December 20, 2018, by Tim Radford
The global water supply is dwindling, even though rainfall is heavier. Once again, climate change is to blame.
LONDON, 20 December, 2018 – Even in a world with more intense rain, communities could begin to run short of water. New research has confirmed that, in a warming world, extremes of drought have begun to diminish the world’s groundwater – and ever more intense rainstorms will do little to make up the loss in the global water supply.
And a second, separate study delivers support for this seeming paradox: worldwide, there is evidence that rainfall patterns are, increasingly, being disturbed. The number of record-dry months has increased overall. And so has the number of record-breaking rainy months.
Both studies match predictions in a world of climate change driven by ever-higher ratios of greenhouse gases in the atmosphere, from ever-increasing combustion of fossil fuels. But, unlike many climate studies, neither of these is based on computer simulation of predicted change.
Each is instead based on the meticulous analysis of huge quantities of on-the-ground data. Together they provide substance to a 40-year-old prediction of climate change research: that in a warming world, those regions already wet will get ever more rain, while the drylands will tend to become increasingly more arid.
As global temperatures creep up – and they have already risen by 1°C in the past century, and could be set to reach 3°C by 2100 – so does the capacity of the atmosphere to absorb more moisture. It follows that more rain must fall. But at the same time more groundwater evaporates, and the risk of damaging drought increases.
“What we did not expect, despite all the extra rain everywhere in the world, is that the large rivers are drying out”
Australian scientists report in the journal Water Resources Research that they studied readings from 43,000 rainfall stations and 5,300 river monitoring sites in 160 countries. And they confirm that even in a world of more intense rain, drought could become the new normal in those regions already at risk.
“This is something that has been missed. We expected rainfall to increase, since warmer air stores more moisture – and that is what climate models predicted too,” said Ashish Sharma, an environmental engineer at the University of New South Wales.
“What we did not expect, despite all the extra rain everywhere in the world, is that the large rivers are drying out. We believe the cause is the drying of soils in our catchments. Where once these were moist before a storm event – allowing excess rainfall to run off into rivers – they are now drier and soak up more rain, so less water makes it as flow.”
The study matches predictions. Just in the last few months, climate scientists have warned that catastrophic climate change could be on the way, and that the double hazard of heat waves and sustained drought could devastate harvests in more than one climatic zone in the same season; and that those landlocked rainfall catchment areas that are already dry are becoming increasingly more parched.
But over the same few months, researchers have established repeatedly that tomorrow’s storms will be worse and that more devastating flash floods can be expected even in one of the world’s driest continents, Australia itself.
Less water available
Of all rainfall, only 36% gets into aquifers, streams and lakes. The remaining two thirds seeps into the soils, grasslands and woodlands. But more soil evaporation means less water is available from river supplies for cities and farms.
US researchers have already confirmed that if soils are moist before a storm, 62% of rainfall leads to floods that fill catchments. If soils are dry, only 13% of the rain leads to flooding.
“It’s a double whammy. Less water is ending up where we can’t store it for later use. At the same time, more rain is overwhelming drainage infrastructure in towns and cities, leading to more urban flooding,” said Professor Sharma.
“Small floods are very important for water supply, because they refill dams and form the basis of our water supply. But they’re happening less often, because the soils are sucking up extra rain. Even when a major storm dumps a lot of rain, the soils are so dry they absorb more water than before, and less reaches the rivers and reservoirs”, he said. “We need to adapt to this emerging reality.”
In the second close look at change so far, researchers based in Germany report in the journal Geophysical Research Letters that they analysed data from 50,000 weather stations worldwide to measure rainfall on a monthly basis.
Climate drives aridity
The US has seen a more than 25% increase of record wet months in the eastern and central regions between 1980 and 2013. Argentina has seen a 32% increase. In central and northern Europe the increase is between 19% and 37%; in Asian Russia, it has been about 20%.
But in Africa south of the Sahara the incidence of very dry months has increased by 50%. “This implies that approximately one out of three record dry months in this region would not have occurred without long-term climate change,” said Dim Coumou, of the Potsdam Institute for Climate Impact Research.
“Generally, land regions in the tropics and sub-tropics have seen more dry records, and the northern mid- to high-latitudes more wet records. This largely fits the patterns that scientists expect from human-caused climate change.”
His colleague and lead author Jascha Lehmann said: “Normally, record weather events occur by chance and we know how many would happen in a climate without warning. It’s like throwing a dice: on average one out of six times you get a six.
“But by injecting huge amounts of greenhouse gases into the atmosphere, humankind has loaded the dice. In many regions, we throw sixes much more often, with severe impacts for society and the environment.
“It is worrying that we see significant increases of such extremes with just one degree of global warming.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.