The standard test for causation in tort is the 'but for' test, i.e. can the claimant prove that the alleged damage would not have occurred but for the defendant's acts or omissions? The strict application of this test would present a claimant with two major hurdles in the context of climate change litigation:
1) Can the claimant show that damage would not have occurred where but for man-made climate change? If so,
2) Can they show that the relevant degree of climate change would not have occurred but for the actions / omissions of the defendant?
Tort law, however, is concerned with both substantive justice and the fair allocation of cost. Where a rigid application of the 'but for' test is inconsistent with these objectives, courts around the world have adopted more flexible approaches. In circumstances of scientific complexity, for example, or where multiple causes are present, courts have adopted alternative tests, such as whether a defendant's acts or omissions made a material contribution to the harm, or materially increased the risk of the harm occurring. More specifically, in the context of pollution from different sources, they have developed the commingled product theory of causation.
Such approaches translate well to the context of climate change litigation, and there is no reason to regard 'causation' as a major obstacle to its success. Demonstrating that the actions of a particular defendant have, for example, 'materially increased the risk' of climate change damage occurring has made made significantly easier by:
i) developments in the science allowing for the probabilistic attribution of specific climactic patterns and weather events to climate change;
ii) the work of Rick Heede, showing that the majority of greenhouse gases can be attributed to just 90 'carbon majors'; and
iii) the work of investigative journalists which appears to show that certain companies deliberately set out to undermine the scientific consensus regarding climate change, with the specific purpose of obstructing the development of appropriate policy responses.
Adapting the 'but for' test in cases of complex causation
This section will review jurisprudence which shows the 'but for' test may be adapted:
a) in cases where there are different, cumulative causes for a single harm; or
b) where it would be unfair or unreasonable to expect a claimant to establish a precise causal link.
Material contribution to harm
In Bonnington Castings Ltd v Wardlaw  AC 613,  UKHL 1 the UK's House of Lords held that it was sufficient to show that a defendant's breach of duty had made a material contribution to the claimant's injury even where other causes had made a more substantial contribution. 'Material', in this context, means a cause exceeding the de minimis threshold.
The case provides, in small scale, an interesting analogue for climate change litigation. The claimant had developed pneumoconiosis as a result of inhaling air containing minute particles of silica. The court found as fact that the defendant's breach of duty resulted in only a small proportion of the inhalation, which could not be precisely quantified. In the words of Lord Reid:
The medical evidence was that pneumoconiosis is caused by a gradual accumulation in the lungs of minute particles of silica inhaled over a period of years. That means, I think, that the disease is caused by the whole of the noxious material inhaled and, if that material comes from two sources, it cannot be wholly attributed to material from one source or the other ... I cannot agree that the question is which was the most probable source of the Respondent's disease, the dust from the pneumatic hammers or the dust from the swing grinders. It appears to me that the source of his disease was the dust from both sources, and the real question is whether the dust from the swing grinders materially contributed to the disease. What is a material
contribution must be a question of degree. A contribution which comes within the exception de minimis non curat lex is not material, but I think that any contribution which does not fall within that exception must be material. I do not see how there can be something too large to come within the de minimis principle but yet too small to be material.
A similar approach may be discerned in Massachusetts v EPA, when the US Supreme Court rejected the US Environmental Protection Agency's argument that its non-regulation of greenhouses gases contributed insignificantly to the claimant's injuries:
Given EPA’s failure to dispute the existence of a causal con- nection between man-made greenhouse gas emissions and global warming, its refusal to regulate such emissions, at a minimum, “contributes” to Massachusetts’ injuries. EPA overstates its case in arguing that its decision not to regulate contributes so insignificantly to petitioners’ injuries that it cannot be haled into federal court, and that there is no realistic possibility that the relief sought would mitigate global climate change and remedy petitioners’ injuries, especially since predicted increases in emissions from China, India, and other developing nations will likely offset any marginal domestic decrease EPA regulation could bring about. Agencies, like legislatures, do not generally resolve massive problems in one fell swoop ...
In the 2015 Urgenda decision, a Dutch court held (paras. 4.79 and 4.90) that despite the Netherlands global emissions totalling only 0.42% of the global total (for 2010), that was nevertheless a relevant a contribution to climate change:
Therefore, the court arrives at the opinion that the single circumstance that the Dutch emissions only constitute a minor contribution to global emissions does not alter the State’s obligation to exercise care towards third parties ...
The fact that the current Dutch greenhouse gas emissions are limited on a global scale does not alter the fact that these emission contribute to climate change.
It is submitted that this approach is more generally applicable to the context of climate change litigation.
Material increase to risk of injury
In certain circumstances courts have made a finding of liability even where the claimant can not prove that the defendant's acts or omissions materially contributed to the injury, as long as he or she can show that they materially increased the risk of injury.
In McGhee v National Coal Board  1 WLR 1 the UK's House of Lords considered the question of causation in circumstances where the claimant, who had developed dermatitis, could show that the defendant had:
The Court held that causation was nevertheless sufficiently established (one of the judges, Lord Wilberforce, going so far as to propose a reversal of the burden of proof):
But the question remains whether a pursuer must necessarily fail if, after he has shown a breach of duty, involving an increase of risk of disease, he cannot positively prove that this increase of risk caused or materially contributed to the disease while his employers cannot positively prove the contrary. In this intermediate case there is an appearance of logic in the view that the pursuer, on whom the onus lies, should fail - a logic which dictated the judgments below. The question is whether we should be satisfied, in factual situations like the present, with this logical approach. In my opinion, there are further considerations of importance. First, it is a sound principle that where a person has, by breach of a duty of care, created a risk, and injury occurs within the area of that risk, the loss should be borne by him unless he shows that it had some other cause. Secondly, from the evidential point of view, one may ask, why should a man who is able to show that his employer should have taken certain precautions, because without them there is a risk, or an added risk, of injury or disease, and who in fact sustains exactly that injury or disease, have to assume the burden of proving more: namely, that it was the addition to the risk, caused by the breach of duty, which caused or materially contributed to the injury? In many cases, of which the present is typical, this is impossible to prove, just because honest medical opinion cannot segregate the causes of an illness between compound causes. And if one asks which of the parties, the workman or the employers, should suffer from this inherent evidential difficulty, the answer as a matter of policy or justice should be that it is the creator of the risk who, ex hypothesi must be taken to have foreseen the possibility of damage, who should bear its consequences. (per Lord Wilbeforce, page 6).
Lord Simon considered that previous case-law had already established a rule (at page 8):
... that where an injury is caused by two (or more) factors operating cumulatively, one (or more) of which factors is a breach of duty and one (or more) is not so, in such a way that it is impossible to ascertain the proportion in which the factors were effective in producing the injury or which factor was decisive, the law does not require a pursuer or plaintiff to prove the impossible, but holds that he is entitled to damages for the injury if he proves on a balance of probabilities that the breach or breaches of duty contributed substantially to causing the injury. If such factors so operate cumulatively, it is, in my judgment, immaterial whether they do so concurrently or successively.
Indeed Lord Simon regarded "material reduction of the risk" and "substantial contribution to the injury" as mirror concepts. Any other conclusion would mean that the defenders were under a legal duty which they could, on the present state of medical knowledge, ignore (page 9).
Lord Kilbrandon expressed a similar conclusion as follows:
When you find it proved (a) that the defenders knew that to take the precaution reduces the risk, chance, possibility or probability of the contracting of a disease, (b) that the precaution has not been taken, and (c) that the disease has supervened, it is difficult to see how those defenders can demand more by way of proof of the probability that the failure caused or contributed to the physical breakdown ... In the present case, the pursuer's body was vulnerable, while he was bicycling home, to the dirt which had been deposited on it during his working hours. It would not have been if he had had a shower. If showers had been provided he would have used them. It is admittedly more probable that disease will be contracted if a shower is not taken. In these circumstances I cannot accept the argument that nevertheless it is not more probable than not that, if the duty to provide a shower had not been neglected, he would not have contracted the disease. The pursuer has after all, only to satisfy the court of a probability, not to demonstrate an irrefragable chain of causation, which in a case of dermatitis, in the present state of medical knowledge, he could probably never do.
Lord Salmon stated:
In the circumstances of the present case, the possibility of a distinction existing between (a) having materially increased the risk of contracting the disease, and (b) having materially contributed to causing the disease may no doubt be a fruitful source of interesting academic discussions between students of philosophy. Such a distinction is, however, far too unreal to be recognised by the common law.
Market share liability doctrine
In the US, Sindell v Abbott Laboratories 26 Cal. 3d 588 (1980) was a class action for personal injuries said to have resulted from pre-natal exposure to the anti-miscarriage drug diethylstilbestrol (DES) which had been manufactured by one of a potentially large number of defendants. The plaintiff could not identify which particular defendant had manufactured the drug responsible for her injuries. However, her complaint alleged that the defendants were jointly and individually negligent in that they had manufactured, marketed and promoted DES as a safe drug to prevent miscarriage without adequate testing or warning of its dangerous side effects; that they had collaborated in their marketing methods, promotion and testing of the drug; that they had relied on each others' test results; that they had adhered to an industry-wide safety standard; and that they had produced the drug from a common and mutually agreed generic formula. The court distinguished Summers on the basis that in that case all the parties who were or could have been responsible for the harm to the plaintiff were joined as defendants, whereas in Sindell there were approximately 200 drug companies which had made DES, any of which might have manufactured the injury-producing drug. The court held that it would be unfair, in such circumstances, to require each defendant to exonerate itself. Further, it said that there might be a substantial likelihood that none of the five defendants joined in the action had made the DES which caused the injury, and that the offending producer, not named, would escape liability. The court surmounted this problem by adapting the Summers rule so as to apportion liability on the basis of the defendant's market share. See pp 593-595, 602-603, 604-605 and 612-613. A very similar case concerning the same drug arose in the Netherlands in B v Bayer Nederland BV (Hoge Raad, 9 October 1992, NJ 1994, 535) which turned on Article 6.99 BW. That provision is in these terms:
Where the damage may have resulted from two or more events for each of which a different person is liable, and where it has been determined that the damage has arisen from at least one of these events, the obligation to repair the damage rests upon each of these persons, unless he proves that the damage is not the result of the event for which he himself is liable ...
[Article 6.99BW] aims to remove the unfairness arising from the fact that the victim must bear his or her own damage because he or she cannot prove whose action caused his or her harm. The victims in the present case are faced with such an evidentiary difficulty . . .
In para 3.7.5 of its judgment the court said:
"It is sufficient for each DES daughter to establish . . . in relation to each of the pharmaceutical companies:
(i) that the pharmaceutical company in question put DES in circulation during the relevant period and can therefore be found liable because it committed a fault;
(ii) that another or several other producers - regardless of whether they are parties to the proceedings or not - also put DES in circulation during the relevant period and can therefore also be found liable because it (they) committed a fault; and
(iii) that she suffered injury that resulted from the use of DES, but that it is no longer possible to determine from which producer the DES originated."
In principle the burden of proof on these issues rests on the DES daughter concerned.
International jurisprudence on causation
In Snell v Farrell  2 SCR 311 at 320, 326-7, Sopinka J, delivering the judgment of the Supreme Court of Canada, said:
The traditional approach to causation has come under attack in a number of cases in which there is concern that due to the complexities of proof, the probable victim of tortious conduct will be deprived of relief. This concern is strongest in circumstances in which, on the basis of some percentage of statistical probability, the plaintiff is the likely victim of the combined tortious conduct of a number of defendants, but cannot prove causation against a specific defendant or defendants on the basis of particularized evidence in accordance with traditional principles. The challenge to the traditional approach has manifested itself in cases dealing with non-traumatic injuries such as man-made diseases resulting from the widespread diffusion of chemical products, including product liability cases in which a product which can cause injury is widely manufactured and marketed by a large number of corporations ...
Reversing the burden of proof may be justified where two defendants negligently fire in the direction of the plaintiff and then by their tortious conduct destroy the means of proof at his disposal. In such a case it is clear that the injury was not caused by neutral conduct. It is quite a different matter to compensate a plaintiff by reversing the burden of proof for an injury that may very well be due to factors unconnected to the defendant and not the fault of anyone.
In March v E & MH Stramare Pty Ltd (1991) 171 CLR 506 at 508, Mason CJ, sitting in the High Court of Australia, did not "accept that the 'but for' (causa sine qua non) test ever was or now should become the exclusive test of causation in negligence cases" and added (at p 516):
The 'but for' test gives rise to a well-known difficulty in cases where there are two or more acts or events which would each be sufficient to bring about the plaintiff's injury. The application of the test 'gives the result, contrary to common sense, that neither is a cause': Winfield and Jolowicz on Tort, 13th ed (1989), p. 134. In truth, the application of the test proves to be either inadequate or troublesome in various situations in which there are multiple acts or events leading to the plaintiff's injury: see, e.g., Chapman v Hearse, Baker v Willoughby  AC 467; McGhee v National Coal Board; M'Kew (to which I shall shortly refer in some detail). The cases demonstrate the lesson of experience, namely, that the test, applied as an exclusive criterion of causation, yields unacceptable results and that the results which it yields must be tempered by the making of value judgments and the infusion of policy considerations.
In Rutherford v Owens-Illinois Inc 67 Cal. Rptr. 2d 16 (1997), in a judgment with which the Chief Justice and all save one member of the Supreme Court of California concurred, Baxter J observed (at p 19):
Proof of causation in such cases will always present inherent practical difficulties, given the long latency period of asbestos-related disease, and the occupational settings that commonly exposed the worker to multiple forms and brands of asbestos products with varying degrees of toxicity. In general, however, no insuperable barriers prevent an asbestos-related cancer plaintiff from demonstrating that exposure to the defendant's asbestos products was, in reasonable medical probability, a substantial factor in causing or contributing to his risk of developing cancer. We conclude that plaintiffs are required to prove no more than this. In particular, they need not prove with medical exactitude that fibers from a particular defendant's asbestos-containing products were those, or among those, that actually began the cellular process of malignancy.
McLachlin J, extra-judicially ("Negligence Law - Proving the Connection", in Torts Tomorrow, A Tribute to John Fleming, ed Mullany and Linden, LBC Information Services 1998, at p 16), has voiced a similar concern:
Tort law is about compensating those who are wrongfully injured. But even more fundamentally, it is about recognising and righting wrongful conduct by one person or a group of persons that harms others. If tort law becomes incapable of recognising important wrongs, and hence incapable of righting them, victims will be left with a sense of grievance and the public will be left with a feeling that justice is not what it should be. Some perceive that this may be occurring due to our rules of causation.
In recent years, a conflation of factors have caused lawyers, scholars and courts to question anew whether the way tort law has traditionally defined the necessary relationship between tortious acts and injuries is the right way to define it, or at least the only way. This questioning has happened in the United States and in England and has surfaced in Australia. And it is happening in Canada. Why is this happening? Why are courts now asking questions that for decades, indeed centuries, did not pose themselves, or if they did, were of no great urgency? I would suggest that it is because too often the traditional 'but-for', all-or-nothing, test denies recovery where our instinctive sense of justice - of what is the right result for the situation - tells us the victim should obtain some compensation.
Professor Christian von Bar (The Common European Law of Torts, 2000, vol 2, pp 441-443) has written:
German law on medical negligence provides the example of the reduced burden of proof of causation in cases of grave treatment errors. Recent environmental legislation has reacted to the problem of scientifically uncertain causal relationships in a similar manner. The reversal of the burden of proof regarding causation is no more than a reduction of the probability required for attribution.
Professor Walter van Gerven (van Gerven, Lever andLarouche: Cases, Materials and Text on National, Supranational and International Tort Law, 2000, p 441), surveying the tort law of, in particular, Germany, France and Britain, wrote:
In many cases, it will be possible for the victim to show that he or she has suffered injury, that it has been caused by someone who must have been at fault, but the author of that fault will not be identifiable. The best that the victim will be able to achieve is to define a class of persons of which the actual tortfeasor must be a member. Strictly speaking, however, the basic conditio sine qua non test will not be met, since it cannot be said of any member of the class that the injury would not have happened 'but for' his or her conduct, given that in fact any other member could have caused the injury. Nonetheless, all the legal systems studied here have acknowledged that it would be patently unfair to deny recovery to the victim for that reason.
In Germany cases of this kind have been held to be covered by the second sentence (to which emphasis has been added) of BGB §830.1 which provides:
If several persons have caused damage by an unlawful act committed in common each is responsible for the damage. The same rule applies if it cannot be discovered which of several participants has caused the damage by his act.
Of this provision Markesinis and Unberath (The German Law of Torts, 4th ed, 2002, p 900) states:
§830.1, second sentence, applies the same rule to a different situation where several persons participate in a course of conduct which, though not unlawful in itself, is potentially dangerous to others. The difference between this and the previous situation lies in the fact that whereas in the former case of joint tortfeasors the loss is caused by several persons acting in consort, in the latter case only one person has caused the loss but it is difficult if not impossible to say which one has done so. (The classic illustration is that of the huntsmen who discharge their guns simultaneously and the pellets from one unidentifiable gun hit an innocent passer-by.) In this case, as well, §830 BGB adopts the same rule and makes all the participants liable to the victim for the full extent of damage.
Article 926 of the Greek Civil Code, entitled "Damage caused by several persons" provides:
If damage has occurred as a result of the joint action of several persons, or if several persons are concurrently responsible for the same damage, they are all jointly and severally implicated. The same applies if several persons have acted simultaneously or in succession and it is not possible to determine which person's act caused the damage.
A similar provision is to be found in the Austrian Civil Code:
1302 In such a case, if the injury is inadvertent, and it is possible to determine the portions thereof, each person is responsible only for the injuries caused by his mistake. If, however, the injury was intentional, or if the portions of the individuals in the injury cannot be determined, all are liable for one and one for all; however, the individual who has paid damages is granted the right to claim reimbursement from the others.
In a Norwegian case (see Nils Nygaard, Injury/Damage and Responsibility, 2000, pp 342-343) F was a passenger on a motor scooter and was injured in an accident caused either by a cable stretched across the street by a construction company or by the motor scooter falling onto him or as a result of collision with a truck, or by any combination of these factors. In giving judgment (RG 1969 p 285 at 293) the Norwegian court said:
As stated in the beforementioned conclusions made by experts, they could not conclude whether the situation that resulted in crushed bones in F's left hip region, was a result of falling on the cobble stones in the street or from the truck's front tyre, that ended up on top of F's left hip region. It is possible that the injuries were partially a result of the fall and being hit by the truck. We cannot say anything definite about this. The court finds that it cannot conclude whether it is the fall or being hit by the truck or a combination of both these factors that caused the injury. After a collective evaluation of the whole event the court finds that A (construction company), the scooter and the truck each have a part in F getting injured and each of them must naturally be seen as adequate cause of injury.
UK asbestos / mesothelioma cases
Of particular interest is a line of cases from the UK relating to asbestos exposure and the development of the cancer mesothelioma. The cases contain a number of features likely to arise in the context of climate change litigation such as:
However it is important to note that these cases concern a difficulty not arising in the context of climate change litigation. Mesothelioma, much like malaria, is understood to be caused by a single exposure rather than by cumulative effect. Consequently where a number of defendants have exposed the claimant to asbestos it is likely that only one was the 'true cause' of the injury. Climate change, by contrast, is caused by the accumulation of greenhouse gases in the atmosphere, and therefore all emitters are technically contributors to the causal chain. It is the other elements of the tort of negligence (duty of care and breach) together with the requirement that the causal contribution is greater than de minimis that limit the scope of claims for liability.
The mesothelioma cases are significant more generally, however, in demonstrating the willingness of the courts to adapt the test of causation where it is in the interests of justice to do so, and where uncertainties in the science obscure identification of the precise causal chain. They also provide guidance on possible approaches to apportionment of damages.
In Fairchild v Glenhaven Funeral Services  the UK House of Lords, addressed the issue of causation in the context of asbestos mesothelioma. The court expressed the problem raised by the case as follows:
(1) C was employed at different times and for differing periods by both A and B, and
(2) A and B were both subject to a duty to take reasonable care or to take all practicable measures to prevent C inhaling asbestos dust because of the known risk that asbestos dust (if inhaled) might cause a mesothelioma, and
(3) both A and B were in breach of that duty in relation to C during the periods of C's employment by each of them with the result that during both periods C inhaled excessive quantities of asbestos dust, and
(4) C is found to be suffering from a mesothelioma, and
(5) any cause of C's mesothelioma other than the inhalation of asbestos dust at work can be effectively discounted, but
(6) C cannot (because of the current limits of human science) prove, on the balance of probabilities, that his mesothelioma was the result of his inhaling asbestos dust during his employment by A or during his employment by B or during his employment by A and B taken together,
is C entitled to recover damages against either A or B or against both A and B?
Lord Bingham concluded that he was, stating:
I do not therefore consider that the House is acting contrary to principle in reviewing the applicability of the conventional test of causation to cases such as the present. Indeed, it would seem to me contrary to principle to insist on application of a rule which appeared, if it did, to yield unfair results.
He emphasised the broad convergence of international jurisprudence on the issue:
Whether by treating an increase in risk as equivalent to a material contribution, or by putting a burden on the defendant, or by enlarging the ordinary approach to acting in concert, or on more general grounds influenced by policy considerations, most jurisdictions would, it seems, afford a remedy to the plaintiff. Development of the law in this country cannot of course depend on a head-count of decisions and codes adopted in other countries around the world, often against a background of different rules and traditions. The law must be developed coherently, in accordance with principle, so as to serve, even-handedly, the ends of justice. If, however, a decision is given in this country which offends one's basic sense of justice, and if consideration of international sources suggests that a different and more acceptable decision would be given in most other jurisdictions, whatever their legal tradition, this must prompt anxious review of the decision in question. In a shrinking world (in which the employees of asbestos companies may work for those companies in any one or more of several countries) there must be some virtue in uniformity of outcome whatever the diversity of approach in reaching that outcome.
Accepting that this broad approach might involve a defendant assuming legal responsibility for an injury he or she had not in fact caused, where the defendant's conduct had been otherwise negligent, he considered such a result less unjust than leaving the victim without compensation:
... there is a strong policy argument in favour of compensating those who have suffered grave harm, at the expense of their employers who owed them a duty to protect them against that very harm and failed to do so, when the harm can only have been caused by breach of that duty and when science does not permit the victim accurately to attribute, as between several employers, the precise responsibility for the harm he has suffered. I am of opinion that such injustice as may be involved in imposing liability on a duty-breaking employer in these circumstances is heavily outweighed by the injustice of denying redress to a victim.
Further he considered it inevitable that the principle would be developed in future cases by analogy:
I would in conclusion emphasise that my opinion is directed to cases in which each of the conditions specified in (1) - (6) of paragraph 2 above is satisfied and to no other case. It would be unrealistic to suppose that the principle here affirmed will not over time be the subject of incremental and analogical development. Cases seeking to develop the principle must be decided when and as they arise.
Lord Nicholls articulated his support for the conclusion in forceful terms (at para. 36):
I have no hesitation in agreeing with all your Lordships that these appeals should be allowed. Any other outcome would be deeply offensive to instinctive notions of what justice requires and fairness demands.
He expressed the threshold test in broad terms (i.e. as long as the breach was 'not insignificant'), and set out the logic for it:
So long as it was not insignificant, each employer's wrongful exposure of its employee to asbestos dust and, hence, to the risk of contracting mesothelioma, should be regarded by the law as a sufficient degree of causal connection. This is sufficient to justify requiring the employer to assume responsibility for causing or materially contributing to the onset of the mesothelioma when, in the present state of medical knowledge, no more exact causal connection is ever capable of being established. Given the present state of medical science, this outcome may cast responsibility on a defendant whose exposure of a claimant to the risk of contracting the disease had in fact no causative effect. But the unattractiveness of casting the net of responsibility as widely as this is far outweighed by the unattractiveness of the alternative outcome.
He suggested that policy considerations would be key to further application of the test:
Policy questions will loom large when a court has to decide whether the difficulties of proof confronting the plaintiff justify taking this exceptional course. It is impossible to be more specific.
Lord Hoffman elaborated on the concepts underpinning rules of legal causation (at para. 56):
The concepts of fairness, justice and reason underlie the rules which state the causal requirements of liability for a particular form of conduct (or non-causal limits on that liability) just as much as they underlie the rules which determine that conduct to be tortious. And the two are inextricably linked together: the purpose of the causal requirement rules is to produce a just result by delimiting the scope of liability in a way which relates to the reasons why liability for the conduct in question exists in the first place.
In Barker v Corus (UK) plc  UKHL 20 the UK's House of Lords addressed two questions undecided by the decision in Fairchild:
Lord Hoffman concluded that it did not matter whether all the sources of risk were tortious, or whether they were man-made:
The purpose of the Fairchild exception is to provide a cause of action against a defendant who has materially increased the risk that the claimant will suffer damage and may have caused that damage, but cannot be proved to have done so because it is impossible to show, on a balance of probability, that some other exposure to the same risk may not have caused it instead. For this purpose, it should be irrelevant whether the other exposure was tortious or non-tortious, by natural causes or human agency or by the claimant himself. These distinctions may be relevant to whether and to whom responsibility can also be attributed, but from the point of view of satisfying the requirement of a sufficient causal link between the defendant's conduct and the claimant's injury, they should not matter.
It was, however necessary to prove that the type of risk to which the defendant contributed was in fact a cause of the injury:
In my opinion it is an essential condition for the operation of the exception that the impossibility of proving that the defendant caused the damage arises out of the existence of another potential causative agent which operated in the same way. It may have been different in some causally irrelevant respect, as in Lord Rodger's example of the different kinds of dust, but the mechanism by which it caused the damage, whatever it was, must have been the same. So, for example, I do not think that the exception applies when the claimant suffers lung cancer which may have been caused by exposure to asbestos or some other carcinogenic matter but may also have been caused by smoking and it cannot be proved which is more likely to have been the causative agent.
Adopting such an approach to the context of climate change implies a two stage test for 'causation':
The Court in Barker, however, preferred to avoid the 'legal fiction' that the test was one of causation. Rather there were circumstances in which risk creation could displace the requirement to prove causation (Lord Scott at para. 53):
It is essential, in my opinion, to an appreciation of the effect of the Fairchild decision to keep firmly in mind that liability was not imposed on any of the defendant employers on the ground that the employer's breach of duty had caused the mesothelioma that its former employee had contracted. That causative link had not been proved against any of them. It was imposed because each, by its breach of duty, had materially increased the risk that the employee would contract mesothelioma. That, coupled with the fact that mesothelioma had been contracted and that it was not possible to tell when the fatal inhalation had taken place, justified, in their Lordships' view, the imposition of liability on each employer who had contributed to the risk.
While Barker ruled in favour of several liability where the Fairchild principle was being applied, the approach was subsequently reversed by the UK Compensation Act 2006 which makes liability in such circumstances 'joint and several'.
In Sienkiewicz v Grief (2011), the UK Supreme Court (the successor to the House of Lords), again considered the test of causation in the context of asbestos exposure. This time the increased exposure created by the defendant's breach of duty was particularly small: the judge at first instance found that the exposure to asbestos over the deceased's working life at Greif’s factory increased the risk to which environmental exposure subjected her from 24 cases per million to 28.39 cases per million – an increase of risk of 18%.
Lord Phillips described the question for the court at para. 11:
Knowledge about mesothelioma is based in part on medical science and in part on statistical analysis or epidemiology. These appeals raise the question of whether, and if so to what extent, the court can satisfactorily base conclusions about causation on the latter, both in mesothelioma cases and more generally.
It was submitted by the defendant that the 'double the risk' test for proving causation should be applied. The theory that causation could be proved on the balance of probabilities by reference to a doubling of the risk of injury was first applied by Mackay J in the oral contraceptive litigation XYZ v Schering Health Care Ltd (2002) 70 BMLR 88. As a preliminary issue, the parties agreed that the judge should examine the epidemiological evidence relating to the risk of deep vein thrombosis arising from two different types of oral contraceptive. The claimant group could succeed only if the epidemiology showed that the risk of harm arising from the type of contraceptive they had been taking (which it was assumed they had not been warned about and would not have taken if warned) was at least twice that arising from the type which they had formerly been taking (which it was assumed they had been warned about and which risk they had accepted). The logic behind this was that, if the risk from potential cause A is x% and the risk from the other potential cause B is 2.1x%, it is more likely than not that the condition which has eventuated has been caused by B.
Lord Phillips explained the circumstance in which he considered the 'doubles the risk' test might be appropriate:
Where there are competing alternative, rather than cumulative, potential causes of a disease or injury, such as in Hotson, I can see no reason in principle why epidemiological evidence should not be used to show that one of the causes was more than twice as likely as all the others put together to have caused the disease or injury.
It does not seem to me that there is any justification for adopting the “doubles the risk” test as the bench mark of what constitutes a material increase of risk. Indeed, if one were to accept Mr Stuart-Smith’s argument that the “doubles the risk” test establishes causation, his de minimis argument would amount to saying that no exposure is material for the purpose of the Fairchild/Barker test unless on balance of probability it was causative of the mesothelioma. This cannot be right.
I doubt whether it is ever possible to define, in quantitative terms, what for the purposes of the application of any principle of law, is de minimis. This must be a question for the judge on the facts of the particular case. In the case of mesothelioma, a stage must be reached at which, even allowing for the possibility that exposure to asbestos can have a cumulative effect, a particular exposure is too insignificant to be taken into account, having regard to the overall exposure that has taken place. The question is whether that is the position in this case.
I do not think that Judge Main would have dismissed the addition that Greif’s wrongful exposure made to the risk that Mrs Costello would contract mesothelioma as statistically insignificant or de minimis.
Commingled product theory
In 2005, the Federal Court for the Southern District of New York, considered the issue of causation in the context of an allegation that a gasoline additive, MTBE, had contaminated water supplies (In re Methyl Tertiary Butyl Ether (MTBE) Prods. Liab. Litig., 379 F. Supp. 2d 348, 377 (S.D.N.Y. 2005)). It noted that some of the factors for applying the market share liability doctrine were absent, but stated:
[F]rom time to time courts have fashioned new approaches in order to permit plaintiffs to pursue a recovery when the facts and circumstances of their actions raised unforeseen barriers to relief.
It developed and applied a new doctrine, 'commingled product theory.' As summarised in 2013 proceedings:
Under this theory, when a plaintiff can prove that certain gaseous or liquid products (e.g., gasoline, liquid propane, alcohol) of many refiners and manufacturers were present in a completely commingled or blended state at the time and place that the harm or risk of harm occurred, and the commingled product caused plaintiff’s injury, each refiner or manufacturer is deemed to have caused the harm. Each defendant is then given the opportunity to exculpate itself by proving that its product was not present at the relevant time or in the relevant place, and therefore could not be part of the commingled or blended product.
Before the rebuttable presumption undergirding this theory is activated, a plaintiff must prove by a preponderance of the evidence that the defendant contributed-in-fact to the injury by showing that each defendant’s product was part of the commingled mass that injured the plaintiff. The theory thus requires the plaintiff to prove that each defendant’s gasoline was part of the commingled product, but relieves the plaintiff of the duty to prove that each individual defendant’s contribution to that product, in and of itself, was sufficient to have caused an injury. “Rather, to establish liability against a particular defendant with respect to an individual well, [the plaintiff] must show that (a) the defendant’s MTBE was present in a commingled product and (b) that the commingled product [rather than defendant’s product alone] caused plaintiff’s injury.”
The combination of 'commingled product theory' with Rick Heede's research into the contribution of 'carbon majors' to total greenhouse base emissions, appears to provide a sound basis for attributing climate change loss and damage to fossil fuel corporations and others.
Climate change litigation
Courts have considered the question of causation in two recent climate change cases. In the 2015 Urgenda case the Government of the Netherlands argued that:
it cannot be seen as one of the causers of an imminent climate change, as it does not emit greenhouse gases [see 4.66].
The court held, however, that:
... it is an established fact that the State has the power to control the collective Dutch emission level (and that it indeed controls it). Since the State’s acts or omissions are connected to the Dutch emissions a high level of meticulousness should be required of it in view of the security interests of third parties (citizens), including Urgenda ...
From the above considerations ... it follows that a sufficient causal link can be assumed to exist between the Dutch greenhouse gas emissions, global climate change and the effects (now and in the future) on the Dutch living climate. The fact that the current Dutch greenhouse gas emissions are limited on a global scale does not alter the fact that these emission contribute to climate change. The court has taken into consideration in this respect as well that the Dutch greenhouse emissions have contributed to climate change and by their nature will also continue to contribute to climate change.
In April 2016,in United States District Court for the District of Oregon, Case 6:15-cv-01517-TC, Coffin Magistrate Judge, again considering the causal relationship between governmental acts and omissions and climate change loss and damage concluded as follows:
As noted above, there must be a causal connection between the injury and the conduct of which plaintiffs complained. In other words, the injury has to be fairly traceable to the challenged action ofthe defendant, and not the result ofthe independent action ofsome third party not before the court. Simon v. Eastern Ky. Welfare Rights Organization, 426 U.S. 26, 41-42 (1976).
The government asserts that the association between the complained of conduct (such as subsidizing the fossil fuel industry, favorable revenue code provisions, allowing transport of fossil fuels, and authorizing fossil fuel combustion in the energy/refinery/transportation/manufacturing sectors) and the associated greenhouse gas emissions that ultimately cause the harm is tenuous and filled with countless intervening actions by unidentified third parties. However, as alleged, without the complained of conduct, the third parties would not be able to engage as extensively in the activities that allegedly cause climate change and the resulting harm ...
In cases where a chain of causation involves numerous third parties whose independent decisions collectively have a significant effect on plaintiffs' injuries, the causal chain may be too weak to support standing at the pleading stage. See Allen, 468 U.S. at 759.
But here, there is an alleged strong link between all the supposedly independent and numerous third party decisions given the government's regulation of C02 emissions. See, e.g., 42 U.S.C. § 7409 (providing the EPA the authority to regulate national ambient air quality standards for the attainment and maintenance of the public health); Massachusetts v. EPA, 549 U.S. 497 (2007) (EPA has power to regulate greenhouse gas emissions). If the allegations in the complaint are to be believed, the failure to regulate the emissions has resulted in a danger of constitutional proportions to the public health. Presumably, sweeping regulations by this agency (the EPA) alone could result in curtailing of major C02 producing activities by not just the defendant agencies, but by the purported independent third parties as well. At this pleading stage, the court need not sort out the necessity or propriety of all the various agencies and individuals to participate as defendants, at least with respect to issues of standing.
For now, it is sufficient that EPA's action/inaction with respect to the regulation of greenhouse gases allegedly results in the numerous instances of emissions that purportedly cause or will cause the plaintiffs harm.
The 'commingled product' theory developed in the US is essentially a variant of the 'material contribution' test.
Applying either approach to the context of climate change loss and damage implies the imposition of a two-stage test:
i) Can the claimant show on the balance of probabilities that loss and damage is attributable / part attributable to the anthropogenic increase in atmospheric concentrations of greenhouse gases? If so,
ii) Has the defendant materially contributed to that increase?
If both questions can be answered in the affirmative a court may well conclude that a defendant has caused the loss and damage in question.
More generally the case-law in the UK regarding asbestos and mesothelioma (although not directly analogous to climate change litigation) illustrates the readiness of courts to ensure uncertainties regarding the chain of causation do not operate unfairly against those who have suffered loss and damage.
Case-law from both the Netherlands and the US offers clear precedent for establishing a causal link between a single defendant and global climate change. Both cases concerned governmental acts or omissions. In the Urgenda case, for example, the Dutch government's annual contribution to aggregate emissions of 0.42% was not considered to be de minimis and the court rejected the government's submissions that its actions were not a cause of climate change loss and damage.
24 April 2017: Record-breaking climate events all over the world are being shaped by global warming, scientists find(Washington Post)
15 December 2016: Scientists are tying more and more extreme events to a changing climate (Washington Post)
15 September 2016: Hesitance to Link Some Weather Events to Climate Change 'No Longer Appropriate'
July 2016, Mitchell et al:Attributing human mortality during extreme heat waves to anthropogenic climate change
Attribution of Extreme Weather Events in the Context of Climate Change (2016), National Academies of Sciences, Engineering, and Medicine, Washington DC.
weather@home—development and validation of a very large ensemble modelling system for probabilistic event attribution(N. Massey, R. Jones, F. E. L. Otto, T. Aina, S. Wilson, J. M. Murphy, D. Hassell,
Y. H. Yamazaki, and
M. R. Allen), Quarterly Journal of the Royal Meteorological Society, Volume 141, Issue 690, pages 1528–1545, July 2015 Part A
June 23, 2015,'Most Extreme Weather Has Climate Change Link, Study Says'
“PRESIDENT TRUMP MAY HAVE HIS ALTERNATIVE FACTS,
BUT ALTERNATIVE FACTS DO NOT WORK IN A COURTROOM”
CHRISTIAN HARTMANN / REUTERS
A man brings baguettes to his mother’s flooded house in Chalette-sur-Loing Montargis, France, on June 1, 2016.
PHILIPPE WOJAZER / REUTERS
View of the flooded Seine near the Eiffel Tower in Paris on June 3, 2016, after days of almost nonstop rain.
LEGAL ACTION FOR THE CLIMATE GOAL: TOWARDS EQUITY, SURVIVAL AND THE RULE OF LAW
João Pereira de Araújo
Bonnington Castings Ltd v Wardlaw  AC 613,  UKHL 1:the UK's House of Lords held that it was sufficient to show that a defendant's breach of duty had made a material contribution to the claimant's injury even where other causes had made a more substantial contribution. 'Material', in this context, means a cause exceeding the de minimis threshold.
Fairchild (suing on her own behalf) etc. v. Glenhaven Funeral Services Ltd and others etc  UKHL 22,  1 AC 32,  3 WLR 89,  3 All ER 305:where the precise causal chain can not be established, the legal test of causation may still be satisfied where a defendant has exposed a plaintiff to a risk of particular type of harm, and the plaintiff has subsequently suffered from that harm.
re Methyl Tertiary Butyl Ether (MTBE) Prods. Liab. Litig., 379 F. Supp. 2d 348, 377 (S.D.N.Y. 2005):the 'commingled product' theory of causation
Barker v Corus (UK) plc  UKHL 20: establishes the concept of 'proportionate liability'
Here, by contrast, plaintiffs' chain ofcausation rests on the core allegation that defendants are responsible for a substantial share of worldwide greenhouse gas emissions. Plaintiffs allege that over the 263 years between 1751 and 2014, the United States produced more than twenty-five percent of global C02 emissions.
At oral argument, plaintiffs explained that their theory of causation has two components. The first relates to defendants' affirmative acts. Specifically, plaintiffs allege that fossil fuel combustion accounts for approximately ninety-four percent of United States C02 emissions ... Defendants lease public lands for oil, gas, and coal production; undercharge royalties in connection with those leases; provide tax breaks to companies to encourage fossil fuel development; permit the import and export of fossil fuels; and incentivize the purchase of sport utility vehicles ... Here, the chain of causation is: fossil fuel combustion accounts for the lion's share of greenhouse gas emissions produced in the United States; defendants have the power to increase or decrease those emissions; and defendants use that power to engage in a variety ofactivities that actively cause and promote higher levels of fossil fuel combustion.
The second component ofplaintiffs' causation theory involves defendants' failure to act in areas where they have authority to do so. Plaintiffs allege that together, power plants and transportation produce nearly two-thirds of C02 emissions in the United States ... (transportation produces approximately twenty-seven per cent of annual emissions)...(power plants produce roughly thirty-seven percent of annual emissions). Plaintiffs also allege DOT and EPA have broad power to set emissions standards in these sectors. So the chain of causation is: DOT and EPA have jurisdiction over sectors producing sixty-four percent of United States emissions, which in turn constitute roughly fourteen per cent of emissions worldwide; they allow high emissions levels by failing to set demanding standards; high emissions levels cause climate change; and climate change causes plaintiffs' injuries.
At the pleading stage, plaintiffs have adequately alleged a causal link between defendants' conduct and the asserted injuries.
Redressability in this case is scientifically complex, particularly in light of the spectre of "irreversible climate change,'' wherein greenhouse gas emissions above a certain level push the planet past "points of no return, beyond which ineversible consequences become inevitable, out of humanity's control." ... This raises a host of questions, among them: What part of plaintiffs' injuries are
attributable to causes beyond this Court's control? Even if emissions increase elsewhere, will the magnitude of plaintiffs' injuries be less if they obtain the relief they seek in this lawsuit? When would we reach this point of no return, and do defendants have it within their power to avert reaching it even without cooperation from third parties? All of these questions are inextricably
bound up in the causation inquity, and none of them can be answered at the motion to dismiss stage.