How will climate change affect rainfall? by KAINAT MUNIBA KHAN

Changes in rainfall and other forms of precipitation will be one of the most critical factors determining the overall impact of climate change. Rainfall is much more difficult to predict than temperature but there are some statements that scientists can make with confidence about the future. A warmer atmosphere can hold more moisture, and globally water vapour increases by 7% for every degree centigrade of warming. How this will translate into changes in global precipitation is less clear cut but the total volume of precipitation is likely to increase by 1-2% per degree of warming. There's evidence to show that regions that are already wet are likely to get wetter, but details on how much wetter and what impacts there will be on a local scale are more difficult to ascertain. The dry regions of the subtropics are likely to get drier and will shift towards the poles. For much of Europe, wetter winters are expected, but with drier summers over central and southern Europe.

It is the changes in weather patterns that make predicting rainfall particularly difficult. While different climate models are in broad agreement about future warming on a global scale, when it comes to predicting how these changes will impact weather – and consequently rainfall – there is less agreement at a detailed level. It is likely that in a warmer climate heavy rainfall will increase and be produced by fewer more intense events. This could lead to longer dry spells and a higher risk of floods.

So far, any impact that climate change may have had generally on regional rainfall cannot be distinguished from natural variations. However, for some specific cases a signal is starting to emerge. A recent study showed that man-made climate change substantially increased the odds of damaging floods occurring in England and Wales in autumn 2000. For the UK, current understanding suggests that increases in heavy rainfall during winter may start to become discernible more generally in the 2020s.

Climate models and observations are improving all the time and the reliability of predictions is likely to improve significantly over the next few years. In particular, new satellites and more detailed models are opening up new possibilities for understanding and predicting how water cycles through the climate system. For example, current climate models typically represent atmospheric processes only down to scales of about 50-100km. This limits their ability to incorporate the effects of mountains and coastlines and means that small-scale processes, such as convection, must be represented by average approximations. In addition, the latest regional climate models capture daily rainfall on large scales but are not good at capturing heavier or more localised events. However, the latest generation of localised weather forecasting models represent scales down to 1km and can capture these localised features. Scientists are now starting to apply these models to climate change studies, raising the possibility of much more confidence in their predictions of changes in extreme rainfall.THANKS KAINAT MUNIBA KHAN