In a Warming World, Evidence of a Human ‘Fingerprint’ on Drought

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Human activity was changing the Earth’s drought and rainfall patterns as far back as the early 20th century, new research shows.

Drying in many regions, the researchers suggested, will get worse, with sobering implications for feeding the planet’s billions of people.

The new paper tracks long-term patterns of moisture levels in soil across regions of the world, including North America, Central America, Eurasia and the Mediterranean. The researchers found a “fingerprint” of human effects from producing greenhouse gases, as distinct from natural variability, as far back as 1900.

Scientists have long known that the planet has shown an overall pattern of warmer temperatures since that time — the phenomenon is the subject of a famous cartoon by Randall Munroe — but the new research shows the effects of that warming correlate with drier soil in some parts of the world and wetter soil in others. Climate scientists have long suggested that in a warming world, dry parts of the planet will become drier and wet parts of the planet will become wetter.

“Climate change isn’t a new thing,” said Kate Marvel, a climate scientist at the NASA Goddard Institute for Space Studies and Columbia University, and lead author of the report, but the new research ties human activity that causes climate change to the drought record.

In the study, published Wednesday in the journal Nature, the researchers combined computer models with archives of tree rings, which “give us a record of global drought going back centuries,” Dr. Marvel said.

She stressed, however, that “we’re not saying we’re seeing a large effect” in the historical record. Instead, the research teases out the effect of human activity from the natural background variability of weather and climate, “a background note against a symphony of other sounds.”

The result, said Benjamin Cook, a co-author of the study who is at the Goddard Institute and Columbia University, is that “we’re beginning to understand the fundamental processes and the basic impact of climate change on drought.”

The scientists found the results fell into three broad phases between 1900 and the 21st century.

A warming period in the first half of the 20th century showed a significant signal of human-caused climate change. From 1950 until 1975, however, there was a midcentury decrease in drought and a muddying of the signal that the scientists suggest was caused byaerosol emissions of pollutants like oxides of sulfur that contribute to smog, and could block some of the sun’s light from reaching Earth and affect weather patterns.

After that, the signal of human-caused climate change picked up again in the 1980s, potentially because of restrictions on pollution in the United States and agreements to reduce aerosol emissions around the world. The fingerprint has become even more obvious since 2000.

The period of relative cooling and diminished human climate signal suggests more study is needed, Dr. Cook said, but he noted that one of the proposals for fighting the effects of climate change involves spraying aerosols containing sulfates into the upper atmosphere, a process similar to what occurred during the middle period of the study. “It was kind of an accidental geoengineering experiment in the middle of the 20th century,” he said.

One way that the researchers detected the climate signal was by looking around the globe. They found that three regions — Australia, Mexico and the Mediterranean — were drying at the same time, even though they react differently to large phenomena like El Niño. “This means that it is harder for natural climate variability alone to produce, by chance, the simultaneous drying across all three regions identified in the fingerprint,” the authors wrote.

As the effects of climate change grow, what happens next, the paper stated, will be dire, particularly since many of the areas headed into lasting drought are agricultural centers today: “The human consequences of this, particularly drying over large parts of North America and Eurasia, will likely be severe.”

Abigail Swann, a climate scientist at the University of Washington who was not involved in the research, said, “It shows a creative way to leverage information especially from the earlier part of the century to figure out what was the cause of droughts in the past.”

Friederike Otto, acting director of the Environmental Change Institute at the University of Oxford, praised the paper in an emailed response to questions. “While the big message of climate change has not changed, we are now making huge progress in understanding what climate change actually means for societies in terms of how global warming affects local variables that are ultimately relevant for food security,” she wrote.

The underlying message, Dr. Otto said, is that “climate change is really here and happening now and not something we can afford (in all meanings of that term) to continue to ignore.”

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