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What do the rising sea levels that all the global warming talk is centered on have to do with hurricanes? Essentially everything, and these tropical storms will continue to become more frequent and more disastrous unless global warming is slowed.

They are already on their way, as Katrina, Rita, and all too many others have illustrated. The intensity of storms has been found to have increased significantly since the 1970s, while the frequency has increased from an annual average of 10 tropical storms (including 5 hurricanes) between 1850 and 1990, up to 15 (with 8 hurricanes) on average between 1998 and 2007. That pattern corresponds to the increase in North Atlantic sea surface temperature, and unless we act now to curb global warming, it will just become worse. (As unproductive as complaining about the failure of Copenhagen is, there's a reason people are so disappointed.) When it comes to tropical storms, it's not the rising sea levels that are to blame so much as the increasing ocean temperatures, but the root cause is the same. Over the last 50 years, oceans have absorbed about 20 times more heat than the atmosphere, meaning the oceans are expanding (both because water expands as it warms as well as because of the melting glaciers and other land ice) and temperatures are rising from the surface down to 1,500 feet below.

The intensification of a tropical cyclone relies on three basic factors: warm ocean temperatures, low vertical wind shear, and high humidity. The Union of Concerned Scientists explains how the storm works:

As warm, moist air rises, it lowers air pressure at sea level and draws surrounding air inward and upward in a rotating pattern. As the water vapor-laden air spirals in and rises to higher altitudes, it cools and releases heat as it condenses to rain. This cycle of evaporation and condensation brings the ocean's heat energy into the vortex, powering the storm.

The increase in surface temperatures, in air temperatures, and in the rate of water evaporation (all consequences of CO2 and the other heat-trapping 'greenhouse gases') all contribute to a greater amount of water vapor that collects over the oceans, which has been found to have increased four percent since 1970.

Normally, the colder water below the ocean's surface serves to weaken a storm. The UCS again:

however, if deeper waters become too warm, this natural braking mechanism is diminished. Data from Hurricane Katrina indicate that while sea surface temperature was warm enough for maintaining the hurricane along its entire path, the storm significantly intensified when it hit the deep pools of warm water in the Gulf of Mexico.

Not too late to act

It's scary, and we may not be able to control hurricanes or other natural disasters directly, but we can—and should—control our contribution to making them worse: by reducing our individual carbon footprints, and then by demanding our Senators fight for strong climate and energy legislation today.

Watch the Focus Earth Episode: Climate Change and the Weather