New study investigates how moisture from irrigation and urban greening affects human heat stress

Recent studies modelling interactions between the land surface and the atmosphere have suggested that adding additional moisture to the atmosphere increases human heat stress.

However, this depends on atmospheric conditions and on how heat stress is defined. A new study by researchers at UCL and the University of Washington explores how measuring heat stress in different ways leads to different conclusions. 

The human body must keep within a certain range of internal temperature to survive. In hot conditions this means losing heat by sweating, but if humidity is high then the cooling effect of sweating is smaller. Heat stress indices, for example, the Heat Index used by NOAA, combine the effects of temperature and humidity into one number.

Heat stress indices usually increase with both temperature and humidity. Heat stress impacts people’s health and ability to work, and is an increasing concern given the warming climate. Irrigation and increased vegetation cover typically reduce air temperature but increase humidity, so could increase heat stress. The question of whether moisture near the land surface increases or decreases heat stress can be thought of as a trade-off: if we can increase humidity to lower the temperature, under what conditions will this reduce heat stress?

Different heat stress indices make different assumptions about how heat stress relates to temperature and humidity. Existing studies use a variety of different heat stress indices, which makes comparison between their results difficult. In their new study published in NPJ Climate and Atmospheric Science, the researchers provide a new method for understanding when different heat stress indices will agree and disagree.

Several recent studies have argued that, in terms of heat stress, the effect of humidity outweighs the effect of temperature. This would mean that higher soil moisture, agricultural irrigation, and increasing vegetation in urban areas might actually worsen thermal comfort and the health effects of extreme heat. But the researchers observed that these studies might have reached different conclusions if they had used different heat stress indices. This suggests that increasing moisture in the environment might be more effective at reducing heat stress than some previous studies found.

Project team member Charles Simpson noted:

“We aimed to achieve two things with this paper. Firstly, we want to encourage researchers to be more critical in how heat stress indices are presented. Secondly, we provide a simple method for understanding the conditions under which different heat stress indices will disagree.

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