Climate commitment studies attempt to assess the amount of future warming that is "committed" by the present levels of climate forcings, even assuming no further increase. This is by no means a new idea [1]; the concept is discussed in the IPCC TAR [2] and in the SAR in 1995. At the time of the TAR there were not yet studies of the levels of unrealized climate commitment that might remain in the current climate.
The basic idea is that if a perturbation - such as an increase in greenhouse gases or solar activity - is applied to the climate system the response will not be immediate, principally because of the large thermal inertia of the oceans. As an analogue, consider the heating of a thick metal block by the sun, or by a flame: the block will warm, more or less slowly, until the entire block has reached equilibrium with the imposed heating. If a thin plate is heated instead, the plate will warm far more quickly because of its lower heat capacity. The oceans, being to some extent vertically mobile, are able to store heat within their depth. Over the land, by contrast, heat penetration beyond the top few meters is very slow because of the lack of mobility. This is why the land surface is observed to warm more than the oceans, and it is predicted that this should continue in the future. It also explains the very large difference in response between "transient" climate prediction runs (in which a full ocean is used) and the climate is out of balance, and "equilibrium" runs in which only a shallow ocean is used and it is assumed that the climate has come to equilibrium.
Recent models (both complex AOGCMs and more simple models) forecast that even in the unlikely event of greenhouse gases stabalising at present levels, the earth would warm by an additional 0.5 °C by 2100, a similar rise in temperature to that seen during the previous century. As ocean waters expand in response to this warming, global sea levels would mount by about 10 centimetres in the next hundred years. But the model does not account for ice cap and glacier melting; a better estimate might be double or triple this value [3].
Present models are incomplete: there are many uncertainties: the effects might be larger or smaller: see climate models.
External links
References
- Meehl G. A., et al. Sciencexpress, 10.1126/science.1106663 (2005).
- Wigley T. M. L., et al. Sciencexpress, 110.1126/science.1103934 (2005).
