Scientists see fingerprint of warming climate on droughts going back to 1900

#human-drivenglobalwarming; #climatechange

Columbia, May 2 (Canadian-Media): In an unusual new study, scientists say they have detected the fingerprint of human-driven global warming on patterns of drought and moisture across the world as far back as 1900, Columbia University research shows.

Rising temperatures are well documented back at least that far, but this is the first time researchers have identified resulting long-term global effects on the water supplies that feed crops and cities. Among the observations, the researchers documented drying of soils across much of populous North America, central America, Eurasia and the Mediterranean. Other areas, including the Indian subcontinent, have become wetter. They say the trends will continue, with severe consequences for humans. The study appears this week in the leading journal Nature.

Image credit: Adapted from Marvel et al., Nature, 2019: Regions projected to become drier or wetter as the world warms. More intense browns mean more aridity; greens, more moisture. (Gray areas lack sufficient data so far.) A new study shows that observations going back to 1900 confirm projections are largely on target. 

In general, scientists agree that as global warming progresses, many now dry regions will become drier, and wet ones will become wetter. Some recent studies suggest that human-induced warming has intensified droughts in particular regions, including a now near 20-year ongoing drought in the southwestern United States. However, the last report by the Intergovernmental Panel on Climate Change says confidence in attributing specific ongoing events directly to humans is still chancy.
The new study combines computer models with long-term observations to suggest that systemic changes in what scientists call the hydroclimate are already underway across the world, and have been for some time. The researchers looked not simply at precipitation, but rather soil moisture, a more subtle measure that balances precipitation against evaporation, and is the quality most directly relevant to farming and forestry. They used tree rings going back 600 to 900 years to estimate soil moisture trends before human-produced greenhouse gases started rising, then compared this data with 20th-century tree rings and modern instrumental observations, to see if they could pick out drought patterns matching those predicted by computer models, amid the noise of natural yearly or decadal regional weather variations.

Image Credit: NASA's Scientific Visualization Studio: Global ‘drought atlas’ data derived from tree rings running from 1400-2005. Green is wetter and brown is dryer. No titles. Note that the timing of the frames slows during the years 1900-2005. 

"We asked, does the real world look like what the models tell us to expect?" said study coauthor Benjamin Cook of the NASA Goddard Institute for Space Studies and Columbia University's Lamont-Doherty Earth Observatory. "The answer is yes. The big thing we learned is that climate change started affecting global patterns of drought in the early 20th century. We expect this pattern to keep emerging as climate change continues."

Lead author Kate Marvel, a climate modeler at Goddard and Columbia University, said, "It's mind boggling. There is a really clear signal of the effects of human greenhouse gases on the hydroclimate."
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Soil moisture is a complex issue, because precipitation and evaporation can work with each other, or against each other. Warmer air can carry more moisture, and thus more rain or snow. But warmer air can also evaporate more moisture from soil and carry it away, outweighing precipitation. That is probably the factor now at work in the drying western United States, and possibly other locations that have seen recent big droughts. "Precipitation is just the supply side," said study coauthor Jason Smerdon, a Lamont-Doherty paleoclimatologist. "Temperature is on the demand side, the part that dries things out." Which part predominates depends on complex factors including wind patterns, seasons, clouds, topography and proximity to the moisture-giving oceans.

​The scientists identified three distinct periods in their study. The first was 1900 to 1949, when they say the global-warming fingerprint was the most obvious. During this time, as predicted by models, drying was seen in Australia, much of central America and North America, Europe, the Mediterranean, western Russia and southeast Asia. At the same time, it got wetter in western China, much of central Asia, the Indian subcontinent, Indonesia and central Canada.