Using studies of tree rings going back centuries, scientists have unearthed clear evidence that the rise of human-generated greenhouse gases was having an effect on global drought conditions as early as 1900.
A new, first-of-its-kind study by scientists at Columbia University’s Earth Institute, published Wednesday in the journal Nature, largely confirms what climate models have shown. In the absence of strong historic data on precipitation, those computer models forecast not only future scenarios, but shed light on historical trends.
The dovetailing of the tree-ring studies, which correlate to soil moisture, with climate models gives scientists the assurance that the computer models are, in fact, correct, they say.
“These tree-ring reconstructions let us go back in time and get a picture of global drought conditions for hundreds of years before the Industrial Revolution,” said Kate Marvel, an associate research scientist at the Earth Institute and NASA's Goddard Institute for Space Studies and the lead author of the study.
Marvel said that the results, while not unexpected, are nonetheless gratifying. “As a scientist, you are always surprised whenever anything you do works,” she said in a phone interview. “There were increased greenhouse gases in the early 1900s and the models say, ‘Hey, you should see a signal.’ But the fact that the signal is really clear in the models and apparent in the tree rings is pretty amazing. We can argue for a detectable human influence.”
Scientists have long predicted that as global warming increases, certain regions of the world, such as the southwestern United States, will become drier, while other areas will grow wetter. But the Intergovernmental Panel on Climate Change has cautioned against ascribing particular rain or drought events to human activity.
The tree-ring data analyzed in the study highlight three periods over the past 120 years in which a human fingerprint on drought and moisture is, to varying degrees, evident. The first, from 1900 to 1949, reveals the strongest signal, mirroring climate models showing that parts of the world from Australia to the Mediterranean were drying as other regions, including swaths of central Asia, were moving in the opposite direction.
The next period, from 1950 to 1975, is more murky, though the tree rings again match the climate models. The study posits that aerosols—particles from car exhaust and the burning of fossil fuels—were so abundant before the advent of anti-pollution measures that they served to block sunlight and thus cool the planet, even while tracking the uptick in greenhouse gases.
The last period, from 1981 to 2017, saw a reappearance of the human influence on drought and moisture. The study concludes that this signal is “likely to grow stronger in the next several decades,” adding that the “human consequences of this, particularly drying over large parts of North America and Eurasia, are likely to be severe.”
The study relied on a series of drought atlases containing tree-ring data from around the globe over the past two millennia. The atlases were assembled primarily by Edward Cook, a scientist at Columbia’s Lamont-Doherty Earth Observatory and the father of Benjamin Cook, one of the study’s co-authors.
Gregory R. Quetin, a postdoctoral researcher in Stanford’s Department of Earth System Sciences, who was not involved in the study, called the findings “powerful,” pointing to the climate record encoded in the tree rings.
“The trees are responding to temperature and the presence of, or lack of, rainfall,” he said, “so they give you these observations—and another path of evidence that this is occurring.”
Abigail Swann, an atmospheric scientist at the University of Washington, agreed: “It’s a creative way to leverage that information from the past.”