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:
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
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