Nov 27 2020
According to an international team of climate scientists, the semi-arid plateau of Mongolia may rapidly become as desolate as parts of the American Southwest because of a “vicious cycle” of heatwaves—that worsens soil drying and eventually creates more heatwaves.
Reporting in the journal Science, the scientists caution that heatwaves and simultaneous droughts have increased considerably over the last 20 years, with worrying repercussions for the future.
The team used tree-ring data, which provide a preview of regional climates from before modern weather logs, and created soil moisture and heatwave records indicating that recent consecutive years of record high temperatures and droughts are unparalleled in over 250 years.
The study’s conclusions reveal that the record high temperatures in the region are exacerbated by soil drying, and together, these deviations are increasing the decline of soil water. According to Deliang Chen, co-author of the study from Sweden’s University of Gothenburg, “The result, is more heatwaves, which means more soil water losses, which means more heatwaves—and where this might end, we cannot say.”
Evaporation cools air at the surface when soil is wet. But when the soil is depleted of moisture, heat transfers straight to the atmosphere. In the article titled “Abrupt shift to hotter and drier climate over inner East Asia beyond the tipping point,” the researchers note that in the last 260 years, only the past few decades “show significant anticorrelation between heatwave frequency and soil moisture, alongside a radical decline in soil moisture fluctuation.”
The researchers observe that a succession of recent heatwaves in North America and Europe expose the connection with soil moisture and near-surface air and propose that “the semi-arid climate of this region has entered a new regime in which soil moisture no longer mitigates anomalously high air temperature.”
At present, lakes in the Mongolian Plateau are undergoing fast reductions. As of 2014, scientists from China had reported a 26% reduction in the number of lakes measuring greater than 1 km2 in size, with an even larger average decrease in size for the largest lakes in the region.
“Now we are seeing that it isn’t just large bodies of water that are disappearing,” stated corresponding author Jee-Hoon Jeong of Chonnam National University in South Korea. “The water in the soil is vanishing, too.”
This may be devastating for the region’s ecosystem which is critical for the large herbivores, like wild sheep, antelope and camels. These amazing animals already live on the edge, and these impacts of climate change may push them over.
Peng Zhang, Study Lead Author and Researcher, University of Gothenburg
According to the co-author of the study Jin-Ho Yoon, from the Gwangju Institute of Science and Technology in South Korea, the hundreds of years of tree-ring data show that the combination of increased summer heatwaves and extreme droughts is unique in the framework of the past 260 years.
Hans Linderholm, the co-author of the study from the University of Gothenburg, stated that the trees used in the study seem to “feel” the heatwaves during the course of their lifetimes.
The conifer trees respond strongly to anomalously high temperatures. By examining their growth rings, we can see their response to the recent heatwaves, and we can see that they do not appear to have experienced anything like this in their very long lives.
Hans Linderholm, Study Co-Author, University of Gothenburg
Tree rings analyzed as part of the study were mostly collected from the Mongolian Plateau, which indicates that the increasing heat is influencing plants even at higher elevations.
Daniel Griffin, from the University of Minnesota’s Department of Geography, Environment and Society, who is not involved in this research but has reviewed the paper, noted that long-term outlook from these tree-ring records shows nuanced imagery of the shifting climate that is now affecting large strips of the inner East Asia region.
“It is one thing to recognize that the ‘normal’ climate conditions are changing. However, what concerns me the most is thinking about the extreme events of the future: how severe might those become?” asked Griffin. “And if the ‘new normal’ is extremely hot and dry by historical standards, then future extremes may well be unlike anything previously witnessed.”
Although drier and warmer trends are detected over Europe and Asia, Mongolia and its neighboring countries are uniquely interesting to climate scientists since this Inner East Asia region has a straight link to universal 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. “The warming climate is amplifying these atmospheric waves, increasing the chance of prolonged or intensified high-pressure to occur over Mongolia and this can also have ramifications across the Northern hemisphere.”
Such large-scale atmospheric force is further amplified by local interactions with the land surface. An even worse problem may have already occurred in which an irreversible feedback loop is triggered and is accelerating the region toward a hotter and drier future.
Hyungjun Kim, Study Co-Author, University of Tokyo
The team has, in fact, noted that recent heatwaves have led to even drier and hotter air, under the strengthened high-pressure ridge, compared to the heatwaves in the past.
The researchers learned that the warming and drying concurrence appears to reach a “tipping point” and is possibly irreversible, which may force Mongolia into a permanent state of drought.
Zhang, P., et al. (2020) Abrupt shift to hotter and drier climate over inner East Asia beyond the tipping point. Science. doi.org/10.1126/science.abb3368.