We're To Blame For Earth's Droughts - And It's Only Getting Worse

The human impact on global drought conditions is unmistakable, a new NASA study concludes ominously, with climate change only likely to worsen conditions unless addressed head-on. The new study is unstinting in its blunt assessment of the impact human behaviors are having on the planet, and the disruptions climate change can provoke on the essential conditions for life to continue.

"Climate change is not just a future problem," warns Ben Cook, climate scientist at GISS and Columbia University's Lamont-Doherty Earth Observatory in New York City, who co-authored the study. "This shows it's already affecting global patterns of drought, hydroclimate, trends, variability – it's happening now. And we expect these trends to continue, as long as we keep warming the world."

The study looked for evidence of potential human impact on global drought patterns through the 20th century. Specifically, it sought to find match between the so-called "fingerprint" of human impact on drying and wetting patterns that theories had predicted, with the actual results across the globe. If human behaviors really were to blame, the global patterns of regional drying and wetting characteristics of the climate should match up.

Sure enough, all the signs were there. The researchers used the Palmer Drought Severity Index, or PDSI, as a gage of water conditions. That takes an average of soil moisture over the summer months, calculating it from data that includes precipitation, runoff, and air temperature. Although NASA is now capable of making soil moisture measurements from space, that data only runs back to 1980.

In contrast, by using "drought atlases" based on tree ring thickness, the researchers could calculate drought conditions over centuries. "Taken together, modern soil moisture measurements and tree ring-based records of the past create a data set that the team compared to the models," NASA says. "They also calibrated their data against climate models run with atmospheric conditions similar to those in 1850, before the Industrial Revolution brought increases in greenhouse gases and air pollution."

Human impact on drought conditions first became significant in the early 20th century. While there was a brief respite into cooler and wetter conditions between 1950 and 1975, which the team believes was down to aerosol gases flooding the atmosphere before environmental regulations clamped down on them, the general trend suggests the model's fingerprint is a true reflection of reality.

The risks of increasing drought conditions are significant. NASA predicts that it could well lead to shortages of both food and water, prompting conflict as people fight for natural resources. Health impacts are likely, too, while parched forests and other previously green areas could fall victim to wildfires.

Importantly, the study tries to cut off arguments based on anecdotal evidence, such as pointing to unexpectedly high rainfall in particular areas. Instead this new NASA study looked to global drought, which can bypass the natural variability experienced by individual regions. "Combining many regions into a global drought atlas meant there was a stronger signal if droughts happened in several places simultaneously," NASA explains.

All the same, the results are likely to be controversial. Previous drought studies, while generally pessimistic in their conclusions, have typically fallen short of directly blaming human actions for the changes in water availability. This new study doesn't shy away from pinning the blame, however.

"Part of our motivation was to ask, with all these advances in our understanding of natural versus human caused climate changes, climate modeling and paleoclimate, have we advanced the science to where we can start to detect human impact on droughts?" Cook suggests. He – and the study – concludes that the answer is an ominous "yes."