Unpacking the Flooding in Europe

Catastrophic flooding inundated Europe last week, and recovery efforts are still underway. At least 189 people are dead, and hundreds more are still missing after widespread swaths of between four and six inches of rain poured over the area, bringing an entire month’s worth of rain (or more) in twenty-four hours. Some locations received even more locally heavy rain, such as in Reifferscheid, Germany, where 8.1 inches of rain fell in just nine hours (according to the European Severe Weather Database)! Andreas Friedrich, a German weather service spokesperson commented that, “In some areas we have not seen this much rainfall in 100 years, in some areas we’ve seen more than double the amount of rainfall which has caused flooding and unfortunately some building structures to collapse.” The death toll in Germany alone is over 100, and Chancellor Angela Merkel has declared July 20 a national day of mourning.

Flooding in Belgium, photo courtesy CNN

The inundation of rain led to bridges, homes, and even large sections of villages completely disappearing, as the rapidly moving floodwaters surged through. Several landslides and mudslides have been reported, accompanied by houses collapsing or being swept away. Landslides occur when masses of rock, earth, or debris move down a slope. Mudslides, also known as debris flows, are a common type of fast-moving landslide that tends to flow in channels. Mudslides develop when water rapidly accumulates in the ground and results in a surge of water-saturated rock, earth, and debris. Flooding or additional slides may occur after a landslide or mudflow, which hampers recovery efforts in these areas.

Translation from German by Google: ++ Breaking news ++ In #Erftstadt -Blessem, houses have been massively undermined and some collapsed. Several people are missing. Emergency calls come from the houses, but rescue is often not possible. Our disaster control is on site. Photos: Rhein-Erft-Kreis

What led to the days of heavy rainfall inundating the region? The answer is a slow moving, persistent low pressure system that hovered in place, as seen in the satellite loop below. Note that low pressure systems in the Northern Hemisphere rotate counter-clockwise, while high pressure systems rotate clockwise.

In some ways, the system was set up in a similar fashion as a Rex block, which is the meteorological term for a blocking pattern that is characterized by a high-pressure system located poleward of a low-pressure system. The term is often used in relation to the United States, where the phenomena most often occurs along or near the West Coast in the spring. The Rex block will remain nearly stationary until one of the height centers changes intensity, unbalancing the high-over-low pattern. Unsettled, stormy weather is usually found near the low pressure while dry conditions are typical with the high-pressure. Strong, particularly persistent Rex blocks can cause flooding near the low-pressure part of the block and short-term drought under the high-pressure part.

The gif below shows the atmospheric conditions over Europe from July 15 through July 18. The cooler, blue shadings represent lower pressure, while the warmer, orange colors represent higher pressure. The airflow pattern is similar to a typical (though tilted) Rex block, which follows a backwards “S” trajectory.

Whenever an extreme event such as this occurs, everyone from the general public to meteorologists to world leaders jumps into the question of how that particular event relates to climate change. Understandably so, as tragedy and extreme events of all natures tend to leave us all wondering why, searching for an answer.

Armin Laschet, the premier of North Rhine-Westphalia and the Conservatives’ candidate to succeed Merkel, said that “We will be faced with such events over and over, and that means we need to speed up climate protection measures, on European, federal and global levels, because climate change isn’t confined to one state.” Angela Merkel correctly noted that, “One flood isn’t an example of climate change.” She went on to say that, “If we look at the last events of recent years, decades, then they are simply more frequent than they were previously — so we must make an effort, a great effort.”

Flooding in Croatia, photo courtesy: nasice.com

As we noted earlier, the extreme rainfall which fell was the result of a slowly moving area of low pressure. While we cannot attribute single events to climate change, we do know that certain factors work together to form sort of a “loaded gun” scenario. We know that our planet is warming, and 93% of that warming has gone into our oceans, primarily at the ocean’s surface. We also know that a warmer atmosphere holds more water vapor (this is why it feels more humid in the summer!), and that an increase in temperatures allows for more water to evaporate from surfaces (such as lakes, oceans, soils, etc.). This means that there is more water available for these storms, which can effectively “supercharge” them. (Reminder: the flip side of this is that while some areas will receive heavier precipitation events, this will also lead to larger, longer droughts in certain regions. Key point: wet areas or seasons will get wetter; dry areas or seasons get drier.)

Thus, we cannot attribute a single storm as being ’caused by’ climate change, though observed changes in our climate may be helping to ‘stack the deck’ in favor of more frequent high impact events such as the recent flooding in Europe. Climate change is serious, but it is solvable. If you have any questions regarding the science of climate change, or what we can do to help combat it, feel free to reach out!

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