WASHINGTON — A new report from the National Academies of Sciences, Engineering, and Medicine provides a comprehensive assessment of the current capabilities of extreme event attribution (EEA) — a scientific field that analyzes whether and to what extent human-caused climate change influenced an individual extreme weather event, such as a specific heat wave or hurricane.

The scientific tools, observational datasets, and methods developed and used for EEA have advanced considerably over the past decade and increased the confidence in EEA results for some types of weather events, says the report, which evaluates progress made since a 2016 National Academies report on the subject.

However, confidence in EEA study results varies significantly across different types of extremes, the report says. The highest confidence is associated with findings for extreme heat and cold events and others strongly influenced by a warming atmosphere, such as large-scale heavy rainfall events. Confidence is lowest for phenomena where observational records are limited and for phenomena that depend on small-scale dynamic processes that are inadequately reflected in global climate models, such as severe convective storms like thunderstorms and tornadoes.

“Significant progress has been made over the last decade, with major advancements in methods and modeling that allow for more robust assessments of extreme events,” said James Hurrell, chair of the committee that wrote the report, and Scott Presidential Chair of Environmental Science and Engineering at Colorado State University.  “But the field still faces challenges, and addressing them is necessary to fully realize the value of attribution science. We hope our recommendations will guide those efforts.”

Advances in tools and methods

Decades of data and research indicate that human-caused climate change is altering the frequency and intensity of several types of extreme weather events, such as hurricanes, record-breaking heat waves, and extreme rainfall. 

EEA allows scientists to move beyond detecting broad trends and instead investigate how climate change influences individual extreme events — yielding information that can help communities and decision-makers better understand the role of climate change in local weather events and inform their planning and policy.  The field has expanded into the nascent field of extreme event impact attribution (EEIA), which studies the extent to which human-caused climate change has influenced the impacts of an extreme event, such as human health impacts or economic losses.

EEA studies compare the observed event’s characteristics, such as its likelihood and intensity in the current climate, with a “counterfactual” world without human-caused emissions. Researchers use a combination of observational data, weather and climate models, and statistical models to make these comparisons and quantify the effect of climate change on the extreme event.=

Over the past decade, increased scientific understanding of the physical drivers of extreme events, expanded observational data, new statistical techniques, and improved models have together contributed to advances in EEA capability, the report says.  For example, radar, satellite, and on-site observations have expanded in terms of area and length of time covered, resulting in additional data to support EEA analyses for many regions. And, the use of larger ensembles of climate model simulations is enabling researchers to more effectively separate the human-caused climate signal from natural variability.

Still, significant challenges remain, the report says — especially for events governed by small-scale processes that most global climate models do not simulate, such as severe thunderstorms. Additionally, in under-resourced and vulnerable regions, such as the Global South, model limitations and lack of consistent, long-term records affect the capacity of attribution science and lower the confidence in attributing many events.

The field of EEIA has significant potential to contribute to the understanding of the extent to which economic, health, and other impacts of extreme events are attributable to climate change, the report adds. The field is still an emerging area, with substantial opportunities to advance the methods and expand investigations internationally and across various impact types.

Driving further advances in attribution science

The report urges a range of actions to further strengthen EEA and EEIA science and their usefulness, including:

• The global community of EEA and EEIA researchers should develop a common overarching framework with recommended best practices. Such a framework would help systematically advance the science, integrate the research communities, build public trust, and enhance the usability of results.

• Higher-resolution global climate models could be transformative for EEA — enabling a more accurate simulation of intense local storms and regional extremes. Developing and applying these models is a critical next step required to mature EEA into a more robust and regionally relevant science.

• More studies are needed that apply multiple EEA approaches to the same event, in order to increase the robustness and reliability of the findings.

• Operational EEA programs — which produce routine, rapid EEA studies — should conduct periodic peer review to enhance confidence in attribution results and provide an opportunity to identify areas where developments might be needed, particularly of understudied event types.

• Researchers should continue to develop and improve the models and data underlying impact attribution, with particular emphasis on regions and hazards that lack reliable tools — especially the Global South and cascading and/or compounding events.

• Significant opportunities exist to expand the usefulness of EEA and EEIA through collaborations with stakeholders. For example, working with local experts to co-develop studies that meet their intended needs, including necessary quantification of uncertainties and targeted communication, will enhance the value of attribution results.

The study was undertaken by the Committee on Attribution of Extreme Weather and Climate Events and Their Impacts and sponsored by the Bezos Earth Fund, National Science Foundation, National Oceanic and Atmospheric Administration, Heising-Simons Foundation, National Academy of Sciences Arthur L. Day Fund, and Robert Litterman.

The National Academies of Sciences, Engineering, and Medicine are private, nonprofit institutions that provide independent, objective analysis and advice to the nation to solve complex problems and inform public policy decisions related to science, engineering, and medicine. They operate under an 1863 congressional charter to the National Academy of Sciences, signed by President Lincoln.

Contact:
Office of News and Public Information
202-334-2138; email news@nas.edu