Dre Sandy P. Harrison - What have we learnt from palaeoclimate observations and simulations?

Résumé / Abstract:

Structural differences among models account for much of the uncertainty in projected climate changes, at least until the mid-21st century. Recent observations encompass too limited a range of climate variability to provide a robust test of simulated climate changes. Climate changes during the geologic past provide a unique opportunity for out-of-sample evaluation of model performance. Analyses of past climate states are now regarded as integral to the evaluation of climate models, and are part of the toolkit used to assess the likely realism of future climate projections. Palaeoclimate assessment has demonstrated that changes in large-scale features of climate that are governed by the energy and water balance (land-sea temperature contrast, high-latitude amplification of temperature changes, changes in temperature seasonality, and the scaling of precipitation changes with temperature) show consistent responses to changes in forcing in different climate states, and these consistent responses are reproduced by climate models. Projected future changes in these features are therefore reliable. However, state-of-the-art models are still unable to reproduce observed changes in climate at a regional scale, including the magnitude of changes in monsoon precipitation and the sign of precipitation changes in mid-continental regions. Differences in performance are only weakly related to modern-day biases or climate sensitivity, and more sophisti¬cated models are not better at simulating climate changes. While palaeoclimate analyses of state-of-the-art climate models suggest an urgent need for model improvement, work is also ongoing to extend and improve palaeoclimate reconstructions through quantifying and reducing both numerical and interpretative uncertainties.