By Tim Radford / Climate News Network


    Caught in monsoonal rain in India. Indians are particularly vulnerable to extreme heat waves caused by a changing climate. (trokilinochchi / Wikimedia Commons)

Researchers have established a direct link between global warming and heat-related deaths from killer heatwaves.

A tiny rise of 0.5°C in mean summer temperatures in India or another comparable tropical developing nation could result in a 146% rise in mass death from the heat.

Average temperature increases by the end of the century for the Asian subcontintent, the Middle East and Africa are likely to be at least 2.2°C and could be as high as 5.5°C. Although the heatwave prediction for India is based on a statistical model, the model itself is based on half a century of carefully-measured temperature, heatwave and heat-related mortality data.

The message is that even moderate increases in mean temperature will have negative effects on human health. And for the poorest – and in India more than 300 million people live on an income of less than $1.25 a day – the effects could be fatal.

“The impact of global climate change is not a spectre on the horizon. It’s real, and it’s being felt now all over the planet,” said Amir AghaKouchak, a civil and environmental engineer at the University of California Irvine.

“It’s particularly alarming that the adverse effects are pummelling the world’s most vulnerable populations.”

Dr AghaKouchak and colleagues from California, Mumbai and New Delhi report in the journal Science Advances that they found their evidence in records from the Indian Meteorological Department for the years 1960 to 2009.

When summer mean temperatures edged from 27°C to 27.5°C, the probability of a heatwave that would kill more than 100 people grew from 13% to 32%: that is, the probability more than doubled to an increase of 146%.

This was reflected in the history of heatwaves in the subcontinent. In 1975, and 1976, when mean summer temperatures reached 27.4°C, there were 43 deaths and 34 deaths respectively. Over the 54-year period, the annual average of heatwave days was 7.3.

In 1998, when heatwaves extended to 18 days, and average summer temperatures went above 28°C, deaths totalled 1,655. In 2003, during 13 days of heatwave, 1,500 died.

The poorest in India have little or no access to electricity or air conditioning, and are inevitably the most vulnerable. Researchers two years ago predicted that extreme heat and humidity could make some parts of the planet uninhabitable if drastic steps were not taken to cut fossil fuel combustion, and the greenhouse gas emissions that are amplifying surface air temperatures to dangerous levels.

“Both rich and poor countries should be ramping up our efforts to combat global climate change instead of turning our backs on commitments we have made to the international community”

The Californian scientists warn that, by mid-century, at least 60% of land areas in tropical Africa and along the rim of the Indian Ocean will be experiencing hotter conditions than any in the 20th century.

And a devastating extreme of heat during April 2016 in mainland southeast Asia – Thailand, Laos, Myanmar, Cambodia and Malaysia – could be blamed on a combination of global warming and the fortuitous timing of El Niño, the notorious cyclical blister of heat that periodically arrives in the tropical Pacific, according to a separate study in the journal Nature Communications.

Temperatures broke all records, humans suffered, crop production was disrupted and energy consumption went up, as cities turned on the air conditioning to keep cool.

The researchers, led by a team from the University of Texas at Austin, report that they looked at the 15 hottest April temperatures of the past 80 years. All of these had occurred since 1980, and all but one had coincided with an El Niño event.

“Though almost 50% of the April 2016 event was due to the 2015-16 El Niño, at least 30% of the anomaly was due to long-term warming, and there’s definitely more to come in the future,” said Kaustubh Thirumalai of the Institute for Geophysics at the University of Texas.

The point of such research of course is to prepare national governments and civic authorities for the dangerous levels of heat on the way. Cities in particular are vulnerable – they are normally significantly warmer than their rural hinterland – and the ability to make long-term forecasts of potentially murderous heat extremes could save lives.

“The silver lining is that these can be predicted a few months in advance since they happen after the peak of an El Niño,” Dr Thirumalai said. But nations need, too, to cut greenhouse emissions by adopting renewable energy supplies, to reduce the risk of mass death in the crowded cities of South Asia.

Donald Trump, President of the US, has announced that he is withdrawing the US from the Paris Agreement to limit climate change to less than 2°C average global warming.

But US scientists have been among the world pioneers in climate research, and have led both the latest studies of Asian heat extremes. They want to see constructive action.

“Given the quantifiable impacts of climate change in India and other developing nations in the coming decades, both rich and poor countries should be ramping up our efforts to combat global climate change instead of turning our backs on commitments we have made to the international community,” said Steven J. Davis, an earth system scientist at the University of California Irvine and one of the partners in the Science Advances study.

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