Compound climate extremes in China: Trends, causes, and projections

Human society and ecosystems are impacted by climate extremes more than by climate averages. In contrast with climate extremes that are driven by individual climatic variables, compound climate extremes stem from a combination of multiple climatic drivers and usually lead to more severe risks than the former extremes do. The influences of the drivers for compound climate extremes are strengthening under global warming. Firstly, this review details the development of the definition of compound climate extremes over recent years and describes different types of events, such as compound drought and heatwave extremes (CDHEs), compound day and night heat extremes (CDNHEs), and compound flooding (CF). Secondly, historical trends in compound climate extremes in China over the past half-century, and projections of future trends under different scenarios, are discussed. For example, this study points out that a large part of China has experienced longer, stronger, and more frequent CDHEs than other parts of the country. CDHEs have followed a significantly increasing trend since the 1990s, and this trend is projected to strengthen further under different scenarios in the future. Thirdly, this study reviews different potential causes for compound climate extremes, including the internal variability of the climate system (e.g., land and atmosphere feedbacks, large-scale circulation patterns) and external anthropogenic forcings (e.g., urbanization and anthropogenic emissions). In this study, we summarize risks from different perspectives by considering interactions between hazards, vulnerability, and exposure. Many studies show that risks to infrastructural damage and population exposure are projected to increase in the future, and that crop yields and ecosystem gross primary production are likely to reduce. Finally, we generalize our study and show that there is an urgent need for a comprehensive study of different combinations of compound events. We argue that it is important that we understand the key dynamic and thermal processes that are modulated by specific drivers and investigate the uncertainty in the projected variabilities for compound climate extremes. This requires interdisciplinary collaboration and will allow appropriate risk adaptation strategies to be developed. There has been great progress in research into compound climate extremes; however, an improved understanding of the mechanisms and risks is necessary as a theoretical basis for more effective climate adaptation policies.