Extreme temperatures and humidity around the Persian Gulf are likely to approach and exceed a threshold of

human tolerance within this century if greenhouse gas emissions continue to increase on their current trajectory, reports a paper published online this week in Nature Climate Change. These findings suggest that human habitability of these regions may be severely impacted by extreme weather events in the future. The human body can shed heat through evaporative cooling of perspiration provided that the wet-bulb temperature-a combined measure of temperature and humidity-remains below a threshold of 35 °C (95 °F). Above this threshold, the body cannot cool itself and the outdoor survival of even fit and healthy individuals is seriously challenged.

Jeremy Pal and Elfatih Eltahir used a number of high-resolution regional climate model simulations to project the incidence of extremes of wet-bulb temperature around the Persian Gulf towards the end of the twenty-first century (2071-2100). They investigated the implications of atmospheric greenhouse gas concentrations using two Intergovernmental Panel on Climate Change (IPCC) Representative Concentration Pathway (RCP) scenarios: RCP8.5, a ‘business-as-usual’ model, and RCP4.5, which considers mitigation. Under the higher-emissions scenario, extreme wet-bulb temperatures are projected to occasionally exceed the threshold for human survival in major cities such as Abu Dhabi, Dubai, Doha, Dhahran and Bandar Abbas. The authors find that such conditions are avoided under the lower greenhouse gas concentration pathway that includes mitigation activities.

Finally, they note that the extreme wet-bulb temperatures predicted under the high-emissions scenario may in some years have a severe impact on the rituals of Hajj, during which millions of Muslim pilgrims pray outdoors from dawn to dusk near Mecca. They suggest that the outdoor rituals are likely to become hazardous to human health, especially for elderly pilgrims, when Hajj occurs during the summer.

In an accompanying News & Views article, Christoph Schar writes that although the elderly and ill have been the most at risk in recent heat waves, the conditions modelled by Pal and Eltahir around the Gulf will also pose a risk to young and fit individuals. “The new study thus shows that the threats to human health may be much more severe than previously thought, and may occur in the current century."