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Coasts

Sea level has been rising, on average, between 1 to 2.5 mm/yr for the last 100-150 years, with 1.8 mm/yr a "best estimate". This historic rise in sea level is most probably linked to the observed global increase in temperature over the same period. IPCC The recent sea-level rise comes from the thermal expansion of the upper ocean layers and melting of mountain glaciers. The contributions from the Greenland and Antarctic ice sheets are not as well-established and may be close to zero at present.

Within the next 100 years, global warming due to greenhouse gas emissions may cause worldwide rates of sea level rise to increase 2 to 5 times over present rates within the next 100 years. Low-lying areas and coastal wetlands would be inundated, beach erosion would intensify, and storm flooding would become more frequent. The effects of regional sea-level rise, and associated coastal hazards, including storm floods and beach erosion, have been examined as part of the MetroEast Coast region climate change assessment.

At present, sea level is rising in New York City at a rate of 2.73 mm/yr, based on tide-gauge measurements (Figure 1). Regionally, this rate now varies between 2.20 and 3.85 mm/yr (Figure 2). Future projections of sea-level rise are based on a suite of global climate model scenarios, adjusted for local land subsidence. The climate models include the Canadian Climate Centre for Modelling and Analysis and the Hadley Centre model (U.K.), with greenhouse gas forcing alone, and with greenhouse gases plus sulfate aerosols. The Hadley Centre projection lies close to the IPCC IS92a "best estimate" projection. An extrapolation of current trends is also shown for comparison. This is the minimal sea-level rise expected, if no additional global warming occurs.

The main threat to coastal areas will come from increased storm flooding, superimposed on the more gradual sea-level rise. Coastal storms striking the New York metropolitan area are of two types:

1. extratropical cyclones ("Nor-easters") and
2. hurricanes. Nor'easters are responsible for major coastal flooding and beach erosion. Although their winds are not as strong as those of hurricanes, they cause considerable damage because they cover a wider area and often last over several tidal cycles at a particular location.

Historic sea-level rise from tide-gauge records, the Battery, New York City (mm per year)

STATION	SEA LEVEL RISE
Willets Pt	2.30
Port Jefferson	2.20
Montauk	2.27
New York City	2.73
Sandy Hook	3.85

Severe coastal flooding and erosion events, such as those following the December 1992 nor'easter, would become more frequent, even if the number and intensity of storms did not change. At the Battery, lower Manhattan, for example, if sea levels increased at current rates into the 2090s, a storm surge of 8.6 feet would produce the same degree of flooding as the 100-year flood today but would recur once every 33 years, on average. It would occur almost once in 2 years, in the worst-case scenario. Even with the relatively modest increases in sea level projected above, a considerable portion of lower Manhattan could be under water every few years. 

Hurricanes inflict even more damage due to extremely high wind velocities (minimum wind-speed of 119 km/hr (74 mi/hr), flooding due to the high storm surge and waves, and heavy rainfall. The storm surge is caused by low barometric pressure and wind shear. The height of the surge is is amplified if it coincides with high tide. At least 9 hurricanes have struck the Metro-East Coast area within the last 200 years, including Donna (1960) and Gloria (1985).

KEY RESULTS FOR THE COASTS

• Sea level is already rising 0.09-0.15 in/yr in the MEC region, due in part to ongoing geologic subsidence following the end of the last glacial period over 6,000 years ago and in part due to the warming trend of the 20th century (0.05"0.03 in/yr). 
• Under projected climate change, sea level in the MEC region may rise by 4.3-11.7 in within the next 20 years, 6.9 to 23.7 in by the 2050s, and 9.5 to 42.5 in by the 2080s.
• Sea level rise would lead to more elevated storm floods and a marked reduction in the flood return period. The 100-year flood would recur, on average, every 80 to 43 years by the 2020s, 68-19 years by the 2050s, and 60 to as often as 4 years by the 2080s.
• Rates of beach erosion would double or triple at sites within the region, increasing 3 to 6 times by the 2050s, and 4 to 10 times by the 2080s, relative to the 2000s.
• Additional sand would have to be placed on the beaches to compensate for these losses. Beach nourishment would remain a viable option through mid-century, but could become significantly more costly, particularly for the high end warming scenarios, by the end of the century.