Fingerprint of Climate Change on Drought Traced to 1900

(CN) – While scientists have known that rising temperatures and the effects of climate change date back to the very early 20th century, researchers revealed Wednesday the fingerprint of climate change on drought and the long-term effect on global water supplies can be traced to 1900.

Scientists studied computer models in conjunction with long-term observations to analyze what scientists call the hydroclimate, finding that widespread changes in water levels have been underway across the world for some time.

The researchers looked at both precipitation and soil moisture levels, using tree rings going back 600 to 900 years to estimate soil moisture trends before human-produced greenhouse gases started rising. They then compared this data with 20th century tree rings and modern instrumental observations.

It is generally accepted that as global warming progresses, dry regions will become drier and wet ones will become wetter. Recent studies suggest that human-induced warming has intensified droughts in some regions, such as the nearly 20-year ongoing drought in the southwestern United States.

Researchers for the study broke their data into three time periods: 1900 to 1949, 1950 to 1975, and 1976 to the present. They found the clearest indicators of warming patterns were visible in the 1900 to 1949 period, which saw drying 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.

From the 1950s to the 1970s the data was less clear, which researchers believe is due to the enormous amounts of industrial aerosols poured into the air without modern pollution controls. But even once governments started instituting clear air laws and aerosols leveled off and dropped, greenhouse-gas emissions continued spiraling up, along with temperatures.

As a result, the researchers say, the global-warming signature on hydroclimate re-emerged beginning around 1981. The signal is not yet as obvious as it was in the early 20th century but it continues to rise, especially since around 2000.

Lead author Kate Marvel, a climate modeler at NASA Goddard Institute for Space Studies and Columbia University, said, “It’s mind boggling. There is a really clear signal of the effects of human greenhouse gases on the hydroclimate.”

Precipitation over much of Central America, Mexico the central and western United States and Europe is projected to stay about the same, or even increase. But according to both the new study and a separate 2018 paper, rising temperatures and the resulting evaporation of moisture from soils in those regions will probably predominate.

The Mediterranean region is expected to be hit with a double whammy of both less rainfall and more heat-driven evaporation. Adding to the drought dynamics of all the affected areas: Earth’ population will continue to rise, adding to water demand. According to an earlier Lamont-Doherty study, a 2006-2010 drought leading up to the disastrous Syrian civil war was probably made more likely by warming climate, and the drought may have helped create the social and economic conditions that sparked the initial rebellion.

Some areas are expected to get wetter, but this may not necessarily be good. India and some surrounding nations are expected to get more rain, because they sit squarely in the path of monsoon winds that pick up moisture from the Pacific and Indian oceans, and those oceans are getting warmer. But the rain may come perhaps more often in overwhelming storms, and not necessarily at times when it is needed.

“This important paper offers new insights into the link between increasing atmospheric greenhouse gases and regional droughts, both in the past and increasingly in the future,” said Peter Gleick, cofounder of California’s Pacific Institute and an expert on climate and water issues. “It also confirms the growing sophistication of our climate models and improves the tools available to detect and identify the fingerprint of human impacts on extreme hydrologic events.”

Study co-authors include Benjamin Cook of the NASA Goddard Institute for Space Studies and Columbia University’s Lamont-Doherty Earth Observatory, Céline Bonfils and Paul Durack of Lawrence Livermore National Laboratory, and A. Park Williams of Lamont-Doherty.

The study appears in the journal Nature.

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