Lower soil moisture conditions are known to positively contribute to warmer temperatures, in particular during heatwaves. Inner East Asia is one of the regions where this coupling is likely significant. Writing in Science, Zhang and co-workers found that the recent coincidence of drought and hot temperatures is unprecedented in a 260 year long record derived from tree rings.
Inner East Asia is one of the regions where the coupling between soil moisture and temperature is believed stronger than the global average (see figure). However putting current observations in a historical context is difficult due to a lack of long-term observations. However yearly tree ring widths are sensitive to both soil moisture conditions and extreme temperatures, and as a result tree ring records can be used to reconstruct past conditions in soil moisture and temperature simultaneously. In an analysis for Inner East Asia, Zhang et al. conclude that the recent conditions are unprecedented, and that they reflect a transition to a new, much drier, state where the temperature is becoming much less sensitive to soil moisture conditions.
Interestingly, the study is seemingly at odds with claims made in other previous studies. In a 2018 study that appeared in Science Advances, Hessl et al. found that recent droughts were extreme from a historical perspective, but they found no indications that future droughts were going to get worse, in spite of the ongoing warming in the region. In a separate study that appeared in 2015 in Scientific Reports, Liu et al. investigated the drivers behind observed soil moisture trends in Northern China. In that study, it was concluded that the drying trends were consistent with the effects of agricultural intensification rather than warming. The study by Zhang et al. highlights the significance of the changes that are going on in the region, but it does not provide clear proof that the increasing temperature extremes are indeed the result of worsening drought through soil moisture-temperature coupling. It remains possible that both are caused by externally-forced changes in atmospheric circulation.
Further reading
Hessl, A. E., et al. (2018), Past and future drought in Mongolia. Science Advances, 4(3), e1701832, doi:10.1126/sciadv.1701832.
Liu, Y. et al. (2015), Agriculture intensifies soil moisture decline in Northern China. Sci. Rep. 5, 11261; doi:10.1038/srep11261.
Miralles, D. G., et al. (2012), Soil moisture-temperature coupling: A multiscale observational analysis, Geophys. Res. Lett., 39, L21707, doi:10.1029/2012GL053703.
Zhang, P., et al. (2020), Abrupt shift to hotter and drier climate over inner East Asia beyond the tipping point. Science, 370(6520), 1095-1099, doi:10.1126/science.abb3368.