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Hurricane Isaac Caps Off America’s Summer of Extreme Weather

This post was co-authored by Forbes Tompkins, an intern with WRI's Climate and Energy Program.

This post is part of WRI's "Extreme Weather Watch" series, which explores the link between climate change and extreme events. Read our other posts in this series.

Almost seven years ago to the day since Hurricane Katrina made landfall, a new hurricane came ashore on the Gulf Coast near New Orleans. While Hurricane Isaac has been much less intense than Katrina, it has caused serious damage, with heavy rains, storm surge, and winds of up to 100 miles per hour.

Hurricane Isaac comes at the end of a U.S. summer season filled with extreme weather events. From heat waves to droughts to wildfires, the United States has seen little in the way of relief from severe events over the last several months. In fact, the majority of the lower 48 states are still facing drought. While Isaac may relieve drought conditions in some areas of the country, recent forecasts from the Climate Prediction Center project drought conditions to continue through large parts of the country at least through November.

America’s Vulnerability to Hurricanes and Tropical Storms

Hurricanes that make landfall disrupt commerce while damaging critical transportation and energy infrastructure, with economic ripple effects throughout the country. Although damage from high winds can be significant, storm surge is often the leading threat to lives and property along the coasts. For example, storm surge was responsible for many of the 1,500 lives lost as a result of Hurricane Katrina in 2005. The combination of Hurricane Ike’s storm surge, winds, and flooding caused property damages in 2008 estimated at nearly $25 billion. Additionally, large amounts of rainfall typically accompany hurricanes, contributing to coastal inundation and oftentimes flooding communities further inland – as we are seeing in the Gulf Coast as a result of Hurricane Isaac.

Each year, the western U.S. Gulf Coast (including Texas, Louisiana, Mississippi, and Alabama) faces an average of $14 billion in damages, driven by high winds and storm surge from hurricanes. The same region has accumulated more than $2.7 trillion in hurricane-related damages over the last century. The Insurance Information Institute estimated at almost $4 trillion the 2004 aggregate value of insured property vulnerable to hurricanes between the Mid-Atlantic and Northeast.

More than half of the nation’s economic productivity occurs in coastal zones. Within the Gulf Coast region alone, 72 percent of ports, 27 percent of major roads, and 9 percent of rail lines are fewer than five feet above sea level. Adding to its vulnerability, the population within the region’s coastal counties has increased 150 percent over the last 50 years, growing to 14 million people. Similarly, more than 41 million Americans live in eastern coastal counties along the Atlantic coast. In New York City alone, 48 major transit facilities reside at 10 feet or fewer above sea level. To put this in perspective, news outlets have reported storm surge associated with Hurricane Isaac (a Category 1 storm) of at least 10 feet in many locations, while the storm surge from Hurricane Katrina (a strong category 3 storm when it made landfall) exceeded 20 to 30 feet across broad areas of Gulfport and Biloxi, Mississippi.

The Climate Change Connection

It is well-established that global warming has already resulted in global mean sea level rise. The IPCC’s most recent science assessment finds it very likely that that rising sea levels will contribute to upward trends in extreme, coastal high water levels, compounding the storm surge impacts of tropical storms and hurricanes. These findings are supported by the U.S. National Academy of Sciences, which recently concluded that the large-scale impacts of man-made climate change include sea-level rise and vulnerability to storm surge.

However, it is too early to say whether or not rising global air and ocean temperatures can be linked to the frequency or intensity of tropical storms – although there is some evidence for the latter. For example, a 2008 assessment report by the U.S. Global Change Research Program concluded that human activity is likely indirectly linked ("through increasing sea surface temperatures, a critical factor for intense hurricanes") to the observed increase in average Atlantic hurricane strength during the past few decades. The report notes that similar trends have not been observed in the tropical West and Eastern Pacific, indicating that more study is needed to more definitively establish a causal link.

Preparing for the Future: Emissions Reductions and Preparedness

One thing is clear: A warmer world with higher sea levels will leave more of the country vulnerable to storm surge from hurricanes. New planning and policy efforts, taking into consideration the best available scientific information, can better prepare communities for future hurricanes. For example, city officials in Norfolk, VA–a city experiencing the highest annual rate of sea level rise along the U.S. East Coast–recently hired a coastal engineering firm to analyze the city’s vulnerabilityto high tides and storm surge. Appropriate city planning can help Norfolk mitigate flooding impacts, particularly for the city’s naval infrastructure.

In addition to planning and new infrastructure, it is essential to keep limiting emissions to slow the pace of climate change and the resulting sea level rise. Curbing carbon emissions—through existing federal authorities, state actions, and new policies—can help reduce the impacts of climate change, protecting cities and especially coastal communities.

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