Since the 1950s, the world's oceans have accumulated close to 95 percent of all the additional heat related to global warming. Because warm ocean surfaces evaporate faster than cool ones and the warmed atmosphere holds more water vapor, the warming and water vapor fuel storms. (MIT climate modeler Kerry Emanuel, for example, has demonstrated that Hurricane Katrina would not have been as strong as it was in 2005 if it had occurred in 1980, when the atmosphere and sea surfaces were cooler.)
So global warming is thus causing climate change, including altered weather patterns, and the engine of change is the heat building up deep inside the world's oceans. Water is warming, ice is melting, and water vapor is rising. How does this help explain tornadoes?
For tornadoes, we have clear ideas on how they form and have some idea about how their strength may be linked to global warming.
It's all about contrasts and gradients. Warmer temperatures over land surfaces create low-pressure systems (since hot air rises, creating "lows"), while cold fronts from the north come with high pressures. Weather "flows downhill," as it were—from highs to lows. When temperature and pressure gradients between highs and lows increase (as they do naturally in spring), the clash can twist to form tornadoes. The greater the contrasts, the greater the force of the twisters.
This spring, especially warm and moisture-laden air from the Gulf of Mexico met up with especially cold fronts from the north, driven by melting Arctic and Greenland ice. (Between 2004-06 and 2007-09, the rate of ice mass loss in the Canadian Arctic Archipelago sped up threefold.)
Warmer air and warmer seas are melting Arctic ice, and the shrinkage has changed North Polar air circulation—allowing "leakage" of cold air outside the Arctic Circle. Cold fronts previously contained within the polar vortex are now slipping out (driving severe winter weather in the U.S. and Europe in the past two years), and melting Arctic ice has also altered the path of the branches of the jet stream. Cold, fresh melt water from the Arctic has set up a blocking high-pressure area in the Atlantic off the Northeast for most of the past 18 months, altering the movement of weather fronts—sometimes hastening them, sometimes stalling them.
In addition, winds have changed globally over the past half-century. Westerly winds have increased in both hemispheres, more so in the south (affecting Australia's weather). These changes in global circulation patterns have climate scientists very worried.
This is complicated, and there are a lot of moving parts. Global warming is affecting many components of the global climate simultaneously, and the result is an increasing propensity for severe storms and other weather extremes.
When scientists are asked about the connections, they've carefully responded that "no one event is diagnostic of climate change." But, as Kevin Trenberth, the head of the climate analysis section of the National Center for Atmospheric Research, explains, questioning whether singular incidents are or are not caused by climate change misses the point. The climate is changing, and all weather events occur in the context of natural variability and the changing climate.
The same set of conditions will not be repeated each year—and tornadoes, like hurricanes, will not increase each year. And the precise dynamics described are open to discussion. But it is clear that changing atmospheric and oceanic conditions underlie the changing patterns of weather—and that the stage is set for more severe storms, including even more punishing tornadoes.
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