The carbon cycle: an update

Carbon cycles continuously through the atmosphere, the oceans, the biota, and the geosphere, as illustrated in Fig 1.3 in the textbook. More recent information (1) on the global carbon cycle allows improvement of this Figure. According to the new estimate (Fig 1), there is more carbon in surface water of the oceans (than previously estimated), and the carbon flux between deep and surface water is larger. Otherwise the alterations are small.

Anthropogenic carbon emissions into the atmosphere amounts to about 6.3 gigatonnes in 1997 (i.e. 31 gt/a of carbon dioxide), which compares with 90 gt/a from the oceans and another 90 gt/a from decaying vegetation. However, the ocean and biosphere emissions are later reabsorbed, along with some of the anthropogenic CO₂. The biosphere takes 1.1-2.9 gt/a, the oceans 0.8-2.6 gt/a, leaving roughly 2.3 added to the atmosphere (3).

A person breathes out something like 0.9 kg of carbon dioxide daily, though the figures depends on age, exercise, etc. If that figure is taken as representative, then a global population of 6 billion means an exhalation of roughly 2 gt/a. That is small compared with the total input to the atmosphere, shown by Fig 1.3 to amount to about 211 Gt/a. On the other hand, people are not the only creatures breathing out carbon dioxide.

Deforestation in the tropics caused carbon emissions of 1.6 (+/- 0.5) GtC/yr (gigatonnes of carbon annually), though some authors quote higher figures. A tentative figure for the net oceanic sink is 2.3 Gt/yr. The climate change resulting directly from an increase of CO₂ is likely to decrease as the concentration rises. But this may be offset largely by a parallel greater reluctance of the warming oceans to dissolve the gas, so that it accumulates in the atmosphere.

A doubling of CO₂ is expected to raise sea levels by about 100 cm eventually, but a rise by only 10 cm is expected within 75 years, on account of the ocean’s slow response to change.

(1) Denman, K., E. Hofmann & H. Marchant,1996: Marine biotic response to environmental change and feedbacks to climate. In Houghton et al. 1996, 483-516.

(2) Houghton, J.T., L.G. Meira Filho, B.A. Callander, N. Harris, A. Kattenberg and K. Maskell, 1996: Climate Change 1995: The Science of Climate Change. Contribution from the Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge Univ. Press, 572pp.

(3) Keeling R. et al. 1996. Nature, 381 , 218.