Climate change is clearly making some regions wetter and others drier. But it’s been difficult for scientists to detect a clear, consistent human role in increasing the frequency and severity of global droughts given natural climate variability, regional differences, and limited data.
A new report in Nature adds evidence to the suspicion that air pollution could be complicating the science, masking the role of greenhouse gases on droughts.
Research has already found that air pollution has likely moderated the level of global warming (see “We’re about to kill a massive, accidental experiment in reducing global warming”). The newest findings suggest this could have played a role in reducing droughts as well, likely by decreasing the level of soil-moisture drying that would have otherwise occurred.
If so, as the world continues to clean up air pollution, the impact of climate change on droughts will get even more severe.
In the new study, researchers at NASA, Columbia University, and Lawrence Livermore National Laboratory analyzed tree ring records to document shifting soil-moisture conditions over time. They found three distinct trends: a clearly detectable human fingerprint on drought levels in the first half of the last century, a diverging trend between 1950 and 1975, and then a return to a positive, though not particularly strong, signal in the years since.
The researchers note that this middle period coincides with an increase in atmospheric aerosols, the tiny particles spewed from planes, cars, coal plants, farms, and even natural events like forest fires and volcanoes. Depending on the particles, they can alter cloud formation, change rainfall patterns, trap heat, or reflect sunlight away from the planet.
Notably, global air pollution levels began to come down sharply in the mid- to late 1970s, with the passage of the US Clear Air Act in 1970 and similar regulations in Europe. That at least correlates with the climate signal pattern noted in the study.
The authors are quick to stress that the findings suggest only a “possible role” for aerosols in moderating drought, and that any connection requires further research.
Jian Lu, a scientist at Pacific Northwest National Laboratory who studies how climate change affects the hydrological cycle, says it’s hard to say whether aerosols are the key factor here, noting that decades-long natural temperature swings originating in the oceans could play a significant role as well.
But if air pollution is a major force on droughts, it could, like so much else in climate change, seriously complicate the problems and solutions.