Global warming's "fingerprint" found in drought trends as early as 1900

Scientists have detected the "fingerprint" of human-caused global warming on drought patterns around the world dating back as long ago as 1900, according to a new study published Wednesday. The research shows how various human influences, from greenhouse gas emissions to pollutants that contribute to smog can influence soil moisture on a global scale.

Why it matters: While global warming has altered temperature and precipitation, and has exacerbated individual droughts in some parts of the world, scientists have been unable to detect a global warming signal in the occurrence or severity of droughts worldwide. This new study, published Wednesday in the journal Nature, attempts to address the detection of a human influence on drought since emissions of planet-warming greenhouse gases began to increase at the start of the Industrial Revolution.

What they did: The researchers, led by Kate Marvel, a climate scientist at NASA, and Benjamin Cook, a tree ring specialist at Columbia University, used large drought atlases of North America, Europe and the Mediterranean, Mexico, parts of Asia and Australia and New Zealand. These drought databases go back to 1400 A.D., and in some cases all the way back to 1100 A.D., based on tree ring data.

• For the study, the researchers constructed a unified global drought atlas, comprised of the data in each of the regional analyses.
• Specifically, they examined the atlases' records of a drought metric known as the Palmer drought severity index, which indicates soil moisture variability. They used the changes in soil moisture to understand how this drought metric has been changing in the 20th and 21st centuries, and to estimate naturally forced and internal climate variability prior to the industrial revolution.
• For the study, researchers sought to detect the expected fingerprint, or pattern, of human-induced drought trends on a global scale, when compared to sources of internal variability, such as ocean climate cycles like El Niño.
• They used computer modeling to get an idea of what global drought patterns they should expect to see in a warming world.

“Fundamentally this paper was asking the question, how well do the observations match what the models predicted should have happened over the course of the 20th century, in terms of hydroclimate. And the answer to that is they actually match really, really well."

What they found: The study found that human emissions of greenhouse gases clearly affected global drought trends from 1900 through 1949, but that emissions of tiny particles known as aerosols from coal-fired power plants, cars and factories reduced this signal from 1950 through 1975.

• Since the Clean Air Act in the U.S. and other clean air laws elsewhere reduced such pollution, the study finds, the fingerprint of emissions of greenhouse gases has reemerged, but it's not yet fully separated from the background noise of natural climate variability.
• The researchers expect the climate change signal on global drought trends to become clearer in the coming years, drying out many regions of the globe with potentially severe societal consequences.

• The researchers found that parts of North America, central America, Eurasia and the Mediterranean have grown drier, while other areas, such as India, have become wetter over time.

“What we call the the fingerprint in the paper is basically the expected pattern, just like if you’re looking for fingerprints at a crime scene, you’re looking for a particular pattern,” Marvel tells Axios.

What they're saying:

"... The essential physics is quite clear," says Michael Wehner, a climate scientist at Lawrence Livermore National Laboratory who was not involved in the new research. "Human-induced temperature increases will lead to drier soils due to increases in evapotranspiration. The impact on ecosystems and agriculture is clearly adverse and a cause for yet more concern about the human interference in the climate system."

The study is not without its critics, however.

• Kevin Trenberth, a senior climate scientist at the National Center for Atmospheric Research in Boulder, Colorado tells Axios that its methods and conclusions are flawed in part because the computer models it uses fail to accurately capture one of the leading causes of natural climate variability, El Niño and La Niña.
• Trenberth also criticized the research questions pursued in the study, saying it would have been more appropriate to examine how droughts caused by natural variability have become more intense, longer-lasting and faster to set in.