenic CO2 emissions.”

    This is a fundamental error: the amount of CO2 exchanged as result of the seasons is 6 to 7 times as large as current anthro CO2 emissions. That is not what is counted as natural variability after a full cycle. How much is exchanged is of not the slightest interest. What is of interest is how much the balance changes each year. And that is +/- 2 GtC variability in sink rate (which is average 4 GtC) for about 8 GtC of anthro releases. Thus the natural variability is much smaller than the human releases.

    Why is the natural variability such small on an average 150 GtC which circulates through the atmosphere? Mainly because the two main exchanges work in countercurrent: in the warm seasons, the oceans release more CO2, while vegetation absorbs more CO2. The net effect is that the global CO2 only changes about 4 ppmv for 1 C global temperature change over a year.

    “If I subtract 56% of the annual anthropogenic emissions (the airborne fraction) from the ice core/instrumental record, CO2 would still have climbed to ~350 ppmv due to the warm-up from the Little Ice Age…”

    Where is that based on? The temperature/CO2 ratio over the past 800,000 years is about 8 ppmv/C. Even with the Moberg (or Huang, boreholes) reconstruction, the increase with 1 C since the LIA would give not more than 8 ppmv increase of CO2…

    • David Middleton Says:


      First off… Thank you for taking the time to read my posts and comment on them. I always enjoy reading your comments at WUWT. Despite the fact that I don’t agree with you, I almost always learn something new from your comments.

      I have been meaning to rewrite this thread. Rather than “natural variability of the Earth’s carbon cycle,” I should have said “annual variability” or “margin of error.”

      I was trying to use Knorr’s finding that 56% of the annual anthropogenic CO2 emissions are taken up by natural carbon sinks to derive a decay factor. Then back-calculate the cumulative anthropogenic CO2 in the atmosphere using CDIAC’s historical emissions estimates.

  5. David Middleton Says:


    Regarding this point: “The net effect is that the global CO2 only changes about 4 ppmv for 1 C global temperature change over a year.”

    I refer you to Frank et al., 2010

    But the magnitude of the climate sensitivity of the global carbon cycle (termed γ), and thus of its positive feedback strength, is under debate, giving rise to large uncertainties in global warming projections. Here we quantify the median γ as 7.7 p.p.m.v. CO2 per °C warming, with a likely range of 1.7-21.4 p.p.m.v. CO2 per °C.

    Frank found that the average CO2 feedback is ~8ppmv/1°C and that the range was 2 to 22ppmv/1°C.

    Based on Frank’s work, a 20 to 40ppmv rise in CO2 since 1600 AD (the nadir of the LIA) is possible. However, Frank derived his feedback number from Antarctic ice cores. If the ice cores are underestimating the rise in CO2 during past interglacials (as the stomata and GeoCarb indicate), the CO2 feedback from warming could be quite a bit higher. If the previous estimate of 40ppmv/1°C is correct, then a 40 to 80ppmv (or more) rise in CO2 since 1600 AD (the nadir of the LIA) is possible

    There are multiple avenues by which to derive a modern CO2 level of 340 to 380ppmv without anthropogenic influence.

  6. Kevin krouse Says:

    I would like to read the Knor paper but I am having difficulty finding it. A search for it, in its stated publication, doesn’t turn it up as it only goes back 12 months. I don’t want to buy a subscription.

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  8. Martin Says:

    This still active? What datasets did you use for your anthropogenic emissions?

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  10. GV Says:

    Earth has been warming up since last Ice Age. I do not know who/what to blame for it. Now, is there anyone who would devote some time to look for the positive side of Global Warming, instead of Al Gore’s gloom and doom? Personally, I do not care if sands of Sahara get warmer as the same time Siberia or waste lands of Northern Canada would produce much needed food. So, there must be some positive side to this, why don’t you people devout some of your brilliant minds to that instead of arguing what if and who done it!

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