Attribution
Did climate change do that..?
Last month I was with a few friends watching a news report about the heat wave and fires in Europe. It was mesmerizing in the way watching a disaster unfold in real time often is, and we were riveted to the TV. In the middle of the broadcast, someone asked,
"Was this caused by climate change?"
As an interested observer and writer on climate change, this is a question I am often asked. It is tempting to just say YES, on the basis that climate change is so pervasive it likely had an impact on the event in question. On the other hand, the answer is NO. Since the question is framed as a binary question looking for a binary answer — yes or no — we can't attribute all of the fire and heat to climate change.
For a fuller answer we turned to the growing field of attribution science. Rather than "did climate change cause this event," attribution scientists ask a different, answerable question: "how did climate change alter the odds or intensity of an event like this?" The field, called extreme event attribution (EEA), analyzes whether or not—and to what extent—human-caused climate change influenced an individual extreme-weather event like a heat wave or hurricane.
To help understand the science, we can look at two peer-reviewed studies. Both centred on heat-related deaths in Europe in the summer of 2022, but asked different questions about the same event.
In the first, a study that appeared in Nature Medicine in 2023 (Ballester et al.)1, answered the question of how many deaths in the summer of 2022 were heat-related (61,672). The study used weekly temperature and mortality data in a complex series of statistical analyses that accounted for the relationship between temperature and death by creating a region-specific "minimum mortality temperature," and then looked at deaths that occurred when the temperature was higher, while accounting for the fact that heat's acute effects usually occur over five days.
The second study (Beck et al.)2 looked at the question of attribution, as in "how many of these heat-related deaths resulted from climate change?" The same data was used as in the 2023 study (Ballester et al.), but Beck et al. (2024) reran their own mortality estimate for the same summer as part of their attribution analysis and got a higher total (68,593, not 61,672). The likely reason is a difference in the epidemiological model's calibration window: Beck et al. used epidemiological models calibrated for the period 2015–2019, whereas Ballester et al. calibrated their minimum-mortality-temperature thresholds using the full 2015–2022 window. Different calibration periods shift the exposure-response curve slightly, which changes the total “heat-related deaths” estimate for the same underlying weather.
To estimate the heat-related deaths resulting from climate change, a counterfactual temperature scenario was used. A counterfactual is used to analyze something under different conditions than what actually occurred. In this case, anthropogenic climate change was removed from the temperature data for the same period, and a mortality rate was calculated based on the lower temperature case. To determine the deaths caused by climate change, the counterfactual was subtracted from the factual results — the result being the excess deaths attributed to climate change. In this case, the result was 38,154 additional deaths as a result of climate change (56% of 68,593, with a 95% confidence interval of 39–77%) .
The proper answer to our friend's question was that climate change resulted in about 38,154 additional deaths from the heat wave.
These study results came out a year or two following the heat event and took considerable research time and effort to complete. As attribution science has become more widely understood, several organizations are working to get the attribution results of major events published within weeks, rather than years. These studies are done using peer-reviewed processes but are published before such a review is done. Timeliness enables reporting on the link to climate change while the event is still in the news.
One organization doing this type of review is World Weather Attribution (WWA), which has published over 100 attribution reports since it formed in 2014. To give a sense of how this works, the WWA published a report on August 6, 2026, on the Canadian wildfires that occurred during June and July of 20263. In it, they concluded that climate change made this extreme fire weather roughly twice as likely overall, with one measure in the Northwest Territories reaching about five times as likely.
Now that science has established that climate change causes damage and deaths, it raises the question as to when the companies and industries responsible for contributing to climate change will be held accountable. There are already legal challenges proceeding on this question in several jurisdictions, notably Boulder County v. Suncor (Colorado) 4: in February 2026, the U.S. Supreme Court granted a decision to review findings of a lower court to determine whether Boulder can proceed with public-nuisance and related claims against fossil fuel companies.
For those of us old enough to remember how the tobacco industry fought anti-smoking regulation and research to protect their business, this all sounds familiar. This shouldn't be surprising given that some of the same organizations (and even the same people) who worked for the tobacco industry were recruited by, and worked on behalf of, the fossil fuel industry. To get a sense of the stakes, the American Petroleum Institute (API) has named stopping "extreme climate liability policy" a top 2026 priority. 5
In case you don't recall, that fight didn't turn out that well for tobacco….
Reading
- Ballester, J., Quijal-Zamorano, M., Méndez Turrubiates, R.F., Pegenaute, F., Herrmann, F.R., Robine, J.-M., Basagaña, X., Tonne, C., Antó, J.M. & Achebak, H. "Heat-related mortality in Europe during the summer of 2022." Nature Medicine 29, 1857–1866 (2023). Published 10 July 2023. DOI: 10.1038/s41591-023-02419-z. Open access (https://www.nature.com/articles/s41591-023-02419-z).
- Beck, T.M., Schumacher, D.L., Achebak, H., Vicedo-Cabrera, A.M., Seneviratne, S.I., … Ballester, J. "Mortality burden attributed to anthropogenic warming during Europe's 2022 record-breaking summer." npj Climate and Atmospheric Science 7, Article 245 (2024). Published 29 October 2024. DOI: 10.1038/s41612-024-00783-2. Open access (https://www.nature.com/articles/s41612-024-00783-2).
- World Weather Attribution. "Climate change means extreme fire seasons in Canada are here to stay." Published August 6, 2026. https://www.worldweatherattribution.org/climate-change-canada-wildfires-2026/.
- Suncor Energy (U.S.A.) Inc. v. Board of County Commissioners of Boulder County. No. 25-170. U.S. Supreme Court, cert. granted February 23, 2026. https://www.scotusblog.com/cases/suncor-energy-inc-v-county-commissioners-of-boulder-county/.
- www.api.org/news-policy-and-issues/energy-agenda