The backdrop of a steadily warming climate has frequently raised questions about whether any given extreme weather event could have been influenced by climate change. It’s a natural question to ask, but answering it in peer-reviewed detail usually takes months or years. In response, researchers started the World Weather Attribution program, which has developed a streamlined analysis pipeline that lets them address questions of climate influence before the public has forgotten the event happened. This technique allowed the group to rapidly determine that climate change played a key role in this summer’s Pacific Northwest heat wave.
Now, the group has attempted to tackle this summer’s European floods, which destroyed communities in Germany, Belgium, and the Netherlands. And here, the answer was a lot more complicated. The floods hit a small area and were extreme enough that they destroyed some of the monitoring equipment that would otherwise have told us just how bad they were. Nevertheless, the team found the climate change likely boosted the chances of an event like that in northwestern Europe.
When the rains came
The weather pattern that produced the rain wasn’t particularly exceptional, and it consisted of a low-pressure system that parked over Europe for a couple of days. The warm, moisture-rich air this drew from the Mediterranean ended up rotating around the low pressure. The Mediterranean air crossed a number of ranges of low hills in northwestern Europe, which caused the sort of atmospheric disruptions that trigger rainfall for a couple of days.
That rainfall turned out to be unusually intense, and its impact was heightened by two factors. One was that the ground was nearly saturated from rainfall in the previous weeks. The second is that the ground in the relevant areas is characterized by less-permeable rock covered by only a thin layer of soil. The result was an incredibly rapid rise in rivers like the Ahr and Meuse. The conditions were obvious enough that several weather services predicted record floods on some of the rivers. Nevertheless, the actual floods that occurred greatly exceeded the warnings and are unprecedented in historic records.
This is the sort of event where we’d expect climate change to have an influence. Warmer air can hold more moisture, and so we’d expect it to enhance precipitation, which it has. But the data indicates that we’re not seeing more rainy days; instead, we’re seeing more intense rain on those days when it does rain.
Finding out whether climate change influenced this particular set of storms, however, turned out to be complicated.
Records, set and lost
One of the challenges was that the floods destroyed the monitoring equipment, so we don’t know how much water ended up in these river basins. But other challenges were distinct to the events themselves. One problem was that the areas affected were quite small, in global terms. The analysis of whether warming influenced an event involves running climate models with and without the influence of human carbon emissions. But most climate models are bad at simulating details down to the size of the individual river basins involved in these floods.
The researchers worked around this by using a set of regional climate models, including some designed specifically for Europe. They also used some convection models that are specific to simulating local atmosphere dynamics.
Another problem was how extreme the floods were. If the researchers compared them to the historic records of flooding on the Ahr, disasters of this magnitude would be expected to have a return period in the neighborhood of once every 15,000 years—so large that the researchers don’t have much confidence in the value. Even if they took the absolute lowest value in the uncertainty range, the return period is once in 700 years.
To get more meaningful statistics, the researchers had to expand the region out to include all of Western Europe north of the Alps while excluding the Nordic countries, the UK, and Ireland. This includes several additional ranges of hills that could potentially spawn a similar weather event.
Within this larger region, you might expect a rain event of similar magnitude about once every 400 years. Climate change has made these events more likely, but uncertainty abounds: flooding like this could be anywhere from 1.2 times more likely to nine times more likely. Obviously, further warming would make the event even more likely.
Pushing the limits
As the people who did the analysis acknowledge, “this study pushes the limits of what current methods of extreme event attribution are designed for.” In other words, we simply don’t have the tools to properly analyze every weather-driven disaster that we’re likely to see. And events of such an extreme magnitude are going to be difficult to understand in any case. So, while we can say Europe’s floods are the sort of events we’d expect to see more of due to climate change, it’s not possible to say how much more likely they are.
Still, this is a problem worth looking at further. The floods killed over 200 people and caused billions of euros in damages. Figuring out how likely we are to see something similar could be essential for appropriate planning.