Tropical storms

The most extensive flooding, shoreline erosion and wetland loss in the Gulf Coast region occurs during hurricanes and lesser tropical storms. An increase in the frequency or intensity of tropical storms entering the Gulf of Mexico could have serious consequences for human settlements and natural ecosystems along this low-lying coastal margin. During Hurricanes Katrina and Rita in 2005, for example, about 300 km2 of land in south Louisiana were converted to open water, according to preliminary surveys (Barras, 2006) (see Figure 8.5, Plate 10). Over 1800 people lost their lives during Hurricane Katrina and the economic losses totaled more than US$100 billion (Graumann et al, 2006). While a single storm cannot be attributed to climate change, the impacts of Hurricanes Katrina and Rita in 2005 illustrate the types of impacts that it is projected would occur more frequently if the Gulf Coast were to experience more category 4 and 5 hurricanes in the future.

An increase in sea surface temperature is considered an important indicator of climate change, and sea surface temperature plays a dominant role in determining the intensity and frequency of hurricanes (Santer et al, 2006). While factors such as wind shear, moisture availability and atmospheric stability also influence tropical cyclone genesis and evolution, increasing sea surface temperature has been correlated with hurricane intensity in the Atlantic tropical cyclogenesis regions where Gulf Coast hurricanes are formed (Emanuel, 2005).

The precise contribution of increasing sea surface temperature to tropical cyclone formation during recent decades is the subject of several recent scientific studies. Some analyses indicate that the increasing intensity of hurricanes since 1970 is driven by natural multidecadal variability (Pielke, 2005, Landsea et al, 2006), but most recent studies support the hypothesis that hurricanes are increasing in the Atlantic cyclogenesis region as a result of sea surface temperature increases related to anthropogenic warming (Emanuel, 2005; Webster et al, 2005; Hoyos et al, 2006; Mann and Emanuel, 2006; Trenberth and Shea, 2006). Some

Figure 8.5 The Mississippi delta, including the Chandeleur Islands (see Plate 10 for color version)

Note: Areas in red were converted to open water during Hurricanes Katrina and Rita. Yellow lines in index map show the track of Hurricane Katrina on right and Hurricane Rita on left. Source: US Geological Survey, Lafayette, Louisiana, modified from Barras, 2006.

studies conclude that the increase in recent decades is due to the combination of natural, cyclic fluctuations and human-induced increases in sea surface temperature (for example, Elsner, 2006). Sea surface temperature has increased significantly in the main hurricane development region of the North Atlantic during the past century (Bell et al, 2007) (see Figure 8.6) as well as in the Gulf of Mexico (Smith and Reynolds, 2004) (see Figure 8.7). Based on modeling, theory and published empirical studies, the IPCC (2007) concludes that the observed increase in intense tropical cyclone activity in the North Atlantic since about 1970 correlates with a concomitant increase in tropical sea surface temperature. The IPCC further projects that tropical cyclone activity is likely to increase during the 21st century.

The greatest damages from hurricanes are caused by the large mass of water pushed ashore by the storms high winds, known as the storm surge. If a category 3, 4 or 5 hurricane makes landfall along the shallow Gulf of Mexico coastal margin when the tide is high and barometric pressure is low, the effects can be particularly severe. An increase in sea surface temperature is likely to increase the probability of higher sustained winds and surge levels per tropical storm circulation (Emanuel, 1987; Holland, 1997; Knutson et al, 1998). Analyses of sea surface temperature, air temperature, wind speed and dew point data from NASA satellites and NOAA buoys in the Gulf of Mexico indicate that Hurricane Katrina reached

1880 1900 1920 1940 1960 1980 2000

Year

Figure 8.6 Sea surface temperature trend in the main hurricane development region of the North Atlantic during the past century

Note: Dark gray line shows the corresponding five-year running mean. Anomalies are departures from the 1971-2000 period monthly means. Source: Bell et al, 2007.

peak intensity when the difference between sea surface temperatures and air temperatures was greatest (Kafatos et al, 2006). Surge during Hurricane Katrina peaked at approximately 8.5 m in coastal Mississippi (Graumann et al, 2006). An increase in storm surge associated with hurricanes that make landfall in the Gulf Coast region could affect the sustainability of natural systems and human developments in Gulf Coast counties because the land surface in most areas is less than 6 m in elevation (see Figure 8.8, Plate 11).

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