What's wrong with climate change

Climate change is now also weather change

Relatively mild temperatures and a lot of rain: This winter it has been easy to believe that the current weather is influenced by climate change. But as soon as it gets really cold again in winter - as for example last January in the USA - this is not so obvious to many people.

The standard answer of the scientists: Weather is not the same as climate and a cold spell does not mean that climate change will not take place. Nevertheless: "When there are cold waves, many people ask why everyone is talking about climate change," says Sebastian Sippel, climate scientist at ETH Zurich Climate reporter °. "That is then also used politically, for example by US President Donald Trump."

He is known for Twitter comments, with which he has already questioned climate change during many such cold spells. At the end of January 2019, he wrote in view of the cold record in the USA: "What's wrong with climate change? Come back, we need you!"

Such "arguments" were also what motivated Sippel and his colleagues to conduct a statistical study, the results of which were recently published in the specialist magazine Nature Climate Changehave appeared.

In it, the scientists have developed a method with which they can detect climate change in the weather on a single day. "We show climate change in the global spatial patterns of temperature and humidity," explains Sippel.

With the help of machine learning, the researchers taught an algorithm to calculate the global mean temperature for a year from these global patterns. "The algorithm uses a so-called fingerprint, which shows in which regions of the world climate change is particularly visible," says Sippel.

"For example, in the tropics, and especially over the oceans, there is a good relationship between the temperature trend signal and relatively low weather variability," he explains. The measured temperature values ​​from these regions are then weighted more heavily.

Climate change visible every day since 2012

In order to determine the influence of climate change on a specific day, the algorithm then receives the measured temperature and humidity data for the day and uses this to calculate the weighted mean across all regions. "He estimates the global annual mean for a whole year from one day. This in turn is a measure of climate change," says Sippel.

Sippel and his colleagues have been able to prove this fingerprint of climate change every single day since spring 2012. If the data for a whole year are taken as a basis, even from 1999.

Strictly speaking, one cannot infer from the results that this climate change is man-made, as study author Sippel puts it, because the researchers did not train the algorithm on this. "However, we already know from other studies that humans are the biggest factor."

"Climate change has now progressed so far that we can determine just from the spatial patterns of a single day how much the earth has warmed up," says physicist Matthias Mengel from the Potsdam Institute for Climate Impact Research Climate reporter °. "That is a big step, because we can now tell people that climate change has already changed our weather significantly and that the weather we experience every day is different than it would be without man-made climate change."

Conversely, however, one cannot say that all weather events in the world are now driven exclusively by climate change. "The weather is a very dynamic process with a high degree of variability," says Mengel. That is why the distinction between climate and weather is still necessary. One could say, however, that climate change is now also weather change.

Climate change is also there during cold spells

The study also wants to build a bridge between so-called attribution studies and long-term evidence of climate change. Attribution studies can demonstrate climate change in individual weather events, such as heat waves, by calculating how likely an event is in today's atmosphere. They then compare that to how likely the event would be without climate change.

For example, researchers have calculated that heat waves like the one in summer 2018 in several European cities are twice or even ten times as likely as they would be without climate change.

Sebastian Sippel sees a main aspect of his study in the communication of climate change. "We see climate change every day, but only on a global level," says the researcher. If there are new cold records in the USA and the US President lets go of a corresponding tweet, one could now prove that climate change is not "gone" despite the cold, as Trump suspected, but still affects weather conditions worldwide.

Whether people like Donald Trump will pay attention to this information is another question.