It started to rain exceptionally heavily in Pakistan in mid-June, and in late August torrential rains were declared a national emergency. The southern part of the Indus River, which crosses the country from end to end, has become an immense lake. Villages have become islands surrounded by putrid water that stretches to the horizon. More than 1,500 people have died. The water can take months to subside.
The flood was compounded by global warming caused by greenhouse gas emissions, scientists said on Thursday, based on a rapidly growing field of research that assesses the influence of climate change on specific extreme weather events shortly after they occur, while the society is still dealing with its devastating consequences.
As their techniques improve, climatologists can assess with increasing specificity and confidence how human-induced changes in Earth’s chemistry affect the harsh climate, adding more weight and urgency to the questions of how countries should adapt.
The floods in Pakistan are the deadliest in a recent string of staggering weather extremes in the northern hemisphere: relentless droughts in the Horn of Africa, Mexico and China; flash floods in West and Central Africa, Iran and the United States, and scorching heat waves in India, Japan, California, the United Kingdom and Europe.
Scientists have been warning for decades about the increasing frequency and intensity of some types of extreme weather events, as more heat-trapping gases are emitted into the atmosphere. As the planet warms, more water evaporates from the oceans. Warmer air also contains more moisture. Thus, storms such as those accompanying the South Asian monsoons can have a greater impact.
But monsoon rains in Pakistan have varied tremendously from year to year for a long time now. For that reason, say the authors of the new study, it is difficult to define how far climate change has worsened the monsoon season this year. Even so, most of the scientists’ computer models indicated that human-caused warming intensified rainfall in some way, which convinced them that this was a contributing factor to the floods.
Pakistan could have suffered disastrously heavy rains this year even without global warming, said the study’s lead author, Friederike Otto, a climatologist at Imperial College London. “But the situation has been exacerbated by climate change,” she said. “And small changes have big effects, especially in these highly vulnerable regions.”
The study was produced by 26 scientists affiliated with the World Weather Attribution, a research initiative specializing in rapid studies of extreme events. Scientists at the organization concluded that the heat that scorched India and Pakistan this year was 30 times more likely to occur due to greenhouse gas emissions. The extreme heat of July in the UK was at least ten times more likely to occur, the group found, whose next research topic will be this summer’s drought in Europe.
Attribution studies seek to link two distinct but related phenomena: meteorology (weather conditions) and climate.
Climate is what happens to weather conditions over long periods and on a planetary scale. Meteorological data only began to be recorded directly a century or so ago in many places. For this reason, scientists use computer models and concepts from physics and chemistry to formulate their view of climate evolution. But weather conditions have always been variable, even without the influence of human activity. Attribution studies seek to distinguish this natural variability from the larger changes being caused by fossil fuel emissions.
The attribution surveys “really help us understand the position of meteorology within long-term climate change,” said Daithi A. Stone, a climatologist at the New Zealand National Institute for Water and Atmospheric Research.
To make an attribution, scientists use mathematical models to analyze the world as it is and what it might be like if humans hadn’t spent decades spewing atmospheric warming gases into the atmosphere. With computer simulations, they can re-enact recent history dozens or even hundreds of times in both worlds to see how often the event and other similar events occur in each of the models. The differences indicate the extent to which global warming was likely responsible for what happened.
Researchers often make this comparison using dozens of climate models to ensure their conclusions are correct. And they verify the simulations, comparing them with the records of real events that happened in the past.
To study this year’s floods in Pakistan, the authors of the new study analyzed two sets of measurements: the 60-day maximum rainfall between June and September across the entire Indus River basin and the 5-day maximum rainfall across the peninsulas. of Sindh and Baluchistan in southern Pakistan, hit hard by the floods.
They found that several of their models did not realistically reproduce the patterns in the actual rainfall data in Pakistan. And the models that did so gave conflicting answers about how much more intense and likely rainfall had become under current levels of global warming.
But the models gave clearer answers when a higher level of warming was considered. In light of this, scientists felt confident in saying that climate change likely worsened this year’s floods, although they did not estimate the extent of worsening.
According to Otto, recent improvements in climate models have helped the authors narrow their estimates. “The uncertainty bars are smaller than they would have been five years ago,” she said, referring to the lines on the stat plots that show ranges of possible values. “But monsoons are still something that models have a hard time calculating.”
According to another study author, Fahad Saeed, an Islamabad-based climatologist with climate group Climate Analytics, Pakistan’s very diverse topography, ranging from the southern coast to the high peaks of the Himalayas in the north, means that the country’s climate is shaped by many physical factors.
“The representation of all these processes can get complicated when you apply a climate model,” he commented.
Meteorological records show that monsoons in South Asia have shown stronger variations between drier and wetter years. This is bad news for farmers who are increasingly forced to deal with either scorched or flooded fields.
Physicist Anders Levermann of the Potsdam Institute for Climate Impact Research in Germany has proposed an explanation. Asian monsoons begin in spring when the land warms and draws in moist air from the Indian Ocean. When this air hits the mountains and cools, its vapor charge condenses, forming rain, and in the process releases heat. The heat draws even more air from the sea to the land, and this conserves leads the monsoon to continue.
On a warmer planet, there is more moisture in this system, which means that rainfall increases. But if something blocks this flow of warmer air to land, such as atmospheric disturbance or heavy smog, its weaker effects on the monsoon could also be magnified, Levermann said.
“That’s the bad thing about climate change,” he said. “It’s not just an increase in one thing or a decrease in another. It’s an increase in variability.”
Originally translated from English by Clara Allain.
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