The climate in inner East Asia may already have reached a tipping point, where recent years' transition to abnormally hot and dry summers can be irreversible. This is the finding of a new international study by researchers at University of Gothenburg now published in Science.
Associated with the ongoing global warming, are changes that impact regional climate and ecosystems. In a worst- case scenario, these can reach what is known as a tipping point, at which point changes are fast and often times irreversible. Examples of tipping points are the sea ice in the Arctic disappearing in the summer or the melting of the Greenland ice-sheet.
Inner East Asia is a sensitive area
Inner East Asia, which includes Mongolia and nearby areas, is a sensitive region that has experienced a clear increase in the number of heat waves during the summer in recent decades. Together with stable high-pressure systems, which raise temperatures, reduced soil moisture can cause intense and long-lasting heat waves because of enhanced interaction between the land surface and the atmosphere.
"What this connection looks like in a longer preindustrial context is, however, unknown since long-term observations do not exist," says Deliang Chen, co-author to the study and leader of the Regional Climate Group (RCG) at the University of Gothenburg.
Annual growth rings from trees provide information on changes that effect their growth and this can be used to study changes in climate in the past.
"By choosing trees whose growth are sensitive to weather variations, the annual rings can be used to reconstruct different climate parameters with annual resolution hundreds of years back in time. Since trees have significant geographic coverage, this data can be used for detailed studies of climate changes over large areas," says Hans Linderholm, co-author of the study and head of the Gothenburg University Laboratory for Dendrochronology (GULD).
Study of tree rings showed a tipping point is close
Long-term observations of soil moisture are rare, but tree rings from trees, which are limited by access to water, can be used as indicators of this parameter. In the same way, trees that grow at high altitudes, where the growing season is short and cool, can be used to provide temperature-related information.
"In this study, we developed a new method for reconstructing both variations in soil moisture and changes in frequency of heat waves in inner East Asia, a region where the interaction between these parameters is very strong," says Peng Zhang, first author of the study and researcher in the Regional Climate Group at the University of Gothenburg.
These new reconstructions allow the scientists to study the recent warm and dry summers in a long-term perspective. The results show that the current high frequency of heat waves and low soil moisture have not been observed during the last 260 years.
"By combining observations, reconstructions and climate model data, we discovered that the link between land surface and atmosphere has become more pronounced in inner East Asia over the last 20 years, along with increased drying of soils. So we argue that reduced soil moisture enhances land-atmosphere coupling contributing to heating of the land surface, which causes more heat waves, which in turn reduces soil moisture and so on," says Peng.
The study's authors found that the recent pattern of increased warming and drought indicate that a tipping point in the climate is close, a change that could be irreversible and lead to a much dryer climate in the region.
"This would increase the stresses of ecosystems and societies in this already vulnerable region," says Peng.
