5.6 Are Atlantic Hurricanes a Growing Threat?

Assess the current and potential vulnerability of the United States to major hurricanes.

The vulnerability of the United States to hurricanes was made clear in 2005, when Hurricane Katrina struck and flooded New Orleans. Because of natural settling and compaction of the sediments on which New Orleans was built, much of the city is slowly sinking and is now below sea level. Three hundred and fifty miles of seawalls and river levees are all that keep the ocean and the Mississippi River out of the city.

The 6 m (20 ft) storm surge generated by Hurricane Katrina caused many of the levees protecting New Orleans to be overtopped by water or collapse as water flowed under them. Within hours, 80% of New Orleans was under water, in some places 25 feet deep. The cost was more than 1,800 human lives and some $128 billion in structural damage. More recently, in 2012, Superstorm Sandy, in which a hurricane combined with a nor’easter, inflicted significant damage in the New Jersey and New York coastal areas (Figure 5.28).

Figure 5.28

Superstorm Sandy, 2012. This satellite image of the East Coast of the United States captures the storm’s extent on the afternoon of October 28, 2012. The storm took over 286 lives, and its financial toll was some $68 billion. Because Hurricane Sandy combined with a nor’easter, the result was a particularly powerful storm.
(NASA Earth Observatory image by Robert Simmon with data courtesy of the NASA/NOAA GOES Project Science team)

Hurricane Activity

About 300 hurricanes have struck the United States since record keeping began, and more than 100 have struck since 1950. Almost all coastal regions in the Gulf of Mexico, in the southeastern United States, and up the eastern coast of North America have been struck by hurricanes (Figure 5.29).

Figure 5.29

U.S. hurricane activity. Each colored dot gives the hurricane return period—the number of years, on average, between major hurricanes (category 3 or higher)—for its stretch of coast since records have been kept. The dots are spaced about 100 km (60 mi) apart. The lower the return period, the more vulnerable a stretch of coastline is. On the southern tip of Florida, for example, category 3 hurricanes have struck about every 19 years.

Except for New Orleans, Miami is the largest metropolitan region in the United States most at risk of flooding and wind damage by a major hurricane. The last time Miami was struck by a hurricane was in 1950, when Hurricane King made a direct hit on downtown. In all, King caused 3 deaths and about 50 injuries. Although damage was considerable, there was little loss of life because fewer than half a million people lived in the Miami metropolitan region at the time. Miami has not suffered a direct strike by a hurricane since, although Hurricane Andrew struck Homestead, a southern suburb, in 1992.

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Since 1950, however, the population of South Florida has grown significantly, and it is projected to be one of the fastest-growing regions in the country for the next several decades. Development now covers much of the coastal area. Some 5.5 million people live in Miami and the surrounding area. Scientists estimate that if a category 4 hurricane were to strike Miami today, it could cause some $90 billion in damage and bring significant loss of human life.

Climate Change and Hurricanes

Atmospheric warming has caused a sea-level rise of about 20 cm (8 in) in the last century. This fact alone makes all coastal cities more vulnerable to the storm surges of hurricanes. But are hurricanes growing stronger because of global warming? Are there more hurricanes because of global warming? Because the air and oceans are warming, hurricanes might be expected to become more frequent and stronger. So far, however, this has not been the case.

There is a natural cyclical pattern of hurricane activity. The 1950s and 1960s was an active period for hurricanes. In the 1970s and 1980s, hurricane activity decreased. Then, in the 1990s, it began to increase again, as shown in Figure 5.30.

Figure 5.30

Atlantic hurricane activity, 1851–2013. This chronology shows all detected hurricanes (872 in total, 308 category 3 and above) that formed in the Atlantic Ocean from 1851 to 2013. Before satellite monitoring, which began in 1966, the record is less reliable. The black line shows long-term trends. Data are from the National Atmospheric and Oceanic Administration (NOAA) Atlantic Hurricane Database (http://www.aoml.noaa.gov/hrd/tcfaq/E11.html).

Taken as a whole, there has been little to no change in the average number or strength of Atlantic hurricanes during the past century. The atmosphere’s temperature began increasing about 100 years ago, but long-term hurricane activity does not reflect this trend. Nor do other ocean basins in which hurricanes form show any significant trend in increasing frequency or intensity of hurricanes.

Earth’s climate is exceedingly complex. There are many factors besides the temperature of the oceans and atmosphere that influence hurricane activity. El Niño, for example, decreases the number of Atlantic hurricanes by causing increased wind shear, which tears hurricanes apart. La Niña, on the other hand, favors Atlantic hurricane development. Similarly, a 2011 study published in the journal Science finds that that wind and wave heights over the oceans have increased by up to 7% in the last two decades worldwide. Windiness and associated wind shear may suppress hurricane formation. Because of these and other complicating factors, scientists are uncertain how climate change will influence hurricanes in the coming decades.

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