Warming since preindustrial times has made the extreme heat in South Asia, now in its third month, at least 30 times more likely.
Global warming has made the severe heat wave that has smothered much of Pakistan and India this spring hotter and much more likely to occur, climate scientists said Monday.
They said that the chances of such a heat wave increased by at least 30 times since the 19th century, before widespread emissions of planet-warming gases began. On average the heat wave is about 1 degree Celsius, or about 1.8 degrees Fahrenheit, hotter than a similar event would have been in those preindustrial times, the researchers said.
“Climate change is a real game changer when it comes to heat waves,” said Friederike Otto, a climate scientist at Imperial College London. “It’s really a major factor.” Dr. Otto is an author of a report on the heat wave by World Weather Attribution, a collaborative effort among scientists to examine extreme weather events for the influence, or lack thereof, of climate change.
The relentless heat, with temperatures soaring beyond 100 degrees Fahrenheit for days, particularly in Northwestern India and Southeastern Pakistan, has killed at least 90 people, led to flooding from glacial melting in the Himalayas, contributed to power shortages and stunted India’s wheat crop, helping to fuel an emerging global food crisis.
The study found that a heat wave like this one now has about a 1 in 100 chance of occurring in any given year. Before warming began, the chances would have been at least about 1 in 3,000. And the chances would increase to as much as 1 in 5, the researchers said, if the world reaches 2 degrees Celsius of warming, as it is on track to do unless nations sharply reduce emissions. The world has already warmed about 1.1 degrees Celsius since the late 19th century.
South Asia is no stranger to heat this time of year, but this heat wave began early, near the beginning of March, and is continuing in some areas where little relief is expected until monsoon rains arrive over the next few months.
The scientists analyzed maximum daily temperatures for March and April, and used computer simulations of the world as it is now and of a fictional world where emissions, and warming, never occurred. While this study has not been peer reviewed, these model-comparison techniques have been peer-reviewed in the past and are now widely used and accepted.
Because of the lack of a long observational record and other uncertainties, the researchers said, the findings are conservative, and the chances of such an event are likely more than 30 times greater than they were before warming began.
The analysis also looked at the effects of the prolonged heat. Arpita Mondal, a climate scientist at the Indian Institute of Technology Bombay in Mumbai and an author of the study, said gathering data about the effects on wheat, a crop that is sensitive to extreme heat, was difficult, despite anecdotal reports of damage.
“But what has been quite startling is that India has banned its wheat exports to the rest of the world,” she said. “That in itself is evidence enough that our agricultural productivity has been affected.”
The ban, coupled with the effects of the Russian invasion of Ukraine on wheat exports from there, has international agencies concerned about the potential of a global food shortage.
Another author, Roop Singh, a climate risk adviser with the Red Cross Red Crescent Climate Center, said that, like other heat waves, this one shows that the effects tend to fall disproportionately on the poor.
She said there have been reports of widespread power outages, in part because the need for more cooling strains the system, and in part because of a coal shortage in India. “This is particularly impactful for the poorest people who might have access to a fan or to a cooler, but might not be able to run it because they can’t afford a generator,” she said.
The findings of the study are consistent with many other analyses of similar events over the past two decades, including an extraordinary heat wave last summer in the Pacific Northwest and Western Canada. This field of research, called attribution analysis, has contributed to a growing understanding among scientists and the public that the damaging effects of global warming are not some far-off problem but are already occurring.
Because emissions have raised the world’s baseline temperature, the link between heat waves and climate change is especially clear. Dr. Otto said that in studies of other extreme events like floods or drought, climate change is usually only one factor among several.
In a recent paper, Dr. Otto and others argued that the influence of global warming on heat waves is now so apparent that it is “fast becoming an obsolete question.” The “next frontier” for attribution science, they wrote, is to provide information to help people decide how to adapt to extreme heat.
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