Receding Glaciers, Advancing Law: Why “Best Available Science” Matters in Climate Litigation

Scientific Evidence in Assessing Causation & Risks: Lliuya v RWE

On 28 May 2025, the Higher Court of Appeal in Hamm, Germany, delivered one of the most highly anticipated judgments in transnational climate litigation: Lliuya v. RWE. The case, brought by a Peruvian farmer against a major German energy company under nuisance law of the German Civil Code (section 1004), illustrates both the potential and the limits of existing legal frameworks in addressing the adverse impacts of climate change.  

Despite ultimately rejecting the claimant’s request for injunctive relief, the Court made several important findings in advancing the private nuisance law that can now give further guidance for the climate science and law research community. First, it confirmed that German private nuisance law is applicable in a transnational context, second, greenhouse gas emissions of a major corporate emitter can be identified as a cause that contributes to climate-related harms occurring thousands of miles away, third, these emissions can increase the risk of a Glacial Lake Outburst Flood (GLOF) that can constitute a property impairment, if the risk qualifies also as being “imminent”. In doing so, the Court reinforced a growing judicial capacity and willingness to engage with corporate responsibility for climate change in the light of advances of climate science.  

Yet the claim failed on a crucial evidentiary threshold: the claimant could not establish that the risk of damage to his property from a GLOF met the legal requirement of an “imminent” threat. This outcome brings into sharp focus a central challenge for climate litigation and the extent that courts interpret and use scientific evidence when assessing causation and risk. 

Climate Causation and Legal Thresholds

At the heart of Lliuya v. RWE lies the question of causation. Climate change is driven by cumulative global emissions, making it difficult to isolate the contribution of any single actor to a particular harm. The Hamm Court nonetheless accepted that such a causal link can be established in principle, marking a significant step forward in climate jurisprudence. 

However, the decision also demonstrates how traditional legal concepts such as “imminence” in nuisance law may need to be conceptualised differently, to give effect to a fuller reading of the physical laws that underlie climate change, the Intergovernmental Panel on Climate Change (IPCC) reports, and attribution science. The advances in our understanding of climate science enables the law to accommodate the probabilistic and forward-looking nature of climate risk. In this case, expert evidence suggested a relatively low probability of flooding (approximately 1% over 30 years), which the Court found to be insufficient to satisfy the threshold of imminent harm. However, if the Court would have considered the increasing risk for harm to higher protected legal interest, beyond the property of the claimant, including the risk to bodily harm and life, then even that risk would have to be considered sufficient in line with established case law.

Apart from the question how the scientific evidence informed the magnitude of the risk, the reasoning of the Court raises an important methodological issue: how should courts evaluate scientific uncertainty amidst conflicting scientific evidence, and considering the physical law of climate change and the assessment reports of the IPCC? And what counts as sufficiently robust evidence or best available science when dealing with long-term, evolving risks?

The Role of “Best Available Science”

Addressing these questions requires closer alignment between legal reasoning and scientific knowledge. A key argument emerging from this research is that courts should anchor their assessment of climate evidence in what can be described as “best available science.”

This baseline includes three interrelated elements:

• the established physical laws governing the climate system;
• the assessments of the IPCC; and
• peer-reviewed scientific evidence consistent with these foundations. 

Using such a baseline helps ensure that case-specific expert evidence is not evaluated in isolation but rather situated within the broader body of high-confidence scientific knowledge. Where evidence deviates from this baseline, courts should require clear justification and carefully consider the implications for evidentiary weight and confidence levels.

Qualitative vs Quantitative Causation

Another critical insight concerns the distinction between qualitative and quantitative statements of causation. Climate science often allows for strong qualitative conclusions, for example, that human emissions have increased the risk of certain extreme events, while precise quantitative attribution remains uncertain.

Legal reasoning, however, frequently depends on numerical thresholds or probabilities. The failure to distinguish between these different forms of scientific knowledge can lead to misunderstandings and, ultimately, to decisions that underappreciate established climate risks.

We explain in our article how recognising this distinction would allow courts to engage more effectively with scientific evidence. Qualitative findings, supported by high levels of scientific consensus, can provide a solid basis for legal conclusions even where precise quantification is difficult. Even where quantification remains uncertain, the science that underpins that qualitative link for causation cannot be set aside based on uncertainties surrounding the quantification, especially not where multiple, independent lines of evidence result in a strong causal link with high confidence. As the case has demonstrated, the legal reading of scientific evidence can lead to a quantification of risk that, in the end, effectively sets aside the qualitative link of causation.

Beyond Lliuya: Implications for Future Litigation

Although the claimant in Lliuya v. RWE did not succeed, the case represents an important step in the evolution of climate litigation and the integration of climate law and attribution science. It demonstrates that a causal link between a specific emitter and a specific climate impact can be established, as the Court found that ‘it had been foreseeable for the optimal observer in the role of an energy producer, since the mid-1960s, that anthropogenic GHG emissions would lead to global warming and the resulting harm’. RWE’s GHG emissions made a traceable and material contribution to global GHG emissions. The Senate was convinced that this causal link existed based on generally accessible scientific information—facts that constituted common knowledge— and that there was no need to engage in further fact-finding regarding causation. At the same time, the Court’s legal assessment of the scientific evidence that demonstrated the risk of an imminent first impairment highlights the need for further development in how legal systems handle scientific uncertainty, especially where the law requires a numerical threshold. The melting of permafrost has been identified by the IPCC as a particularly high risk for humans and ecosystems in the near term, and any additional warming will further amplify permafrost thawing (high confidence). This evidence that relates to glaciers in a warming world and it provides relevant scientific information for the expansion of glacial lakes. It may not yet directly support a quantification of risk over a given period. Therefore, assuming a specific timeframe and a concrete, constant risk factor over 30 years, considering contradicting evidence, downplays the uncertainties that exist. Without a clearer framework for integrating best available science, courts risk setting evidentiary thresholds that remain misaligned with the realities of climate change.

Looking ahead, greater interdisciplinary collaboration between legal scholars and climate scientists will continue to be essential. By refining concepts such as causation, probability, and imminence considering current scientific understanding, legal systems can become better equipped to respond to the challenges posed by a warming world.

Concluding Reflections

Lliuya v. RWE sits at the intersection of law, science, and justice. It shows both how far climate litigation has come and how far it still must go. As glaciers recede and climate risks intensify, the need for legal frameworks that can meaningfully engage with scientific complexity becomes ever more urgent.

Embedding “best available science” at the heart of legal decision-making is not simply a technical adjustment but a necessary step towards ensuring that the law addresses one of the defining challenges of our time.