March 11, 2016 by Canadian Underwriter
It is now possible to estimate the influence of climate change on some types of extreme events, such as heat waves, drought and heavy precipitation, says a new report from the Washington, DC-based National Academies of Sciences, Engineering, and Medicine.
The relatively new science of extreme event attribution has advanced rapidly in the past decade owing to improvements in the understanding of climate and weather mechanisms and the analytical methods used to study specific events, the National Academies said in a statement issued on Friday. But more research is required to increase its reliability, ensure that results are presented clearly and better understand smaller scale and shorter duration weather extremes, such as hurricanes and thunderstorms, said the committee that conducted the study and wrote the report.
“An increasingly common question after an extreme weather event is whether climate change ‘caused’ that event to occur,” said committee chair David W. Titley, professor of practice in meteorology and founding director of the Center for Solutions to Weather and Climate Risk at Pennsylvania State University. “While that question remains difficult to answer given all the factors that affect an individual weather event, we can now say more about how climate change has affected the intensity or likelihood of some events.”
Scientists cautioned in the past that individual weather events couldn’t be attributed to climate change, notes the overview of the report, titled Attribution of Extreme Weather Events in the Context of Climate Change. “Now, with advances in understanding, the climate science behind extreme events and the science of extreme event attribution, such blanket statements may not be accurate. The relatively young science of extreme event attribution seeks to tease out the influence of human-cause climate change from other factors, such as natural sources of variability like El Niño, as contributors to individual extreme events.”
Extreme event attribution is a fairly new area of climate science that explores the influence of human-caused climate change on individual or classes of extreme events compared with other factors, such as natural sources of climate and weather variability, the statement explained. The science typically estimates how the intensity or frequency of an event has been altered by climate change and provides information that can be used to assess and manage risk, guide climate adaptation strategies and determine greenhouse gas emissions targets. For example, in the wake of a disastrous event, communities may need to make a decision about whether to rebuild or relocate and need input on how much more likely or more severe this type of event is expected to become in the future, the National Academies said.
Some extreme event attribution studies use observational records to compare a recent event with similar ones that occurred in the past, when the influence of human-caused climate change was much less. Other studies use climate and weather models to compare the meteorological conditions associated with an extreme event in simulated worlds with and without human-caused climate changes. The report finds that results are most reliable when “multiple, different methods are used that incorporate both a long-term historical record of observations and models to estimate human influences on a given event.”
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, the report notes. For example, a warmer climate increases the likelihood of extremely hot days and decreases the likelihood of extremely cold days. Long-term warming is also linked to more evaporation that can both exacerbate droughts and increase atmospheric moisture available to storms, leading to more severe heavy rainfall and snowfall events. “However, temperature alone does not fully determine the probabilities of extreme events,” the statement said. “Attributing specific extreme events to long-term climate change may be complicated by factors such as natural long-term fluctuations in the ocean surface temperatures.”
Event attribution is retrospective, but the report calls for the development of predictive weather-to-climate forecasts of future extreme events that account for natural variability and human influences.