This blog post was written by Hunter Cutts as part of the Spring 2018 UGA Urban Ecology class.
Sea level rise has been documented since the early 20th century and continues to be one of the most visible and talked about results of global warming. Hundreds of millions of people in the United States live in coastal areas that are vulnerable to flooding at the current sea level, and will become even more vulnerable as sea levels continue to rise. Extensive studies have been done in order to attempt to predict and manage sea level rise in the next hundred years, but has a conclusion been reached regarding the seriousness of the threat? Is there scientific consensus about the speed and magnitude of sea level rise, and what is the point at which sea level rise makes major coastal cities uninhabitable? What has been done and what can be done in order to respond to and mitigate the effects of sea level rise in the next century?
Assessments of sea level data taken throughout the 20th century conclude that the mean sea level rise between 1900 and 1990 was around 1.1mm per year, and from 1993 to 2012 the average rise was closer to 3mm per year (Dangendorf et al.). This rise is attributed to the combination of melting glaciers and the thermal expansion of warming seawater. The increase in the rate of sea level rise is correlated with the increased ice melt from the ice caps and other glaciers in recent years. The melted runoff from the Greenland ice sheet can account for more than 0.7mm of sea level rise per year, nearly ⅓ of the total (Mordret et al.). Antarctic melting accounts for a large fraction of the sea level rise as well. These melts are increasing in volume as global temperatures rise, which accounts for both the current rate as well as the increased predicted rate of sea level rise. There is no doubt in the scientific community that sea level rise is a real trend, and that the rate at which it is happening has increased within the last 3 decades. There is controversy outside of the scientific community over the effects that climate change may have on future sea level rise. These projections focus mainly on the rate of CO2 and greenhouse gas release over time causing a steady increase in global average temperature. The NASA models range from a rise of 0.6 meters to 2 meters between now and 2100. The wide range of predicted values has caused some skepticism outside of the scientific community and this skepticism has led to inaction. Prevention and preparation efforts have failed to get off of the ground in some areas that could be majorly affected by future sea level rise.
Warming sea temperatures also affect the number and power of tropical cyclones. Grinsted et al. surge index shows a trend between increased temperature in sea water and the most extreme cyclone events in the past 10 years. As these cyclone events become more frequent and more powerful, the potential for flooding coastal areas increases. The powerful wind, heavy sustained rainfall, and the storm surge accompanying a cyclone put enormous pressure on coastal city infrastructure. Buildings can become structurally unsound and may contain harmful molds and other pathogens after repeated flood events. Combined sewer systems that are overloaded during floods spill raw sewage into the floodplain which cause a list of negative health effects throughout the floodplain. As coastal populations grow, the potential for damage increases. Aside from hurricane Katrina, which struck in 2005, the two most costly hurricanes to hit the U.S., Maria and Harvey, occurred in 2017. These storms were devastating in their power, but as the economies of the coastal areas booms, so does the potential cost of storm damage. This coupled with the rising sea level and warming water temperature puts an enormous amount of pressure on these areas to adapt and respond to the threat of sea level rise.
The effect of sea level rise is often imagined as the ocean reaching into cities and rendering them permanently underwater, but before that becomes a reality, cities will have to deal with the gradual onslaught of tidal flooding. Unlike flooding due to extreme weather events, tidal flooding occurs at regular intervals as the moon cycles through its phases. Communities are categorized as “effectively / chronically inundated” by flooding when the experience floods 26 times a year or more in at least 10% of the community area (Dahl et al.). Despite this high frequency of inundation, these communities are not necessarily uninhabitable.
Image Credit: Dahl et al 2017
There are communities in coastal Maryland that already fall under the category of effectively inundated. Much of the land that is projected to be effectively inundated is rural land that is not directly used by human populations. Land that is both vulnerable to sea level rise and important to human populations should be made a priority when designing flood prevention and mitigation strategies. Some coastal cities that are exceptionally vulnerable to sea level rise, such as Annapolis, have already begun implementing zoning and building ordinances for structures within the floodplain. These plans are typically aimed at not just the current floodplain, but the projected 50 or 100 year floodplain accounting for future sea level rise. Any new structure within Annapolis must have its first floor set at least 8 feet above mean sea level. (Annapolis Climate Plan 2008). Other proposed measures for flood mitigation include backflow preventing stoppers on storm drains and raising the elevation of important roadways within the floodplain. These protection and mitigation efforts can yield positive results, but not all communities have the power and wealth available in order to enact the appropriate measures. Over half of the projected effectively inundated communities by the year 2035 are socioeconomically vulnerable, poverty stricken and largely minority communities (Dahl et al.).
Image Credit: Dahl et al 2017
Many of these communities are on the Gulf Coast in Louisiana and Texas, areas that are especially at risk for chronic flooding due to the large floodplains and frequent extreme weather events. The monetary cost of prevention and protection can often far exceed what is available to a community.
The National Oceanic and Atmospheric Administration NOAA has a sea level rise viewer program on their website NOAA SEA LEVEL RISE VIEWER. The viewer has a moveable slider that changes the sea level from the current level up to 6 feet above current day. This tool can be used to visualize and understand where the threat of sea level rise is highest, and whether certain communities are subject to future flooding events.
References:
Dahl, K. A., Spanger-Siegfried, E., Caldas, A., & Udvardy, S. (2017). Effective inundation of continental United States communities with 21st century sea level rise. Elem Sci Anth, 5.
Grinsted A, Moore JC, Jevrejeva S (2012) Homogeneous record of Atlantic hurricane surge threat since 1923. Proc Natl Acad Sci USA
Idris, S., & Dharmasiri, L. M. (2015). Flood risk inevitability and flood risk management in urban areas: A review. Journal of Geography and Regional Planning, 8(8), 205-209.
Mordret, A., Mikesell, T. D., Harig, C., Lipovsky, B. P., & Prieto, G. A. (2016). Monitoring southwest Greenland’s ice sheet melt with ambient seismic noise. Science advances, 2(5), e1501538.
Annapolis, Maryland Climate Action Plan (2008). https://www.annapolis.gov/DocumentCenter/Home/View/406