Guest post: How attribution can fill the evidence ‘gap’ in climate litigation

In November 2015, a Peruvian farmer and mountain guide, Saúl Luciano Lliuya, launched a pioneering bid for climate justice in a German court more than 10,000km from his home in the Andean city of Huaraz. 

Lliuya claimed that climate change had driven the Palcaraju glacier to retreat 2km up a mountain, expanding a large lake in the wake of its retreat. That lake threatens to burst onto Huaraz with catastrophic consequences.

The case argues that RWE, a German energy company and one of the world’s biggest greenhouse gas emitters, is responsible for part of the costs of protecting the city due to their contribution to climate change. His case is one of the growing numbers of climate-related lawsuits filed in courts around the world.

Just weeks after Lliuya filed his lawsuit, the world’s governments agreed the terms of the Paris Agreement. However, it left individual countries to determine what action they would take to contribute to its long-term goal of limiting warming to 1.5C or “well-below” 2C.

Taken together, current national pledges to cut emissions fall short of the Paris goals. And since the agreement does not directly confer binding targets on individual states, lawyers began to ask: can the courts compel countries, and even companies, to set and meet climate targets aligned with the Paris Agreement?

Increasingly, the answer to that question is yes. However, cases that seek damages for the impacts of climate change – like Lliuya’s – have largely been less successful.

In our new study, published in Nature Climate Change, we find that the evidence used in these cases often does not reflect the most recent advances in climate science.

We show how climate change attribution – the field of research that quantifies how human-caused climate change alters the likelihood and intensity of severe extreme weather events – could be used to fill the evidence “gap” in climate litigation.

Climate before the court

This year has already seen major legal successes on climate change in courts around the world.

At the end of April, judges from Germany’s highest court condemned the country’s Climate Protection Act as inadequate for failing to protect future generations from the impacts of climate change and demanded that the government strengthen the Act. A week later, the German government announced major changes to its climate targets, including cutting emissions by 65% by 2030 and eliminating net emissions by 2045.

In recent weeks, courts have ruled that the Australian government must take reasonable care to ensure that new coal mines do not harm children and that Belgium’s climate policy is inadequate and violates human rights. 

In the Dutch courts, Shell was ordered to align its business with the Paris Agreement and cut its CO2 emissions by 45% by 2030, relative to 2019 emissions. This includes the emissions associated with all the oil and gas it produces, even if it is burned by other companies or consumers.

The power of the courts as a force for climate action can no longer be in dispute. However, these successful cases are still limited in number and most climate-related lawsuits that rely on evidence linking greenhouse gas emissions to climate change impacts have been unsuccessful. 

So, if cases seeking compensation for losses suffered due to climate change are also to be successful, what needs to change?

The role of scientific evidence in climate litigation

In our study, we considered the evidence provided by litigants to support their arguments in over 70 lawsuits, filed in 14 jurisdictions, and the courts’ interpretation of this evidence. 

In many cases we studied, plaintiffs sought financial remedies from high-emitting corporations for losses suffered due to climate change. Courts rely on scientific evidence to evaluate the causal link between the defendant’s behaviour and the plaintiff’s losses. Across jurisdictions, courts evaluate that evidence by applying legal tests that set evidence-based thresholds for establishing causation – but the stringency of these tests varies. 

When more stringent tests are applied, claimants may have to show that the defendant’s actions constituted “material contributions to harm”, whereas more flexible standards merely ask if damages are “fairly traceable” to the defendant’s actions.

Lawyers have long championed the crucial role for attribution science as evidence for causal claims in climate lawsuits. However, we found that the evidence submitted and referenced in these cases still lags considerably behind the state-of-the-art climate science, impeding causal claims.

Our analysis revealed that, in many cases, the evidence plaintiffs provided was not sufficient to overcome even the more flexible causation tests. While accepting that greenhouse gas emissions cause climate change, courts typically found that plaintiffs failed to demonstrate that defendants’ emissions caused the alleged impacts. 

In jurisdictions as diverse as the US, Germany and New Zealand, courts found that plaintiffs were unable to establish that their injuries would not have occurred in the absence of defendants’ emissions. In some cases, for example, courts pointed towards defendants’ small contribution to climate change, the high number of emitters responsible, or the absence of a method for discerning entities responsible for impacts.

Our assessment is that it is not due to scientific limitations that the evidence provided failed to convince judges. Instead, we found that at least some of these obstacles could be overcome if plaintiffs made better use of climate science – particularly in the field of attribution science.

Attribution science

The field of attribution science has been applied to a growing number of extreme events and provides, in many cases, the necessary tools and understanding to evaluate the full causal chain from greenhouse gas emissions of countries and companies to losses suffered by individual communities.

Attribution can bridge the gap identified by judges between a general understanding that human-induced climate change has many negative impacts and providing concrete evidence of the role of climate change at a specific location for a specific extreme event that already has led or will lead to damages. Attribution of causal relationships in science can therefore be translated into legal causality.

As a result, it can provide concrete causal evidence for specific lawsuits. A further implication of the growing body of attribution research is a broader scientific understanding of which damages from natural hazards today are made worse by climate change and which are not. 

Attribution research identifies the types of hazards for which human-caused climate change is, or is not, a key driver. It also highlights where scientific gaps exist. For example, in some locations and for some types of weather events, it is not yet possible to assess the role of climate change. For instance, climate model limitations restrict our ability to provide attribution assessments for some winter storms. Nonetheless, this body of literature makes it possible to put specific causal claims into context and has the potential to enhance their effectiveness in meeting courts’ standards for evidence.

Based on our analysis, we identified three areas where attribution science research could address gaps in the evidence provided to lawsuits on climate change impacts.

First, evidence linking the emissions of individual entities – such as countries or companies – to specific impacts of climate change demonstrates how defendants’ emissions contribute to harms.

Second, researchers can provide a more complete assessment of how climate-related hazards interact with plaintiffs’ vulnerability and exposure to help identify the legally relevant causes of losses.

Finally, attribution studies show how an event of given magnitude was made more likely to occur, and/or how an event of given probability was made more intense. In some settings, the numbers for these two approaches to conducting attribution can be sensitive to the exact definition of the hazard. Attribution science can help to identify these sensitivities and lead to a more robust causal argument overall.

The way forward

So, if attribution science can provide the evidence, what is needed for this research to become more widely used?

Greater scientific awareness in the legal community, and vice versa, could ensure that cases filed focus on impacts that are in fact attributable to climate change – and that the evidence submitted to courts clearly substantiates the alleged relationship between defendants’ emissions and plaintiffs’ losses. In many past lawsuits, this has not been the case. 

Improved dialogue between the legal and scientific communities on the basis for causal claims would ensure that lawyers are aware of – and able to request – evidence that can be used to robustly evaluate causal claims. Lawyers can identify the key questions around evidence that plaintiffs need to answer if cases are to be successful. Scientists can then assess whether these causal links exist.

Previously seen as a speculative attempt to get the courts to enforce climate action or merely to draw attention to the issue, lawsuits are forcing governments and companies to step up their climate ambition. It does not seem far-fetched anymore to suggest that – supported by the right scientific evidence – future cases will compel companies to pay compensation to communities impacted by climate change.