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…

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.

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…

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…

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)…

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

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…

Or in the last thousand years…

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