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Cause and effect?

The ice-core trends of temperature and greenhouse gases match so precisely that there has been room for doubt as to what is cause and what is effect. Thus, could the temperature changes be driving CO2/methane levels in the atmosphere (by altering patterns of global biomass production and storage, say) rather than the other way around? If this was true, then the currently increasing levels of CO2 and methane need not give rise to significant global warming: they would be a consequence, rather than a cause.

The record of greenhouse gas links to climate is further muddied by records of climate changes which were not global in extent. It is becoming increasingly clear, for instance, that a severe, millennium-long cooling event in the northern hemisphere which saw an ice-cap grow over northern Scotland some 12 000 years ago, saw warming (by a kind of global see-saw effect) in the southern hemisphere. Another uncertainty of the recent record is that we are already at a temperature high of the current Ice Age, and so, climatically, are heading towards uncharted territory.

One track into this uncharted territory is to model, mathematically, the effects of increasing greenhouse gases on temperature. In these models, the earth and its various parameters need to be simplified, and there also remain considerable uncertainties, such as whether increased water vapour produced during warming will lead to further warming (water vapour being a greenhouse gas) or cooling (if the water vapour condenses to produce light-reflecting clouds). Most current models suggest global warming of between 2 and 6 degrees by the end of this century, to levels unprecedented in earth history over the past few million years.

The most authoritative assessment of climate change in the near future is provided by the Inter-Governmental Panel for Climate Change (IPCC) who list much information on their website, also summarized in:
  • Houghton, J.T. et al. (eds) 2001. Climate Change 2001: The Scientific Basis. Cambridge University Press.
  • For a report of a recent IPCC meeting, see a news feature in Science (2004, vol. 305, pp. 932-934).
  • See also: Royer, D.R. et al. 2004. CO2 as a primary driver of Phanerozoic climate. GSA Today, vol. 14, pp. 4-10.
  • O’Neill, B.C. & Oppenheimer, M. 2004. Climate change impacts are sensitive to the concentration stabilization path. Proceedings of the National Academy of Sciences, vol. 101, pp. 16411-16416.