Record rainfall spurred Pakistan’s worst flooding in more than a decade, destroying more than a million homes, killing nearly 1,500 people and affecting an additional 33 million people. Now, an analysis released Thursday showed that climate change probably intensified the rain by 50 to 75 percent.
“That would have been disastrously high rainfall event without climate change, but it’s worse because of climate change,” said Friederike Otto, a climate scientist at Imperial College London. “Especially in this highly vulnerable region, small changes matter a lot.”
Otto and more than two dozen scientists with the World Weather Attribution project quantified the influence of climate change on the heavy rainfall. The group analyzed weather data and ran computer models to simulate the rainfall in a world without climate change compared to today’s climate, which has warmed about 1.2 degrees Celsius since the 1800s. The findings, which are not yet peer-reviewed, use well-established methodologies that have been peer-reviewed and used in past analyses.
The researchers looked at rainfall for 60 days across the entire summer nationwide as well as the five-day heaviest period over Baluchistan and Sindh. They found that climate change probably intensified the five-day total by up to 75 percent and increased the intensity of the 60-day rain by 50 percent.
They also found that having rain in such amounts today has about a 1 percent chance of happening in a year, known as a 1-in-100 year rain event.
“In the world without climate change, it would have been less likely,” said Otto.
Otto emphasized that the findings have large uncertainties because of the difficulty of modeling rainfall in the region, which experiences highly variable rainfall from year to year and numerous factors that can affect the monsoonal rainfall.
However, the team’s findings agree with recent assessments from the United Nations Intergovernmental Panel on Climate Change, which project more intense rain in the area. Historical weather records and past studies also show the number and frequency of extreme rain events has increased in recent decades.
While climate change is a factor, the team said there are several factors — some even more important than climate change — that set the unprecedented flooding in motion.
Scientist Fahad Saeed, who was part of the analysis, said several meteorological factors played a role in the heavy rainfall. The current La Niña conditions, characterized by cooler surface water in the equatorial Pacific Ocean, instigated more monsoonal rainfall to the region. A negative dipole in the Indian Ocean, creating warmer-than-average sea surface conditions in the eastern Indian Ocean, also made more moisture available for transport over the land.
Saeed said intense heat waves over the country in the spring also primed the season. Record heat waves in March and April — made 30 times more likely due to climate change, according to a different analysis by the World Weather Attribution group — increased the differential between land and water temperatures, prompting more winds from the ocean to flow to land and drop more moisture.
Yet perhaps the most important catalyst for the fatal flooding was the country’s vulnerability and preparedness.
“The historic levels of rainfall that we’ve just heard about, especially in Sindh and Baluchistan, meant that the country had obviously been dealing with an unprecedented hazard,” said Ayesha Siddiqi, a researcher at the University of Cambridge who was involved in the analysis. “It’s not really possible for any country, anywhere, to be entirely prepared.”
Even so, Siddiqi said water management along the Indus River delta fed into the widespread flooding. For instance, she said much of the damage from the catastrophic 2010 flooding was not caused by the rainfall but issues related to dams. Sedimentation reduced the capacity for water channels to carry water while water levels continued to rise. Drainage issues in the lower Indus basin prevented upstream water entering the sea.
Construction of buildings near riverbeds, which block natural water courses, has also allowed water to stagnate.
“It is important to remember that this disaster was the result of a vulnerability that was constructed over many, many years and shouldn’t be seen historically as the outcome of one sporadic sudden event,” Siddiqi said.