Destination of atmospheric carbon dioxide

E. Linacre and Ray Langenfelds


A recent study of the trend of the atmosphere's content of oxygen (O2) allows estimation of the amounts of anthropogenic carbon dioxide CO2 (formed from the oxygen) being dissolved in the oceans, and taken up by terrestrial vegetation, respectively (1). This is explained below.

The rate of CO2 emission to the atmosphere is obtained from estimates of fossil fuel production and consumption. These estimates are based on coal/oil/gas companies' reports of production and are independent of any atmospheric measurements. The average release of carbon from fossil fuel combustion and cement manufacture for the period 1977 - 1995 was 5.63 gigatonnes of carbon (GtC/a). This figure does not include carbon release from land use change.

The growth of CO2 in the atmosphere is known quite precisely from various international measurement programs, for instance on Mauna Loa in Hawaii or at Cape Grim, Tasmania. They give an average growth rate of 1.47 ppm/yr for the period 1978 - 1997. This equates to an increase of 3.13 GtC/yr, thus implying that, on average for this period, 2.50 GtC/yr was removed from the atmosphere. The only known carbon reservoirs capable of absorbing carbon at this rate are the oceans and the terrestrial biosphere. This is well documented. What is more difficult to work out is how the missing carbon is partitioned between these two reservoirs. This problem cannot be solved with measurements of the CO2 trend alone. Measurement of the O2 trend are used (1) to obtain a unique solution. This can be done because the amount of O2 dissolved in the ocean can be treated as constant on interannual timescales.

The rate of O2 removal from the atmosphere that accompanies the fossil fuel CO2 emissions can be calculated from the stochiometry of the combustion process, allowing for the proportions of different fuels consumed and their different O2/CO2 exchange ratios. The main fuels are coal, oil and natural gas with O2/CO2 exchange ratios (mole/mole) of -1.17, -1.44 and -1.95 respectively. Thus, for a 2 mole O2 combustion, natural gas produces just 1 mole of CO2, whereas coal produces nearly 2 moles of CO2 (the exact numbers are 1.03 and 1.71, respectively). The weighted average O2/CO2 exchange ratio for 1977 - 1995 is -1.38.

Emission of 5.63 GtC/a equates to 2.64 ppm CO2/a. This then will remove 1.38 x 2.64 = 3.65 ppm O2 per year. This in turn equates to a decline in atmospheric O2/N2 ratio of 17.4 per meg/yr using the units described in (1). The measured rate of atmospheric O2/N2 decline is 16.7 per meg/yr, indicating that after allowance for O2 loss through fossil fuel combustion, the atmosphere has gained 0.7 per meg/yr in O2/N2.

We now have to consider the way in which CO2 and O2 are exchanged between the atmosphere and the two linked reservoirs, the oceans and the terrestrial biosphere. Terrestrial exchange through photosynthesis, respiration or biomass burning is characterised by an O2/CO2 exchange ratio of -1.1. By contrast, the oceans have a large capacity for uptake of CO2 (the oceans contain about 60 times as much carbon as the atmosphere) but relatively little capacity to alter atmospheric O2. While there is some O2 dissolved in the oceans, this represents only about 1% of the atmospheric O2 content. Therefore as atmospheric CO2 increases and equilibrium of the atmosphere-ocean carbon system is disturbed, inorganic seawater chemistry is expected to absorb some of the atmospheric carbon excess in trying to re-establish equilibrium. The corresponding adjustment for O2 is negligible in its implications for the carbon budget. While there is a possibility that second order effects could produce a change in oceanic O2 inventory that is significant for these calculations, (1) assumes that any change in the atmospheric O2 trend that is not related to O2 consumption by fossil fuel combustion is due only to a change in the size of the terrestrial biosphere.

The 0.7 per meg/yr O2/N2 anomaly is therefore interpreted as a net growth of the terrestrial biosphere, corresponding to net uptake of 0.2 Gt/yr of carbon. The rate of oceanic carbon uptake is then simply calculated as 2.5 - 0.2 = 2.3 GtC/yr.

Finally, the net terrestrial uptake of 0.2 GtC/yr is calculated with no account yet taken of carbon emission through deforestation. This means that if there is a substantial contribution to atmospheric CO2 by deforestation, the rest of the terrestrial biosphere is growing fast enough to effectively compensate for that deforestation. The best estimate of carbon emission to the atmosphere from deforestation is about 1.6 GtC/yr although this figure carries a large uncertainty. If one assumes that 1.6 is correct and if it is included in the carbon budget, then this result says that the terrestrial biosphere is acting as a sink of 1.8 GtC/yr as a result of reforestation or fertilisation by higher levels of CO2, nitrogen deposition or climate change. Of course this study does not allow a distinction between these possibilities.

In summary, nearly half of the fossil fuel CO2 emissions disappear from the atmosphere: they are largely absorbed by the oceans, although a small fraction of the 'disappearing' CO2 (about 10%) is taken up by the terrestrial biosphere. This net biosphere uptake is small compared to CO2 emissions due to deforestation (estimated independently), suggesting that increased CO2 uptake by plants (either by reforestation, or extra plant growth) readily compensates for CO2 emissions by deforestation.

There are still many uncertainties about the transfer rates of CO2 and other chemical species between the atmosphere, the oceans, the biosphere and the solid Earth. To improve estimates of the various fluxes, groups such as the Carbon Dioxide Information Analysis Center and the Geochemical Earth Reference Model (GERM) have been established.


(1) Langenfelds, R.L., R.J. Francey, L.P. Steele, M. Battle, R.F. Keeling and W.F. Budd, 1999: Partitioning of the fossil CO2 sink using a 19-year trend in atmospheric O2, Geophysical Research Letters, 26, 1897-1900.