A powerful tornado as much as three-kilometers wide devastated the Oklahoma City suburb of Moore on May 20. The twister reportedly boasted winds above 300 kilometers-per-hour as it tore through homes and schools, leaving a path of large-scale destruction and killing dozens of people, including many children. Oklahoma is ground zero for tornadoes for geographic reasons. The state is located precisely where warm air from the Gulf of Mexico collides with colder weather from the west and north. Many scientists wonder if the recent increase in concentrations of greenhouse gases in Earth’s atmosphere—and the extra heat they trap—intensified this particular extreme weather phenomenon and will continue to worsen other tornadoes as the season continues through August. Scientific American corresponded with climate scientist Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colo., to get his thoughts on whether global warming is making tornadoes stronger or more intense. In a July 2007 story for Scientific American, Trenberth predicted that climate change could lead to more intense hurricanes in the Atlantic Ocean. [An edited transcript of the interview follows.] I know this kind of extreme weather is part of the territory in the middle of the country, but is climate change going to make such extreme weather more likely or more powerful? Of course, tornadoes are very much a weather phenomenon. They come from certain thunderstorms, usually supercell thunderstorms that are in a wind shear environment that promotes rotation. That environment is most common in spring across the U.S. when the storm track is just the right distance from the Gulf [of Mexico] and other sources of moisture. The main climate change connection is via the basic instability of the low-level air that creates the convection and thunderstorms in the first place. Warmer and moister conditions are the key for unstable air. The oceans are warmer because of climate change. The climate change effect is probably only a 5 to 10 percent effect in terms of the instability and subsequent rainfall, but it translates into up to a 33 percent effect in terms of damage. (It is highly nonlinear, for 10 percent it is 1.1 to the power of three = 1.33.) So there is a chain of events, and climate change mainly affects the first link: the basic buoyancy of the air is increased. Whether that translates into a supercell storm and one with a tornado is largely chance weather. How does such a little global warming input translate into such a big effect in terms of damage? There is a large level of natural variability, but it is when the natural variability is going in the same direction as climate change effects that we suddenly break records. We cross thresholds and record new extremes. A small effect can translate into a large impact under those conditions. It's the straw that breaks the camel's back. How does the warming affect the buoyancy of the air? I thought it had more to do with water vapor levels? It is the combination of temperature and moisture. The vertical temperature structure of the atmosphere is a critical ingredient: adding more heat and moisture adds buoyancy to the lowest levels. The moisture part comes in mainly when the air starts to condense, giving up latent heat in the process. That heat, which came from the original heat used to evaporate the moisture, adds to the buoyancy. Given this potential, why did the U.S. enjoy a tornado drought recently? Climate change does not change the weather or patterns of weather (much). Last year there were anticyclonic conditions and subsiding air aloft, and the jet stream was pushed way north. There are almost no storms under those circumstances. The variability from year to year is large and the El Nino phenomenon plays a big role. This year El Nino or La Nina is not in play allowing the weather to be more normal and variable. The pattern is not locked in place as it was in the past two years (in very different ways). Is the problem climate change or is the problem development in the path of extreme weather, whether in Tornado Alley or coasts exposed to hurricanes and the like? No doubt the biggest issue is people putting themselves in harm's way, building in coastal areas and flood plains particularly. What can we expect in the future given ever-increasing levels of greenhouse gases in the atmosphere that will presumably trap even more heat? The heat has to go somewhere. We expect more extremes in the water cycle in particular. Stronger droughts, bigger heat waves and much greater wildfire risk, but also stronger storms and heavier rainfalls where the rain is occurring. Managing water will be a major challenge. Follow Scientific American on Twitter @SciAm and @SciamBlogs. Visit ScientificAmerican.com for the latest in science, health and technology news.
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