In recent years, so-called attribution research has rapidly developed into its own branch of climate research. This only became possible after rapid advances in computer technology produced ever more powerful processors and memory space ceased to be a luxury good. Specifically, the question is: How likely is an extreme weather event in a region in a world undergoing climate change (with an increased concentration of greenhouse gases) and in a world without climate change? It sounds harmless, but it is incredibly complex to calculate. Exactly what kind of extreme weather could an atmosphere that is not pumped full of carbon dioxide (CO2) produce? And how often?

To do this, it is not enough to "simply use a climate model and simulate possible weather once or twice on every single day of the past ten years", Otto writes in her book "Wütendes Wetter - Auf der Suche nach den Schuldigen für Hitzewellen, Hochwasser und Stürme" (Ullstein, 240 pp., 18 euros), which entered its fourth edition in 2019. Instead, she said, it requires up to 1,000 simulations. "If we were to simulate only two conceivable summers, for example, it would be like picking two clovers, one three-leaf and one four-leaf. And without ever having seen any clover leaves before, you wouldn't know that three-leaved is the normal case and four-leaved is the extreme case. That's the reason we have to go through possible weather so insanely often."

When the first attribution studies started, their initiators were still outsiders. They had little money and no free ride for computing time on the world's most powerful supercomputers. A solution had to be found for the gigantic power consumption alone. What to do? One inspiration came from alien hunters who used a software called BOINC, developed in 1999, to join forces in the SETI@home project to help scientists search for extra-terrestrial life. Millions of people joined in and practised digital citizen science by donating computing time on their home PCs and, of course, by providing electricity. Whenever the private screensaver came on, it was SETI time. At the end of March 2020 (after 21 years), the SETI project was discontinued due to financial constraints. Especially the operation of the antennas, which were used to listen for extra-terrestrial audio messages, swallowed up a lot of money.

The attribution scientists took a similar approach. Climateprediction.net is the name of their project and their website. Download the BOINC software and - in the next step - donate computing time and electricity to the weather climate change signal search. 700,000 people worldwide have joined in so far. "In 2015 alone, the working time of all those processors added up to 120,000 years. If we were to buy this time in the cheapest cloud, we would have to spend six billion dollars," Otto reports.

The fact that we know with certainty that today’s extreme weather conditions are the result of human-induced climate change is also thanks to the generosity of the many people who have donated electricity. The speed with which attribution research has become established can also be seen in the fact that the last and sixth assessment report of the UN Intergovernmental Panel on Climate Change (IPCC) for the first time included a chapter on extreme weather. Frederike Otto was one of the lead authors.

Yet it is by no means the case that every weather event that deviates from the norm and becomes extreme has been pushed by climate change. The attribution researchers have provided remarkable insights, for example into what is happening in East Africa. According to the findings, the droughts in Ethiopia (2015) and Kenya (2017) had mixed causes: on the one hand, a loss of rainfall as part of natural climate variability, and on the other, inappropriate land usage. Intense dry phases are part of meteorological variability. When does the absence of a rainy season turn into drought and famine? The farmers of East Africa prefer to grow maize and rice as crops. These not only require a lot of water but are also prestigious in Ethiopia, as opposed to the more undemanding millet. In addition, in the country, forests have increasingly been turned into fields and the government has helped with irrigation systems, but this has increased vulnerability in the event of a drought.

Otto writes that it is important for local scientists to be involved in such studies. "This helps politicians and society to accept the results" - especially when they do not meet political expectations because there is no scientific evidence against climate change. The world can be divided up like this: Those who cause climate change tend to live in the West (= industrialised countries), the victims in the global South (= developing countries). Sceptics and supporters of attribution studies are distributed accordingly.

The growing number of attribution studies will foreseeably also influence global climate lawsuits, provided some are admitted in court. The number is growing. The global registry at the Sabin Center for Climate Change Law at Columbia University in New York now lists more than 1500 climate lawsuits, more than half in the USA. Lawsuits are filed primarily against governments that do too little to reduce greenhouse gas emissions and against companies that continue to operate their fossil fuel business models against their better judgement. National laws offer different levers for legal action, but all actors are entering new legal territory. A major shortcoming until now is how to find a specific defendant for a case of specific extreme weather damage if the whole world is the cause.

20 years ago, the environmental organisation Germanwatch said that human-induced climate change had the potential to "redefine the perfect crime". Perpetrators, motives, witnesses - it's all there. "And yet: no one can be held accountable as a perpetrator" because everyone remains anonymous in a diffuse mass. When the Arctic village of Kivalina sued the energy company Exxon Mobil in 2009 because (as the charge goes) its greenhouse gases had destroyed Kivalina's livelihood high in the north, the judge in San Francisco had to pass: It could not be clarified beyond doubt whether the cause was really US greenhouse gases or not Chinese, Russian or German ones.

Other judges and courts were reluctant to even allow lawsuits because they did not feel they had jurisdiction and did not want to take on politicians' duties. Referring to the "Julia v. United States" lawsuit, attorney Jeffrey H. Wood, representing the US government, told the New York Times: "This lawsuit is an unlawful attempt to use a district court in Oregon to control energy and climate policy across the nation." The lawsuit had been filed in 2015 by the non-governmental organisation Our Children's Trust on behalf of 21 young people. The Obama administration fought it as vehemently as that of Donald Trump. It was already considered a success if the courts dealt with it at all. After a legal tug-of-war over jurisdiction that lasted seven years, the erstwhile young people now want to try a second time with new grounds of appeal.

The wind has changed in the courts in some countries, with plaintiffs changing their strategies. In the meantime (2015), more than 195 countries had signed a climate agreement in Paris that is binding under international law, but does not provide for sanctions for national violations. Then in the Netherlands, in October 2018, the government was sentenced in the last instance to reduce its country's CO2 emissions by 25 per cent by 2020 - and not only by 23 per cent, as planned. The ruling was a success for the environmental organisation Urgenda, which had already taken around 900 private plaintiffs to court in 2013. Ultimately, the speed limit of 100 kilometres per hour on Holland's motorways can also be traced back to the decision of The Hague Court of Appeal. This had overruled the government's criticism that judges should not give direction to elected politicians. In other words: a direction for which there were no democratically legitimised political majorities. In any case, the Dutch judges were the first in the world to find that a promise made by a government under the Paris Climate Agreement is enforceable in court.

The German Federal Constitutional Court also intervened in politics and made headlines around the world when it ruled in May 2021 that the German Climate Protection Act was unconstitutional in parts because it weighted the freedom of those alive today too highly and restricted that of those born later. The CO2 reduction burdens should be distributed fairly across the generations. The logic behind this is that the CO2 release budget is too high: Because the CO2 release budget for achieving German climate goals is limited, freedom must ultimately also be budgeted. Accordingly, the policy is demanding too little from current generations in terms of CO2 savings and is burdening future generations with foreseeable radical reduction burdens. "Virtually all freedom is potentially affected by these future emission reduction obligations," the judgment states, "because almost all areas of human life are still associated with the emission of greenhouse gases and are thus threatened with drastic restrictions after 2030."

Then, in June 2021, the British-Dutch Royal Dutch Shell became the first multinational oil company to be found guilty - again by a Dutch court. Shell must reduce its CO2 emissions by 45 per cent by 2030 compared to 2019. According to the judges, this also applies to all Shell suppliers. Now it is going to appeal.

One case that is being meticulously monitored globally is that of Saúl Luciano Lliuya, a Peruvian smallholder and mountain guide from Huaraz, a small town below Lake Palcacocha in the Andes. Because the glaciers are melting, the lake is filling up; if the dam breaks, Lliuya's house would be destroyed by the flood and many lives might be lost as well. The German energy company RWE is being sued. To prevent a judge from arguing like the one in San Francisco in the Kivalina case, lawyer Roda Verheyen is demanding that RWE contribute 0.47 per cent of the cost of construction measures to stabilise the dam. 0.47 per cent: that is RWE's share of global CO2 emissions to date. 0.47 per cent of the construction costs would correspond to 17,000 euros. Not an amount that would scare RWE - but it would be a precedent that could subsequently mean millions or billions not only for RWE but for all companies in the fossil complex.

The Foundation for Sustainability and Germanwatch support Verheyen and Lliuya. Germanwatch Executive Director Christoph Bals says: "The Huaraz lawsuit has already had the effect of re-reading section 1004 in the Bürgerliches Gesetzbuch (German Civil Code). The right to removal and injunctive relief against interference with property by a troublemaker is normally used in neighbour disputes." Initially, the Regional Court of Essen had dismissed the Huaraz case in 2015, while the Higher Regional Court (OLG) of Hamm ruled that climate change with its transboundary effects had brought about a kind of global neighbourhood relationship, so that Section 1004 of the German Civil Code also applied here. This alone made legal history. Then it entered into the taking of evidence. On site, too: the official trip to Huaraz, repeatedly postponed because of the Coronavirus pandemic, was due in May. Three judges flew to the glacial lake in Peru.

Professor Claudio Franzius, Director of the Research Centre for European Environmental Law in Bremen, refers in a research report to an evidentiary decision of the Higher Regional Court of Hamm, according to which the court found "taking into consideration the large number of alleged contributors to climate change, it cannot be inferred from the existence of several disturbing parties that it is impossible to eliminate the disturbance". The court did not accept the defendant's objection that it was not a 'disturbing party' in the legal sense, since RWE operated its power plants lawfully with official authorisation."

In the meantime, attribution science had evolved. In February 2021, a study on the endangerment situation at Lake Palcacocha had appeared in Nature Geosciences, involving researchers from the UK and the US, notably including Professor Myles Allen of the Environmental Change Institute at Oxford University, one of the pioneers of attribution research. It was Allen who had brought Friederike Otto to Oxford. According to the results of the study, the influence of humans on the measured warming in this Andean region is over 85 percent; it is more than 99 percent certain that the retreat of the glacier, which leads to the flood risk, cannot be explained by natural changes alone. The plaintiffs hope that this will close a gap in the evidence in the "Lliuya vs RWE" case. How will the OLG judges judge this? A decision is expected in 2023.

The assumption that allocation researchers facilitate the "business" of the climate plaintiffs with their expertise by providing scientific evidence - so far missing for a concrete case - remains only an assumption for the time being. The study "Closing the evidence gap in climate processes" by researchers from Oxford University, published in the journal Nature Climate Change, was sobering. In summary, the scientists come to the conclusion that lawyers hardly use the progress in attribution research. The evidence provided by plaintiffs in 73 court cases in 14 countries fell well short of the latest research. Plaintiffs' lawyers provided even less quantitative evidence of a link between defendants' emissions and plaintiffs' damages. Some 73 per cent of cases did not refer to peer-reviewed evidence. And 48 per cent of the cases that did refer to extreme weather events claimed that the weather was due to climate change, but without providing any evidence.

Rupert Stuart-Smith, lead author of the climate lawsuit study, said, "Most recently, successful lawsuits in the Netherlands, Germany and elsewhere have required countries and companies to drastically increase their climate targets." This makes "the power of climate litigation" increasingly clear, he said, although many have been unsuccessful. "If litigation to compensate for losses suffered due to climate change is to have the best chance of success, lawyers need to use scientific evidence more effectively" - and long before a claim goes to court. Attribution research can now answer questions raised by courts in earlier cases, but which scientists were unable to answer at the time, he said. That is different now.

Meanwhile, many hope that attribution evidence will one day trigger a revolution in the courtroom similar to the one that genetic fingerprinting once did. (Original text: Wolfgang Wiedlich / Translation: Jean Lennox)