Detection of anthropogenic influence on a summertime heat stress index

Abstract

One of the most consequential impacts of anthropogenic warming on humans may be increased heat stress, combining temperature and humidity effects. Here we examine whether there are now detectable changes in summertime heat stress over land regions. As a heat stress metric we use a simplified wet bulb globe temperature (WBGT) index. Observed trends in WBGT (1973WBGT ( -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 analysis suggests that there has been a detectable anthropogenic increase in mean summertime heat stress since 1973, both globally and in most land regions analyzed. A detectable increase is found over a larger fraction of land for WBGT than for temperature, as WBGT summertime means have lower interannual variability than surface temperature at gridbox scales. Notably, summertime WBGT over land 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.

Recent Very Hot Summers in Northern Hemispheric Land Areas Measured by Wet Bulb Globe Temperature Will Be the Norm Within 20 Years

“…The influence of anthropogenic forcings (i.e., ANT, GHG, or OANT) is clearly detectable in the observed summer WBGT, but the influence of NAT is not detected (Figure c). This is consistent with Knutson and Ploshay (). The best estimate of the scaling factor for ANT is slightly larger than one but the 90% uncertainty range of scaling factor includes one (pink bar in Figure c), suggesting that the modeled response to ANT may be somewhat underestimated but it is still consistent with the observations.…”

“…This trend pattern is similar, in terms of both spatial distribution and magnitude, to that of summer air temperature ( Figure S2a), suggesting that the widespread intensification of summer WBGT is largely a result of rising summer temperatures. This is consistent with Knutson and Ploshay (2016) who found that the global mean summer WBGT trend changes little if fixed climatological RH was used to compute WBGT. In fact, RH trend is small across the covered land ( Figure S2b).…”

“…Due to the lack of reliable wind and sunlight data, we compute WBGT ( ∘ C) as a weighted sum of wet bulb temperature (WBT, ∘ C) and air temperature (TA, ∘ C) such that WBGT = 0.7 × WBT + 0.3 × TA (Liljegren et al, 2008). The resulting temperature represents a lower limit for WBGT in well shaded or indoor conditions, similar to that studied by Dunne et al (2013) and Knutson and Ploshay (2016). We compute WBT (and thus WBGT) based on TA and relative humidity (RH) following the method of Stull (2011).…”

A Review of Recent Advances in Research on Extreme Heat Events

“…However, the challenge is that the traditional approach to attribution is intractable for any single heat event. It can be used, however, on climatologies of heat events or indices (e.g., [94]). …”

Surface air relative humidities spuriously exceeding 100% in CMIP5 model output and their impact on future projections

“…The water vapor capacity of air is largest in high temperatures, and accordingly, the impacts of RH have been studied most widely in warm conditions. To mention some examples, RH contributes to the growth of crops [e.g., Asseng et al, 2015], human comfort and heat stress [e.g., Willett and Sherwood, 2012;Knutson and Ploshay, 2016], wildfires [e.g., Lehtonen et al, 2014;Vajda et al, 2014], and surface evapotranspiration and soil moisture [e.g., Seneviratne et al, 2010;Ruosteenoja et al, 2017]. Even so, air humidity has multiple implications in areas with cold climate as well.…”