“Even if we manage to keep warming to the limits agreed in Paris, the impacts on human health from heat stress are likely to increase substantially as global temperatures climb.”
Bengaluru: Sweltering under the blazing sun, sweat streaking down my forehead, I ran into the cool interiors of a taxi, vowing never to step out during the day until the rains came. It was barely summer, but the headlines of the discarded newspaper already warned of an impending heatwave. I looked up from the newspaper for a moment to see construction workers outside labor on, probably unaware of the heat warnings.
If you think summers are getting worse each year, you’re not alone. Those studying the climate for a living are thinking the same thing. This steady warming stresses humans more. Projections have suggested that in some regions the heat may even exceed human tolerance levels by the end of the century, with such episodes of dangerously high heat becoming more frequent than they are now. There have been efforts globally to limit warming, but simply meeting these targets may not be enough for large cities, especially in Southeast Asia.
As far back as 1896, the Swedish chemist Svante Arrhenius deduced a relationship between temperature and global carbon dioxide levels. Carbon dioxide and water vapour maintain Earth at an average of 15° C because they absorb the Sun’s heat and act as a natural greenhouse. Arrhenius suggested that, if carbon dioxide levels doubled, Earth would become warmer by 5° C.
Since then, there have been numerous studies warning of a warming earth and its serious consequences. Because of this growing concern, the United Nations adopted a framework for climate change mitigation in 1992 that all UN member states have ratified. This paved the way for the negotiation of various global climate change treaties such as the Kyoto Protocol adopted in 1997, and the more recent Paris Agreement in 2015. The main aim of the Paris Agreement is to limit the rise in global average temperature to less than 2 °C above pre-industrial levels, and pursue efforts to limit it to less than 1.5 °C above pre-industrial levels.
The heat index
But that may not be enough for the highly populous Southeast Asian cities, claims a new study led by Tom Matthews of the Liverpool John Moores University, Liverpool. “Our study shows that even if we manage to keep warming to the limits agreed in Paris, the impacts on human health from heat stress are likely to increase substantially as global temperatures climb,” said Matthews.
Matthews and his team used both temperature and humidity in their projections, a measure called the heat index, which is a better measure of how we feel than simply using the temperature. For example, it is generally much more difficult to tolerate the stickiness of Chennai than the dry heat of New Delhi, implying that the heat index of Chennai is higher than that of New Delhi. This is because it is much harder to cool down by evaporation of sweat when it is very humid, making the heat stress on humans much greater.
And at higher temperatures, the heat index increases significantly more than the corresponding increase in temperature. “We were suspicious that the warming permitted by the Paris targets could therefore result in large increases in heat stress,” said Matthews, explaining why they undertook the study. They looked at the temperature increases permitted by the Paris Agreement, and higher values, taking into account a scenario where the agreed limit could not be met, and projected the global heat stress burden.
The heat stress burden for a 1.5 °C rise in temperature is projected to be 5.7-times that experienced during 1979-2005, rising to about 12-times if there is a 2 °C warming. If we fail to limit warming to the targets agreed and there is a 4 °C rise, this burden may reach fully 75-times that of the reference period.
What this means is that heatwaves, like the deadly one in 2015 over northern India and Pakistan will not be a one-off event, but will occur every year, and possibly be more intense.
Mapping the projected heat stress values to the world’s most populous cities, the study estimated an additional 350 million people could face the burden of heat stress by the end of the century compared to the 1979-2005 period. Indian cities such as New Delhi, Kolkata, Chennai and Ahmedabad that already experience large heat stresses will experience consistently larger increases. Of the top 10 cities that are projected to have the highest heat stress burden, six are in India, Pakistan and Bangladesh. Ho Chi Minh city, Vietnam, is projected to have one of the largest increases in heat stress burden because of a combination of dramatic population increase and sharp increases in the heat index.
Of the increases permitted by the Paris Agreement, we have already used up 0.8 °C between 1986 and 2015. That only allows 1.9- (1.5 °C) and 3.4- (2 °C) times the warming already experienced, which are ambitious targets. This in turn translates to a significantly greater increase in the heat stress because of its non-linear relation with temperature.
Under a dire scenario when the planet warms by 4 °C, places such as New York City and Bangalore can become heat-stressed, developing heat-index values similar to Ahmedabad and Hanoi, respectively.
Heat action plans
Krishna AchutaRao, a climate scientist at IIT Delhi, said, “As expected, especially in the tropics, big cities, the megacities, are all going to get a) more populated, and b) they are going to get hotter and more humid. Therefore there are going to be more people exposed to the kind of deadly heat [waves] that we’ve experienced in the recent past in India.” He also added that the had study assessed hazards and exposure but hadn’t assessed vulnerability. “Not everybody who is exposed to this heat is going to be equally affected by it” Somebody out on the street selling their wares – or construction workers labouring in the heat, the old, very-young or sick – will evidently be more vulnerable than somebody working in an office building.
This is something Matthews is interested in working on in the future. “I see much potential for working on heat stress adaptation–helping vulnerable communities live with more frequent episodes of extreme heat,” said Matthews. Apart from limiting the earth from getting any warmer, there are a range of adaption methods that can be implemented, like making cities greener or painting urban roofs white. “While promising, these design solutions principally reduce excess urban heat. They can’t eliminate the danger of extreme heat driven by large-scale high temperature and humidity events.”
He added, “Beyond engineering solutions, improved preparedness – of individuals to city authorities to emergency response teams – can help reduce the impacts of extreme heat.”
One such action in India is the Ahmedabad Heat Action Plan. It strives to make people, hospitals and emergency response teams aware of extreme heat and ways to tackle it. It was first drafted in 2013 by the Ahmedabad Municipal Corporation with the help of national and international academic experts, and is said to have reduced heat-related deaths by up to 25%.
Other cities, like Nagpur and Bhubaneswar, are also interested in developing such a plan, according to AchutaRao. Although the India Meteorological Department issues heat-wave warnings and long-term forecasts, the Ministry of Earth Sciences has also been urging states and cities to develop their own heat actions plans, he added.
Although the study is only an indication of what we may be in for in the future, the increase in the frequency and intensity of heatwaves over the Indian subcontinent in the last few years, along with their occurrence earlier in the year, serves as a warning. We can no longer afford to deny the important role of mitigation and helping societies adapt to an intensely warming world.
Lakshmi Supriya is a freelance science writer based in Bangalore.