Flash flooding occurs when rain falls faster than the ground can absorb it. It is “flash” because of the rapid onset; water levels can rise feet/meters in minutes.
While there are many factors that can worsen the impacts from heavy rainfall — ground type, such as soil or concrete, and how much moisture was in the ground to being with — the most important variable is how much rain falls over a period of time, or the rainfall rate.
Human-caused climate change has already fueled extreme rain events in hurricanes and in non-tropical flash flood events like we have seen this week in Europe. This is because of a simple physical relationship — known as the Clausius-Clapeyron equation — between temperature and humidity.
“Simply put, warmer air holds more water vapor,” said Jim Kossin, an atmospheric research scientist at NOAA’s National Centers for Environmental Information. With more water vapor in a warmer atmosphere, rainfall rates can increase and flash flooding is more likely to occur.
Drought can compound this effect. Very dry soil cannot efficiently absorb water (think of trying to wet a very dry sponge). While the rain is ultimately beneficial, if a region that has been experiencing intense drought gets hit with heavy rain, flash flooding is more likely to occur.
While the overall amount of rainfall may not change over the course of the year in any given location, more of the rain is expected to fall in shorter bursts, which would tend to increase the frequency of flooding events. This was noted by scientists with the European Environmental Agency, who said that “the projected increase in frequency and intensity of heavy precipitation over large parts of Europe may increase the probability of flash floods, which pose the highest risk of fatality.”
This week’s flooding in Belgium and Germany provides an all-too perfect demonstration of this in action. It was only a few months ago that historically low water levels on the Rhine in Cologne, Germany, were disrupting shipping along the river, but now the river is swelled by two months of rain falling in just one day.
Scientists are increasingly able to quantify the impact that a warming climate is having on individual weather events.
A similar, though at first-take not as extreme, flooding event in Western Europe in 2016 that killed 18 in Germany, France, Romania and Belgium, was analyzed by scientists to see if climate change played a role in the floods. They found that a warmer climate made the flooding 80-90% more likely to occur than it was in the past before man-made climate change.