CLIMATE WATCH
Prediction of desert floods
People in desert and tropical regions alike would be smart to invest in a good umbrella. As climate change worsens, both dry and wet areas are becoming wetter, a new study from Australian and American researchers suggests.
In a finding that challenges the widely held assumption that arid regions will become drier as the world warms, the study, published in Nature Climate Change in March, concludes that many dry areas are receiving more rain or snow overall, not less – and that this trend will continue in the decades to come.
Sound like good news? Far from it. The extra moisture will often arrive in the form of intense deluges and snowstorms, causing major flooding, the study found. Hundred-year floods could strike every two or three years.
Previous analyses that suggested a drier future for arid areas were based on large-scale simulations that include complex shifts in evaporation and atmospheric cycles across continents and oceans. But the new study, conducted by climate scientists from the University of New South Wales and the Massachusetts Institute of Technology, took oceans out of the equation and focused on how precipitation will change at the regional level, in wet and dry land areas specifically.
The team, led by Markus Donat, a climate scientist at the University of New South Wales in Sydney, examined 60 years of weather observations – from 1951 to 2010 – and pinpointed the most extreme precipitation events (the most rain or snow that fell in a day in a given year) in arid regions across the continents. They dug up the same data from wet, tropical areas for the same period.
The observations lined up with computer simulations for the same period, the researchers found. Combining that data into their model, they projected trends for the same areas through the end of the 21st century.
They found the lush wet tropics would, as expected, receive more rain as climate change worsens.
But according to their model, desert areas, too, will become wetter – particularly western and central Australia, California, central Asia and southwestern Africa. Those regions have already witnessed a 1-2% increase in precipitation per decade between 1951 and 2010, according to the study.
This counterintuitive shift is explained in part by the increase in water vapour that comes with rising temperatures. The so-called Clausius-clapeyron formula predicts that for every 1°C of warming, atmospheric moisture content should increase by 6-7%.
But more heat also means more soil evaporation, so rather than replenishing moisture, the additional water will most likely run off parched earth, further swelling streams and reservoirs.
William Ingram, a climate scientist at Oxford University, says that the projections help show “how risks will change – which is precisely the information needed by emergency planners”.
Many communities in arid regions lack the infrastructure to handle the increase in flooding, Donat says.
The tropics are likely to receive more rain, too, although exactly how much is unclear, he adds. A lack of historical data from those areas makes such estimates difficult.
One thing is for sure, though, Donat says: “Future precipitation will change in every corner of the world with global warming.”
THE WATER WILL MOST LIKELY RUN OFF PARCHED EARTH, FURTHER SWELLING STREAMS AND RESERVOIRS.