Detection of Anthropogenic Influence on a Summertime Heat Stress Index

Has there been a detectable anthropogenic influence on summertime heat stress over land regions? This is an important issue, since climate change projections suggest that one of the most consequential impacts of anthropogenic warming on humans will be increased heat stress, combining temperature and humidity effects. Here we examine an index of heat stress which combines such temperature and humidity effects: a simplified index of wet bulb globe temperature (WBGT). While one might initially assume there has been a detectable increase in the WBGT index owing to the previously detected human influences on global and regional surface temperatures, several complicating factors must also be considered. For example, there has been an observed decrease in relative humidity over land since the late 1990s, and there has been an apparent slow-down in global mean warming over this same period. Furthermore, the global data set of in situ surface humidity only extends back to 1973, meaning that we do not have as long a global record of combined surface air temperature and surface humidity as we have for temperature alone. In our analysis, observed trends in WBGT (1973-2012) are compared to trends from CMIP5 historical simulations (eight-model ensemble) using either anthropogenic and natural forcing agents combined or natural forcings alone. Our findings suggest that there has been a detectable anthropogenic increase in mean summertime heat stress since 1973, both globally and in most land regions analyzed. Notably, summertime WBGT over land regions has continued increasing in recent years--consistent with climate models--despite the apparent ‘hiatus' in global warming and despite a decreasing tendency in observed relative humidity over land since the late 1990s.