Sea level rise (SLR) represents one of the most profound environmental and economic challenges facing coastal regions globally. For a densely populated, low-lying coastal area like Long Island, escalating water levels from the Atlantic Ocean and Long Island Sound are a serious concern. This phenomenon is a scientific reality already altering coastlines and infrastructure. Understanding when low-lying areas may become permanently inundated requires examining the global causes and the unique local factors that make the region susceptible to rising tides.
Drivers of Sea Level Rise
The fundamental mechanism driving the increase in global mean sea level is planetary warming. This global rise is attributed to two distinct processes occurring simultaneously. The first is the thermal expansion of seawater, known as the steric effect, which occurs as ocean water absorbs excess heat. Since warmer water occupies a greater volume, even a small temperature change leads to a noticeable rise in sea level across the vast ocean.
The second, and increasingly dominant, factor is the influx of water from melting land-based ice masses. This includes mountain glaciers and the massive ice sheets covering Greenland and Antarctica. When this ice melts, the water flows directly into the ocean basin, adding mass and causing the sea level to rise. The contribution from melting ice has accelerated in recent decades, now accounting for about 60 percent of the total global sea level increase.
Geographic Factors Making Long Island Vulnerable
Long Island’s unique geology and physical setting amplify the threat posed by global sea level rise. The island is characterized by low elevation, particularly along the south shore, where large sections of land sit just above current high-tide lines. For example, communities like Seaford are located only about 10 feet above sea level. This low-lying topography means a small vertical increase in water level translates to a dramatic horizontal loss of land.
The island also contends with land subsidence, which is the gradual sinking of the land mass itself. This sinking is caused by the compaction of sediments and the over-extraction of groundwater from underlying aquifers. Long Island is part of a coastal “hotspot” where the combination of rising water and sinking land has resulted in a sea level increase rate three to four times the global average in the past. This localized sinking compounds the problem, making the relative sea level rise faster than the global average.
Coastal erosion further exacerbates the vulnerability, particularly for the island’s fragile barrier island system, including areas like Fire Island and Jones Beach. Rising sea levels increase the energy of coastal storms and waves, leading to the rapid retreat of sandy shorelines. As the sea rises, these natural buffers are unable to migrate fast enough, threatening the primary line of defense for the mainland.
Scientific Projections and Timelines
The timeline for chronic inundation depends heavily on future global emissions, which dictate the rate of sea level rise. Near-term projections are more certain, with the U.S. coastline generally expected to see an additional 10 to 12 inches of sea level rise by 2050. Specific to Long Island, state assessments predict a rise of 15 to 21 inches by the 2050s, compared to a 1995–2014 baseline. This intermediate rise is sufficient to shift the threat from occasional storm damage to chronic tidal flooding.
By the 2040s, areas along the Long Island Sound that currently experience about four high-tide flooding events per year could see that number increase dramatically to between 50 and 90 annual events. This “sunny-day flooding” will become the new normal, causing frequent disruption long before permanent submergence occurs. The long-term future, by the end of the century, presents a much wider range of possibilities based on climate action.
Under scenarios with moderate emissions, Long Island’s sea level is projected to rise by 32 to 54 inches (2.7 to 4.5 feet) by 2100. If global emissions remain high and major ice sheets destabilize rapidly, worst-case projections suggest a rise of up to 114 inches, or 9.5 feet, by the end of the century. These upper-end forecasts represent the difference between chronic flooding and the permanent, widespread inundation of low-lying coastal communities.
Near-Term Impacts on Infrastructure and Ecosystems
Even before permanent inundation, rising sea levels create immediate problems for Long Island’s infrastructure and natural environment. A primary concern is saltwater intrusion into the island’s freshwater aquifers, which are the sole source of drinking water for the region. As the sea level rises, the heavier saltwater is pushed further inland and upward, contaminating the underground freshwater supply. This threat to drinking water security is compounded by the fact that the land is also sinking in many areas.
The increasing frequency of high-tide flooding severely stresses existing infrastructure. Coastal roadways, rail lines, and the electrical grid are increasingly exposed to saltwater damage and disruption. Wastewater systems are vulnerable, with septic systems and cesspools in low-lying areas already experiencing back-ups and contamination during high-tide events. This results in public health hazards and requires costly upgrades or relocation of facilities.
Long Island’s natural coastal ecosystems, which provide services, are also facing rapid degradation. Salt marshes and coastal wetlands act as natural buffers, absorbing storm surge and filtering pollution. As water levels rise, these ecosystems are squeezed between the rising sea and developed land, preventing them from migrating inland and leading to their loss. The erosion of the island’s sandy beaches, such as those at Jones Beach, threatens the local tourism economy that relies on these assets.