While either accelerated sea level rise or an increase in the intensity of tropical storms could significantly increase the potential for coastal disasters in the Gulf Coast region, it is likely that both will occur. Even if hurricanes do not increase in frequency or intensity over the next century, flooding is likely to increase along much of the low-lying Gulf of Mexico shoreline due to ongoing sea level rise and land surface subsidence, which is presently greater than the current rate of sea level rise in some regions. The projected acceleration in the rate of sea level rise will increase the current rate of loss of land and the widespread, episodic flooding that is already challenging the sustainability of some ecosystems and human settlements in the region. As climate change intensifies, the natural flood defenses (for example, barrier islands and forests) of the region will decline more rapidly,

Figure 8.7 Sea surface temperature trend in the Gulf of Mexico region derived using the ERSST v.2 database

Note: The plot displays the sea surface temperature anomalies averaged annually, as well as the anomalies determined from the averages for August only and for the July-September peak of the hurricane season. Source: Smith and Reynolds, 2004.

Figure 8.8 Land surface elevation and roads in the central Gulf Coast region (see Plate 11 for color version)

Note: Areas in red are all below 6 m in elevation.

Source: Elevation data from US Geological Survey, National Wetlands Research Center. Road data from US Department of Transportation, Bureau of Transportation Statistics.

thereby serving as a positive (amplifying) feedback to the effects of climate change on coastal systems. That is, the deterioration of natural coastal environments caused by sea level rise and storm surge increases the potential for more intense flooding and land loss due to sea level rise and storm surge in the future.

The likelihood that tropical storms will increase in intensity and the unavoidability of future sea level rise conflicts with present-day development patterns on the Gulf Coast. Historical damage due to flooding is high in the Gulf Coast region compared to many other US coastal areas because of the elevation relative to sea level, intensity and proximity of development to the shoreline, deterioration of natural coastal defenses, and the lack of infrastructure design features that would accommodate a changing environment. In addition to the direct impacts of human development on coastal ecosystems, the buildings, roads, seawalls, flood control levees and revetments that have been constructed along the Gulf Coast will prevent the natural inland migration of coastal landforms in response to sea level rise. Another consequence of intensive development in this low-lying coastal zone is the exposure of human communities and infrastructure to storm surge and increasing mean water levels. Exposure to flooding is virtually certain to increase in most Gulf Coast counties if sea level rise accelerates and tropical storms intensify. Damages will likely be most severe in places that are lowest in elevation, have insufficient natural or engineered features to buffer against storm surge, and are already deteriorating due to factors other than climate change.

Collectively, the effects of human development, accelerated sea level rise and increased tropical storm activity in the Gulf Coast region have important implications for coastal landforms, the natural habitats that they support, and the human communities that depend upon them. Other aspects of climate change will affect the ecological structure and sustainability of coastal habitats globally, such as the effects of increasing temperature on coastal plant communities and the effects of changing rainfall patterns on runoff. In the Gulf Coast region, however, increasing sea level rise and tropical storm activity appear to be the most important climate-related factors affecting the sustainability of ecosystem services and human settlements.

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