Report Says Science Can Estimate Influence of Climate Change on Some Extreme Events
By Stephanie Doster, Institute of the Environment
A National Academies of Sciences committee that includes a University of Arizona professor has produced a report tackling the commonly asked question: Did human-induced climate change cause a specific flood, drought or other extreme-weather event?
“While a definitive answer to whether climate change caused a particular event cannot usually be provided because natural variability almost always plays a role, it is now possible to assess whether human-caused climate change has influenced the intensity or frequency of some types of extreme weather events,” said Kathy Jacobs, director of the UA’s Center for Climate Adaptation Science and Solutions at the Institute of the Environment.
“So a better way to ask this question is, to what extent was the event intensified or weakened because of climate change?” she said. Jacobs, who led the third National Climate Assessment, is also a professor in the Department of Soil, Water and Environmental Science.
The National Academies Committee released the report, Attribution of Extreme Weather Events in the Context of Climate Change, on Friday. The report evaluates the rapidly growing science of event attribution, examining the degree to which extreme events such as heat waves, droughts, cyclones and extreme precipitation can be linked to human-caused climate change.
Recently developed scientific methods allow for estimation of how the intensity, duration or frequency of an event has been altered by climate change and provide information that can be used to assess and manage risk, guide climate adaptation strategies, and determine greenhouse gas emissions targets, according to the report.
The most dependable attribution findings are for those events related to an aspect of temperature, for which there is little doubt that human activity has caused an observed change in the long-term trend.
“We are more confident that extreme events related to temperature are influenced by climate change than other types of extremes,” Jacobs said. “This has implications for the Southwest because temperature has a direct relationship to impacts on river flows and soil moisture. The relationship between anthropogenic climate change and some extreme droughts is an important conclusion for this region and the people who live here.”
Because increasing concentrations of greenhouse gases are affecting the entire planet's climate system, scientists cannot rule out some influence of climate change on any individual extreme event, Jacobs said. But each event has a variety of possible natural and human-related causes.
Attributing extreme events in the context of climate change can be approached from different scientific perspectives, Jacobs said. One approach is a statistical analysis of the likelihood of such events occurring in the absence of climate change, based on our understanding of the frequency and intensity of such events in the past. Another way to evaluate the human footprint on extreme events is to use models to study the physics of individual events and compare them to previous analogous events.
“We have the greatest confidence in attributions that are based on sound physical principles, a good long-term observation record, and when computer models can accurately re-produce the extreme event,” said committee chair Admiral David W. Titley, a professor of practice in meteorology at Pennsylvania State University. Titley also works with the UA in a project funded by the U.S. Department of Defense to identify climate-related impacts on and adaptation options for military installations.
Until recently, scientists were reluctant to link climate change to any single weather event. But their understanding of climate and the forces that drive extreme events has improved and methods used in event attribution have advanced, Jacobs said. Still, the committee wrote, more research is required to increase the reliability of event attribution, ensure that results are presented clearly, and better understand smaller scale and shorter duration weather extremes such as hurricanes and thunderstorms.
Concerns about the increasing costs of extreme events has spurred strong public interest in understanding the underlying causes of the events.
"Event attribution is important because of its relationship to managing risk," Jacobs said. "Human behavior can either exacerbate or mitigate the impacts of extreme events and that is why understanding the social and ethical issues, as well as the science, of extreme events is such an important research need."