Climate researchers have long documented something about a warmer world that¡¯s hard even for nonscientists to miss: It doesn¡¯t rain, it pours.
Intense, concentrated rainstorms have been on the rise for decades. And those bigger storms turn out to have a counterintuitive effect. The more that rain comes down in deluges, the less water is available on land, according to a study published Wednesday in the journal Nature.
By the time the world has warmed by 2 degrees Celsius, 27% of the human population could live under "abnormally dry conditions¡± brought about by the shifting timing and intensity of rainfall. The Earth¡¯s average temperature has already risen about 1.3 C since industrialization.
"Intensification of rainfall is just a given, given global warming,¡± said Justin Mankin, a climate scientist at Dartmouth College and senior author of the paper. "What does it imply for the amount of water going into the land? Does the land essentially have to drink from a firehose?¡±
In a word, yes. The researchers¡¯ results hold "broad implications for future water availability,¡± Mankin and his co-author Corey Lesk write.
Major river basins already face drying from the concentration of rainfall, including the Amazon, Nile, Mississippi, Ganges and Yangtze. The relative effect is biggest in arid and very humid regions, but the scientists¡¯ findings hold for all climates globally.
Conceptually, what¡¯s happening is straightforward. Hotter air can hold more water vapor, about 7% more for every rise of 1 C in the global temperature average. Extra moisture comes down less regularly, in greater amounts. The paper looks at this trend across three precipitation datasets.
When rain comes regularly and at a lower volume, the ground is able to absorb the water, feeding plants and replenishing aquifers. But after massive storms, the water pools at the surface and evaporates back into the atmosphere before it can even run off into rivers, recharge soil and aquifers or transpire from plants.
Adding insult to injury, when rainy days are farther apart, that means there are more days of sun to evaporate more water ¡ª a secondary effect of the concentrated precipitation.
A linchpin of the new study is a version of the century-old measure of income inequality called the Gini coefficient. Mankin and lead author Lesk, a Dartmouth postdoctoral researcher, used the same approach to gauge how rainfall has been shifting.
Their scale measures the difference between a hypothetical "equality¡± of rain ¡ª where every day sees the same amount ¡ª and the reality of rain bunching up in fewer drizzles and bigger downpours. A score of 0 would mean every day has equal rainfall and a score of 1 would mean it all falls on one day.
They cite Phnom Penh, Cambodia, as an extreme example of what happened globally between 2002 and 2022. The city¡¯s year of least concentrated rainfall during that period came in 2003, when its score was 0.69. Its most concentrated year was 2016, when the score jumped to 0.93.
"I think this is the first study to tease this relationship ¡ª that more concentrated precipitation can decrease terrestrial water storage ¡ª out from observations,¡± said Flavio Lehner, a climate scientist at Cornell University who was not involved with the research. Follow-up work, he added, might focus on better understanding why the magnitude varies for different precipitation datasets, although without affecting the headline results.
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