Pakistan began receiving abnormally heavy rains in mid-June, and in late August the torrential downpours were declared a national emergency.
The southern part of the Indus River, which runs through the entire country, became a large lake. The villages have become islands, surrounded by putrid water that stretches to the horizon.
More than 1,500 people have died.
The floods could take months to recede.
The deluges were made worse by global warming caused by greenhouse gas emissions, scientists said Thursday, drawing on a rapidly growing field of research that measures the influence of climate change on specific extreme weather events soon after they occur, and while societies are still dealing with its devastating consequences.
As climate scientists’ techniques improve, they can assess, with greater confidence and specificity, how human-induced changes in Earth’s chemistry are affecting severe weather outside our windows, adding weight and urgency to the questions about how nations should adapt.
The floods in Pakistan are the deadliest in a recent series of mind-boggling extreme weather events across the Northern Hemisphere:
relentless droughts in the Horn of Africa, Mexico and China; flash floods in West and Central Africa, Iran and the interior of the USA; scorching heat waves in India, Japan, California, Britain and Europe.
Scientists have warned for decades that some types of extreme weather are becoming more frequent and intense as more heat-trapping gases are pumped into the atmosphere.
As the planet warms, more water evaporates from the oceans.
Warmer air also contains more moisture.
So storms like the ones that come with the South Asian monsoon can have an imajor impact.
But Pakistan’s monsoon rains have varied widely from year to year, making it hard to pin down precisely how much more severe this season was due to climate change, the authors of the new study said.
Still, most of their computer models indicated that human-caused warming had intensified the rainfall to some degree, convincing them that it was a contributing factor.
The country could have experienced disastrously high rainfall this year, even without global warming, said study lead author Friederike Otto, a climate scientist at Imperial College London.
“But it is worse due to climate change,” Otto said.
“And especially in these highly vulnerable regions, small changes matter a lot.”
The study was produced by 26 scientists affiliated with World Weather Attribution, a research initiative that specializes in rapid studies of extreme events.
This year, scientists in the group found that the heat that scorched India and Pakistan this spring was 30 times more likely to occur due to greenhouse gas emissions.
Extreme July heat in Britain had been at least 10 times more likely, the group found.
The following is a study on this summer’s drought in Europe.
Attribution studies aim to link two distinct but related phenomena: climate and weather.
Climate is what happens to the weather over long periods and on a planetary scale.
Direct weather records only go back a century or so in many places, which is why scientists use computer models and concepts from physics and chemistry to develop their understanding of evolving climate.
But the climate has always been variable, even without the influence of human activity.
Attribution studies try to separate this natural variability from the larger changes that fossil fuel emissions are causing.
Attribution research “really helps us understand how climate fits into long-term climate change,” said Daithi A. Stone, a climate scientist at New Zealand’s National Institute of Water and Atmospheric Research.
Nearly two decades ago, Stone worked on the first study to estimate the fingerprints of climate change on a single event, in that case the brutal 2003 heat wave in Europe, which killed tens of thousands of people.
Since then, scientists around the world have published 431 attribution studies on 504 extreme events, according to an informal tally of research in English by the climate news site Carbon Brief.
The field is still expanding rapidly, by Carbon Brief’s count: three-fifths of these studies were published in 2017 or later.
A fifth was published this year or last.
“The diversity of tools that we have at our disposal to analyze it now,” Stone said, “goes beyond what we could have imagined back then.”
To make an attribution, scientists use mathematical models to analyze both the world as it is and the world as it might have been, if humans hadn’t spent decades pumping planet-warming gases into the atmosphere.
With computer simulations, they can play back recent history dozens, even hundreds, of times on both worlds to see how often the event, and others like it, occur on each.
The differences indicate how much was likely to be responsible for global warming.
Researchers often make this comparison using dozens of climate models to ensure their conclusions are robust.
They also check the simulations against the records of actual events that occurred in the past.
To examine the floods this year in Pakistan, the authors of the new study looked at two metrics: the maximum 60-day rainfall each year between June and September across the Indus River basin, and the maximum five-day rainfall each year in the badly hit provinces. southerners of Sindh and Baluchistan.
The researchers found that several of their models did not realistically reproduce the patterns of actual rainfall data from Pakistan.
And those that did gave divergent answers about how much more intense and likely this year’s rains had become under current levels of global warming.
However, the models gave clearer answers when considering a greater heating level.
This gave the researchers confidence to say that climate change had likely made this year’s flooding worse, though they refrained from estimating how much.
Recent improvements in climate models helped the authors lower their estimates, Otto said.
“The uncertainty bars are smaller than they would have been five years ago,” he said, referring to the lines on statistical graphs that show ranges of possible values.
“But the monsoon is still something models really struggle with.”
Pakistan’s highly varied topography, from its southern coast to the towering Himalayan peaks in the north, means its climate is shaped by many physical drivers, said another study author, Fahad Saeed, an Islamabad-based climate scientist with Climate research group. Analytics.
“Representing all these processes can be difficult when applying a climate model,” said Saeed.
Scientists often find that storms, droughts and forest fires they are more difficult to attribute to global warming compared to periods of extreme heat or cold.
Those events involve not only temperatures, but also air circulation and complex interactions between land, sea, and the atmosphere.
Still, new and improved models, plus larger amounts of data, are helping to close the gaps.
“For us as climate scientists, our laboratory is our climate models,” said Andrew Hoell, of the National Oceanic and Atmospheric Administration in Boulder, Colorado.
“And they have advanced in ways that have allowed us to do more robust attribution studies.”
Today, models continue to get better at capturing climate and its drivers on progressively smaller scales, Hoell said.
Scientists can begin to think not only about drought in a large area, but also about evaporation in specific watersheds and reservoirs. Not just average precipitation, but individual tornadoes and thunderstorms as well.
Climate scientists have also started using artificial intelligence and other computational techniques to search weather data for new insights, said Dim Coumou, a climate researcher at the Dutch university VU Amsterdam.
These methods can help scientists uncover the hidden mechanisms that drive complex weather patterns, leading to better attributions and forecasts of extreme events.
“There is a lot of data that is becoming more accessible to scientists,” Coumou said.
Weather records show that the South Asian monsoon is moving more between the driest and wettest years, unwelcome news for farmers who must deal with increasingly dry or flooded fields.
Anders Levermann, a physicist at the Potsdam Institute for Climate Impact Research in Germany, has proposed an explanation.
The South Asian monsoon begins each spring when the land warms and draws in moisture-rich air from the Indian Ocean.
When this air hits the mountains and cools, its vapor load condenses into rain, releasing heat in the process.
The heat draws even more air towards the land from the sea, which keeps the monsoon going.
On a warmer planet, there is more moisture in this system, which means the rains are amplified.
But if something blocks this input, such as an atmospheric disturbance or heavy air pollution, then its weakening effects on the monsoon could also be amplified, Levermann said.
“That’s the bad thing about climate change,” he said.
“It’s not just an increase in something or a decrease in something. It’s an increase in variability.”
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