Climate in the pavement, attribution in climatology

Climate in the pavement, attribution in climatology

In terms of weather, large parts of the world have experienced harsh summers. At the end of June, heat waves swept the United States and Canada with record temperatures, and in mid-July, local torrential rains in Germany, Belgium, the Netherlands and Luxembourg led to significant flooding. The latter killed more than 200 people, at least 180 of them in Germany and a few dozen in Belgium.

Listen to the science! But what can you tell us?

Mika Baumeister, via Unsplash

Shortly thereafter, the latest report from the Intergovernmental Panel on Climate Change (IPCC) was released.International Panel on Climate Change), the United Nations climate panel that monitors global warming and its impact on us. Global warming, due to greenhouse gas emissions, melting ice at the poles, and sea levels rising, has been a worrying but not surprising result.

But it is also becoming clear that warming leads to changes in the weather – and climatologists have not written this with such certainty before. The report said rising temperatures will lead to more and more intense heat waves, more frequent heavy rains, more frequent droughts, and more intense tropical cyclones. Were the extreme weather events of last summer also under the influence of climate change?


To answer these kinds of questions, there is now global weather ratios initiative, Created by an international consortium of climatologists, including a number of KNMI (Royal Netherlands Meteorological Institute).

This working group wrote in July of this year that heatwaves in June in America and Canada would not have been quite up to date with potential near certainty without the warming of greenhouse gases. Records went far beyond historical measurements. In Lytton, Canada, a temperature of 49.6°C was exploited, after which the village fell prey to one of the many wildfires in the area. Even in today’s world, which is on average 1.2 degrees warmer than it was before the Industrial Revolution, this is a rare occurrence. It is estimated that such a heat wave occurs on average once every thousand years. Scientists estimate that, without warming, this chance would have been at least 150 times less.


Before the floods in Europe, the question was much more difficult to answer. The main cause of the torrential rain that fell here was the Bernd, an area of ​​low pressure that would not move, forcing the clouds to drain all their water into the same area. Because of the heavy rain at the beginning of July, the soil was saturated and could no longer absorb much of that moisture.

In the Netherlands, this has resulted mainly in large tracts of water, areas of water already overflowing, and dams that have barely held. In parts of Limburg, especially in Belgium, Germany and Luxembourg, where elevation differences are greater and many villages are located in narrow mountain valleys, the waters gained unprecedented strength and things went wrong. There, the current swept away people, cars, and buildings.

“The weather is unpredictable, in which case the timing and location of prolonged heavy rain turned out to be particularly bad,” says Sjoukje Philip, a climate scientist at KNMI in De Bilt. She concluded with 38 other climate scientists from the world that it is impossible to determine whether rainfall in July would be very heavy without global warming. Refer the weather in the worldWorking Group in a huge report at the end of August. But the chance of such an amount of rain has increased.

“This was such a private and local event that it was difficult to model it or compare it to other rains,” says Philip. “That’s why we arrived at a great deal of uncertainty in our study.” In the end, the scientists chose to zoom out and investigate the possibility of similar rainfall in the area between the Netherlands and the northern border of the Alps. They divided the study area into sixteen regions, and concluded that under current conditions, these regions receive such an amount of rain on average once every four hundred years. Without overheating, it was 1.6 to 9 times lower. The chance is small, the uncertainty is high.

Floods in Telf, Belgium, June 16, 2021

Regine Fabry, CC-SA-4.0, via Wikimedia Commons


Arnaut van Delden, a weather and climate researcher at the Institute of Sea and Atmospheric Research at Utrecht University, has some criticisms about these types of “attribution studies” – in which the impact of climate change on the likelihood of certain events is calculated.

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He says these types of studies are governed by statistics. “But you can’t prove much with statistics, especially when it comes to rare events.” Statistics companies with future forecasts for models completely completely shaky Van Delden says: “These models themselves are by no means perfect, especially when it comes to forecasting rain and clouds.”

The problem is that to properly mimic precipitation, you need accurate and detailed models. However, this does not allow you to look very far, as you do with climate models. Van Delden: “For weather models the limit is ten days, for climate models at least until the year 2100. That’s simply too long computing time.” For this reason, researchers use lower accuracy for the calculation in climate models, but this actually makes them unsuitable for this type of calculation.

People want numbers

You can never say the weather we’re experiencing is caused by climate change, Philip agrees. But people want to know how often certain weather conditions are likely, in order to prepare themselves if necessary. They want numbers and we give them. But with bandwidth, we don’t make it any more accurate than we think. The uncertainty in the results is just as important in our studies as the results themselves.”

She stressed that this summer’s heavy rain was on the brink of what the statistics could handle Refer the weather in the worldsame consortium. Although this time the researchers used accurate models for the first time, due to the rarity of the event, they looked at a larger area.

It rained again and again…

Frame Wires, Unsplash


Philip explains that we start from the observations in our attribution studies. To draw conclusions from them, you need to understand how they came. Computer models help with this, they calculate the interaction between known mechanisms and physical laws – albeit in a simplified version. “Statistics is then a tool for finding out how well observations and models fit in.”

Forms that do not reflect current observations are not used. A weighting is assigned to the other models, depending on the uncertainty in the model results. Ultimately, the uncertainties in the models and the data together determine the margins of the results. Philip: “We also check if the models agree to some extent. If so, we are confident, otherwise we add an ‘uncertainty model’ to the results.” In this way, the consortium hopes to show which connections are really clear and which aren’t.

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“Sometimes the result is that an exceptional climatic phenomenon is almost never confirmed by climate change,” emphasizes Philip. This was the case, for example, with the water shortage in Brazil in 2015, which appeared to be caused by increased population and water consumption, and with the drought in 2016 in Somalia, which was most likely related to natural changes that would have occurred without climate change.


However, it is better to choose a different angle, says Van Delden. He argues in favor of shifting the focus back to the physics behind climate, such as the dynamics of air circulation patterns. How is it affected by high temperature? We are learning more than that from statistics using models, as this will help us better understand the mechanisms that determine weather, he said in a lecture to the German research network ClimXtreme last May.

Van Delden: “During the heavy rain in July, we had a very special situation, as the hurricane remained very cold over Germany. This caused a constant flow of moist air from the Baltic Sea to the Ardennes and Eifel, as clouds crashed into the hills and rained. It continued again and again. We’ve seen it before.For example, in 2002, Prague was flooded in the same way, and then water reached the second and third floors of the houses.These cyclones – cold craters – are separated from the so-called polar vortex, and cold air flows around the North Pole Thus, whether it occurs more often is more important for this type of flood than the amount of rain that falls more often than before. But there is very little knowledge about that.”

In the end, both are necessary, Philip answers. “If you want to put in place a policy to reduce the consequences of flooding, it doesn’t really matter what the meteorological condition is. Then you just want to know how big the chance of that much water is.”

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