Global warming will dump rain on dry areas – but not in a helpful way

New research challenges the view that drier areas will get drier with global warming. Climate scientists suggest that as the world warms, dry regions will get more rain. Drought-stricken farmers, rejoice!

Hold the champers. While there will be more rain overall on populated areas, it’s unlikely to be useful, and may make life harder for those unused to regular drenchings.

In a study published in Nature Climate Change, climate scientists from Sydney’s University of New South Wales and the Massachusetts Institute of Technology looked at 60 years of climate observations and modelled future rainfall.

They found the tropics will receive more rain with climate change – as will arid areas such as western and central Australia, California, central Asia and southwestern Africa.

Most of the rainfall will be tied up in massive storms that could lead to flash floods. Areas used to little rainfall may not be able to handle such deluges.

“The concern with an increased frequency and in particular intensity of extreme precipitation events in areas that are normally dry is that there may not be infrastructure in place to cope with extreme flooding events,” lead author Markus Donat says.

And when it’s not pouring, the added heat in the atmosphere will lead to more evaporation.

While climate modelling studies have suggested that wet parts of the world will become wetter while dry parts of the world will become drier, this, the authors argue, holds for large-scale simulations over oceans, not land.

To determine how wet and dry parts of the world will fare under climate change, the researchers decided to steer clear of comparing wet places to dry and try to work out the complexities between the two. Instead, they compared like with like.

When the modelling was extended into the late 21st century, he saw that rate continue for arid regions.

They took the most extreme rainfall (as in, the most that fell in a day in a year) from similarly dry land in Australia, Asia, Africa and elsewhere from 1951 to 2010. They repeated it with similarly wet regions across the world.

They averaged, separately, the extreme rainfall events across the wetter and drier areas. Over the 60-year period, they saw the fraction of annual rain that falls on the wettest day of the year matched what’s known as the Clausius-Clapeyron rate.

Named after German Rudolf Clausius and Frenchman Benoît Clapeyron, both physicists known for their work in thermodynamics, the Clausius-Clapeyron rate predicts those days of extreme rainfall should increase by 6 to 7% per 1°C of warming.

Donat’s simulations, using a general climate model, of that period matched the observations. When the modelling was extended into the late 21st century, he saw that rate continue for arid regions.

And while the tropics will receive more rain, Donat admits exactly how much is as yet unclear. This could be because there’s simply less historical data from those areas.

William Ingram, a climate scientist at Oxford University, writes in a News and Views article that while the work won’t help local meteorologists forecast days of extreme rainfall, it tells us “how risks will change – which is precisely the information needed by emergency planners”.