This year ushered in a series of intense weather events that impacted communities across the globe: record-breaking heat waves; super-charged cyclones; intense flooding; coastal waters hitting a balmy 38°C (1–4). Attributing extreme weather to climate change has become the norm when reporting on these seemingly more frequent and intense events. But beyond simply acknowledging weather to be more violent or destructive than it was in the past, how is it that climate experts are able to determine if increasing greenhouse gas levels are the culprit behind these extreme weather events? The answers can be found in climate attribution science.

What is Climate Attribution Science?

Climate attribution science is a relatively new discipline that employs advanced statistical methods and climate models to assess to what extent human-caused climate change has influenced specific weather events (6). In this field of study, meteorologists, climatologists, statisticians and computer scientists gather and analyze data from historical records, observational data and climate simulations. By comparing the likelihood that an extreme weather event would occur under our current climate conditions versus a hypothetical world without human-induced climate change, researchers can statistically discern how climate change impacts—or doesn’t impact—these events.

The first instance of using statistical models to attempt to attribute a weather event to human-caused climate change was from a 2003 heat wave in Europe (6,7). In this case, scientists concluded that human influence doubled the risk of this sort of heat wave event. Before then, most experts cautioned that climate models could only point to broad trends, not discrete weather events. But better statistical and climate models have changed that. Now, scientists have the tools to contextualize extreme weather and estimate if and how much those events are impacted by climate change.

Storyline Versus Risk

There are two main approaches to carrying out a climate attribution study: a storyline approach and a risk-based approach (6). In a storyline approach, experts estimate how much specific natural and human causes contribute to an extreme event. Different factors (such as warmer ocean surface temperatures) are first linked to the event in question (such as a cyclone). The degree to which human activities influences those factors is then assessed. In a risk-based approach, the probability that human factors changed the likelihood of an extreme event is determined. Historical records and data are important for calculating these probabilities.

Is It Climate Change?

Some recent attribution studies that found that climate change played a role include:

• A 2021 heatwave in South Korea was at least four times more likely to occur because of human influences on climate.
• Human influences and La Niña increased risk of drought and high temperatures in California by sixfold in 2021.
• Record rainfalls in the United Kingdom in 2021 were made 1.5 times more likely due to human-induced climate change impacts.
• A severe drought in Iran that lasted from 2020–2021 was 50% more likely due to human sources of greenhouse gas emissions.

In contrast, one example of an extreme weather event that was not found to be influenced by climate change was a 2011 Texas heatwave (6,8). Using a storyline approach, researchers examined factors that influenced the severity of the heatwave. The main contributor was a lack of rainfall during the previous spring and winter, which was, in turn, linked to a change in the surface temperature of the Gulf of Mexico. However, the change in surface temperature was within the normal variation of historical temperatures. Human activity did not significantly influence the heatwave.

Proceed with Caution

Though climate change is certainly influencing the severity of weather events across the globe, reports that any specific extreme weather event is influenced by climate change should be backed up with peer-reviewed analysis that properly accounts for various sources of uncertainty (such as uncertainty about observational data or the suitability of various climate models) (6). These studies typically are published weeks or months later than the original weather event, and people should be cautious when climate change impacts are cited in the immediate aftermath of a specific extreme weather event.

A related challenge climate attribution scientists face is effectively communicating their findings to the public and to policymakers. The statistical nature of attribution studies is complex, and conveying uncertainties and relative risk can be difficult (as seen when attempting to communicate risk in the midst of a pandemic). Effective communication of attribution science is crucial for informed public discourse and action on climate change.

Putting Climate Attribution Science to Work

The insights gained from climate attribution science could deeply impact climate policy and disaster preparedness (6). By linking extreme weather events to climate change, policymakers can make more informed decisions on mitigating greenhouse gas emissions and implementing strategies to better protect regions most vulnerable to extreme weather.

Attribution science also helps insurance companies and others assess the risk associated with extreme weather events. It’s likely that climate attribution studies will eventually be used as evidence in lawsuits against defendants who allegedly contribute to an extreme weather event and its impacts (9).

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References

1. Dickie, G. (2023, July 27) July 2023 set to be world’s hottest month on record. Reuters. Retrieved Aug. 9, 2023, from: https://www.reuters.com/business/environment/july-2023-set-be-worlds-hottest-month-record-scientists-2023-07-27/
2. World Meteorological Organization (2023, March 10) Tropical cyclone Freddy may set new record. Retrieved Aug. 9, 2023, from: https://public.wmo.int/en/media/news/tropical-cyclone-freddy-may-set-new-record
3. O’Malley, I, et al. (2023, July 10) Deadly flooding is hitting several countries at once. Scientists say this will only be more common. Associated Press. Retrieved Aug. 9, 2023, from: https://apnews.com/article/flooding-rainfall-climate-change-warming-atmosphere-d207e68ba3374bdc2df3d36e97a84a1b
4. Tabuchi, H. (2023, July 26) 101 degrees in the ocean off Florida: Was it a world record? New York Times. Retried Aug. 9 2023, from: https://www.nytimes.com/2023/07/26/climate/florida-100-degree-water.html
5. Patricola, C.M. and Wehner, M.F. (2018) Anthropogenic influences on major tropical cyclone events. Nature. 563, 339–346. doi: 10.1038/s41586-018-0673-2
6. Haskett, J. D. (2023) Is that climate change? The science of extreme event attribution. Congressional Research Service. R47583. Retried Aug. 9, 2023 from: https://crsreports.congress.gov/product/pdf/R/R47583
7. Stott, P. A. et al. (2004) Human contribution to the European heatwave of 2003. Nature. 432, 610–614. doi: 10.1038/nature03089
8. Hoerling, M. et al. (2013) Anatomy of an extreme event. Am. Meteor. Soc. 2811–2832. doi: 10.1175/JCLI-D-12-00270.1
9. Joselow, M. (2021, June 30) Climate lawsuits are using old science. E&E News. Retrieved Aug. 9, 2023, from: https://www.scientificamerican.com/article/climate-lawsuits-are-using-old-science/

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Jordan Nutting

Jordan is formerly a science writer at Promega Corporation. She earned her PhD in Chemistry at the University of Wisconsin-Madison and worked as a science reporter at the Milwaukee Journal Sentinel as a AAAS Mass Media Fellow. Jordan loves reading and is always looking for book recommendations. In her spare time, Jordan also enjoys knitting, going on hikes and gardening.

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