In this case, however, it is not clear how big the role of climate change has been.
It’s relatively easy to conduct an attribution study assessing the influence of warming in heat waves, with higher average temperatures pushing up the baseline where such sweltering events originate. The group calculated exactly how much climate change has changed the likelihood of blistering Heat Wave Pacific Northwest last year (such conditions would be “at least 150 times rarer without human-induced climate change”), the recent British heat wave (climate change made it “at least 10 times more likely”), and those in Pakistan and India earlier this year (“30 times more likely”).
But using climate models to pinpoint the role of global warming in amplifying the entire monsoon season proved trickier, the researchers noted in a press statement. World Weather Attribution attributed the uncertainty to a combination of the wide variability in heavy rainfall patterns over long periods, natural processes at work that the models may not fully capture, and the area’s weather characteristics. The Indus River Basin is located at the western edge of the region’s monsoon area, where there are large differences in precipitation trends between the dry west and wet east.
PAULA BRONSTEIN/ GETTY IMAGES
On the other hand, weather data clearly shows that the heaviest rainfall periods in the region have intensified in recent decades, with about 75% in the two worst affected provinces. Some models found that climate change increased rainfall by as much as 50% during the five wettest days of the two-month monsoon season in those areas.
“So while it’s difficult to give a precise figure for the contribution of climate change, the fingerprints of global warming are clear,” said Friederike Otto, senior lecturer in climate science at Imperial College London and one of the leaders of World Weather Attribution. said in a statement.
In a scientific paper released Thursday, the team of researchers noted that a combination of meteorological forces caused the extreme rainfall. They include a La Niña event, which cools the upper ocean waters and rains more than usual over much of the world, coupled with unusually hot spring and summer weather across Pakistan. Those simmering temperatures also accelerated the melting of the thousands of glaciers that feed the Indus River, although it’s not known how much that contributed to the flooding.
Climate scientists have long warned that global rainfall patterns will become more erratic as the planet warms, making both very wet and very dry spells more common. Warmer air, among other things, retains moisture, sucks the water from the soil and plants and changes the atmospheric pressure systems. The UN Climate Panel projects that the South Asian monsoons will fluctuate from year to year in the coming decades, but will generally increase in intensity in the 21st century.
Pakistan’s heaviest rainy days are likely to become even more extreme as temperatures rise, World Weather Attribution found. That underscores the need for the country to strengthen its riverbanks, homes and other infrastructure to protect citizens — and for rich countries that have caused a hugely disproportionate share of climate pollution, to do everything they can to help.