When swathes of Pakistan were flooded last month due largely to unprecedented monsoon rainfall, many suspected that climate crisis was to blame.

Now an international team of climate scientists has worked through historical weather data to show that rising global temperatures likely made the monsoon rainfall more intense.

Gathered as part of the World Weather Attribution research group, the scientists have just released a report that articulates the link between climate and Pakistan's historic flooding. 

An unprecedented deluge

While resonating with extreme flooding in Australia and Germany over the last year, the deluge in the South Asian nation has been unprecedented in terms of its scale with tens of millions of people displaced and facing food insecurity, 1.7 million homes destroyed and more than 1,300 people killed.

Record-breaking rainfall fell for over two months between June and August, causing the Indus River that runs through Pakistan to burst its banks and flood thousands of square kilometers.

An extreme summer heat wave also melted glaciers that feed into the Indus, exacerbating the flooding that created a 100-kilometer (62-mile) wide lake in the southern province of Sindh.

But a vast majority of the floodwater came from extreme monsoon rains over a 5-day period that researchers say has increased in intensity by up to 50% in the last 100 years. 

This increase would have been less likely, say the World Weather Attribution researchers, without a rapid increase in greenhouse gas emissions caused largely by burning fossil fuels. Similar events could become more frequent as the climate warms.

Impact of climate on flooding getting clearer

Due to the natural variability from year to year in monsoon rains in the Indus River basin, the scientists said the model contained inherent uncertainties and that they weren't able to quantify exactly how much more likely the event was made by climate change. 

This is unlike more consistent heatwaves across the region that can be clearly modeled, noted report co-author Fahad Saeed, a researcher at the Center for Climate Change and Sustainable Development in Islamabad, Pakistan.

Heatwaves are now 30 times more likely due to climate change, Saeed said in a briefing on Thursday.

"Fingerprints of climate change in exacerbating the heatwave earlier this year, and now the flooding, provide conclusive evidence of Pakistan's vulnerability to such extremes," he said.

Friederike Otto, a co-author and senior lecturer in climate science at the Grantham Institute, Imperial College London, said that even if monsoon variability makes it difficult to quantify the exact impact of climate change on rainfall, Pakistan's extreme flooding is "exactly what climate projections have been predicting for years." 

"It's also in line with historical records showing that heavy rainfall has dramatically increased in the region since humans started emitting large amounts of greenhouse gases into the atmosphere," she added. "And our own analysis also shows clearly that further warming will make these heavy rainfall episodes even more intense."

The record rainfall that fell in August over Sindh and Balochistan, the worst affected regions in Pakistan's south, were 726% and 590% higher than the monthly average.

Pakistan as a whole received near 230% more rainfall than usual, making it the wettest August since records began in 1961.

Vulnerable communities most affected

Pakistan is one of the countries that has contributed least to climate change but bears the brunt of extreme weather. With climate vulnerable yet low-polluting countries in the Global Southcalling for reparations from richer, high-emitting nations, the report especially focused on the impact of the flooding on marginalized communities. 

"This exceptionally high monsoon precipitation hit a country with both high population density and high rates of poverty, creating significant vulnerability to climate-related hazards and potential changes in likelihood and intensity of such events," wrote the report authors.

Top-down water management practices which, from colonial times, have diverted streams into mega rivers for irrigation purposes, have reduced drainage that could mitigate the impact of flooding, said Ayesha Siddiqi, assistant professor at the University of Cambridge's geography department. 

Local communities who better understand "the natural ecology of rivers" have been excluded from flood management in the face of bureaucracy and mega projects, increasing their vulnerability to climate change, she added.

For Siddiqi, climate vulnerability is also historically rooted and is not simply "the outcome of a single weather event." Infrastructure and land conversion, among other factors, play a role, too. 

Fahad Saeed said that the report is further evidence of Pakistan's vulnerability to worsening climate change.

While the scale of the flooding and its link to climate change should inspire immediate emission reductions, he implored "developed countries to take responsibility" and pay for adaptation.

"Populations bearing the brunt of climate change" also need to compensated, Saeed said. 

Edited by: Jennifer Collins