Science can now simulate what the climate would look like without human influence. This has far-reaching legal consequences: US states and companies are already being accused of climate disasters - and convicted.
Last August, a modest county court in the US state of Montana was the setting for a landmark ruling. In a trial that lasted two months, 16 young people were suing the state. They argued that its energy policies were fuelling climate change and therefore violating the state’s constitution, which enshrines the right to "a clean and healthful environment".
The lead plaintiff in the case, Rikki Held, grew up on a ranch in a region of Montana that is rich with reserves of coal - and mining companies. The state also has 5000 gas wells, 4000 oil wells and four oil refineries. Held testified that, in heat waves that topped 43 Celsius, wildfires had burned down power lines, disabling the pumps the family used to extract water for their cattle.
"That's my life," she told the court, tears running down her face. "My home is there, and it impacts the well-being of myself, my family, my community."
Held and her fellow plaintiffs, who were represented by a legal non-profit group, made a compelling emotional case. But key to their suit would be the testimony of scientists who are increasingly able to identify the causes of extreme weather and the disasters that they cause, including floods and fires.
"Attribution science" has become a powerful tool in both advancing popular and political understanding of the impacts of climate change, and - now - in confronting the governments and corporations who stand accused of either enabling or contributing to it.
In the Montana case, several scientists gave evidence to show that, for example, increasingly common temperature extremes, which can cause wildfires, are in large part the results of anthropogenic climate change.
Ultimately, the court found in the young plaintiffs’ favour, ruling that the state’s Environmental Policy Act was unconstitutional because it failed to properly consider the effects of permitted fossil fuel projects on the climate. It was one of thousands of cases being brought worldwide that lawyers and campaigners have hailed as a vital new front in the fight to save the planet.
"This court ruling is a step toward climate justice," said Delta Merner, lead scientist at the Science Hub for Climate Litigation at the US-based Union of Concerned Scientists, in a statement after the ruling. "Whether it's governments, fossil fuel companies or industry trade groups - holding polluters and their enablers accountable is a critical piece of meaningful climate action."
Climate litigation is often being used to defend and compensate traditionally underrepresented groups. In another recent case, a community in Peru sued a German energy company by using attribution science to reveal the extent to which climate change was causing a lake fed by a rapidly melting glacier to burst its banks, increasing the risk of floods.
But how does such attribution work? It’s a field that dates back more than 20 years and has been shaped by disasters. After the deadly 2003 European heatwave, which resulted in tens of thousands of deaths, scientists at the Met Office, the UK’s weather and climate agency, and the University of Oxford concluded that human influence had at least doubled the risk of such extremes of heat.
Key to determining that link, and attempting to measure it, is a kind of retrospective climate modelling. In simple terms, climate scientists create computer models that include emissions of greenhouse gases alongside sea surface temperatures. They then run two simulations. One includes emissions to create a reference that matches the current climate. In the second simulation, human input is removed to create a picture of the climate as it would have been had the world not industrialised and pumped carbon into the atmosphere.
This modelling cannot say what the weather would have been like day to day without man-made climate change. But the increasing computing power used to create it means that scientists can compare the simulations and say - with growing speed and confidence - that, for example, the heatwaves that struck Europe again in 2023 would have been “virtually impossible” without human influence.
The modelling can also be extended to predict future risk. In the UK last year, for example, where temperatures hit 40 Celsius for the first time in recorded history, the Met Office warned that, in a continuing high emissions scenario, 40 Celsius summers could come round every three to four years by the end of this century.
These prognoses are being used outside of courtrooms to inform decision making. In industries such as agriculture or transport infrastructure, climate data is vital when investments are possibly worth billions of Euros. If the increasing likelihood of heatwaves exacerbated by manmade climate change means upgrading power cables and railways, for example, then the potential cost of such work can be weighed against the cost of investing in the kind of climate change mitigation that might help preserve existing infrastructure.
Sometimes, of course, the evidence says otherwise; scientists last year concluded that a devastating drought in Argentina and Uruguay would have happened regardless of man-made emissions. Either way, being able to determine the cause of a catastrophic heatwave or flood, at least to within a reasonable range of probability, means that such events can no longer be compared with historic events or dismissed as "acts of God".
Ultimately, campaigners, lawyers and the people they represent want this new threat of potentially costly legal action to become another reason for corporations and governments to ramp up efforts to meet climate targets. "Such litigation may become an important driver of both mitigation of greenhouse gas emissions and adaptive action by both public and private sectors," an article in Nature Geoscience said in 2017.
In Montana, the success of the young people whose case found favour with a judge is inspiring similar suits in other US states with high emissions. “It’s monumental,” Badge Busse, 15, one of the Montana plaintiffs, told the New York Times after the ruling. “It’s a completely beautiful thing. Hopefully this will continue this upward trend of positivity."
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