Finding New Ways to Link Weather Events to Climate Change

By CIRCulator Editorial Staff

Reprinted from: The Climate CIRCulator

FROM HURRICANE SANDY to this year’s drought in California and Oregon, extreme weather is now frequently associated with climate change in the popular imagination. For researchers, attributing these individual events to climate change has been tricky, though not impossible.

How can researchers know whether a given event is attributable to human-caused climate change? That is, did climate change increase the risk of an individual event occurring? A recent paper published in the Journal of Climate provides a rigorous mathematical framework for risks attributable to climate change. Researcher Gerrit Hansen and colleagues lay out a way to assess that risk using either observations or a climate model.

Alternative “attribution” studies use climate models to develop, in effect, “fingerprint” tests for individual events. When joined with historical weather station data, climate models can simulate historical conditions, similar to the methods reviewed in October’s CIRCulator article on coastal hazards. The advantage of this method is twofold: It increases the number of data points available for more robust statistical analyses, and it provides a way to “control” for the role of human influence by modeling a past with increasing levels of CO₂ and one with stable CO₂ levels.

OCCRI and CIRC researchers have used different attribution approaches for investigations into the role of CO₂ in the following events:

• The Texas drought of 2011
• The central U.S. drought in 2012
• Northern Hemispheric changes is spring snow cover
• Pacific Northwest warming trends in the 20th century

Recently, the Bulletin of the American Meteorological Society’s annual special issue on extreme events contained three articles on the 2013-14 California-Oregon drought. None attempted a rigorous attribution, but two of the three compared features of the drought with comparable features in climate simulations. An article by Daniel Swain and colleagues noted that the ridge of atmospheric high pressure over the Pacific Ocean — the direct cause of the drought — was unprecedented in magnitude, and they estimated it had roughly a 400-year return period. They compared the frequency of such events in preindustrial climate model simulations with “climate change” simulations and found that such events became more likely. Hence, even without the direct effects of warming on moisture balance, the drought was made more likely by rising CO₂.

NOTE: Hansen and colleagues used a “stochastic point process” or representation of randomly occurring events. They suggest that one can think of the baseline point process that would have occurred without climate change, and an additional point process of events that were in a sense “caused” or added by climate change.

Hansen G., M. Auffhammer, and A.R. Solow (2014) On the Attribution of a Single Event to Climate Change, J. Climate, 27, 8297–8301. doi:10.1175/JCLI-D-14-00399.1

Swain, D.L., M. Tsiang, M. Haugen, D. Singh, A. Charland, B. Rajaratnam and N.S. Diffenbaugh (2014) The extraordinary California Drought of 2013/2014: Character, Context and Role of Climate Change, [in “Explaining Extremes of 2013 from a Climate Perspective”].

Bulletin of the American Meteorological Society, 95(9), S3–S7. Full Report.

A version of this story originally appeared in the November/December issue of The Climate CIRCulator.