Warmer and drier irreversible climate over the interior of East Asia

According to an international group of climatologists, Mongolia’s semi-arid plateau may soon become as barren as parts of the American Southwest due to a “vicious cycle” of heat waves – which makes soil more dry and dry. eventually produces more heat waves.

Write in the journal Science, researchers warn that heat waves and accompanying droughts have increased dramatically over the past two decades, with worrying implications for the future. Using tree rings data, which provides insight into regional climates from before modern weather logs, the researchers developed records of heat waves and soil moisture that suggest that in recent consecutive years of temperatures and record droughts are unprecedented for more than 250 years.

According to the results of the study, the record temperatures in the region are accelerated by the drying out of the soil, and together these changes amplify 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 loss from the ground, which means more heat waves – and where it might end, we can’t tell. “

When the soil is moist, evaporation cools the air at the surface. However, when the soil is free of moisture, the heat is transferred directly to the air. In their paper, Sudden Shift to Hotter, Drier Climate in East Asia Beyond Tipping Point, the authors state that over the past 260 years, only the past decades “show significant anticorrelation between frequency of heat waves and soil moisture, along with a drastic drop in soil. fluctuation in humidity. ”Scientists note that a series of recent heat waves in Europe and North America reveal the link with air and soil moisture near the surface and suggest that “the semi-arid climate of this region has entered a new regime in which soil moisture is no longer abnormally attenuating. high air temperature. “

Already, the lakes of the Mongolian plateau have seen rapid reductions. In 2014, Chinese researchers documented a 26% decrease in the number of lakes larger than one square kilometer, with even larger average reductions in the size of the region’s largest lakes.

“Now we are seeing that it is not just large bodies of water that are disappearing,” said correspondent author Jee-Hoon Jeong from National Chonnam University in South Korea. “The water in the soil also disappears.”

“This can be devastating for the region’s ecosystem which is essential for large herbivores, such as wild sheep, antelopes and camels,” Peng Zhang, lead author of the study and researcher at Gothenburg University. “These amazing animals are already living on the edge, and these impacts of climate change can push them.”

Co-author Jin-Ho Yoon, from the Gwangju Institute of Science and Technology in South Korea, noted that the hundreds of years of dark circles data clearly show that the confluence of rising waves of summer heat and severe droughts is unique in the context of the past. 260 years old. Co-author Hans Linderholm, University of Gothenburg, said the trees used in the analysis appear to “feel” heat waves throughout their lives.

“Conifers react strongly to abnormally high temperatures,” Linderholm said. “By looking at their growth rings, we can see their response to recent heat waves, and we can see that they don’t seem to have experienced anything like this in their very long lives.”

The rings of the trees examined in the study were mainly collected from the Mongolian plateau, suggesting that the increased heat affects plants even at high altitudes.

Daniel Griffin, of the Department of Geography, Environment, and Society at the University of Minnesota, who is not involved in this study but has reviewed the document, said the long-term perspective for these tree rings illustrates a nuanced picture of climate change now afflicting vast swathes of the interior region of East Asia.

“It’s one thing to recognize that ‘normal’ weather conditions are changing. However, what worries me most is thinking about extreme events in the future – how serious could they become?” Griffin asked. “And if the ‘new normal’ is extremely hot and dry by historical standards, then future extremes may well be unlike anything seen before.”

While warmer and drier trends are observed in Europe and Asia, Mongolia and its surrounding countries are of particular interest to climatologists because this region of interior East Asia has a very direct connection to global atmospheric circulations. .

“Summer atmospheric waves tend to create a pattern of high pressure ridges around Mongolia that can linger for weeks, triggering heat waves,” explained co-author Simon Wang of Utah State University in the States. -United. “Global warming is amplifying these atmospheric waves, increasing the risk of prolonged or intensified overpressure over Mongolia and this may also have ramifications in the northern hemisphere.”

“Such 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 taken place in which an irreversible feedback loop is triggered and accelerates the region towards a warmer, drier future.”

Indeed, the researchers observed that the recent heat waves arrived with even drier and warmer air, under the enhanced high-pressure peak, than the heat waves of the past.

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

This research appears in the November 27, 2020 issue of Science.