INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE,
TWENTIETH SESSION
Paris, 19-21 February 2003
Background
One of the most important parameters in climate science is the 'climate sensitivity', broadly
defined as the global mean temperature change (°C) for a given forcing, often that of a doubling of atmospheric carbon dioxide. Climate sensitivity has played a central role
throughout the history of IPCC in interpretation of model outputs, in evaluation of future climate changes expected from various scenarios, and it is closely linked to attribution of currently observed climate changes. An ongoing challenge to models and to climate projections has been to better define this key parameter, and to understand the differences in computed values between various models. Throughout the last three IPCC assessments this basic parameter of the Earth’s climate system has been estimated as being in the range 1.5 to 4.5°C (i.e., uncertain by a factor of three), making this parameter central to discussions of uncertainty in climate change. WG I is concerned to sharpen understanding of the differences between general circulation models used in climate change research.
Currently the primary reason for the substantial range in model based estimates of climate sensitivity is widely believed to be differences in their treatment of feedbacks – particularly cloud feedbacks, but systematic intercomparisons have not been done to confirm that this is so for the current generation of models. Within international climate modeling projects, the development of new models together with both formal and informal mode intercomparison exercises that are currently taking place by various groups suggest that a renewed focus on the reasons for different model estimates of climate sensitivity may be particularly useful at this time.
In addition, some recent studies suggest that new insights into the likely range of climate
sensitivity may be possible through comparisons of models and observational data – both
contemporary and historical or paleoclimatic. Other recent studies raise issues regarding the limitations of applicability of forcing/response relationships in the climate system - such as questions regarding the predictability of climate and its relevance for estimates of climate sensitivity, and the degree to which forcings such as those due to solar, well-mixed greenhouse gases, or aerosols may produce different responses. A review of these questions about the interpretation of climate sensitivity could also sharpen scientific understanding and would hence be of benefit to the WG I AR4.
In summary, there is broad interest for a carefully planned workshop on climate sensitivity.
Given the importance of the climate sensitivity parameter, it is likely that the outcome of this
workshop will provide a major focus for the discussion and treatment of climate models in
the WG I contribution to AR4.
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