Substantial Factor Causation in Climate Public Nuisance Suits

Across the United States, local governments are increasingly turning to the courts to address the mounting costs of climate change. In the absence of significant state and federal action, some municipalities have begun directly suing major greenhouse gas emitters for damages tied to extreme weather events, rising sea levels, and the expense of climate-proofing local infrastructure [1]. Many of these suits invoke “public nuisance”: a tort in state common law governing unreasonable interferences with a plaintiff’s access to public rights and services [2]. In these cases, municipalities generally argue that greenhouse gas emissions have infringed on the public’s right to a healthy environment—the paradigmatic “public right” under this tort—and that fossil fuel companies are therefore legally liable on public nuisance grounds [3].

One such case is Board of County Commissioners of Boulder County v. Suncor Energy (U.S.A.) Inc. (2018) (“Suncor”) [4]. In this case, several local Colorado governments sued major fossil fuel companies (including Suncor) for climate-related damages, asserting, among other things, that the companies had contributed to a public nuisance [5]. Importantly, in May 2025, the Colorado Supreme Court ruled that such state-law claims are not preempted by federal statutes or federal common law, setting Suncor to be among the first such public nuisance cases to proceed to a ruling on the merits [6]. This ruling will turn on the question of causation: given the diffuse nature of emissions, can a single producer be said to have legally caused the harm? The answer largely depends on the standard for causation that courts adopt. This essay argues that the court should use the “substantial-factor” causation standard instead of relying on strict “but-for causation,” and that under this standard companies such as Suncor are liable for the relevant harms.

Like other torts, public nuisance claims require proof of causation. In most cases, courts use a “but-for” standard: someone’s action causes a harm when the harm would not have occurred if the action had not occurred [7]. This makes sense in the context of a normal negligence suit. For example, I can easily say that someone crashed into me because they were driving irresponsibly, since if they had not driven irresponsibly then we would not have crashed. Causation in climate change suits, however, operates entirely differently. Even though the courts have noted that man-made emissions as a whole are causally responsible for rising temperatures and disasters, it is in practice impossible to establish that a specific harm caused by global warming would not have occurred if a given company had not polluted [8]. This is partly due to the quantity of emitters involved—virtually every person and corporation emits to some degree—and partly because it is near-impossible to attribute any specific event to one emitter (or even to climate change itself), since events like floods and hurricanes can occur even without global warming [9].

The courts, then, should not use the but-for test in global warming suits, since fossil-fuel companies are clearly responsible for global warming even though strict but-for causation is not satisfied. Modern science has accurately estimated in painstaking detail the cumulative greenhouse gas emissions for the world’s largest emitters, and we now know that only one hundred companies are responsible for over 50% of the world’s emissions [10]. Moreover, defendants in cases like Suncor have conceded that climate change has caused a dramatic increase in the rate of extreme weather events and that emissions have contributed to climate change [11]. Therefore, even if it is nearly impossible to show that a particular storm or wildfire resulted from a single company’s actions, the evidence clearly demonstrates that major emitters have each materially increased the likelihood and severity of such events in general. Thus, even a specific event may have been made more likely by a single company’s conduct. Additionally, we now know that fossil fuel companies knew as early as 1968 that their products would disrupt the climate and yet either said nothing about this or, worse, engaged in a campaign to actively mislead the public about climate change [12]. As such, a company like Suncor has been a major factor in creating an environment that is substantially responsible for the relevant harms, and must therefore be held responsible. That the but-for test cannot show this does not mean that companies like Suncor are not liable; rather, it just means that the but-for test is not well-suited to climate change suits.

Thankfully, tort law has developed a different causation standard applicable to climate torts: the substantial-factor test. According to the Restatement of Torts, Second, conduct can be a legal cause of harm if it is “a substantial factor in bringing about the harm” [13]. Courts have found in a variety of cases that defendants caused harm under this standard, even when actions clearly failed the but-for test. For example, in Anderson v. Minneapolis (1920), two independent fires, one negligently started by the defendant, merged and destroyed the plaintiff’s property [14]. Although the defendant’s actions were not a but-for cause since the other fire would have caused the harm alone, the Minnesota Supreme Court nevertheless held the defendant liable because his actions would have caused the harm, reasoning that any narrower standard “absolve[s] all wrongdoers” whenever multiple causes exist [15]. Moreover, later cases demonstrate that each defendant’s actions do not even need to be independently sufficient. In Rutherford v. Owens-Illinois (1997), a worker’s family sued multiple asbestos manufacturers (including Owens-Illinois) after he died from cancer following decades of exposure. His family could not prove that the fibers from Owens-Illinois’s specific products had themselves caused the cancer— which would have defeated causation under the but-for test—but the California Supreme Court held Owens-Illinois liable under the substantial-factor test, arguing that Owens-Illinois’ actions had substantially increased the risk of cancer [16]. Importantly, the Court held that the substantial factor standard requires “only that the contribution of the individual cause be more than negligible or theoretical”—not that each cause must by itself be sufficient [17].

Under the substantial-factor test, the actions of fossil fuel companies like Suncor plainly satisfy this burden of causation. As discussed above, major fossil fuel companies have contributed measurably to climate change, which drives the increased rate of extreme weather events. Crucially, even though end-users like car owners or power plants perform the actual combustion, the fact that major fossil fuel companies actively misled the public about the harms of greenhouse gasses makes these companies responsible for the harmful burning of fossil fuels [18]. After all, the tort system routinely imposes liability on manufacturers who market products knowing that their ordinary use will cause injury [19]. Indeed, the Colorado Supreme Court has already signaled in Suncor that the case boils down to a dispute over deceptive marketing and failure to warn [20]. As such, the court will likely deem the burning of Suncor-produced fossil fuels a result of Suncor’s actions, so the relevant question becomes whether we can tie the harms suffered by Boulder to these fossil fuels. The answer, I have argued, is a resounding yes.

Yet even if it is doctrinally correct for courts to use the substantial-factor standard here, several commentators have argued that the courts have strong normative reasons not to. David Dana points out that global warming is a complex resource management problem, whose solution requires striking a delicate balance between energy needs and emissions concerns through careful economic and scientific modeling. Because the courts lack the legislative expertise necessary to adequately plan in this way, he concludes that solutions should be left to the legislatures [21]. Amicus briefs filed by the American Petroleum Institute and several U.S. states in Suncor echo this and also claim that successful public nuisance claims in state common law would invite a patchwork of regulation across the various states, throwing the already-volatile energy industry into chaos and obstructing effective federal legislation [22].

These objections fail, however, because they incorrectly assume that successful suits will position courts as policymakers. Boulder seeks damages, not an injunction. As such, ruling for Boulder does not require the courts to invent carbon taxation or emission trading schemes, but instead just forces Suncor to pay for the damage it has caused—a familiar judicial function that does not require a “patchwork of regulation” [23]. Moreover, cases like People v. ConAgra Grocery Products Co. (2017) and In re MTBE Products Liability Litigation (2013) show that courts can draw from technical expertise to apportion damages with technical precision, so the payments required will be rationally calculated [24]. Finally, the claim that legislative solutions would be preferable ignores the reality of legislative inaction: judicial action now would not suppress legislation but fill a dangerous gap. Indeed, as the history of tobacco and opioid legislation demonstrates, litigation and legislation are not exclusive. Rather, litigation often plays an important role in shaping public opinion and thereby catalyzing legislation [25]. Therefore, the prospect of legislative action is not a reason for judicial restraint but a reason for judicial engagement.

The courts, in conclusion, should use the substantial-factor standard for causation in Suncor and hold Suncor Energy liable for its contributions to climate change. A decision to the contrary would grossly disregard both the logic of cumulative causation and a century of precedent recognizing substantial-factor liability. Even more, a ruling for Boulder in this case could encourage a wave of essential climate legislation, incentivizing a faster transition away from fossil fuels and financing climate-proofing across the country. The courts, then, not only face doctrinal pressure from Anderson v. Minneapolis and Rutherford v. Owens-Illinois to apply the substantial-factor framework but also have strong normative reasons to do so—both because it is the goal of the legal system to deliver justice, and because the fate of the planet may depend on it.

Works Cited

[1] Albert C. Lin, “The Second Wave of Climate Change Public Nuisance Litigation,” American Bar Association, September 1, 2019, https://www.americanbar.org/groups/environment_energy_resources/resources/trends/2012-2021/second-wave-climate-change-public-nuisance-litigation/.

[2] Jack Wold-McGimsey, “Climate Change and Modern State Common Law Nuisance and Trespass Tort Claims,” Colorado Law Review 94, no. 3 (n.d.), accessed October 4, 2025, https://scholar.law.colorado.edu/lawreview/vol94/iss3/6.

[3] Lin, “The Second Wave.”

[4] “Board of County Commissioners of Boulder County v. Suncor Energy (U.S.A.), Inc.,” accessed October 4, 2025, https://www.climatecasechart.com/collections/board-of-county-commissioners-of-boulder-county-v-suncor-energy-u-s-a-inc-_be5980.

[5] Ibid.

[6] John Watson, “Colorado Supreme Court Rules That Boulder’s Tort-Based Climate Change Damages Lawsuit Will Proceed in State Court,” Spencer Fane, May 14, 2025, https://www.spencerfane.com/insight/colorado-supreme-court-rules-that-boulders-tort-based-climate-change-damages-lawsuit-will-proceed-in-state-court/.

[7] “Causation in Environmental Law,” Harvard Law Review 128, no. 8 (2015), https://harvardlawreview.org/print/vol-128/causation-in-environmental-law/.

[8] “Tort Law and Climate Change” in Law and Climate Atlas, February 2024, https://lawclimateatlas.org/resources/tort-law-and-climate-change-2/; David Bullock, “Public Nuisance and Climate Change: The Common Law’s Solutions to the Plaintiff, Defendant and Causation Problems,” The Modern Law Review 85, no. 5 (2022): 1136–67, https://doi.org/10.1111/1468-2230.12732.

[9] Bullock, “Public Nuisance and Climate Change.”

[10] Paul Griffin et al., The Carbon Majors Database: CDP Carbon Majors Report 2017 5 (CDP Worldwide, Jul. 2017), https://cdn.cdp.net/cdp-production/cms/reports/documents/000/002/327/original/Carbon-Majors-Report-2017.pdf.

[11] Lin, “The Second Wave.” For scientific evidence, see Attribution of Extreme Weather Events in the Context of Climate Change, Committee on Extreme Weather Events and Climate Change Attribution et al. (National Academies Press, 2016), https://doi.org/10.17226/21852.

[12] Daniel Lungren, Donald Kochan, Patrick Parenteau, Richard Faulk, David Bookbinder, John S. Baker “Originally Speaking: Climate Change and Common Law Public Nuisance,” FedSoc Blog, December 7, 2018, https://fedsoc.org/commentary/fedsoc-blog/originally-speaking-climate-change-and-common-law-public-nuisance-opening-response.

[13] Restatement (Second) of Torts §433 (A.L.I, 1965).

[14] Anderson v. Minneapolis, 146 Minn. 430 (1920).

[15] Ibid.

[16] Rutherford v. Owens-Illinois, 16 Cal. 4th 953 (1997).

[17] Ibid.

[18] Drew Tharp, “Fighting Over Forum: How State Common Law Public Nuisance Claims Will Shape the Future of Climate Change Litigation,” Indiana Law Review 56, no. 3 (2023): 623–51, https://doi.org/10.18060/27246.

[19] Bookbinder and Parenteau, “Originally Speaking.”

[20] Casey Alan Coyle and Stafnie Pitcavage Mekilo, “Clearing the Air on Public Nuisance and Preemption: A Look at Climate-Change Litigation in Pa. and Beyond,” Babst Calland: Attorneys at Law, August 1, 2025, https://www.babstcalland.com/news-article/clearing-the-air-on-public-nuisance-and-preemption-a-look-at-climate-change-litigation-in-pa-and-beyond/.

[21] David A. Dana, “The Mismatch Between Public Nuisance Law And Global Warming,” Supreme Court Economic Review 18 (February 2010): 9–42, https://doi.org/10.1086/659980.

[22] Brief for American Petroleum Institute et al. as Amicus Curiae Supporting Respondent, Board of County Commissioners of Boulder County v. Suncor Energy (U.S.A.) Inc., No. 2023SC76 (Colo. May 12, 2025); brief for Alabama and 25 Other States as Amici Curiae Supporting Respondent, Board of County Commissioners of Boulder County v. Suncor Energy (U.S.A.) Inc., No. 2023SC76 (Colo. May 12, 2025).

[23] Tharp, “Fighting Over Forum.”

[24] In re MTBE Products Liability Litigation, 725 F.3d 65 (2d Cir. 2013); People v. ConAgra Grocery Products Co., 17 Cal. App. 5th 51 (2017).

[25] Nora Freeman Engstrom and Robert L Rabin, “Pursuing Public Health Through Litigation: Lessons from Tobacco and Opioids,” Stanford Law Review 73 (2021).