Soil Water - Climate Feedbacks in Europe in the 21st Century (SWELTER-21)

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Whilst computer predictions of future climate agree that rising concentrations of greenhouse gases will warm the earth's surface over the 21st century, there is less concensus concerning how this will affect the climate in a region such as Europe, in particular when considering changes in rainfall. Broadly speaking, northern Europe is likely to get wetter, particularly during the winter, and central and southern Europe are expected to experience drier and hotter summers. Although regional changes in rainfall are difficult to predict with confidence, these are critical features of a changed climate which decision makers need to plan for now. For example, the likelihood of the UK experiencing more frequent summer droughts in the future has implications for the planning and building of new reservoirs, for agriculture and for the maintenance of key habitats. Uncertainty in our predictions of the water cycle arises from inadequate representation of key processes in climate models. When considering the likelihood of future droughts, one key area is the relationship between the atmosphere and the land. During summer, soils dry, which in many parts of Europe limits evaporation of soil moisture into the atmosphere. When this happens, there is a change in the partition of solar energy absorbed by the land surface; less energy is used for evaporation and more is used to warm the atmosphere directly. This 'feedback' can affect the development of clouds and rainfall, especially the occurrence of local summertime storms which develop during the afternoon. Furthermore, when soil dries out over a large region, as happened for example across much of Western and Central Europe during 2003, the lack of land evaporation can affect much larger-scale weather systems. Warmer air temperatures are expected to produce notably drier soil conditions in the summers of the late 21st century through increased spring-time evaporation. As a result, we would expect the drier soils to start to feed back on the atmosphere earlier in the summer, and in more northerly regions which are currently wet. Our lack of detailed knowledge about how this feedback between soil wetness and precipitation operates provides one of the major uncertainties in predicting the likelihood of droughts in the coming decades. Studies have suggested that the drought of 2003 may have illustrated the shape of things to come, with dry spring soils implicated in the drought and associated heatwave which followed. This implies that future European summers could become more variable from year to year, as droughts become 'locked in' by favourable soil conditions. This project will use a state-of-the-art computer model of the land surface and atmosphere combined with new compilations of data obtained from satellites to improve our understanding of how soil wetness influences rainfall. We will use observations from periods of drought to see directly how temperatures rise as soil water declines. We will use this knowledge to better represent evaporation over land in the UK Met Office climate model. We will also examine where, within drought-affected regions, clouds and storms preferentially develop, over relatively wetter or drier landscapes. This will allow us to predict the conditions where dry soils suppress rainfall, thus prolonging drought. From our detailed observations we will evaluate and improve climate models and their representation of soil wetness feedbacks. These improvements will feed into new Met Office predictions of climate change for Europe. We would also expect to improve the Met Office capability to predict whether the forthcoming summer will be hot and dry, as these seasonal predictions use the same computer model. Any improvements in prediction on either time scale would have direct benefits for the UK economy.