It’s important to firstly realise that there is no climate expert who is an expert across all of climate, any more than a medical doctor who is expert in all areas of medicine, or a lawyer who is an expert in all areas of law. For instance, a climate scientist might be world-class in the processes that explain rainfall changes and know nothing about sources of greenhouse gases. Similarly, one might have an intimate knowledge of carbon accounting, or how to do regional carbon budgets, but know nothing about risks associated with climate change on accelerating emissions from natural sources of carbon. A meteorologist might never think about carbon dioxide; a climate scientist might never worry about peak wind speeds affecting specific infrastructure.

Next, there is a vast array of sources of information on climate change science that requires different degrees of expertise to read and understand. The definitive source of the science is the assessment reports by the Intergovernmental Panel on Climate Change – the IPCC. There are many reports. Some focus on science, some on climate impacts, some on adaptation and mitigation, and there are some special reports. A given report, for example, the newly released Sixth Assessment Report by the IPCC on physical climate science, is split into three basic sections.

The main report, which only a deep-subject expert can read, is 1,000 pages long, and a world-leading expert might have a good understanding of three of 15 chapters. There is also a Technical Summary that might be 80 pages, and a world-leading expert could read all of that and provide comments. Finally, there is a 25-page “Summary for Policymakers,” which is not as easy to read as one might hope but can be read – with care – by anyone to get a sense of what the 1,000-page report finds.

The IPCC report examines and assesses thousands of science publications and reaches a sense of what is right, wrong, robust, flawed, and so on, and teases out the essence of the evidence. It tries to provide a balance of evidence on any issue. It is the best source of evidence, but it is not perfect. It’s a few years out of date, and you need to be a specialist to understand it. For a legal audience, quoting from the IPCC reports brings with it all of the processes, reviews, rigour and balance imposed by the assessment procedures. It uses precisely calibrated language like “virtually certain” or “likely,” which have quantified precision associated with them.

Since the IPCC reports assess the published literature, they do not assess ideas that are not explored and examined in the scientific literature. So, the IPCC reports touch on “known unknowns” (for example, we know climate projects are not robust and reliable at certain scales). By definition, the IPCC cannot assess “unknown unknowns” because if we knew what they were, they would not be unknown.

A climate expert providing advice to a legal case is therefore unlikely to be across all climate science. If, in giving advice, they fall back on the IPCC in areas they are non-expert, they are likely on safe ground. If they choose to take specific science publications and scaffold advice from that, this is more courageous unless they can assess the robustness of those papers.

So, how should a lawyer, whether in a large or boutique firm, approach climate change matters?

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• Context is king: Evidence taken out of context becomes contentious and can set a dangerous precedent if its interpretation isn’t correct. For instance, take “tipping points,” which are very real risks associated with collapses of ice sheets, major ocean circulation changes, and failures of large-scale ecosystems. These are real, and there are risks of one of these tipping points activating this century. Unfortunately, the consequences of activation can be truly catastrophic and represent an existential risk for some countries. This is entirely different from the now established and increasing risk of heatwaves, extreme rainfall, extreme fire weather, etc. These are not “predictions” or things climate scientists “think” will emerge; these are already observed and are reflected in independent and robust observations. As such, balancing the low risk but massive consequences of tipping points with high-risk and high-consequence climate extremes and communicating this with precision and balance to a court requires both broad and deep expertise.
• Avoid generalisations: Further to the above, one of the easiest errors is to generalise and assume that a climate expert is an expert across all climate areas. To use an analogy: you would not ask a cardiovascular surgeon to do brain surgery. You would do your homework and find the person with niche expertise to suit your needs. It is harder, it’s complex, but exactly the same philosophy applies in finding expert witnesses and advisers in climate science.
• Identify domain-specific experts: It’s prudent to obtain counsel from climate change experts and, as far as possible, domain-specific expertise. For example, use qualified meteorologists in the weather forecasting area and use experienced, widely published climate scientists in climate change space. If models (used for climate predictions into the future) are used as part of a case, find a scientist with a track record of publications in model development or model use. Also, recognise that you may require several experts in complex cases and that expertise in climate change dates rapidly.
• Know it’s complicated: Asking a climate scientist whether carbon dioxide is heating the planet is a little like asking a medical doctor if smoking 20 cigarettes a day is a health concern. A legal office might need advice that starts at very basic levels and builds to something very targeted and advanced, but the more the lawyer understands about the basics, the more efficient this can be.
• The science is non-negotiable: Where the science is settled, and in particular where an expert falls back on foundations like the IPCC, expert opinion should be robust and non-contestable. And yet:
• The science is uncertain: Climate change is uncertain for many reasons but uncertain within well-constrained limits. The climate science community has developed ways to manage this uncertainty with heavily calibrated language, using probabilities, confidence intervals and other technical language. To this point, an awareness of language is essential. For instance, “precautionary principal” or “supporting evidence” mean very different things to a scientist versus a lawyer.