3. Expertise, Reliability and Assessing the Probative Value of Evidence

In this section we look at when expert opinion evidence is needed, how expert opinion evidence is and can be evaluated, and as we move into the final section of this chapter, raise questions about who will be recognised as an expert in climate litigation cases. A fundamental principle of the adversarial trial (whether criminal or civil) is that witnesses offer their evidence by way of a narrative of fact(s) and refrain from offering their opinion or interpretation of those facts. This principle is codified under the UEL in s 76, which prohibits witnesses from offering their opinion if the purpose of adducing that opinion is proving the truth of the facts asserted in that opinion.^[1] However, s 79 offers an exception to this exclusionary rule, allowing courts to hear from a witness whose opinion is considered relevant and where that opinion is based on the witness’s specialised knowledge, which in turn must be based on their ‘training, study or experience’. Australian appellate courts, when interpreting the sections of the UEL dealing with the admissibility of expert opinion evidence, have tended to emphasise or prioritise the autonomy of the fact finder by limiting the role of the trial judge to that of ‘gatekeeper’. In line with the approach in IMM v The Queen,^[2] courts have clarified that the UEL does not contain any expectation or requirement that the evidence led from an expert must be shown to be valid or reliable to be admissible. This is in contrast to the approach in the United States and Canada, which do incorporate an expectation that expert testimony and evidence should be shown to be reliable before it is admitted.^[3] Consequently, for judges in Australia, the key question, once the expert opinion evidence has been shown to be relevant, will be how to weigh or assess the evidence, rather than whether it is admissible per se.

Expert evidence sits somewhat awkwardly within the conventional adversarial fact-resolution process.^[4] The underlying rationale of the exception allowing experts to give evidence in the form of an opinion acknowledges that there will be matters which will sit outside of the capacity of the fact finder (whether this is a jury or a judge) to resolve without the assistance of an expert. So, for example, in Nature Conservation Council, the court explored a similar question in the context of the requirements of the Uniform Civil Procedure Rules 2005, which state that expert opinion evidence needs to be shown to be ‘reasonably necessary’.^[5]

Notwithstanding the underlying justification for the admission of expert opinion evidence, the fact finder remains at liberty to reject such evidence and indeed in some situations will be expressly permitted to substitute their own interpretation of the evidence for that of the expert’s. Even where the evidence is complex or of a technical nature, a specialist court may be more readily inclined to reject the conclusions of an expert witness based on their own views as to the state of the science or accepted facts.^[6] For example, in a Land and Environment Court case David Kettle Consulting Pty Limited v Gosford City Council (‘David Kettle’)^[7] the judge made the following observation when evaluating an expert’s evidence about the likely environmental impact of increasing the rate of groundwater extraction for commercial bottling:

32 In response to a question in relation to ground water recovery rates, Mr Lane [expert witness for the applicant] confirmed that he had assumed a continuation of past rainfall and aquifer recharge patterns.

33 The most recent information published by the Intergovernmental Panel on Climate Change makes it clear that the validity of such an assumption is improbable. Recent observational data show that relative to the worst-case scenario model developed by the IPCC, climate change is occurring more rapidly and at a greater magnitude than anticipated. These recent significant upwards increases in climate change rates coupled with an inherent uncertainty associated with the limited temporal data elucidating the ground water-extraction relationship, direct us to consider the matter with caution.

The approach of the judge in this case, in relying on the IPCC report apparently independent of the evidence presented by the parties, implies that in that court at least, the existence and impacts of climate change have reached the status of information available to the court in a manner akin to a court taking ‘judicial notice’ of climate change and its impacts.^[8] The principle of judicial notice, which allows a court to take into account facts without the need to hear evidence to support those facts, is expressed in s 144 of the UEL .

The approach in David Kettle offers a contrast to the far narrower approach to the question of whether it is acceptable for a judge to rely on external published research as expressed in the case of Aytugrul v The Queen.^[9] In Aytugrul, the High Court was highly critical of an appellate court judge who incorporated reference to well-established psychological research and stressed that this was an unacceptable departure from the traditional role of the judge in an adversarial system. This variation points to some differences between specialist courts (where judges may develop their own subject matter expertise) and courts of general jurisdiction, when it comes to assessing expert evidence. These differences may be more apparent where there is variation in the application of the rules of evidence in different cases heard in the same court; a judge working on a court that can ‘inform itself on any matter in such manner as it thinks appropriate’ in some matters will necessarily carry that experience and knowledge into other cases where the rules of evidence are applied. It may also promote more holistic or multidisciplinary decision-making.^[10]

One of the consequences of the restrictions placed on the capacity of the judge to exclude evidence that is unreliable, or of unknown reliability, is that this can facilitate the admission of weak or speculative evidence. Potentially this could extend to the admission of expert evidence denying the existence of anthropogenic climate change itself. Though this does not seem to have arisen so far in the Australian context, related strategies such as exaggerating levels of ‘uncertainty’ or the social and economic benefits for particular projects, are very much apparent.^[11] So, while it may not be possible to challenge the admissibility of weak, speculative or unreliable evidence (especially noting the variable application of the formal rules of evidence), lawyers working in this area should none the less be attentive to the underlying assumptions utilised by experts, as well as how to understand the strengths and weakness of expert evidence as adduced. Even if there is a limited scope for the exclusion of evidence, there is still an expectation that the underlying facts or assumptions upon which the opinion is based have been established as sound. At the very least, the underlying facts or assumptions should be exposed to the judge making the decision about admissibility.^[12] These principles, even if not explicitly expressed into s 79, create space for the court to consider the basis of the evidence when deciding which expert evidence to accept. This approach can inform decisions made in the ‘shadow of the law’ even in cases where the formal rules of evidence may not apply. An example of this approach can be seen in the David Kettle decision discussed earlier, and even more clearly in the detailed consideration of the expert evidence in Gloucester Resources. For example, in rejecting expert evidence proffered by the appellant (GRL), Preston CJ found that the underlying assumptions relating to ‘market substitution’ and ‘carbon leakage’ upon which the expert had based their opinion were ‘unproven’, and consequently, that they were ‘unreliable and unhelpful’.^[13]

Given the trends in climate litigation in Australia and overseas, generalist as well as specialist courts will increasingly be called upon to assess complex and contested technical evidence. This will especially be so if the types of cases brought in Australia increase in line with trends overseas, including cases seeking compensation for the negative impacts of climate change.  The implications of the removal of a requirement for ‘reliability’ as a condition of admissibility under the UEL have predominantly been explored in the context of criminal trials.^[14] Correspondingly, there is limited jurisprudence in Australia that has addressed this question directly in the context of climate change litigation, but this may shift as more cases come before the courts where the relevance, accuracy and reliability of attribution evidence linking a specific negative impact to a specific GHG emitter is at issue.^[15] As noted, the key questions that are likely to arise will relate not to the existence of anthropogenic climate change itself, but rather to the scope of the impacts in a particular case, or whether the expert evidence is sufficient to link, or attribute, a specific impact to a particular project and its associated emissions. These will raise complex questions for the courts around how to frame the substantive question of causation, as well as how to assess whether evidence can meet the relevant tests.^[16]

Climate litigation is, of course, not the first context in which courts have been called on to address complex questions of causation drawing on (competing) scientific evidence, especially when we look at the history of representative or class actions in what became known as ‘toxic tort’ litigation.^[17] These tort claims relied on novel medical and scientific evidence and on epidemiological, probabilistic reasoning to establish a causal link between, for example, exposure to a carcinogen and the illness(es) experienced by the claimants.^[18] Similarly, the attribution evidence being proffered in cases in other jurisdictions, where claims are being made that specific effects can be attributed to the actions of a specific polluter (or government), means that plaintiffs must rely on complex modelling and attribution science evidence to establish a causal connection between specific conduct (or omission) of the defendant and the plaintiff’s loss or damage. For many of these cases to date, Stuart-Smith et al argue that the evidence presented was not the strongest available, often lagging behind the ‘state of the art’ of climate science.^[19] Further, Stuart-Smith et al argue that this was in many respects a function of the ways that cases were framed and of the existing legal tests rather than a limitation in the evidence available.^[20] In this sense, the ways that expert evidence is being introduced and managed in climate litigation shows strong parallels with the history and experiences of plaintiffs in toxic tort class actions, and is similarly consistent with critical insights that scientific evidence generated for courts will in many respects be shaped or ‘co-produced’ by and through that court process.^[21] This has implications for the way(s) that courts receive and assess expert opinion evidence adduced in climate litigation, as well as related environmental law actions.

While courts rarely appreciate the complex role that law, legal processes and legal systems play in co-producing scientific evidence and knowledge, they are attuned to the problem of ‘partisanship’ in the production and presentation of expert evidence. Concerns about partisan experts underpin many of the more recent procedural reforms in this area, including the development of expert codes of conduct,^[22] the rise of single or court-appointed experts, and the use of concurrent evidence.^[23] Dealing with the reality and the perceptions of expert partisanship will be an ongoing challenge for lawyers working in this area, especially those acting for clients seeking remedies for the negative impacts of climate change or to prevent developments that will increase GHG emissions. Attention also needs to be paid to whether certain parties, or witnesses, are more likely to be found to have engaged in deliberate partisanship or could be accused of advocacy (or activism) over others.^[24] At the same time, while the capacity of the substantive law to recognise novel claims may in the end be both limiting and determinative, the strength and independence of the expert evidence, including that relying on attribution science, being adduced in these cases will also be critical.