Extreme Weather Events: Connecting the Dots

by Bill Chameides | September 13th, 2012
posted by Erica Rowell (Editor)

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New paper finds that isolated extreme events around the globe may have the same cause.

When it comes to weather, you don’t need a weatherman to know which way the extremes are headed. Witness the summer we’ve had this year (see here and here) and indeed the weather the past couple of years (see here, here and here). The most common questions swirling around the topic are perhaps: What’s causing all this crazy weather — is it global warming or just anomalies reflecting natural variability?

Rolling a weather-related snake eyes

Answering those questions has been difficult because the events themselves represent weather while the questions are essentially focused on climate. Nevertheless, scientists have been making progress on this problem — taking an individual event like a flood or regional drought, and using statistics and probabilities to separate out natural and global-warming influences. The results are just beginning to flow in from this type of approach and they tend to show that global warming is indeed playing a role in fostering extreme weather events. Jim Hansen of NASA’s Goddard Institute for Space Studies likes to describe what is happening as “loading the dice.” With global warming, the chances of rolling “snake eyes” — a devastating heat wave, drought or flood — become more and more likely. (This Marc Roberts cartoon runs with the analogy.)

What about multiple events?

Writing in the Journal of Geophysical Research, Kevin Trenberth and John Fasullo of the National Center for Atmospheric Research take a slightly different tack on the problem of understanding the origin of extreme events. Rather than focusing on trying to attribute a single extreme event to a cause like global warming, they wondered if there were underlying global connections between a seemingly disparate series of extreme events. For example, could high temperatures in the ocean — such as might arise from global warming — cause a “clustering of extremes”? To answer that question, the authors examined the impact of record-high sea surface temperatures on extreme weather patterns around the globe in the summer and fall of 2010.

The year 2010 was a doozy of a year. Among the weird weather going around that year were these events:

Trenberth and Fasullo looked at all these events, but the focus of their analysis was on the Pakistani flood and the Russian drought. The authors used a variety of meteorological data, sea surface temperatures and data on the intensity of long- and shortwave radiation to discern patterns and relationships between the conditions that fostered each of the events of interest.

The authors’ analysis suggests that a common denominator for all of the events was the conditions and changes that occurred in the Pacific Ocean. During the first half of 2010 the surface water temperatures of the South Pacific were at record highs indicating that there was a good deal of extra heat energy stored in those waters. These high temperatures were due, in large part, to a strong El Niño (the warm phase of the El Niño-Southern Oscillation or ENSO) overlaid on top of global warming. Then in the late spring of 2010 the Pacific experienced a rapid transition from the warm El Niño phase to the cool La Niña phase. This transition changed circulation patterns causing the excess energy stored in the surface waters of Pacific to be pumped outward into the Atlantic and Indian Oceans. This in turn drove changes to precipitation and evaporative weather patterns across the globe that spawned the cluster of extreme events in the later half of 2010.

Interesting, but what, if anything, does it have to do with the effects of global warming on extreme events? Maybe a lot. At least the authors think so; they stated that their results are consistent with the findings of Stefan Rahmstorf and Dim Coumou of the Potsdam Institute of Climate Research that “the clustering of extremes occurs when natural variability (e.g. ENSO) creates anomalies that are in the same direction as global warming.”

Connections, we keep finding tight connections.

filed under: climate change, drought, El Nino, El Nino-Southern Oscillation, faculty, global warming, rainfall, weather
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