Carbon dioxide (CO2)
The atmospheric carbon dioxide (CO2) concentration was
has risen continuously since around 1750, reaching 367 ppm in 1999. The
rate of increase over the past century is unprecedented, at least during
the past 20,000 years.
The global carbon cycle and the earth's climate system are intimately
linked due to the radiative forcing properties of CO2 and the rise in
CO2 concentration is believed to be the main cause of climate change.
The increase in atmospheric CO2 concentration is caused by anthropogenic emissions of CO2. About three-quarters of these emissions are due to fossil fuel burning, releasing 5.4 ± 0.3 PgC/yr during 1980 to 1989, and 6.3 ± 0.4 PgC/yr during 1990 to 1999. Land use change is responsible for the rest of the emissions.
Figure 1: Increase in atmospheric CO2 concentration at Mauna Loa, Hawaii between 1958 and 1993.
The rate of increase of atmospheric CO2 content was 3.3 ± 0.1 PgC/yr during 1980 to 1989 and 3.2 ± 0.1 PgC/yr during 1990 to 1999. These rates are less than the emissions, because some of the emitted CO2 dissolves in the oceans, and some is taken up by terrestrial ecosystems. Thus around 45% of emitted carbon remains in the atmosphere, and around 2.0 Gt C yr-1 can be accounted for by oceanic uptake. The remainder is assumed to be added to the store of organic matter in vegetation and soils on land. This assumption may be correct because the increasing atmospheric CO2 concentration and rates of nitrogen deposition have been shown to accelerate the rate of photosynthesis. The important consequence is that a carbon sink may exist on land which is acting as a 'brake' on the rate at which the CO2 concentration is increasing in the atmosphere.
Ecosystem models and scenarios of climate and environmental change may be coupled to predict the magnitude of the terrestrial carbon sink in the future. One such model, Hybrid (Friend and White 1999, White et al. 1999) driven by climate predictions from the UK Met. Office general circulation model (Hulme et al. 1999) suggests that the sink will increase to around 3 Gt C yr-1 over the first half of the next century, continuing to absorb around 20% of the carbon emitted by anthropogenic processes. However, after around 2050, the terrestrial carbon sink dramatically decreases, disappearing entirely by 2080. The main reason for this collapse is a decrease in tropical forest biomass due to extreme changes in temperature and precipitation. The global consequence is that more of the CO2 emitted by human activities will remain in the atmosphere, increasing the magnitude of radiative forcing.
References
Friend A. D. and White A. (2000): Evaluation and analysis of a dynamic terrestrial ecosystem model under pre-industrial conditions at the global scale. Global Biogeochemical Cycles. 14, 1173-1190.
Houghton J. T., Meira Filho L. G., Bruce J., Lee H., Callander B. A., Haites E., Harris,N. and Maskell K. (1995): Climate change 1994. Radiative forcing of climate change and an evaluation of the IPCC IS92 emission scenarios. Cambridge University Press, Cambridge.
Hulme M., Mitchell J., Ingram M., Johns T., New M. and Viner D. (1999): Climate change scenarios for global impacts studies. Global Environmental Change, 9, S3-S19 Suppl. S.
White A, Cannell M.G.R. and Friend A.D. (1999): Climate change impacts on ecosystems and the terrestrial carbon sink: a new analysis. Global Environmental Change. 9, S21-S30 Suppl. S.