Is Extreme Weather
Linked to Global Warming?

In the past year, the world has seen a large number of extreme weather events, from the Russian heat wave last summer, to the severe flooding in Pakistan, to the recent tornadoes in the U.S. In a Yale Environment 360forum, a panel of experts weighs in on whether the wild weather may be tied to increasing global temperatures.

That global air and ocean temperatures are rising, and that human activity is largely to blame, is no longer a subject of debate among the vast majority of the world’s climate scientists. But there is no such consensus when talk turns to another important question: Is climate change already causing more extreme weather events, including worsening downpours and flooding, intensifying heat waves, and more powerful hurricanes?

Yale Environment 360 asked eight leading climate experts whether they think there is growing evidence that human-caused global warming is contributing to an increased incidence of extreme weather — and to cite specific recent examples in their answers. Their responses varied, with some contending that rising temperatures already are creating more tempestuous weather and others saying that more extreme weather may be likely but that not enough data yet exists to discern a trend in that direction. Scientists in both camps said two physical phenomena — warmer air holds more moisture, and higher temperatures exacerbate naturally occurring heat waves — would almost by definition mean more extremes. But some argued that the growing human toll from hurricanes, tornadoes, floods, and heat waves is primarily related to burgeoning human population and the related degradation of the environment.

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Kevin Trenberth, senior scientist at the U.S. National Center for Atmospheric Research’s Climate Analysis Section.
Yes, undoubtedly. The environment in which all storms form has changed owing to human activities. Global warming has increased temperatures and directly related to that is an increase in the water-holding of the atmosphere. Over the ocean, where there are no water limitations, observations confirm that the amount of water vapor in the atmosphere has increased by about 4 percent, consistent with a 1 degree F warming of sea surface temperatures since about the 1970s. The human component does not change much from year to year and affects all storms.

In the absence of water, during a drought, the extra heat goes into raising temperatures and creating a more intense drought and heat waves, no doubt contributing to the 2010 Russian heat wave. However, the most spectacular events over the past year have been extreme heavy rains: flooding in India, China, and Pakistan in July and August, and then Queensland, Australia in December 2010 and January 2011. Further, very heavy rains in the U.S. in April 2011, along with snow melt, have also led to extensive flooding. In all these cases, very high sea surface temperatures have undoubtedly contributed to extra moisture flowing into the storms that produced the heavy rains and likely contributed to the strength of the storms through added energy. While perhaps a major part of these high sea surface temperatures was related to natural variability such as ENSO [El Nino Southern Oscillation], a component is related to global warming. It is when global warming and natural variability come together that records are broken. Our current work is documenting the link between the Asian flooding and the Russian heat wave and why the blocking anticyclone that led to it was so persistent. 

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Kerry Emanuel, director of Massachusetts Institute of Technology’s Program in Atmosphere, Oceans and Climate.
There is some evidence that hydrological events are becoming more extreme. This is not so easy to estimate, because rainfall is often quite local, so a good network of observing stations is required. In the U.S., where we have an excellent precipitation network, several published papers present evidence that rainfall is becoming increasingly concentrated into less frequent but more intense events.

Worldwide, satellite-based observations indicate that the most intense tropical cyclones are becoming more frequent even though the far more plentiful weak storms are occurring somewhat less often. In the North Atlantic region, where we have the best observations of such storms, there is a close correlation between their power and the temperature of the tropical Atlantic in summer. This correlation, which was first pointed out in 2005, has grown stronger since. Evidence is mounting that the tropical Atlantic sea surface temperature has been influenced by sulfate aerosol “global dimming” as well as by greenhouse gas increases, while evidence for a “natural cycle” in ocean temperature and storminess is correspondingly less persuasive.

Although this has been an exceptional year for tornadoes in the U.S., strong variations in reporting such events over time prevent us from deducing any long-term trends. Work is under way to examine trends in objective measures of thunderstorm rotation detected by Doppler radar since about 1988, when such radars were first widely deployed. But so far, there has been little theoretical work on how global warming might influence severe local storms, including tornadoes and hail storms.

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William Hooke, director of the American Meteorological Society’s Policy Program.
We live on a planet that does much if not most of its business through extreme events. What we call "climate" reflects the summing-up of these extremes to find averages. And what we call climate "variability" or "change" will therefore be reflected in variability or change in the locations, timing, patterns, intensity, and duration of these extremes.

That said, the statistics of the extremes are inherently noisy. Teasing out long-term changes in the relationships linking the extremes and the averages merits concerted and sustained scientific attention, but will remain a multi-year aspiration. Scientists — let alone non-experts — will be debating any findings for some time to come. (The most likely place to be convincing earliest may prove to be the statistics for heat waves and cold snaps.)

In the meantime, society has to deal with extremes. The truth is that we're not proving very adept at coping with the extremes and hazards we're facing today. Furthermore, the biggest change in our experience with hazards in

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http://e360.yale.edu/feature/forum_is_extreme_weather_linked_to_global_warming/2411/