Melting glaciers and early rain
In spring 2022, a severe heat wave – also favored by La Niña – hit India and Pakistan, and with it the glaciers in the Indus river basin. The meltwater ran into the rivers or caused lakes at the foot of the glaciers to swell – and even overflow. Pakistan recorded several spectacular eruptions of such glacial lakes in the spring and summer of 2022. In May, such a flood damaged an important bridge and two power plants in the Hunza Valley, and another cut off the way home for thousands of visitors to Pakistan’s largest sports festival in July.
© NASA Earth Observatory, Joshua Stevens (detail)
Flooded areas in Pakistan | Large areas of the flat alluvial plain of the Indus are flooded by heavy monsoon rains.
More important than such individual events, however, is the sum of the water flowing out of the mountains, which caused the rivers to swell and reduce their absorption capacity even before the monsoon began. In addition, there were unusually early rains in southern Pakistan, caused by a large low-pressure system over the Arabian Sea – that too is unusual. It is unclear whether the preceding heat wave in April and May has something to do with it.
But while these factors played their part in the flooding, their main cause was the heavy, long-lasting monsoon rains. In August fell in Pakistan five times as much rain as normal. There is much to suggest that climate change made these rains particularly intense and the floods more likely – Pakistan is one of the most vulnerable countries in the world to climate change.
The monsoon and climate change
The simplest effect for the heavy rains is that higher temperatures allow more water to evaporate and a warm atmosphere allows more water to be held. As early as spring 2022, experts suspected that the very high temperatures in April and May would carry more moisture into the atmosphere and thus increase rainfall. Both data and simulations also show that heavy rain events are becoming more violent in Southeast Asia as a result of climate change.
Rising temperatures over the land masses, which are warming up faster than the sea as a result of climate change, also increase the difference in air pressure between the continent and the ocean, so that more humid air is drawn in. Climate models show that the impact of global warming on the summer monsoons in South Asia is significant. According to a study by the Potsdam Institute for Climate Impact Research (PIK), rainfall will then increase by around five percent per degree.
While it is generally clear that climate change is amplifying the current catastrophe, it will be some time before the contribution of global warming is properly disaggregated. The South Asian summer monsoon is complex – to this day it is difficult to reliably predict the annual rainfall, the forecasts are always wrong. That’s because other factors play a role alongside climate, including land use and urbanization.