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 Human CO2 is a tiny % of CO2 emissionsAs human CO2 emissions are much smaller than natural CO2 emissions, man's impact on climate is minimal. What the science says...Manmade CO2 emissions are much smaller than natural emissions. However, the CO2 that nature emits (from the ocean and vegetation) is balanced by natural absorptions (again by the ocean and vegetation). Human CO2 emissions upsets the natural balance. The carbon cycle Consumption of vegetation by animals & microbes accounts for about 220 gigatonnes of CO2 per year. Respiration by vegetation emits around 220 Gt. The ocean releases about 330 Gt. In contrast, human emissions are only around 26.4 Gt per year. Land plants absorb about 440 Gt of carbon per year and the ocean absorbs about 330 Gt. This keeps atmospheric CO2 levels in rough balance.
Human CO2 emissionsAs for human CO2 emissions, about 40% is being absorbed, mostly by the oceans. The rest remains in the atmosphere. As a consequence, atmospheric CO2 is at its highest level over the past 800,00 years (Brook 2008). A natural change of 100ppm takes 5,000 to 20,000 years. The recent increase of 100ppm has taken just 120 years. Carbon isotopes - the human "fingerprint"How can we know the rising CO2 levels are due to human activity? The carbon atom has several different isotopes (eg - different number of neutrons). Carbon 12 has 6 neutrons, carbon 13 has 7 neutrons. Plants have a lower C13/C12 ratio than in the atmosphere. If rising atmospheric CO2 comes fossil fuels, the C13/C12 should be falling. Indeed this is what is occuring (Ghosh 2003) and the trend correlates with the trend in global emissions.
The ocean's diminishing ability to absorb CO2While the ocean absorbs around half of human CO2 emissions, empirical observations reveal the oceans are losing their ability to absorb CO2. A study released in May 2007 found that the Southern Ocean has reached its saturation point, diminishing its ability to absorb more CO2 (Quéré 2007). Similarly, CO2 absorption by the North Atlantic has dropped even more dramatically, halving over the past decade (Schuster 2007). If this trend continues, it potentially leads to a positive feedback where the oceans take up less CO2 leading to CO2 rising faster in the atmosphere leading to increased global warming. Further readingBoth graphs from this page are taken from Chapter 2 of the IPCC AR4 report. Real Climate goes in-depth into the science and history of C13/C12 measurements. It's not particularly relevant to this argument but World Resources Institute have posted such a great resource, I had to put it somewhere. It's the World GHG Emissions Flow Chart, a visual summary of what's contributing to manmade CO2 (eg - electricity, cars, planes, deforestation, etc):
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The skeptic argument...