Relative importance of increased atmospheric CO₂ concentration and local moisture deficit to hot extremes

Abstract. This study identifies which factor, increased atmospheric CO₂ concentration or local moisture deficit, dominates the temporal occurrence of hot extremes at the global scale. The wavelet decomposed GRACE Terrestrial Water Storage (TWS) is for the first time applied in examining the relationship between soil moisture (θ) and number of hot days in the hottest month (NHD). It reveals stronger θ–NHD relationships over larger areas than other commonly used soil moisture proxies (i.e., standardized precipitation index (SPI) and model derived product). During the study period 1985–2015, hot extreme occurrence with a dominant influence from increased atmospheric CO₂ concentration is mainly observed in South America, Africa and Asia, while soil moisture deficit dominates the occurrence of hot extremes in larger areas, including parts of North America, West Europe, Australia, Southeast Asia and South Africa. Global action in reducing emissions will support combating hot extremes. In addition, important attention should be directed to address, e.g. by adaptive land management, the increasing moisture deficit in some regions.