Flooding paralyzes lower Manhattan during Superstorm Sandy (Photo courtesy of Flickr)

On August 29, 2005, Hurricane Katrina slammed into the Gulf Coast, unleashing winds of 125 mph, record-breaking storm surge and heavy, persistent rainfall. The storm was particularly devastating for New Orleans, where conditions overwhelmed the city’s inadequate flood-protection system, inundated entire neighborhoods and displaced up to 400,000 residents. Seven years later, Superstorm Sandy roared up the East Coast, bringing extreme storm surge and eventually becoming the second costliest natural disaster in U.S. history behind Hurricane Katrina. As damaging as Sandy was, however, the storm could have been many times more destructive than Katrina. The reason? While Katrina brought with it great storm surge and heavy precipitation, Sandy actually produced very little rainfall. The significance? New research now suggests that major U.S. coastal cities are increasingly at risk of storms that produce both strong storm surge and rainfall. Consequently, over 123 million people, or nearly 40 percent of the U.S. population who live along the coast are now much more likely to experience destructive storms more like Katrina than Sandy.

In a new study published in Nature Climate Change, researchers have found that the odds of an event similar to Hurricane Katrina have since doubled for cities like New Orleans and New York in the past 30 years. Based on historical data, the study finds that storm surge and heavy precipitation, two mechanisms by which flooding in coastal zones can occur, are not only increasing, but are also occurring together as a result of climate variability and change. According to the study, the frequency of this phenomena, known as compound flooding events, has been rising over the past three to four decades in six major U.S. coastal cities (New York City, San Francisco, San Diego, Boston, Los Angeles and St. Petersburg).

Hurricane Katrina was a drastic example of a compound-flooding event that caused inadequate infrastructure such as levees and floodwalls to breach and crumble due to the dangerous combination of heavy storm surge and significant rainfall. Meanwhile, Superstorm Sandy, which had significant storm surge as well, hardly produced any rainfall and was therefore not a compound flooding event. If Superstorm Sandy had been one, the loss of life and damage could therefore have been significantly worse for millions who reside on the East Coast. By the same token, however, because compound flooding events are indeed occurring twice as often in the last 30 years for coastal cities, then NYC and other coastal cities should expect and prepare for Katrina-like events.

Despite this meteorological difference, both Katrina and Sandy also demonstrated that the impacts of severe flooding, whether caused a by compound event or not, are disproportionately borne by low-income and minority communities. Following Katrina, one-fifth of those displaced by the storm were likely to have been poor and African Americans alone were estimated to have accounted for 44 percent of the victims. Following Sandy, African Americans and Latinos were found to have disproportionately resided in census tracts within three miles of the storm surge. Storm surge is especially damaging since it also causes overflow of industrial waste sites and contamination of water supplies-many of which belong to low-income and minority groups. When combined with a heavy precipitation event, where rainfall overflows sewers and storm drainage systems, the impacts of storm surge on low-income and minority communities can be exacerbated.

In the aftermath of both storms, these two cities have responded by improving resiliency including efforts building community preparedness against future storms and promoting long-term sustainability. After 10 years of rebuilding since Hurricane Katrina, the city of New Orleans has bounced back, regaining up to 90 percent of its population since the storm hit and building its resilience against the impacts of future storms. Meanwhile, NYC is undertaking rigorous efforts to build a stronger, more resilient community to prepare for future risk. Yet, as this study illustrates, it is becoming increasingly likely that NYC will soon have a Katrina-type event in its future. Therefore, NYC and other major cities must quickly learn from the lessons of Katrina, recognize the disproportionate impacts that people of color face from extreme weather events and continue to strengthen resilience to meet these increasing risks. With a more accurate gauge of the risk, cities should therefore reflect these growing understandings of climate change and its subsequent impacts.