Analysis of climate variables associated with extreme climate events. (A) September 5-d precipitable water associated with the 2013 Colorado floods. (B) Summer 500-hPa geopotential height pattern associated with the 2010 Russia heatwave. (C) September Arctic sea ice extent. HIST, Historical Climate Model Simulations; PI, Pre-Industrial Control Simulation. Graphic: Diffenbaugh, et al., 2017 / PNAS

By Ellie Bowen
25 April 2017

(Stanford Daily) – Using a four-pronged framework, Professor of Earth System Science Noah Diffenbaugh ’96 M.S. ’97 and his research team have found a direct connection between extreme weather events and human impact.

The team’s study, published in the Proceedings of the National Academy of Sciences magazine, outlines an objective approach to determining whether or not extreme weather events – such as the flooding in northern India in June 2013 or the slowly-subsiding California drought that began in 2012 – can be linked to climate change over the course of several years. The researchers discovered that, for a substantial number of recent extreme weather cases, there is indeed a connection.

“Our results suggest that the world isn’t quite at the point where every record hot event has a detectable human fingerprint, but we are getting close,” Diffenbaugh stated in an interview with Stanford News.

While scientists have historically avoided directly linking specific extreme weather events to climate change due to the variability of weather, according to Diffenbaugh and his research team, it is more important than ever to determine global warming’s role in causing record-breaking events.

The study’s multi-pronged approach reveals not only surface-level weather conditions but also underlying meteorological phenomena that contribute to these events.

The team analyzed climate change observations with advanced statistical models, a framework which the researchers say can be used to inform decisions in fields as diverse as farming, insurance and infrastructure.

With this framework, the researchers found that global warming from human emissions has made extreme hot weather events more likely across over 80 percent of the areas of the globe for which observations are possible.

“Our approach is very conservative,” Diffenbaugh said. “It’s like the presumption of innocence in our legal system: The default is that the weather event was just bad luck, and a really high burden of proof is required to assign blame to global warming.”

Researchers show connection between extreme weather and climate change

ABSTRACT: Efforts to understand the influence of historical global warming on individual extreme climate events have increased over the past decade. However, despite substantial progress, events that are unprecedented in the local observational record remain a persistent challenge. Leveraging observations and a large climate model ensemble, we quantify uncertainty in the influence of global warming on the severity and probability of the historically hottest month, hottest day, driest year, and wettest 5-d period for different areas of the globe. We find that historical warming has increased the severity and probability of the hottest month and hottest day of the year at >80% of the available observational area. Our framework also suggests that the historical climate forcing has increased the probability of the driest year and wettest 5-d period at 57% and 41% of the observed area, respectively, although we note important caveats. For the most protracted hot and dry events, the strongest and most widespread contributions of anthropogenic climate forcing occur in the tropics, including increases in probability of at least a factor of 4 for the hottest month and at least a factor of 2 for the driest year. We also demonstrate the ability of our framework to systematically evaluate the role of dynamic and thermodynamic factors such as atmospheric circulation patterns and atmospheric water vapor, and find extremely high statistical confidence that anthropogenic forcing increased the probability of record-low Arctic sea ice extent.

SIGNIFICANCE: Extreme climate events have increased in many regions. Efforts to test the influence of global warming on individual events have also increased, raising the possibility of operational, real-time, single-event attribution. We apply four attribution metrics to four climate variables at each available point on a global grid. We find that historical global warming has increased the severity and probability of the hottest monthly and daily events at more than 80% of the observed area and has increased the probability of the driest and wettest events at approximately half of the observed area. Our results suggest that scientifically durable operational attribution is possible but they also highlight the importance of carefully diagnosing and testing the physical causes of individual events.

Quantifying the influence of global warming on unprecedented extreme climate events



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