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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

What the science says...

Measurements of carbon isotopes and falling oxygen in the atmosphere show that rising carbon dioxide is due to the burning of fossil fuels and cannot be coming from the ocean.

We can be confident the extra CO₂ in the atmosphere has come from the oxidation of fossil fuels and not from outgassing from the ocean or from soil/land sources by using two key observations.

1. Oxygen decrease

Atmospheric oxygen is going down by the same amount as atmospheric CO₂ is going up. Oxygen is so abundant at about 21% (209,500 ppm) that we are in no danger of running out; the change in oxygen simply shows that whatever the source of CO₂ in the atmosphere, the carbon part of it has come from the oxidation of reduced carbon compounds and the oxygen has come from oxygen gas in the atmosphere. That is, the extra CO2 was not released in the form of CO₂ from an unknown source but instead some reduced carbon compound was burnt in the atmosphere to produce CO₂. See: AR3WG1 Section 3.5.1, especially Figure 3.4.

2. Known fossil fuel CO2 emissions

Most obviously, any alternative explanation for the source of the CO₂ in the atmosphere has to also come up with where the 30 billion tonnes of CO₂ known to be released by fossil fuel burning each year goes.

Atmospheric CO₂ is currently increasing at about 2 ppmv per year (or 16 billion tonnes). That is, only around half of the CO2 we release remains in the atmosphere. The pH decrease in the oceans corresponds to most of the “missing” CO₂, so we can also be confident that land use changes etc are not a major source/sink. Caveat: Land use and biomass changes certainly soak up a lot of CO₂, some it simply regrowth of forests etc, but the point is that the increasing CO₂ in the atmosphere clearly demonstrates that they do not soak up enough.

In summary:

Amount of increased CO2
in the atmosphere

Amount of increased CO2
in the oceans

Amount of known fossil
fuel emissions of CO2

Acknowledgements: this post was written by New Zealand chemical oceanographer, Doug Mackie.

Updated on 2010-06-26 by Doug Mackie.

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What the science says...

1. Oxygen decrease

2. Known fossil fuel CO2 emissions