Observational constraints on climate projections

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The objective of the proposed research is to use observations of climate change over the past century to improve climate model predictions of future climate change. Climate models are the best available tools for predicting future climate change, providing key information necessary to inform decisions on reducing greenhouse gas emissions, and adaptation to future climate change. However, due to the many complexities of the climate system and uncertainties in its properties, climate model projections of future change in many aspects of the climate differ strongly between models. For example the last United Nations IPCC assessment report concluded that, even given a specific trajectory of 21st century emissions, 21st century global warming could be anywhere from 2.4°C to 6.4°C. While the impacts on society and the environment from a warming of 2.4°C might be manageable, the impacts of a warming of 6.4°C would likely be very severe. Thus it is important to try to reduce these uncertainties, if climate predictions are to be more useful to policymakers and society at large. The proposed research focuses on using observations of past climate change to improve our predictions of future climate change. A large number of climate model simulations are currently being carried out around the world in preparation for the next report of the UN IPCC report. By analysing simulated climate change over both the historical period (1850-2010) and the future, it should be possible to identify robust relationships between simulated past climate change and simulated future climate change. To take a simple example, we might find that models which warm more in the past tend also to warm more in the future. We can then use observations of past climate change, either to rule out some models as being either too sensitive to greenhouse gas increases or not sensitive enough, or we can rescale model projections so that the match to observations is improved. Such research carried out to date has had limited success in constraining future projections. But the availability of new simulations with better models, many finishing in 2010, presents a promising new opportunity to improve our climate projections in this way.