DEEP: Climate Change Primer - Sea Level Rise and Coastal Storms

Climate Change Primer - Sea Level Rise and Coastal Storms

Global Sea Level Rise

Sea level has been rising since the end of the last ice age. The Intergovernmental Panel on Climate Change (IPCC) estimates that global mean sea level rose by 17 cm in the 20th Century. They estimate with high confidence (greater than an 8 in 10 chance) that the rate of change was greater than in the 19th century. They also report an observed increase in the rate of global mean sea level rise over the 20th Century. The rate from 1961 to 2003 was 1.8 mm/year, while the rate from 1993 to 2003 was 3.1 mm/year.

Future sea level rise will be influenced by two primary factors: thermal expansion of water in the ocean (as water warms, it expands and takes up more room) and the melting of land-based ice, most of which is contained in the Greenland and Antarctic ice sheets. While that much is known, Rahmstorf (2007) describes the challenges of forecasting sea level rise:

"Understanding global sea-level changes is a difficult physical problem, because complex mechanisms with different time scales play a role, including thermal expansion of water due to the uptake and penetration of heat into the oceans, input of water into the ocean from glaciers and ice sheets, and changed water storage on land. Ice sheets have the largest potential effect, because their complete melting would result in a global sea-level rise of about 70 m. Yet their dynamics are poorly understood, and the key processes that control the response of ice flow to warming climate are not included in current ice sheet models [for example, meltwater lubrication of the ice sheet bed or increased ice stream flow after the removal buttressing ice shelves.] Large uncertainties exist even in the projection of thermal expansion, and estimates of the total volume of ice in mountain glaciers and ice caps that are remote from the continental ice sheets are uncertain by a factor of two."37

The 95% confidence interval of 2007 IPCC estimates of global mean sea level rise is 18-59 cm by 2090-2099, or 1.5 to 9.7 mm/yr.38 Several individual studies, however, have estimated global mean sea level rise by 2100 to be an order of magnitude larger. Overpeck et al. (2007) predict global mean sea level rise by 2100 to be on the order of meters, with high uncertainty of the actual value because of high uncertainty about how ice sheets will respond to warming.39

Although the IPCC considers the collapse of the West Antarctic Ice Sheet or the rapid loss of the Greenland Ice Sheet unlikely, they state "the occurrence of such changes becomes increasingly more likely as the perturbation of the climate system progresses."40 

Sea Level Rise in Connecticut
Although observations and forecasts of global mean sea level rise (also called eustatic sea level rise) are important, local sea level rise (referred to as relative sea level rise as it includes not only changes to water levels but also changes to the landmasses) is the measure that should be used for planning purposes. Gornitz et al. (2004) explains the main reason why relative sea level rise in Connecticut differs from the global mean:

"During the last glaciation, the weight of the ice sheet caused the Earth's crust to warp and Connecticut to be slightly uplifted. Now that the ice sheet has melted, the Earth's crust is evening out, and Connecticut is slowly sinking at approximately 0.03-0.035 inches per year [0.76-0.89 mm/yr]".41

Rates of relative sea level rise are available from NOAA tide gauges in coastal Connecticut at New London and Bridgeport.

  • Over the period 1938-2006 (69 years), monthly tide gauge analyses at New London, CT show a mean sea level rise trend of 2.25 mm/yr +/-0.25 mm at the 95% confidence interval (alternatively, 0.09 inches/yr, +/- 0.01 inches.)
  • Over the period 1964-2006 (43 years), monthly tide gauge analyses at Bridgeport, CT show a mean sea level rise trend of 2.56 mm/year +/-0.58 mm at the 95% confidence interval (alternatively, 0.10 inches/year, +/-0.02 inches.)42

These numbers exceed the eustatic sea level rise rate of 0.5 – 1.5 mm/yr (0.02 - 0.06 in/yr) due to Connecticut's rate of subsidence.43

Regardless of the magnitude, coastal hazards in Connecticut will likely increase as sea level rises. Even the conservative estimates will directly lead to the inundation of low-lying coastal lands, damaging natural flood protection such as dunes, beaches and wetlands, and further contributing to flood hazards.

Coastal Storms
Coastal storms have historically been a threat to Connecticut’s coastline. Looking through the lens of climate change, these events can be expected to take a more significant role in coastal hazards. For example:

  • The IPCC, shows that we can expect an increase in the intensity, but not necessarily the number, of tropical storms in the North Atlantic.44
  • The Union of Concerned Scientists’ 2007 synthesis report of Northeast Climate Impacts Assessment (NECIA), suggests that we may see an increase in the number of winter storms received in coastal Connecticut, especially late in the winter.45

Researchers from NECIA have investigated the average return time of coastal flooding (using the 100-year flood as a benchmark) at 5 cities (Atlantic City NJ, New York City NY, New London CT, Boston MA, and Woods Hole MA) and coupled that with sea level rise scenarios to estimate future flooding.  The results indicate:

  • By 2050:
  • Substantial increases in the maximum elevations of major coastal floods can be expected.
  • Substantial increases in the frequency of today’s 100-year flood events. In the case of New London, CT, one might expect the effects of a 100 year storm by today’s standards (2010) to recur every 56 to 61 years.
  • By 2100:
    • Most locations are projected to experience increases over the current 100-year-flood elevation of roughly 1.5 feet under the higher-emissions scenario and roughly 1.0 feet under the lower emissions scenario.
    • Today’s 100-year floods will recur even more often. In the case of New London, CT one might expect the effects of a 100 year storm by today’s standards (2010) to recur every 17 to 32 years based on the higher or lower emissions scenarios, respectively.

In 2010, the Adaptation Subcommittee Working Groups from the Governor’s Steering Committee on Climate Change produced a Final Report outlining the anticipated impacts of climate change on Infrastructure, Natural Resources, Agriculture, and Public Health. Focusing on those elements relevant to Connecticut’s coastal environment yields:

  • Most of the agricultural features assessed would be highly negatively impacted. Shellfish production was included among top five most imperiled agricultural planning areas or features in Connecticut.
  • The infrastructure items most likely impacted are coastal flood control and protection, dams and levees, stormwater, transportation, and facilities and buildings. These were most affected by changes in precipitation and sea level rise.
  • The Natural Resources at the highest risk may be Cold Water Streams, Tidal Marsh, Open Water Marine, Beaches and Dunes, Freshwater Wetlands, Offshore Islands, Major Rivers, and Forested Swamps.  The degree of impact will vary, but likely changes include conversion of rare habitat types, loss and/or replacement of critical species dependent on select habitats, and the increased susceptibility of habitats to other on-going threats.
  • The impacts to Public Health, while not specifically focused on any one particular coastal aspect, include public health infrastructure, environmental justice communities, air quality and extreme heat ailments and vector-borne diseases.
Content Last Updated January 3, 2012