A few days ago, record rainfall caused rivers to burst their banks, leading to devastating floods in parts of western Europe. In the hardest hit parts of Germany, two months’ worth of rain fell in 24 hours, according to Deutscher Wetterdienst, Germany’s meteorological agency. Heavy rain affected not only Germany but also Belgium, the Netherlands, Luxembourg, and France. So, what happened to cause this once in a hundred-year type deluge? Much like how a “blocking” in the atmosphere caused a heat dome to build over the Pacific Northwest of the US last month, leading to unprecedented heat, similar type “blocking” occurred here, but led to unprecedented rainfall instead.
A storm system or, area of low pressure, over Central Europe became trapped between flanking areas of high pressure to the west and east. The low-pressure system tapped the Mediterranean Sea for moisture – and because the low was sandwiched by these highs, a weather system “traffic jam” occurred, allowing heavy rain to persist. The total precipitable water – or, the amount of moisture in the atmosphere – was comparable to that seen along the U.S. Gulf Coast when hurricanes make landfall – that’s a LOT of water!
So, why the exceptional intensity of the rain? This is consistent with rising temperatures caused by human-induced climate change. Higher temperatures lead to more evaporation, which places more water in the atmosphere to later get rained out. A recent study that investigated rainstorms in Europe found that intense and slow-moving storms across the continent may become much more frequent by the end of the century (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL092361). Storms may slow due to a weakening of upper-level winds in the atmosphere. Climate change is expected to decrease wind speeds as high latitudes warm more quickly than the mid-latitudes, thereby reducing the north-south temperature differences that drive the wind. This, in turn, may lead to an increase in “blocking” patterns in the atmosphere, in which storms get “stuck.” Unless we get rid of our dependency on fossil fuels and rapidly transition to a carbon-free society, these types of events – and headlines – will soon become the norm, but it should not be a norm that we want. There are solutions out there – now where’s the leadership?