6 February 2023

The water cycle has changed. And it favors floods and droughts

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Extreme rainfall concentrated in short periods is becoming more frequent just like months characterized by exceptionally low rainfall, scientists from the Global Water Monitor Consortium explain. Growth in duration and severity of heat waves causes “flash droughts” especially in Europe and China

by Matteo Cavallito

 

Global warming is altering the water cycle with obvious consequences for the environment and humans, according to a report released by the Global Water Monitor Consortium, an initiative of the Australian National University (ANU). “Globally, the air is getting hotter and drier, which means droughts and risky fire conditions are developing faster and more frequently,” said Albert Van Dijk, a professor at ANU and co-author of the research.

“Our research team watches the global water cycle closely. We analyse observations from more than 40 satellites that continuously monitor the atmosphere and Earth’s surface,” he said in an article published by local network The Conversation. By combining the information gathered with data from thousands of weather stations, the authors were able to paint a comprehensive picture of the phenomenon globally.

A change in rainfall

Today, Van Dijk says, the data collected do not show a change in average global rainfall. Some trends, however, are beginning to emerge clearly and cause some concern. On the rise, for example, is the incidence of the most extreme phenomena. That is, to simplify, times when rainfall is either too abundant, to the point of increasing flood risk, or excessively scarce, such that it triggers droughts.

“The number of times high monthly precipitation records were broken in one of the 4687 river catchments worldwide was the highest since 2010 and 14% higher than around 2000 (1995–2000)”,” the study says. The research “has found that increasing trends in extreme precipitation over shorter periods (five days or less) have become more common than decreasing trends”.

As for the driest periods, the authors note, “The number of times low monthly precipitation records were broken was the third highest since 1979 and 12% higher than the average around 2000. The highest number of low rainfall records broken annually were in the last three years, and there appears to be a long-term trend towards more months with record low rainfall.”

A global problem

La Niña, the phenomenon of water cooling in the central and eastern tropical Pacific that is associated with warming in the western Pacific, characterized 2022. The result has been widespread flooding over a huge area between Iran and New Zealand.

“The most devastating floods occurred in Pakistan, where about 8 million people were driven out of their homes by massive flooding along the Indus River,” Van Dijk writes.

By contrast, rainfall has been significantly lower on the other side of the Pacific. Not surprising in this regard are the droughts experienced in the western United States and parts of Central America. With lake levels dropping to their historic lows.

Nel 2022 il numero di volte in cui sono stati battuti i record negativi di precipitazioni mensili è stato il terzo più alto dal 1979. Fonte: © Australian National University, 2023, licensed by the Global Water Monitor Consortium under a Creative Commons Attribution 4.0 International licence

In 2022, the number of times negative monthly precipitation records were broken was higher than the 1995-2005 average for the 11th consecutive year. Source: © Australian National University, 2023, licensed by the Global Water Monitor Consortium under a Creative Commons Attribution 4.0 International licence

Hot air and droughts

Rsearchers are also concerned about the rising air temperature. Over the past year it has averaged 0.56 degrees Celsius above the 1995-2005 average. This is the lowest figure since 2018. However, this number should not be misleading: 2022, in fact, has been the seventh warmest year since 1979.

“The number of times high monthly average temperature records were broken in the 4687 river catchments worldwide was lower than the preceding four years but nonetheless above the 1995–2005 average for the 11th year in a row, by 168% or 2.7 times,” the study says.

The consequences are already evident. One of these is the increasing of duration and severity of heat waves causing the so-called “flash droughts,” – which occurs when warm air causes rapid evaporation of water in the soil – in Europe and China, in particular. The phenomenon affects soil crops but also the resilience of forests, which are increasingly prone to wildfires. A vicious cycle that is set to continue without a reversal in climate change. “Until then,” says Van Dijk “global temperatures will continue to increase. New records will continue to be broken: for heatwaves, cloudbursts, flash droughts, bushfires and ice melt,”