“We are basically turning climate science upside down”

“We are basically turning climate science upside down,” says Friederike Otto, physicist and executive director of the Institute for Environmental Change at the University of Oxford. She is co-initiator of the World Weather Attribution initiative and one of the faces of the attribution research branch. In 2017, Otto was able to prove that climate change probably made the heat waves in the Mediterranean at that time 100 times more likely, or at least ten times more likely. She calculated that the scorching heat last July in Western Europe was 10 to 100 times more likely due to climate change. Otto will be awarded the German Environmental Prize this year for her research. Together with the timber construction entrepreneur Dagmar Fritz-Kramer, she received the prize worth 500,000 euros, which is awarded by the German Federal Environmental Foundation (DBU).

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On the occasion of the award, we are republishing this text, which comes from issue 7/2020 MIT Technology Review (available as a PDF).

Climate is what you expect and weather is what you get. Because the weather is so capricious, meteorologists have always recited a standard saying when they were asked to explain whether the climate crisis was to blame for extreme weather events such as heat waves or floods: “Climate change only shows up in long-term trends; individual events cannot be blamed on it lead back.”

But the sentence no longer applies. In recent years, a scientific field has been established that achieves exactly this classification – attribution research. It not only revolutionizes climate science, but also has direct consequences for climate protection: it closes the last link in the chain of evidence to bring to justice the companies that supply the raw materials for the crisis. Chevron, Gazprom, ExxonMobil, BP and Shell, but also RWE and Ruhrkohle AG would then have to pay. There are billions of dollars in damages and fatalities caused by hurricanes, forest fires, heat, droughts or heavy rain.

How can Germany become climate neutral? How can AI make climate models better? And: What is behind negative emissions? The current climate special issue of MIT Technology Review revolves around these and other questions (the PDF of the issue is available in the heise shop).

Link environmental damage directly to climate change

But attribution science is only one end of a causal chain that is now becoming apparent. If scientists can determine how much more likely or worse an extreme weather event has become due to climate change, the resulting damage can also be directly linked to climate change. Those companies that have been mining and selling coal and oil for decades could now find themselves in product liability. As a sister science to attribution research, so to speak, a number of legal experts have thought about how this liability for the damage caused by coal and oil could be enforced in court proceedings. The questions of guilt and responsibility that arise could also help to establish new climate laws.

Three years ago, the Peruvian farmer and mountain guide Saúl Luciano Lliuya made headlines when he sued the energy company RWE with the support of the environmental organization Germanwatch. Lliuya lives in the Andes below a glacial lake into which large chunks of ice keep falling. Due to global warming, the glacier is melting faster, the water level is rising, and larger chunks are breaking off. One day, a particularly severe one could cause the lake to overflow and trigger a devastating tidal wave. A protective wall would prevent this, and Lliuya has now sued RWE, a major producer of carbon dioxide, for the costs of building it. More precisely, RWE’s corresponding share of the total greenhouse gases worldwide: just under 0.5 percent or the equivalent of 17,000 euros.

The Higher Regional Court in Hamm declared in an oral hearing that Lliuya’s claims were fundamentally justified and that RWE would therefore have to be liable if the facts presented by the plaintiffs turned out to be correct. The court is currently negotiating with the Peruvian authorities as to whether an on-site visit to take evidence would be possible. “This shows that lawsuits are quite promising,” says Friederike Otto. However, the feared extreme event of a major glacier collapse has not yet occurred here. RWE would therefore not be liable for any damage caused, but rather for eliminating an impending danger.

But what would a continuous chain of evidence look like? The World Weather Attribution has defined certain rules for this: First, the researchers decide which parameters are important for the event. For example, the temperature when investigating a heat wave. They then define which area they want to investigate exactly, such as the European Mediterranean region. The duration of the event, the weather data recorded during the event and any other special circumstances are also recorded in detail. The scientists then obtain as much historical weather data from the selected area as possible; for the heat waves in Europe, this would be the temperature measurements from the beginning of records in 1900 to the present day.

Now climate models come into play

The climate models calculate possible weather using physical equations, taking into account boundary conditions such as greenhouse gas concentrations or land use data. Using the historical weather data, the researchers are now checking which models can actually simulate the real weather events of the past 120 years well. “If they don’t do that, we can’t use them for this attribution study,” explains Otto. With the remaining climate models, Otto now simulates the weather events in the area under consideration – beyond the period from which the real weather data comes. Calculated over many centuries or even millennia, it becomes clear how common large swings up or down are, i.e. the extreme events that attribution researchers are after. Heat or cold waves, for example. The computing capacity for such long model runs has only been available for a few years.

Now comes the real trick: the climate models can also be adjusted so that they simulate exactly the same world, only without the man-made increase in greenhouse gases. Climate scientists call this the “counterfactual world.” And in this world too, the frequencies of major deviations from the norm can be calculated.

10 to 100 times

Climate change made the scorching heat in Western Europe in July 2021 more likely.

30 percent

The Australian forest fires at the turn of the year 2019/2020 have become more likely due to climate change – at least.

2-mal

The heat wave in the Indian state of Andhra Pradesh in 2015 with over 1,800 deaths became more likely.

3-mal

Climate change made the heavy rains during Hurricane Harvey in Houston more likely. 1,000 liters per square meter fell over three days. The hurricane claimed 83 lives and caused $125 billion in damage.

17.000 Euro

The Peruvian farmer and mountain guide Saúl Luciano Lliuya wants to sue the energy company RWE because a glacial lake is threatening to overflow due to global warming.

840 million

Dollar damage has been caused by climate change through floods and droughts in New Zealand in the last ten years.

“If you now compare the frequency of a certain extreme event in the simulations with and without climate change, you can determine how strongly climate change contributed to the fact that this exact event actually occurred,” explains Friederike Otto. It is very important here that this is not a general statement that, for example, heat waves are becoming more common due to climate change. “Because at the beginning we define exactly what we want to look for in the simulations based on the event that actually took place, we then receive concrete statements about exactly this event.”

It may turn out that climate change has made the event more likely – or even less likely. Or that he had no influence at all. “Sometimes, such as in a drought we studied in Brazil, two opposing effects of climate change cancel each other out,” says Otto. Here, the greenhouse effect favored both precipitation and evaporation, and the risk of drought therefore remained the same.

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