Warming to boost deadly humidity levels across South Asia

Matt McGrath BBC 3 Aug 17;

Millions of people living in South Asia face a deadly threat from heat and humidity driven by global warming according to a new study.

Most of India, Pakistan and Bangladesh will experience temperatures close to the limits of survivability by 2100, without emissions reductions.

The research says the fraction of the population exposed to dangerous, humid heat waves may reach 30%.

South Asia is home to one-fifth of the world's inhabitants.

Wet bulb threat

Most official weather stations around the world measure temperature with two thermometers.

The first, or "dry bulb" instrument, records the temperature of the air. The other, or "wet bulb" thermometer, measures relative humidity in the air and the results are normally lower than just the pure air temperature.

For humans, this wet bulb reading is critically important.

While the normal temperature inside our bodies is 37C, our skin is usually at 35C. This temperature difference allows us to dissipate our own metabolic heat by sweating.

However, if wet bulb temperatures in our environment are at 35C or greater, our ability to lose heat declines rapidly and even the fittest of people would die in around six hours.

While a wet bulb 35C is considered the upper limit of human survivability, even a humid temperature of 31C is considered an extremely dangerous level for most people.

Recorded wet bulb temperatures on Earth have rarely exceeded 31C. However, in 2015 in Iran, meteorologists saw wet bulb temperatures very close to 35C. In the same summer, a deadly heat wave killed 3,500 people in India and Pakistan.

This understanding of the potentially deadly impact on humans of wet bulb temperatures is key to this new study.

The researchers involved came to their conclusions by using a high resolution climate model, that was tested against observations.

They projected wet bulb temperatures to the end of this century using two different climate change scenarios.

When the model examined a high emissions future, the wet bulb temperature would approach the 35C threshold "over most of South Asia, including the Ganges river valley, north eastern India, Bangladesh, the eastern coast of China, northern Sri Lanka and the Indus valley of Pakistan".

According to the scientists, around 30% of the population is projected to live in a climate characterised by a median of the maximum annual wet bulb temperature of 31C or more. At present, the number of people facing this level of threat is essentially zero.

"The valleys of the Indus and the Ganges rivers are where the water is, they're where the agriculture is and they're where the population has exploded," author Prof Elfatih Eltahir from the Massachusetts Institute of Technology (MIT) told BBC News.

"Our map that shows where the temperature extremes are, it's the same place that you have relatively poor people who predominantly have to work in agriculture and there are so many that they happen to coincide in a region where the hazard is maximised."

Impacts of Paris

If the rise in global temperatures is contained to just over two degrees, roughly in line with the Paris Climate Agreement, the fraction of the population exposed to humid heat above 31C drops to 2%.

Heat waves up to and beyond 31C are projected to become much more frequent if little action is taken on cutting carbon. In most locations, the once-every-25-year heat wave in the present climate is projected to become an approximately once-a-year occurrence. If the limitations agreed in Paris are met, these heat waves are likely to happen every two years.

"Climate change doesn't look like an abstract concept if you look at India," said Prof Eltahir.

"This is something that is going to impact your most vulnerable population in ways that are potentially pretty lethal. But it is avoidable, it is preventable."

Around 30% of the population of the region will be facing dangerous humid heat waves by 2100 according to the study

Other researchers say the "damaging and downright deadly" conditions described in this study are likely to occur if the world doesn't embrace rapid and substantial cuts in carbon emissions.

"This study provides a crucial glimpse of the future," said Prof Matthew Huber from Purdue University, US, who wasn't part of the research team.

"Either we - the whole world - decide to reduce carbon emissions substantially or we face a highly dangerous scenario in one of the most populous regions in the world, with a deep history and culture, and also a history of political instability."

According to Prof Christoph Schaer from the Institute of Atmospheric and Climate Science at ETH Zurich, the work is "alarming".

"The study is credible as extremely hot and humid heat waves already occur under current climatic conditions in some of the areas considered," he said.

"As conditions are close to a critical health threshold already today, a warming of a few degrees could strongly increase the risk of deadly heat waves."

The study has been published in the journal Science Advances.


Climate change to cause humid heatwaves that will kill even healthy people
If warming is not tackled, levels of humid heat that can kill within hours will affect millions across south Asia within decades, analysis finds
Damian Carrington The Guardian 2 Aug 17;

Extreme heatwaves that kill even healthy people within hours will strike parts of the Indian subcontinent unless global carbon emissions are cut sharply and soon, according to new research.

Even outside of these hotspots, three-quarters of the 1.7bn population – particularly those farming in the Ganges and Indus valleys – will be exposed to a level of humid heat classed as posing “extreme danger” towards the end of the century.

The new analysis assesses the impact of climate change on the deadly combination of heat and humidity, measured as the “wet bulb” temperature (WBT). Once this reaches 35C, the human body cannot cool itself by sweating and even fit people sitting in the shade will die within six hours.

The revelations show the most severe impacts of global warming may strike those nations, such as India, whose carbon emissions are still rising as they lift millions of people out of poverty.

“It presents a dilemma for India between the need to grow economically at a fast pace, consuming fossil fuels, and the need to avoid such potentially lethal impacts,” said Prof Elfatih Eltahir, at Massachusetts Institute of Technology in the US who led the new study. “To India, global climate change is no longer abstract – it is about how to save potentially vulnerable populations.”

Heatwaves are already a major risk in South Asia, with a severe episode in 2015 leading to 3,500 deaths, and India recorded its hottest ever day in 2016 when the temperature in the city of Phalodi, Rajasthan, hit 51C. Another new study this week linked the impact of climate change to the suicides of nearly 60,000 Indian farmers.

Eltahir said poor farmers are most at risk from future humid heatwaves, but have contributed very little to the emissions that drive climate change. The eastern part of China, another populous region where emissions are rising, is also on track for extreme heatwaves and this risk is currently being examined by the scientists.

Their previous research, published in 2015, showed the Gulf in the Middle East, the heartland of the global oil industry, will also suffer heatwaves beyond the limit of human survival if climate change is unchecked, particularly Abu Dhabi, Dubai, Doha and coastal cities in Iran.

The new work, published in the journal Science Advances, used carefully selected computer climate models that accurately simulate the past climate of the South Asia to conduct a high resolution analysis of the region, down to 25km.

The scientists found that under a business-as-usual scenario, where carbon emissions are not curbed, 4% of the population would suffer unsurvivable six-hour heatwaves of 35C WBT at least once between 2071-2100. The affected cities include Lucknow in Uttar Pradesh and Patna in Bihar, each currently home to more than two million people.

Vast areas of South Asia – covering 75% of the area’s population – would endure at least one heatwave of 31C WBT. This is already above the level deemed by the US National Weather Service to represent “extreme danger”, with its warning stating: “If you don’t take precautions immediately when conditions are extreme, you may become seriously ill or even die.”

However, if emissions are reduced roughly in line with the global Paris climate change agreement, there would be no 35C WBT heatwaves and the population affected by the 31C WBT events falls to 55%, compared to the 15% exposed today.

The analysis also showed that the dangerous 31C WBT level would be passed once every two years for 30% of the population – more than 500 million people – if climate change is unchecked, but for only 2% of the population if the Paris goals are met. “The problem is very alarming but the intensity of the heatwaves can be reduced considerably if global society takes action,” said Eltahir.

South Asia is particularly at risk from these extreme heatwaves because the annual monsoon brings hot and humid air on to the land. The widespread use of irrigation adds to the risk, because evaporation of the water increases humidity. The projected extremes are higher in the Gulf in the Middle East, but there they mostly occur over the gulf itself, rather than on land as in South Asia.

The limit of survivability, at 35C WBT, was almost reached in Bandar Mahshahr in Iran in July 2015, where 46C heat combined with 50% humidity. “This suggests the threshold may be breached sooner than projected,” said the researchers.

Prof Christoph Schär, a climate scientist at ETH Zurich, Switzerland, and who was not involved in the study, said: “This is a solid piece of work, which will likely shape our perception of future climate change. In my view, the results are of concern and alarming.”

The report demonstrates the urgency of measures to both cut emissions and help people cope better with such heatwaves, he said. There are uncertainties in the modelling – which Schär noted could underestimate or overestimate the impacts – as representing monsoon climates can be difficult and historical data is relatively scarce.

Prof Chris Huntingford, at the UK Centre for Ecology and Hydrology, said: “If given just one word to describe climate change, then ‘unfairness’ would be a good candidate. Raised levels of carbon dioxide in the atmosphere are expected to cause deadly heatwaves for much of South Asia. Yet many of those living there will have contributed little to climate change.”

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