Hurricanes And Global Warming

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A hurricane is an intense tropical storm in which sustained wind speed exceeds 74 miles (119 km) per hour. According to the 2007 Fourth Assessment Report of the IPCC, it is "more likely than not" (meaning better-than-even odds) that there is a human contribution to the observed trend in the rise of hurricane intensity since the 1970s. The IPCC states that "it is likely (more than 67 percent odds) that future tropical cyclones (hurricanes) will become more intense, with larger peak wind speeds and heavier rain associated with continually rising sea surface temperatures."

Scientists at NOAA recognize two factors that contribute to more intense hurricanes: ocean heat and water vapor. They also recognize that these two factors have increased over the past 20 years because of human activities, such as burning fossil fuels and deforestation. Both activities have significantly raised the CO2 levels in the atmosphere.

They also warn that the world's oceans have already absorbed 20 times as much heat as the atmosphere over the last 50 years, which has warmed the waters to depths of 1,500 feet (457 m) in places. As the oceans warm, the water expands. Atmospheric humidity over the oceans has risen 4 percent since 1970, and because warm air holds more water vapor than cold air, this explains an increase in air temperature. They believe this is a visual, measurable result of global warming.

Another impact from this is higher storm surges, the height and amount of water that washes up on shore during a storm. Rising sea-levels mean that storm surges will be higher and more destructive, causing coastal flooding and erosion. Many people who live along coastal areas will have their property damaged and lost. The following table illustrates the resultant storm surge of different category hurricanes.

Saffir-Simpson Hurricane Scale


wind speed (miles per hour/ kilometers per hour)

storm surge (feet/meters)
















Beach erosion is a serious problem. This erosion was caused by a northeaster on the Outer Banks, North Carolina. (Richard B. Mieremet, Senior Adviser, NOAA)

Some scientists believe that the strength of a storm and the length of time it lasts may be increasing as global warming emissions increase in the atmosphere. Rising sea levels intensify the damage along coasts from storms. Warmer ocean temperatures (>80°F, or >27°C) signify the potential for more hurricanes. Because natural cycles cannot explain recent warming trends in the oceans, some scientists are focusing more on global warming because the CO2 levels in the atmosphere are much higher than they have been in the past 400,000 years.

As reported on MSNBC on June 16, 2005, Kevin Trenberth of the National Center for Atmospheric Research claims that warmer oceans and increased moisture could intensify the showers and thunderstorms that fuel hurricanes. He said: "Trends in human-influenced environmental changes are now evident in hurricane regions. These changes are expected to affect hurricane intensity and rainfall, but the effect on hurricane numbers remains unclear. The key scientific question is how hurricanes are changing."

Chris Landsea of NOAA counters by saying there is evidence for natural swings between high and low hurricane activity that extend for 25 to 40 years. He comments: "The last 10 years have been busy for the U.S.—similar to what we experienced between the 1920s and 1960s."

As reported by National Geographic News on August 4, 2005, a study run by Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology, that appeared in the journal Nature, found that hurricanes and typhoons have become stronger and longer lasting over the past 30 years, and that these upward swings correlate with a rise in sea surface temperatures. Kerry also concluded that the duration and strength of hurricanes have increased by about 50 percent over the last three decades.

According to the Union of Concerned Scientists, a study conducted in 2005 determined that over the past 30 years, hurricanes' destructive power has increased approximately 70 percent in both the Atlantic and Pacific Oceans. Another study in 2005 by P. J. Webster, J. A. Curry, and H. R. Chang of the Georgia Institute of Technology and G. J. Holland from the National Center for Atmospheric Research in Boulder, Colorado, determined that the number of hurricanes classified as category 4 or 5 (based on satellite data) has increased over the same period. These findings correlate with the rise in observed sea surface temperatures in regions where tropical cyclones usually originate.

In 2004, Ruth Gorski Curry of the Woods Hole Oceanographic Institution presented a study on hurricanes and climate change and observed that in both the Eastern and Western Hemispheres, tropical storm activity had been well above normal for several years. In addition, in March 2004, the South Atlantic experienced its first hurricane ever recorded. She cited several factors that have helped fuel the increased number and intensity of tropical storms in recent years:

• the absence of El Niño conditions in the Pacific

• stratospheric winds in the equatorial belt

• a wet season in the African Sahel

• unusually warm water temperatures in the Atlantic, western Pacific, and Indian Oceans

Tropical cyclones, such as this one off the coast of Brazil, are some of the most deadly storms on Earth. (NOAA)

Both climate models and observations support the idea that as global temperatures rise, the oceans are getting warmer. The models also agree that greenhouse warming will enhance the frequency and intensity of hurricanes in the coming century. T. R. Knutson and R. E. Tuleya at NOAA have looked at the potential for future storm trends and determined that if there is a 1 percent annual increase of CO2 concentrations over the next 80 years, it would produce more intense storms and the amount of rainfall would increase 18 percent.

One major problem with rising ocean temperatures is that it will fuel hurricanes. Under ordinary conditions, the surface winds will churn up cooler water from the ocean, which will help slow the storm, but if the subsurface is too warm, it will only fuel the storm.

According to USA Today, experts had this to say about global warming:

• Kerry Emanuel, Massachusetts Institute of Technology: "Storms are lasting longer at high intensity than they were 30 years ago. Hurricane reported durations have increased by about 60 percent since 1949, and average peak storm wind speeds have increased about 50 percent since the 1970s.

• The scientists who support the link between global warming and hurricanes believe that a warming world has caused the oceans to heat up over the past several years, which is causing an increase in the number and intensity of hurricanes.

• The number of category 4 and 5 hurricanes has increased sharply over the past few decades, from about 11 per year in the 1970s to 18 per year since 1990. Rising sea surface temperatures encourage stronger hurricanes because they draw energy from warm ocean waters and release it in huge storms.

• Scientists believe that about half of the ocean's extra warmth in 2005 was due to global warming.

• According to the IPCC in one of its reports, the scientists said that during the 21st century, hurricanes, typhoons, and Indian Ocean and South Pacific cyclones are likely to produce higher winds and heavier rain in some areas, but there is no way to tell whether the frequency and locations of these storms could change.

A 2005 study by Kerry Emanuel published in Nature suggests that storm intensity and duration is linked to global warming and recent ocean warming trends. Using a measurement called the power dissipation index (PDI), which measures how destructive a storm is, Emanuel and his team came to the conclusion that in the last 30 years, the destructive power of storms has doubled in the Atlantic and Pacific Oceans, the bulk of this destruction occurring in the last 10 years.

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