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droughts

Tree rings shed light on past—and future—droughts

Researchers have developed a climate record stretching 2,060 years into Mongolia’s past by using the natural archive of weather conditions stored in the rings of Siberian pines.

The researchers also combined information on past climate from the tree rings with computer models that can project future regional climate.

According to the researchers, the extreme wet and dry periods Mongolia has experienced in the late 20th and early 21st centuries are rare but not unprecedented and future droughts may be no worse.

The most recent extended drought in Mongolia lasted from 2000 to 2010 and resulted in major livestock die-offs and a massive migration of nomadic herders to the capital city.

Khorgo lava field (droughts in Mongolia)
Researchers Kevin Anchukaitis and Amy Hessl walk across Mongolia’s Khorgo lava field to collect cross-sections of dead trees. (Credit: Neil Pederson/U. Arizona)

“We were able to quantify how unusual this drought was,” says coauthor Kevin J. Anchukaitis, an associate professor in the School of Geography and Development at the University of Arizona. “The drought was not unprecedented, but it has a 900-year return interval. It’s a once-in-a-millennium drought.”

Future droughts

Finding that future droughts would likely be no worse than those of the past was a surprise, says Anchukaitis, who led the modeling team. In other semi-arid regions of the world that he has studied, such as California and the Mediterranean, global warming already has changed precipitation and temperature patterns and thereby increased the risk of long-term drought.

Mongolia, a landlocked country in central Asia, has long, cold winters and short summers. Much of the country is cold, semi-arid grasslands that resemble eastern Montana.

“You would expect, based on everything we’ve been thinking about and reading as climate scientists, that elevated temperatures are going to lead to more severe droughts in semi-arid regions. But the models did not project increased frequency or severity of droughts,” says lead author Amy Hessl of West Virginia University.

The timing of the rainy season is the likely cause, the researchers say. Mongolia’s rainy season is in the summer, the warmer time of the year, whereas California and the Mediterranean have winter rains and dry summers. As global temperatures increase, continental regions with summer rains may get more precipitation, offsetting the effects on plants of higher temperatures.

Anchukaitis, who also has appointments in geosciences and in the Laboratory of Tree-Ring Research at UA, is interested in how past and current civilizations dealt with drought and climate variability. This new research is an outgrowth of previous research he, Hessl, and their colleagues conducted to figure out how past climate influenced the Mongol civilization.

Siberian pine forest (droughts in Mongolia)
Researchers constructed a drought history of Mongolia using tree rings from the Siberian pines growing on the Uurgat lava field, shown here with the Khangai Mountains in the background and seasonal homes of nomadic herders in the lower right. (Credit: Scott Nichols/U. Arizona)

Anchukaitis and his colleagues used their tree-ring record of past climate in Mongolia to reconstruct what the annual Palmer Drought Severity Index, or PDSI, would have been going back in time 2,060 years. The PDSI combines both temperature and precipitation to get a measure of soil moisture, one measure of how water-stressed a plant would be.

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He and his colleagues combined the reconstruction of past annual PDSI measures with a set of climate model simulations called the Community Earth System Model to understand what influenced the Mongolian droughts for the period from 850 to 2100.

‘Tug of war’

The model incorporates information about past solar variability, volcanic eruptions, land use changes, and carbon dioxide emissions. For projections to the end of the 21st century, the model uses the future emissions scenario called RCP 8.5, in which the rate of emissions of greenhouse gases continues to increase.

Even with the highest level of greenhouse gas emissions and rising global temperatures, the model simulations indicate that future droughts in Mongolia would be no more severe than those of the past.

“There’s a tug of war between trends toward increased rainfall and more evaporative demand because of hotter temperatures. There’s uncertainty about which will win this tug of war,” Anchukaitis says.

“The simulations say that Mongolia dries between now and about 2050 because of higher temperatures, but then it turns around because of the increase in precipitation,” he says.

Many people in Mongolia are nomadic pastoralists. The vagaries of weather and climate particularly affect them, because the combination of winter and summer temperatures plus rainfall controls the number of cattle the grassland can support.

“What to me stands out is this deep uncertainty about the future, particularly when you have a society that is so vulnerable to climate variability,” he says, adding that uncertainty makes it hard to plan for coping with future climate change.

Anchukaitis says one of his next steps is translating the team’s estimates of future soil moisture into estimates of the future productivity of Mongolia’s grasslands.

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The researchers report their findings in the journal Science Advances. National Geographic and the National Science Foundation funded the research.

Additional coauthors are from West Virginia University in Morgantown; the Lamont-Doherty Earth Observatory, Palisades, New York; NASA Goddard Institute for Space Studies, New York City; the National University of Mongolia, Ulaanbaatar; Harvard University; and Auburn University, Alabama.

Source: University of Arizona

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