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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