The unusually active 2005 Atlantic hurricane season sparked a debate as to whether we were glimpsing into future conditions brought on by global warming or it was part of a natural cycle of hurricane intensity. Heat that drives global warming can provide the trigger for the development of hurricanes and thunderstorms. A disturbing outcome of global warming may be the increased likelihood of severe weather in the form of stronger and more frequent storm systems.
Coastal inhabitants from the the midlatitudes to the tropics live with the threat of tropical cyclones. Two factors are important in determining future hurricane activity, high ocean surface temperatures and the effect of El Niño/La Niña. As discussed in this chapter, hurricanes derive their power from water ocean water. If climate change continues to warm the oceans, we should expect more active hurricane seasons and powerful storms.
Measures of the destructive potential of hurricanes have shown a substantial increase, though the cause is still debated in the climate science community. Evidence seems to indicate that the severity of hurricanes has increased over the last several years as a result of increasing ocean temperatures. The number of category 4 and 5 hurricanes has increased 75% since 1970, with the largest increases in the North Pacific, Indian, and Southwest Pacific oceans. Model simulations indicate the trend toward more severe hurricanes continuing over the next century.
Complicating the ability to predict future hurricane activity is the influence of El Niño/La Niña. Generally, Atlantic hurricane activity is lower during an El Niño, and increases during La Niña. The influence of global warming on El Niño, however, is not well-understood and thus prediction of hurricane activity related to it remains problematic.
Thunderstorms and Tornadoes
The processes that create thunderstorms and tornadoes occur at geographic scales too small to be reproduced in most climate models. Geoscientists can, however, project the number of days with conditions conducive to thunderstorm development from their models. Global warming will likely lead to an increase in humid conditions near the ground, the fuel for thunderstorms. Humid coastal and regions that already experience a significant number of storms, will increase their chances for experiencing thunderstorms. Favorable conditions for thunderstorms are predicted to occur during the same seasons that they do today, leading to a more intense storm season.
Tornado formation is not well-understood, making future prediction practically impossible. Hurricanes that make landfall often spawn tornadoes. Recent modeling by climate scientists has shown that four factors are good predictors of hurricane-induced tornado formation, hurricane size, intensity, track direction and the moisture gradient strength at midlevels in the storm. They found a 35 percent increase in the size of tropical cyclones occurred between 1948 and 2008 from the Gulf than between 1924 and 1948, resulting in a doubling of the number of tornadoes produced per storm. When tested against actual storms, their model correctly predicted the 33 tornadoes spawned by Hurricane Ike in 2008 and 56 of the 58 tornadoes produced by Katrina in 2005.
An unusually mild winter followed by devastating tornadoes early in the spring ushered in 2011. Raging wildfires and devastating drought gripped much of the United States in 2012. Listen to Kevin Trenberth, distinguished senior scientist at the National Center for Atmospheric Research, discuss the correlation between climate change and extreme weather with NPR Talk of the Nation host Neil Conan (April 5, 2012).