Climate change increases the risk of summer drought in our region, there is more evaporation and precipitation will become more erratic in the long term. But reports that direct the drought to us in connection with the rapid warming of the Arctic are incorrect, experts say to NU.nl.
Last week, various Flemish media knew of the extreme drought affecting the Netherlands and Belgium due to a slower jet stream. That is a strong west wind at an altitude of 10 kilometers in the atmosphere, which can sometimes wind like a river.
If such meanders (also known as rosy-wave waves) stay in place for a while, they are often the cause of all kinds of extreme weather, including heat waves, snowstorms or drought.
In one of those articles, which was also circulated and widely read in the Netherlands, it was argued that there is scientific consensus that the oscillation of the jet stream increases due to warming of the Arctic. Because the polar region heats up relatively quickly, the temperature contrast decreases with lower latitudes, making the air circulation patterns shift more easily. At least that is the hypothesis.
The weakening Gulf Stream has a greater influence on us
KNMI is not convinced. That is to say: it is true that the weather extremes are associated with wave movements in the jet stream, but not that those meanders increase due to warming of the polar region. According to climate researcher Geert Jan van Oldenborgh, spring - which was extremely dry - is really a specific season in which different relationships play a role than in winter and summer.
"The water in the North Atlantic is relatively cold, for example. You often see that a high-pressure area near the Azores has foothills to the north towards us."
According to Van Oldenborgh, that effect is reinforced by another effect of global warming, namely the weakening of the warm Gulf Stream. In winter, this gives us more low-pressure areas and westerly winds with rain, but from spring the effect reverses, and the chance of high-pressure areas and thus dry weather increases.
Moreover, an increase in solar radiation and thus drought need not be due to an increase in high-pressure areas. The Dutch springs have become measurably sunnier, but Van Oldenborgh does not find a clear trend for air pressure. KNMI's Climate Explorer statistics also show no decrease in the temperature difference between Western Europe and the polar region north of it, on which the jet stream hypothesis is based.
"Finally, due to climate change, we do see an increase in the temperature difference between the tropics and poles at high altitudes in the atmosphere. This is because the tropics become warmer and more humid, so that more condensation heat is released at high altitudes. jet stream. "
See also: Drought in Achterhoek can already be attributed to climate change
'A nice hypothesis, but not well substantiated'
Van Oldenborgh is supported by Tim Woollings, a jet professor of climate at the University of Oxford who specializes in jet streams. "There is actually no evidence in the global weather data that jet stream meandering has increased in any season."
"For several years, the observations seemed to indicate changes in jet stream, especially in the fall and winter. But an update to those studies shows no significant change. The hypothesis was interesting, but is ultimately not supported by a strong physical justification or simulations of climate models. "
See also: Paradoxical: Our summers can get drier or wetter