Irreversible warmer and drier climate over Inner East Asia

Heatwaves push Mongolian weather to a tipping point

Mongolia’s semi-arid plateau may soon become barren like parts of the American Southwest due to a “vicious circle” of heat waves – which exacerbate soil drying and eventually produce more heat waves – according to an international group of climate scientists.

Writing in the diary Science, the researchers warn that concomitant heatwaves and droughts have increased significantly over the past two decades, with worrying implications for the future. Using tree-ring data, which offers a glimpse of regional climates before modern weather records, the researchers have developed records of heatwaves and soil moisture that suggest the last consecutive years of record high temperatures and droughts are unprecedented. in over 250 years.

According to the study’s findings, record high temperatures in the region are accelerated by soil drying and together these changes are amplifying the decline in soil water. “The result,” said co-author Deliang Chen of the Swedish University of Gothenburg, “is more heat waves, which means more water losses into the soil, which means more heat waves – and we can’t say where this might be. to end”.

When the soil is moist, evaporation cools the air on the surface. However, when the soil runs out of moisture, the heat is transferred directly to the air. In their paper, Abrupt Shift to Warmer and Drier Climate in Inner East Asia Beyond the Turning Point, the authors argue that in the past 260 years, just the last few decades “show a significant anticorrelation between heat wave frequency. and soil moisture, along with a radical drop in soil moisture fluctuation. “Scientists note that a series of recent heat waves in Europe and North America reveal the connection with near-surface air and the soil moisture and suggest that “the semi-arid climate of this region has entered a new regime in which soil moisture no longer fades to abnormally high air temperatures.”

The lakes of the Mongolian plateau have already undergone rapid reductions. As of 2014, Chinese researchers had documented a 26% decrease in the number of lakes larger than one square kilometer, with even larger average reductions in size for the region’s largest lakes.

“Now we are seeing that not only large bodies of water are disappearing,” said corresponding author Jee-Hoon Jeong of Chonnam National University in South Korea. “Soil water is also disappearing.”

“This can be devastating to the region’s ecosystem which is critical to large herbivores, such as wild sheep, antelope and camels,” Peng Zhang, lead author of the study and researcher at the University of Gothenburg. “These wonderful animals already live on the fringes, and these impacts of climate change could push them further.”

Co-author Jin-Ho Yoon, of the Gwangju Institute of Science and Technology in South Korea, noted that hundreds of years of tree ring data make it clear that the confluence of increasing summer heatwaves and severe droughts is unique in the context of the past 260 years. Co-author Hans Linderholm of the University of Gothenburg said the trees used in the analysis appear to “feel” heat waves throughout their life.

“Conifers respond strongly to anomalous temperatures,” Linderholm said. “By examining their growth rings, we can see their response to recent heatwaves and we can see that they don’t seem to have experienced anything like this in their very long lives.”

The tree rings examined in the study were collected primarily from the Mongolian plateau, which suggests that the increasing heat is affecting the plants even at high altitudes.

Daniel Griffin, from the University of Minnesota Department of Geography, Environment, and Society, who is not involved in this study but reviewed the article, said the long-term perspective of these tree-ring records illustrates a picture. nuance of the changing climate that is now plaguing large areas of the Inner East Asia region.

“It is one thing to recognize that ‘normal’ climatic conditions are changing. However, what worries me most is thinking about the extreme events of the future: how serious can they get? “Griffin asked.” And if the “new normal” is extremely hot and dry by historical standards, then the future extremes may be different from anything previously observed. “

While warmer and drier trends are observed in Europe and Asia, Mongolia and surrounding countries are of particular interest to climate scientists because this region of Inner East Asia has a very direct link with global atmospheric circulations.

“Summer atmospheric waves tend to create a high-pressure ridge pattern around Mongolia that can persist for weeks, triggering heatwaves,” explained co-author Simon Wang of Utah State University in the United States. “Climate warming is amplifying these atmospheric waves, increasing the possibility of sustained or intensified high pressure on Mongolia and this may also have ramifications in the Northern Hemisphere.”

“This large-scale atmospheric force is further amplified by local interactions with the Earth’s surface,” said co-author Hyungjun Kim of the University of Tokyo in Japan. “An even worse problem may have already occurred where an irreversible feedback loop is triggered and is accelerating the region into a warmer, drier future.”

In fact, the researchers observed that the recent heat waves have come with even drier and warmer air, under the reinforced crest of high pressure, than the heat waves of the past.

The research team found that the simultaneity of heating and drying appears to be approaching a “tipping point” and is potentially irreversible, which could push Mongolia into a state of permanent aridity.

Reference: “Sudden transition to warmer, drier climate over Inner East Asia beyond the turning point” by Zhang, P., J.-H. Jeong, J.-H. Yoon, H. Kim, S.-Y. Wang, HW Linderholm, K. Fang, X. Wu, D. Chen, November 27, 2020, Science.