Cause and effect: the science of climate attribution

For a long time it has not been clear in climate research how much climate change affects individual extreme weather events. That has changed now: scientists can link climate change to its causes more precisely.

If it’s raining, or the sun is shining, or there’s a hurricane, that’s the weather. In a specific place at a specific time. There has always been extreme weather, that’s undeniable. But now the impact of climate change is becoming increasingly clear. Our climate system is affected by many different factors: by natural factors such as solar intensity or volcanic eruptions, but also by additional emissions of greenhouse gases.

Therefore, it has been difficult for a long time to attribute human impact to extreme weather events such as heatwaves, droughts, heavy rains and floods. But that has changed now. Scientists can more accurately determine the causes of various climate changes and extreme weather events. The research discipline behind this is called attribution research.

Climate change dynamics

Sebastian Sippel is Junior Professor in Climate Attribution at the Leipzig Meteorological Institute. “We can imagine the fluctuations in our weather like a dice,” he explains. “There are six different numbers: one represents a very cold day, and six represents a heat wave. So what we are asking is relatively random.”

A warm, sunny day at 20°C in mid-November in Germany is just as common as a rainy day at 0°C. “Climate change affects our dice and affects the probabilities. Now it can happen more often that we roll a six and a little less than a one. Our dice are marked, so to speak,” says the expert.

Likely And the intensity

In addition to extreme weather events potentially occurring more frequently due to climate change, their intensity is also changing. Circa 2021 in Canada. The heatwave brought temperatures of up to 49.6 degrees Celsius. An unprecedented temperature record in the region. Scientists at World Weather Attribution have now determined that this heatwave would have been about 2 degrees Celsius cooler if not for climate change. “An event like this, which currently occurs only once every 1,000 years, will occur approximately every five to 10 years in this future world with global warming of about 2 degrees Celsius,” the accompanying attribution study says.

Twin worlds As climate models

To find out something like this, researchers use climate models. It allows them to compare two worlds: one with man-made climate change and one without. The first model considers the probability of a real extreme climate event occurring under the conditions under which it occurs. The simulation reproduces the event several thousand times, taking all critical factors into account. The result is a probability distribution: this is how likely such an extreme weather event is to occur on that day under given conditions. “In the second step, we look at a model in which we take CO2 concentration and other human-made climate factors into account,” Siebel says.

The parameters are adjusted according to the reference period before industrialization, approximately 1850–1900. Here again, the model performs thousands of tests and calculates the statistical distribution of a potential extreme weather event without a climate change factor. “We can then compare these two distributions and see: This is how the probability of such an event occurring has changed due to climate change,” Siebel explains.

The limits of Attribution research

In order to accurately attribute climate changes to climate change, climate models must take a large number of factors into account, says Jakob Zechsler. He heads the Working Group on Composite Weather and Climate Events at the Helmholtz Center for Environmental Research in Leipzig. “When it comes to heatwaves, we can now say specifically how much climate change is affecting them,” says the scientist. “This can easily be reflected in climate models. Almost every heatwave is exacerbated by climate change.”

“Temperature and precipitation are fairly well observed. Here we have good information about historical climate changes. This is not necessarily the case for other factors,” adds Katja Freiler from the Potsdam Institute for Climate Impact Research. Other extreme weather events are therefore somewhat more complex to reconstruct and map. Such as heavy rain events, floods or periods of drought.

But here too, the impact of climate change has become increasingly clear in recent years, says Zechsler: “When we talk about drought, that is, dry soil, we now know that it is increasingly affected by rising temperatures. Usually, the first thing that comes to mind is “There will be a water shortage. “But because temperatures are rising, a lot of it is evaporating.” This makes the drought much worse. His current study shows that 25% of the severity of the drought in 2022 in Europe can be attributed to climate change and associated rising temperatures.

This is why research is needed

Attribution research reveals complex links between human-caused global warming and extreme weather events. A somewhat less prominent branch of research here is climate damage attribution: “For example, we see an increase in damage from flood events around the world. Then we determine: to what extent is climate change driving this forward? Is it perhaps also Because “more people are moving into the danger zone,” Freiler explains.

The consequences of climate change can be attributed to the economy, environment, health or social structures. Freiler stresses that such understanding is crucial to addressing and adapting to climate change: “Climate change will continue to get worse. In order to influence it and drive improvements, we always first have to understand the causes. This is exactly what the mission of attribution research is,” she summarizes.