The likelihood of experiencing hot extremes has drastically increased due to global warming. Using 554 ground-based air temperature stations, changes in the number of hot days (NHD) in the Chinese mainland during 1960 to 2011 was investigated. We found that the NHD of the current period (1991–2011) was 70% higher than that of the base period (1960–1990). This NHD increment was attributed to the increased summer air temperature mean and its extended hot tail length—defined as the range of the mean and 90th percentile of summer air temperature. To distinguish the relative contribution of air temperature mean and hot tail length change to the NHD occurrence, a numerical-based frame work was proposed. Results showed that global warming (mean temperature increase) contributes approximately 75% of the total NHD increment. Although the average contribution of the hot tail length change is relatively small, it dominates the NHD change in regions where global warming is insignificant, e.g., the central part of China. Results suggest that frameworks and models that fail to capture the air temperature tail changes will result in biased low hot extreme occurrence estimations. This low bias would be particularly significant in regions where the mean temperature change is marginal. Hence, capturing the tail change should be the key model evaluation criterion in regions where the annual mean temperature change is relatively insignificant.
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