A question of life and death | Expert opinions

July 7, 2024

We know that climate change is affecting the weather, which in turn is destroying lives and livelihoods. But what we don’t discuss enough is how these weather extremes affect human health. In this desperate season, when temperatures rose unbearably, we learned how heat can kill. We also learned that the rise in low temperatures – the nighttime heat – can be a killer. It is crucial that we recognize the connections between the seemingly distant crisis of climate change and the impacts on our health.

This year, the world has experienced scorching temperatures. And that heat has claimed lives—in Delhi, an estimated 270 people died of extreme heat by the end of June. But I repeat that figure with caution. We don’t know how many died from heat alone, because heat is an aggravating factor for existing health conditions like cardiovascular or kidney disease. Many more people may have died from heat this summer, but doctors would have attributed that to underlying conditions. We know that those most vulnerable are those exposed to heat because of their working conditions—from construction workers to farmers. It is also the poor who have no access to electricity and no devices to cool them down. But their deaths are not recorded as heat stress deaths; only that they are either poor or old and died of “unknown cause.” Heat is not listed among the country’s reported diseases, which also means it does not need to be registered or provide information for further action. So we must admit that we know little about the health burden and deaths caused by the devastating fires we have experienced recently.

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However, research is now pointing to the dynamics of deadly heat. First, the increase in nighttime heat is thought to be the cause of the highest number of deaths. A 2022 article in The Lancet, a British medical journal, found “that the relative risk of death could be 50 percent higher on days with hot nights than on days with cooler nighttime temperatures.” The reason, the authors explain, is that heat interferes with sleep and does not allow the body to repair itself; and this in turn worsens health stress. Second, we know that evaporation is the method our bodies use to cool themselves; but this becomes ineffective when humidity levels rise above 75 percent – also known as the wet bulb phenomenon. So it’s about understanding not just temperatures, but also thermal discomfort.

Worryingly, we are seeing a rising trend in all three killer factors, especially in urban centres, where temperatures are rising beyond human endurance, humidity is increasing and so is nighttime heat. A recent report by my colleagues at the Centre for Science and Environment tracked heat trends in India’s major cities and found that ambient temperatures in cities are rising compared to the national average. Hyderabad, Delhi, Mumbai, Kolkata and Chennai are experiencing wetter summers – an increase of 5-10 per cent in the last decade (2014-23) compared to 2001-10. Only Bengaluru has not seen a rise in summer humidity and this needs further study.

The report also finds that cities are not getting cooler at night – across all climate zones. It finds that during the summers of 2001-2010 (across all cities), night temperatures fell by 6.2 to 13.2 degrees Celsius from the daytime maximum. But over the last decade, this difference between day and night temperatures (maximum and minimum) is narrowing. Hyderabad has seen a 13 percent drop in temperatures, Delhi 9 percent and Bengaluru 15 percent. Kolkata, which already had the dubious distinction of having a smaller temperature difference between day and night temperatures, is now even worse due to higher humidity.

We know that all of this is part of the double whammy we are experiencing in our world. On the one hand, there is a warming planet – this year has broken all previous temperature records. Worse, there are changes in weather patterns ahead in the form of erratic rain, intense heat and changing wind patterns. All of this makes heat more stressful and deadly. On the other hand, our cities are experiencing a dramatic change in microclimate – the heat island effect is being amplified as concrete takes over open and green spaces; and transport and the use of energy for cooling are contributing to heat being trapped in the air.

This summer has taught us new lessons about heat stress. The fact is that climate change will bring us many such surprises in terms of human health impacts. Even now, that science is not understood. What climate change will do is bring the impacts closer to our bodies and our health. That’s why planetary health is about human health. It’s time we made that connection. It’s time we understood why climate change is an existential crisis; it’s literally a matter of life and death.

The author can be contacted at the Centre for Science and Environment at [email protected], X: @sunitanar