In June and July of 2020, Eastern China was hit by extreme Meiyu rainfall, with a particularly long rainy season and frequent extreme rainfall events. The accumulated Meiyu rainfall in the Yangtze River Valley in 2020 was 54% greater than normal, breaking the record since 1961. Having caused extensive floods and economic and societal losses, it is highly concerned whether human activities have played a role in the occurrence of such extreme events.
Human activities have reduced the occurrence probability of persistent heavy Meiyu rainfall events in the lower reaches of the Yangtze River Valley as that in observations in 2020 by approximately 46%, according to a study recently published in SCIENCE CHINA Earth Sciences. This research reveals the anthropogenic influence on the occurrence of extreme Meiyu rainfall events, particularly the tug of war between greenhouse gases and anthropogenic aerosols.
To assess the human influence, scientists used multi-model simulations that participate in the Detection and Attribution Model Intercomparison Project (DAMIP) in the Coupled Model Intercomparison Project Phase 6 (CMIP6). These simulations are driven by different external forcings, e.g., anthropogenic forcing, greenhouse gas forcing and aerosol forcing, and are thus useful tools in attribution studies.
“Comparing model simulations with and without anthropogenic forcing, we show that human activities have reduced the probability of persistent heavy rainfall events as that in observations in the lower reaches of the Yangtze River in 2020,” said ZHOU Tianjun, the leading and corresponding author on the paper. Zhou is a senior scientist at the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences. He is also a professor at the University of Chinese Academy of Sciences.
The two major anthropogenic forcing components – greenhouse gases and aerosols – had opposite influences, though. On one hand, greenhouse gas emissions would have increased the probability by approximately 44%, as a result of atmospheric warming and moistening. On the other hand, this effect was offset by anthropogenic aerosols, which reduced the probability by approximately 73%, by reducing atmospheric moisture and weakening the East Asian summer monsoon circulation.
“The attribution analysis demonstrates that human influences on extreme events rely heavily on the components of external forcing, in particular, the relative contributions of greenhouse gases and aerosols,” added Dr. Wenxia Zhang, the co-author of the study. “It is worth noting that the attribution of present changes is not simply analogous to future climate, with the changing greenhouse gas and aerosol emissions in the future.”
With the continuous emissions of greenhouse gases and reductions in aerosols in the future, similar persistent heavy rainfall events are projected to occur more frequently. Higher greenhouse gas emissions are expected to lead to higher occurrence probabilities. Thus, efficient mitigation measures will help to reduce the impacts related to extreme rainfall.
See the article: Zhou T, Ren L, Zhang W. 2021. Anthropogenic influence on extreme Meiyu rainfall in 2020 and its future risk. Science China Earth Sciences, https://doi.org/10.1007/s11430-020-9771-8
Editor: GAO Yuan