Harmful heatwave events that cause serious damage to human society and the natural environment have occurred with increasing frequencies worldwide. In this paper, we classify summer heatwave events in the Northern Hemisphere through a new method considering more com-prehensive event characteristics of the intensity, duration, affected area and humidity amplification. Apart from low-severity events in which all event characteristics are at normal levels, there are also five main different high-severity event types in nature, including high-intensity, high-humidity-amplification, long-duration, large-area and long-duration-large-area events. Different types of high-severity events usually occur at different locations in the Northern Hemisphere. The proportion of high-severity events, which is calculated as the ratio of the high-severity event number to the total heatwave number, increased significantly after 1989, and these increases mainly came from large-area and long-duration-large-area events. Further model analyses using the Detection and Attribution Model Intercomparison Project and Scenario Model Intercomparison Project of CMIP6 show that greenhouse gas-related external forcing is the leading cause of the significant increases in these two types of heatwave events. Anthropogenic activities are suggested to cause more rapidly increasing occurrences of extreme heat-waves with large affected areas and heatwaves with both long durations and large affected areas.
 

Summer heatwave,heatwave classification,global warming,anthropogenic forcing,CMIP6