In the current global warming scenario, extreme rainfall events can be expected to increase in both dry and wet land regions of the world, according to a recent study published in the journal Nature Climate Change.

The water vapour concentration increase in the atmosphere per degree Celsius rise in temperature is 7 per cent according to the Clausius-Clapeyron theory. Over the ocean, where evaporation is greater than precipitation (rainfall), such as in dry areas, the atmosphere will get drier with increased global warming as the evaporated water, in the form of vapour, is carried away by winds, leaving behind a dry atmosphere. And where the precipitation is greater than evaporation such as in wet areas the areas will get wetter.

But over the land masses it has been less clear as to how the rainfall patterns change with global warming. Based on models and observations it has been found that global average precipitation increases only about 2 per cent. In some way the atmosphere produces less rain.

One way that this could happen is if rain increases at the Clausius–Clapeyron rate when it does fall, but falls less often, making precipitation events more extreme. This is what is seen typically in general circulation models (GCM), the most detailed models of the climate system.

Another point is that the heat released by the condensation of water itself tends to pull more moisture into a precipitating system. This suggests that intense rain might instead increase with warming at even higher rates than the Clausius–Clapeyron rate— perhaps twice as fast, as some observations for short timescales (minutes to hours) seem to show. On the other hand, as the atmosphere’s capacity to evaporate moisture from arid regions and to transport it away will increase at the Clausius–Clapeyron rate, arid regions are expected to become drier still, and it seems plausible that this would reduce all precipitation (from light to heavy) in these regions.

In the study, Dr Markus Donat of the Climate Change Research Centre, University of New South Wales, Australia and colleagues analyse the changes in observations of annual total and extreme (the most to fall on any single day in the year) precipitation over land for 1951–2010. They find that the fraction of the year’s rain that falls on the wettest day increases consistently with the Clausius–Clapeyron rate across both these regions.

GCM simulations of the same period are consistent with these observations, and those of the late 21st Century show that the trend continues. Therefore, we can have some confidence that extreme precipitation risk will increase not only in wet regions but also in dry ones.

The study, though significant for its findings has the drawback that the tropics were poorly covered. The global warming effects are most severely felt in the tropics where complex physical interactions make prediction hardest. Also, extreme rainfall data is widely available only on the daily timescale. It cannot be known how it will change on shorter and longer timescales, which will indicate the flood risk in different places.