Increasing drought, torrential rain, heat waves and, on the contrary, a decrease in skating winters. Climate change not only raises the average temperature, but extreme weather also changes. And those changes often have greater consequences than the shift in the average. How does this work exactly? Dim Coumou from the VU University Amsterdam and Geert Jan van Oldenborgh from the KNMI provide answers.

Coumou investigates how climate change and extreme weather are related, looking among other things at changes in the jet stream (a high-altitude wind in the atmosphere). After the end of extreme weather conditions, Van Oldenborgh investigates whether and how these are related to global climate change.

Will climate change make it wetter or drier?

The effect on heat waves and cold waves is simple. As a result of global warming, heat waves are intensifying almost everywhere. In many places, temperatures are reached that were virtually impossible in the past. Cold waves are decreasing, but this is less clear because in winter the variations from year to year are naturally very large. Yet we can also measure this and notice: in the Netherlands we get less ice and snow. The chance of an Elfstedentocht is now much smaller than before.

Heat waves occur almost everywhere and also become hotter. There are a few exceptions: heat waves are not increasing in a number of areas where today much water is being pumped up for agriculture. Examples are the eastern half of the United States and the north of India. Due to evaporation of that moisture it stays cooler during summer heat and the local heat waves there are no hotter than in the past when these areas were much drier. This is called the Irrigation Cooling Effect.

This map shows the change in the highest temperature of the year, measured at official weather stations. The colors indicate whether the local heat waves rise in temperature faster or slower than global warming. In most places on earth, heat waves quickly become hotter (orange / red). There are a few exceptions, such as the eastern half of the US (blue). This is due to the Irrigation Cooling Effect. (Data: GHCN-D & KNMI Climate Explorer)

In our part of Europe, on the other hand, the temperature of heat waves has risen sharply in the last fifty years. The rise is even faster than we expect based on climate models. We do not yet know where this extra rapid rise is due.

This card shows the opposite: the change of the coldest days. Here the connection is even stronger: the average coldest days become much less cold almost everywhere. (Data: GHCN-D & KNMI Climate Explorer)

Is it getting wetter or drier?

In the Netherlands we get increasingly heavy summer showers, and even in winter it rains harder. This trend towards more intense showers can be seen in large parts of the world and is due to the fact that warmer air can retain more water vapor and that rains out in one go.

That also makes sense: French thunderstorms are more intense than Dutch ones, and our weather is becoming more and more like that. France, in turn, gets more tropical showers, which can give even more rain in a short time. We think that these more intense showers also give more lightning and hail, but the measurements are not yet good enough to confirm that.

Will it become drier next to wetter, like last summer? There is currently no trend in the Netherlands for drier summer weather, but we expect that in the future. Climate models predict that the western winds will weaken in the summer, partly due to the increasing heat in the Mediterranean and the weakening of the Gulf Stream in the Atlantic Ocean. As a result, the Netherlands is more often confronted with dry east wind.

The warming of the Arctic can also play a role in this. This change in weather types may lead to more prolonged periods of dry summer weather - as we saw in 2018 - but we do not yet know exactly how that works.

What we do know is that the drought in 2018 in the Netherlands was exacerbated by the fact that more water evaporated as a result of the warming. Drought therefore not only has to do with the amount of rain, but also with the rise in average temperature. This effect plays a role worldwide. Elsewhere you also see more drought due to a decrease in rainfall: for example around the Mediterranean Sea and near Cape Town in South Africa. But this varies greatly from area to area. In East Africa, for example, no change is noticeable, and in the Sahel the chance of drought may decrease.

An increase in the number of record rainy days is being measured worldwide. (Source: Lehmann et al, Climatic Change, 2015)

Does climate change lead to stronger hurricanes?

In the Netherlands we see little change in winter storms above the sea. In the future, we will be bothered by more hurricanes in the fall that can come much more to the north than before. They can then hit Western Europe as an intense autumn storm. An example was hurricane Ophelia, which landed in Ireland with wind force 12 in 2017.

Climate change does not cause more but stronger hurricanes. We expect more hurricanes of the toughest categories 4 and 5 - and also a strong increase in the damage of these heavy hurricanes. This is because most of the damage is caused by water: rain and storm floods. The amount of rain from hurricanes is increasing strongly. And due to the sea level rise the storm tides are also higher, which brings the seawater further on land during hurricanes.

Strangely enough, storms over land are decreasing in strength, but that has nothing to do with climate change. This is probably caused by the roughening of the landscape: more tall buildings, and more trees elsewhere in Europe, slow down the wind on the surface.

And how do we all know this?

There are three important pillars for research into weather extremes. Firstly, a good understanding of meteorology and physics: how the weather works. In addition, the statistics of observations: many extremes show a clearly rising trend.

Finally, there are the climate and weather models. If the results of all three of these pillars point in the same direction, climate researchers have confidence in the outcome. For some extremes that is not yet the case. That applies, for example, to tornadoes, but also hail and lightning, and climate scientists are discussing this strongly. For many other weather extremes it is now clear: they are increasing due to climate change. And that determines a lot of the consequences worldwide.

This question is part of a special series of climate questions from NU.nl. Do you also have a climate question that you would like a climate researcher to answer? Then set this in the Nujij comments.

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In 60 seconds: Global warming

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