Scientists see the fingerprint of global warming on droughts dating back to 1900

In an unusual new study, scientists claim to have detected fingerprints of man-made global warming on drought and moisture patterns around the world as early as 1900. The temperature rise is well documented at least as far back as it may have been, researchers have identified the long-term global effects on the water resources that feed crops and cities. Among the observations, the researchers documented the drying up of soils in much of the population of North America, Central America, Eurasia and the Mediterranean. Other regions, including the Indian subcontinent, have become wetter. They say the trends will continue, with serious consequences for humans. The study appears this week in the main newspaper Nature.

In general, scientists agree that as global warming progresses, many dry areas will become drier and wetlands will become wetter. Some recent studies suggest that man-made warming has worsened droughts in some areas, including a nearly 20-year drought in the southwestern United States. However, the latest report of the Intergovernmental Panel on Climate Change indicates that confidence in the attribution of specific ongoing events directly to humans remains uncertain.

The new study combines computer models with long-term observations to suggest that systemic changes in what scientists call hydroclimate have already been underway worldwide for some time. The researchers looked at not only rainfall, but rather soil moisture, a more subtle measure that balances precipitation and evaporation, and is the most directly relevant quality for the rainy season. agriculture and forestry. They used 600- to 900-year-old tree rings to estimate trends in soil moisture prior to the increase in man-made greenhouse gases. They then compared these data with twentieth-century tree rings and modern instrumental observations. predicted by computer models, in the midst of the noise of regional annual or decennial natural weather variations.

"We asked if the real world looked like what models tell us to wait." said co-author of the study, Benjamin Cook, NASA's Goddard Institute for Space Studies and the Lamont-Doherty Earth Observatory of Columbia University. "The answer is yes, what we have learned is that climate change has begun to affect global patterns of drought in the early 20th century, and we expect this pattern to continue to emerge as climate change continues. "

Lead author Kate Marvel, a climate modelist at Goddard and Columbia University, said, "It's staggering. The effects of greenhouse gases on the hydroclimate are very clear.

Soil moisture is a complex problem because precipitation and evaporation can work with each other or against each other. Warmer air can carry more moisture and therefore more rain or snow. But warmer air can also evaporate more moisture from the soil and carry it off, compensating for precipitation. This is probably the state factor drying in the western United States and perhaps in other areas with severe droughts. "Precipitation is only on the supply side," said co-author of the study, Jason Smerdon, a paleoclimatologist from Lamont-Doherty. "The temperature is on the demand side, the part that dries things up." The predominant part depends on complex factors such as wind patterns, seasons, clouds, topography and the proximity of moisturizing oceans.

Scientists have identified three distinct periods in their study. The first was from 1900 to 1949, when they said that the fingerprint of global warming was the most obvious. As predicted by the models, dewatering has been observed in Australia, much of Central America and North America, Europe, the Mediterranean, Western Russia and Southeast Asia. At the same time, western China, much of Central Asia, the Indian subcontinent, Indonesia, and central Canada were wetter.

From 1950 to 1975, the model was dispersed into seemingly random events. Scientists believe that this could be related to the huge amounts of industrial aerosols spilled into the air without control of modern pollution. These can affect the formation of regional clouds, precipitation and temperature, including blocking solar radiation and providing nuclei for moisture droplets. Researchers believe that the complex effects of aerosols have probably wrenched the climate in many places, masking the effects of greenhouse gases, even though these gases have continued to increase.

Then, from the 1970s, many industrialized countries, including the United States, began to adopt increasingly stringent air purification laws. Although industrial activities continued to grow, aerosols quickly stabilized or declined slightly in many places. But at the same time, greenhouse gas emissions continued to rise at the same time as temperatures. As a result, according to researchers, the signature of global warming on the hydroclimate began to reappear around 1981. The signal is not yet as obvious as at the beginning of the twentieth century, but it continues to increase, especially since the 2000s.

"If we do not see it getting stronger in, say, the next 10 years, maybe we should ask ourselves if we are right," Marvel said. "But all models predict that you should see unprecedented drying soon, in many places."

Many of the dewatering areas are agricultural production centers and could become permanently arid. "The consequences for humans, especially drying out in large parts of North America and Eurasia, will likely be severe," the study says.

Precipitation over much of Central America, Mexico, Central and Western United States and Europe is expected to remain about the same or even increase. However, according to the new study and a separate document from 2018, rising temperatures and evaporation of soil moisture in these areas will likely be predominant. The Mediterranean region is expected to be affected by half as much rainfall and increased evaporation from heat. Adding to the drought dynamics of all affected areas: populations should continue to increase, adding to the demand for water. According to a previous study by Lamont-Doherty, a drought in 2006-2010 that led to the disastrous Syrian civil war was probably aggravated by the warming of the climate and the drought could have helped to create the social and economic conditions in the first place of the initial rebellion.

Some areas should become wetter, but this may not necessarily be good. India and some surrounding countries are expected to receive more rain as they sit in the direction of the monsoon winds that accumulate moisture in the Pacific and Indian Oceans, and these oceans are warming up. But rain may come more often in extreme storms, and not necessarily when necessary.

The new study was made possible in part by the recently published atlases of tree ring chronologies from thousands of sites around the world, dating back more than 2,000 years. These provided researchers with a baseline of temperature variation before humans began to affect it heavily. The atlases are largely the work of Lamont-Doherty's scientist, Edward Cook, father of co-author of the study, Benjamin Cook. The atlas of drought in North America was published in 2004, followed by that of the monsoon in Asia in 2010 and compilations for Europe and the Mediterranean, Mexico and Australia / New Zealand in 2015. (One for South America is en route; much of Africa remains uncovered.)

"This important document offers new information on the link between the increase in greenhouse gases in the atmosphere and regional, past and future droughts," said Peter Gleick, co-founder of the Pacific Institute of California , expert on climate and water. "This also confirms the increasing sophistication of our climate models and enhances the tools available to detect and identify the fingerprint of human impacts on extreme hydrologic events."

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The study was also co-authored by Celine Bonfils and Paul Durack of the Lawrence Livermore National Laboratory and by A. Park Williams of Lamont-Doherty.

contacts:

Kate Marvel [email protected] | 212 678 6038

Benjamin Cook [email protected] | 212 678 5669

Jason Smerdon [email protected] | 845 365 8493