Climate crisis: Wet-bulb heat is a threat we cannot afford to ignore

And since it was an outlier among other mercury readings across Delhi, a central minister said that the weather department has been asked to check if the thermometer was okay. Whether or not it has its notches correct, the scorching heat this year makes anything around the 50° mark seem believable. The oven-like conditions that have gripped large parts of our landmass are all too real, with people groaning that this summer is the hottest they have ever experienced. 

The tragic irony of such groans is that unlike a ‘once in a century’ event like the covid pandemic, it’s likely that the same attestation of misery will be made year after year as we go along. After all, the blazing sun is only the most visible cause. Air pollution that traps heat on the planet has put us on a path whose perils are only just starting to show.

An analysis of data from India Meteorological Department by the Centre for Science and Environment (CSE) reveals that it’s not just regular heat, the hot season has gradually become more muggy too. In Delhi, the average relative humidity in summer months was 52.5% in the first decade of the millennium, but recent years have seen it go above 60%, pushing us into a sweltering zone. 

Perspiration cools our bodies down as sweat evaporates, but if the ambient air is too humid—with high moisture, i.e.—then we take longer to dry and this cooling effect weakens. Exposure for hours on end to extreme humid heat can thus induce stress and even prove fatal. While heat-stroke deaths are few and far between right now, what we need to recognize is how soon global warming and rising humidity could combine to make the fatality count shoot up. 

Global human tolerance studies suggest that five hours or more spent outdoors in ‘wet-bulb heat’ of above 35° Celsius could kill people (with variation on account of other factors, of course). With such a risk staring at millions of workers without air-conditioned work spaces, we need to wrap our heads around wet-bulb heat as a concept. 

It refers to readings of a thermometer whose bulb is covered with a cloth soaked in water. As it’s wet, its reading will be much lower than a dry bulb’s, but the gap can start closing rapidly if high humidity does not let the wet bulb cool. In other words, while the usual dry heat readings have begun reaching scary levels, with rising moisture in the air, we may see much worse to come.

Several issues demand attention. For one, we must not fall for the myth of hardy Indian toilers, ready to work under harsh conditions. On current trends, the bulk of our population may be vulnerable to wet-bulb heat by mid-century. For another, the energy needed to keep Indians safe—let alone cool—would be enormous, which could push us into a vicious cycle if we don’t plan properly. 

Even as we adopt clean-tech options for electricity, carbon neutrality by 2070 will not be easy to achieve. As with all climate science, nothing can be said with certainty. But still, the risks we face are too severe to kick this can further down the road. As lives are involved, we must prepare for a worst-case scenario. We shouldn’t face a fatality curve someday that’s too late to flatten.