Past trends in public health relevant characteristics of U.S. extreme heat events

Using the newest United States Historical Climatology Network dataset, trends from 1930-2010 were investigated in summertime extreme heat events (EHEs) over the continental United States (CONUS). The trends in the typical characteristics of EHEs were quantified (e.g. number per summer, days per summer) along with the characteristics that have been previously associated with mortality such as duration, intensity and the number of early season EHEs.

We investigated in those trends over the CONUS, whether there was spatial structure, temporal structure and dependence on EHE definition. The results showed that trends existed since 1930 in the CONUS average but depended on the EHE definition to whether they were positive, negative or essentially neutral. However, nearly all trends displayed coherent regional variability as well as variability between subsets of the 1930 - 2010 record. The existence and strength of relationships between trends of different EHE definitions varied on the definitions being compared.

Over the 81 years the center of the CONUS exhibited decreasing trends in all typical characteristics of EHEs, and the more southern and coastal regions displayed increasing trends. The decrease in EHEs with definition requirements upon the daily highs was stronger in the earlier half of the period than the increase in the recent half of the period; causing an overall decrease in daily high EHEs. Conversely, EHEs based on the daily lows had more consistent increasing trends in the recent half of the period compared to the older period; which caused an overall decrease in daily low EHEs.

Also the relationships between these EHE trends and the trends of products that can be more confidently predicted by global climate models were quantified. Our results suggested that the trends in summer average daily extreme temperatures relate well to EHE trends based on the same daily extremes.