Photo: AP

Climate change used to be thought of as a future problem. Now people are finally starting to view it as a present problem, but new research looking at drought shows it’s been stalking humanity for much longer.

The findings, published in Nature on Wednesday, use tree rings and climate models to take a global look at drought back to 1400 and compare it to what’s happened in the past 120 or so years. The results show that a clear human influence on global drought is apparent as early as 1900 and the influence is likely to become even more clear in the coming decades if carbon emissions keep rising.

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“Climate change—despite its new prominence—is not new,” Kate Marvel, a climate scientist at the NASA Goddard Institute for Space Studies who led the new study, told Earther. “It’s something that has been happening for a long time.”

Drought is a surprisingly tricky topic to understand. Sure, at its base it means things are dry. But drought in the Amazon and drought in the western U.S. are different. Ditto for what causes them, their timing, and their impacts. But it’s still vitally important to understand if drought is changing in general, because no water means no civilization.

In an effort to understand if humans are causing drought patterns to shift, scientists at NASA took a regional and global look at drought to sift through the noise of natural variability and the rising signal of climate change. Marvel likened the whole thing to listening to a symphony. Natural variability—your random weather events, things like El Niño, volcanoes, and more—have been playing a pretty standard tune for centuries. But climate change has altered the composition.

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Marvel and her colleagues listened for that human-driven note in the noise of regional drought records from tree rings as well as climate models. Their results show that drought has changed during three distinct periods, starting in 1900.

In the first few decades of the 20th century, Marvel told Earther “we can pick out that really subtle, really soft note [of climate change] against this sort of background of the symphony of natural climate variability.”

That period was marked by a clear change in global patterns of drought using an atmospheric metric called the Palmer Drought Severity Index, which is observable in the growth of tree rings. The change doesn’t mean the world just got drier, but rather that certain areas got drier while others got wetter in ways that scientists would expect as the climate warmed.

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But that note started to fade in the middle of the 20th century, a period that was also marked by a huge uptick in industrial activity and with it, the release of tiny particles called aerosols that reflect sunlight back into space. They actually cool the planet and may have helped slow global warming during that period, which may also be the reason climate change’s influence on drought got quieter.

But as the Clean Air Act and other regulations cleaned up the sky of those pollutants while letting carbon dioxide emissions run rampant, the human drought signal began to re-emerge in the 1980s. And it will get clearer still in the 21st century.

The findings present a global look at drought, but obviously there’s huge variability between regions and even within them. Interestingly, the signal of climate change isn’t nearly as loud in specific regions as it is when looking at the globe as a whole, but it’s certainly there.

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“For the U.S., these findings are less definitive, because it’s only by combining the data from a larger area that the climate change signal becomes clearly event,” Connie Woodhouse, a tree ring researcher at the University of Arizona, told Earther in an email. “However, the same general findings are likely to apply: that anthropogenic climate change has had an influence on drought over the 20th and 21st centuries, primarily as warming. This make sense to me, and coincides with what I’ve seen in my work [showing] that temperatures are having an increasing impact on other hydroclimatic measures, like annual streamflow.”

More broadly, Marvel said the findings show that climate models are pretty damn good tools at explaining changes in the past and the role humans have played. That means they’re likely to be pretty damn good tools at projecting the future, too.