Flash droughts emerge

With Mathilde Fontez, editor-in-chief of the scientific magazine Epsiloon, we are now talking about the question of the level of groundwater, which is low almost everywhere in France, according to the latest readings. We risk drought this summer.

franceinfo: And as a bonus, researchers are now discovering a new type of drought:

Mathilde Fontez: Yes, droughts of a new kind, more brutal and intense. A new climatic scourge: flash droughts. The phenomenon has been studied by climatologists and meteorologists for only a few years. And a big study from the University of Hong Kong is now painting a portrait of it, for the first time.

Do they last less long than normal droughts?

Much less. Normal droughts set in over months. We see it today in France, we are at the end of April, and we are already preparing for the drought this summer.

There, it is heat strokes over a few weeks, or even more often a few days, which fall on a well-defined region. This is also why they have gone unnoticed for so long, they are difficult to detect. There is nevertheless an emblematic example: the drought which affected the center of the United States in August 2012. It only lasted a few days, but it suddenly exhausted all the water in the soil.

The example of 2012 shows the impact of these flash droughts: they are short, therefore less severe in a sense. Except when they fall at a key time for agriculture. In the American Midwest, they totally withered corn crops, and caused a loss that has been estimated at more than 35 billion dollars. A colossal loss.

Do we now know how they are formed?

The study begins to detail the causes: it is not only a rise in temperatures that causes them. You need a heat stroke, but you also need the atmospheric pressure to be at its lowest. Then, the vegetation, the ground, see all their water suddenly escaping.

By analyzing all the global hydroclimatic data – all the soil humidity measurements taken by satellite over the past 20 years – the researchers have also been able to study how it is changing: they see that these flash droughts are getting worse. They are getting faster and more intense. Those that dry out the soil in just 5 days, increased from 3 to 19%. The increase even reaches 20% in the most sensitive regions. Obviously, we suspect an effect of global warming.

So there are regions that are particularly affected?

Yes, this is the other discovery of the researchers. Flash droughts occur mostly in areas where there are large seasonal variations in humidity. Regions that are often particularly fertile: Southeast Asia, the Amazon basin, eastern North America.

Hence the importance of trying to predict them, in order to be able to protect the crops. Except it’s not obvious. As we said, flash droughts don’t warn before they hit. The analysis of retrospective climate simulations, for that of 2012 in the United States, showed that there is almost no precursory signal.

But the researchers have nevertheless found an indicator, the vapor pressure deficit, which could be measured more finely to help predict the arrival of these little climatic monsters.