Human activity has been impacting global drought conditions as early as the year 1900, according to a study published today in the journal Nature. The findings, which analyzed data from past studies on tree rings, suggest that rising global temperatures will likely continue to sap moisture from soil in many regions, potentially imperiling several of the world’s major agricultural centers.
These results broadly confirm the predictions of several earlier climate change models, and thus might not be terribly surprising. But they provide concrete evidence for the idea that particular events in the drought record can be confidently ascribed to human activity.
“The fact that the signal is really clear in the models and apparent in the tree rings is pretty amazing,” study author Kate Marvel of Columbia University’s Earth Institute and NASA's Goddard Institute for Space Studies told Lisa W. Foderaro at National Geographic. “We can argue for a detectable human influence.”
Tree trunks might not keep written records per se—but enshrined in their rings is an archive of the past that’s almost as good. With each passing year, a tree ages and adds a new, distinct circle of dead wood around its trunk. The patterns in these rings record an environmental progress report from the tree’s perspective, revealing, for instance, precipitation, temperature, and even the conditions of the surrounding soil.
“[This is] 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,” Abigail Swann, a climate scientist at the University of Washington who was not involved in the research, told John Schwartz at The New York Times.
Using tree ring data as a proxy for soil moisture, the researchers mapped out climatic conditions from the last 600 to 900 years and compared them to what scientists have observed in modern times. The patterns they were looking for were subtle, but it’s thought that human-driven climate change will amplify the world’s extremes, making some areas drier and others wetter.
The resulting timeline revealed three broad phases within the past 120 years. The first, from 1900 to 1949, showcases a period in which humans clearly made their mark, increasing aridity in places like Australia, Mexico, and the Mediterranean, while regions in Central Asia accumulated more moisture. The second, beginning around the middle of the 20th century, saw a slight dip in this trend as aerosols released from industrial activity entered the atmosphere, blocking sunlight and cooling the planet even as greenhouse gases continued to accumulate.
Finally, between 1981 and 2017, the repercussions of human activity returned in full force—perhaps due to regulations that banned aerosol emissions—launching our world back onto a trajectory that it’s still hurtling along today. If drought persists in many of the vulnerable regions identified, including large parts of North America, the researchers write, the consequences, which would likely include lower crop yields, could be both dire and lasting.
However, studying the milder period of the mid-20th century could someday be fruitful, study author Benjamin Cook of NASA’s Goddard Institute and Columbia University’s Lamont-Doherty Earth Observatory told Schwartz at The New York Times. To combat the effects of climate change, some scientists have proposed spraying aerosols containing sulfates into the upper atmosphere, which might mirror what happened in the 1950s.
That solution, if it works at all, won’t be enough on its own. But addressing these issues will require a lot of creativity, especially as the world continues to shift—and forestalling climate change in our future will mean confronting that it’s very much reflected in our past.