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PalaeoQUMP is part of the QUEST program funded by NERC. QUEST's Theme 2 is "Natural changes in the atmosphere, ocean and land," which is comparing climate model predictions with data from glacial-interglacial and longer time scales.

PalaeoQUMP aims to use these comparisons to constrain the predicted range for climate sensitivity. Climate sensitivity is the rise in global temperature following a doubling in atmospheric CO2 concentration from pre-industrial levels.

Aim

To constrain the uncertainty in climate sensitivity using reconstructions and model simulations of past climates.

Scientific issues and rational

Estimates of the climate sensitivity to a doubling of CO2, based on multiple climate models, are in the range of 1.5-4.5°C (Houghton et al., 2001). One source of uncertainty in these estimates comes from uncertainties in the values assigned to key processes that are parameterised in current models.

The DEFRA-funded Quantifying Uncertainties in Model Predictions (QUMP) project has perturbed parameters in the HadSM3 model to determine the range of climate changes consistent with the uncertainties in modelling key processes. The resulting probability density function for the sensitivity of the climate to a doubling of CO2 has been constrained by weighting different model versions using estimates of reliability from a climate prediction index (CPI) based on a broad range of observed modern climate variables. On the basis of a 53-member ensemble of perturbed-physics model versions, the 5-95% probability range for climate sensitivity is 2.4-5.4°C (Murphy et al., 2004). This means that the constraints supplied by recent observations of mean climate are insufficient to limit uncertainties in prediction.

A similar study in which many more ensemble experiments were made has shown the possibility of very high climate sensitivities (Stainforth et al., 2005: climateprediction.net). Stronger constraints may be derived from records of different climatic conditions in the past.

Our aim is to derive further constraints based on the existence of robust, quality-controlled global palaeoenvironmental datasets for the mid-Holocene (MH: 6000 yr B.P.) and Last Glacial Maximum (LGM: 21,000 yr B.P.), which show strong, regionally-coherent signals in response to large and well-known changes in climate forcing, specifically changes in the latitudinal and seasonal distribution of insolation during the MH and the existence of large ice sheets, cold oceans and lowered greenhouse gases at the LGM. The signals of regional climate change are sufficiently robust to provide ideal sources of additional constraints on model uncertainties.

References

Houghton JT, Ding Y, Griggs DJ, Noguer M, van der Linden PJ, Dai X, Maskell K, Johnson CA (2001) Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, U.K. and New York, NY, USA.

Murphy JM, Sexton DMH, Barnett DN, Jones GS, Webb MJ, Collins M, Stainforth DA (2004) Quantification of modeling uncertainties in a large ensemble of climate change simulations. Nature 430: 768-772.

Stainforth DA, Aina T, Christensen C, Collins M, Faull N, Frame DJ, Kettleborough JA, Knight S, Martin A, Murphy JM, Piani C, Sexton D, Smith LA, Spicer RA, Thorpe AJ, Allen MR (2005) Uncertainty in predictions of the climate response to rising levels of greenhouse gases. Nature 433: 403-406.