The High Water Line marks the 10-feet above sea level line around Brooklyn and Lower Manhattan. Some of the science behind this line:
Impacts of sea level rise in the New York City metropolitan area (2001) Vivien Gornitz, Stephen Couch, Ellen K. Hartig
The current 100-yr flood height in New York City and environs is 9.7 feet - very close to the area outlined by the 10-ft contour.
...the likelihood of a 100-yr flood could become as frequent as once in every 43 years by the 2020s, once in every 19 years by the 2050s, and once in 4 years by the 2080s, on average, in the most extreme case.
Because of the highly developed nature of the coast within the Metro East Coast region, a large population and considerable private property and infrastructure will be potentially at risk to inundation and flooding.
...the effects of a hurricane for a worst-case scenario Category 3 hurricane. Maximum surge levels could reach 25 feet...at JFK airport; 21 feet at the Lincoln tunnel entrance; 24 feet at the Battery; 23 feet at Liberty Island, NJ; 18 feet at West 96th Street; flooding the West Side Highway...These figures do not include the additional heights of waves on top of the surge.
Modest rises in sea level of 4.3 to 7.6 inches could occur by the 2020's at current rates.
By the 2050's, sea level could rise by 6.9 to 12.1 inches under current trends...
from the ICPP Working Group Group I Fourth Assesment Report Summary for Policy Makers, 2007
Observations since 1961 show that the average temperature of the global ocean has increased to depths of at least 3000 meters and that the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise.
There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater.
It is very likely (greater than 90% chance) that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent [emphasis by IPCC.]