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

虽然对未来气候的计算机预测一致认为，温室气体浓度的上升将在21世纪使地球表面变暖，但关于这将如何影响欧洲等地区的气候，特别是考虑到降雨的变化，人们的共识较少。总的来说，北欧可能会变得更潮湿，尤其是在冬季，而中欧和南欧预计将经历更干燥、更炎热的夏季。虽然很难有把握地预测降雨量的区域变化，但这些是气候变化的关键特征，决策者现在需要对此作出规划。例如，英国未来可能会经历更频繁的夏季干旱，这对新水库的规划和建设、农业和主要栖息地的维护都有影响。我们对水循环预测的不确定性源于气候模式中关键过程的不充分代表。在考虑未来干旱的可能性时，一个关键领域是大气和土地之间的关系。在夏季，土壤干燥，这在欧洲许多地区限制了土壤水分蒸发到大气中。当这种情况发生时，陆地表面吸收的太阳能分配发生了变化；用于蒸发的能量更少，而用于直接加热大气的能量更多。这种“反馈”可以影响云和降雨的发展，特别是在下午形成的当地夏季风暴的发生。此外，当大片地区的土壤干涸时，就像2003年西欧和中欧大部分地区发生的那样，土地蒸发的缺乏会影响更大范围的天气系统。由于春季蒸发量增加，预计21世纪后期夏季气温升高将导致土壤明显干燥。因此，我们预计干燥的土壤会在夏季早些时候开始反馈到大气中，并且在目前潮湿的更北部地区。我们缺乏关于土壤湿度和降水之间的反馈如何运作的详细知识，这是预测未来几十年干旱可能性的主要不确定性之一。研究表明，2003年的干旱可能已经说明了未来的情况，干旱的春季土壤与干旱和随之而来的热浪有关。这意味着，随着有利的土壤条件“锁定”干旱，未来欧洲的夏季可能会逐年变化。该项目将使用最先进的陆地表面和大气计算机模型，并结合从卫星获得的新数据汇编，以提高我们对土壤湿度如何影响降雨的理解。我们将利用干旱时期的观测结果，直接观察气温是如何随着土壤水分的减少而上升的。我们将利用这些知识在英国气象局的气候模型中更好地代表陆地上的蒸发。我们还将研究，在干旱影响的地区，云和风暴优先发展，相对潮湿或干燥的景观。这将使我们能够预测干燥土壤抑制降雨的情况，从而延长干旱。根据我们的详细观测，我们将评估和改进气候模式及其对土壤湿度反馈的表示。这些改进将为英国气象局对欧洲气候变化的最新预测提供依据。我们还希望提高气象局预测即将到来的夏季是否炎热干燥的能力，因为这些季节性预测使用相同的计算机模型。在这两个时间尺度上，预测的任何改进都将给英国经济带来直接好处。

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.