Higher Seas Endanger Coastal Communities

Higher seas endanger coastal communities—where 40 percent of the world's population lives—and threaten groundwater supplies.

New York skyline and harbor  See how sea-level rise from global warming puts New York City at risk—and find other hot spots threatened by rising seas on the Climate Hot Map.

Take action on global warming now! Two major mechanisms are causing sea level to rise. First, shrinking land ice, such as mountain glaciers and polar ice sheets, is releasing water into the oceans. Second, as ocean temperatures rise, the warmer water expands. Trapped within a basin bounded by the continents, the water has nowhere to go but up. In some parts of the world, especially low-lying river deltas, local land is sinking (known as subsidence)—making sea levels that much higher. The consequences of sea level rise include:

Threats to coastal communities. Some 40 percent of the world's population lives within 62 miles (100 kilometers) of the ocean, putting millions of lives and billions of dollars' worth of property and infrastructure at risk.

High tides and storm surges riding on ever-higher seas are more dangerous to people and coastal infrastructure.

Natural protections against damaging storm surges are increasingly threatened. Barrier islands, beaches, sand dunes, salt marshes, mangrove stands, and mud and sand flats retreat inland as sea level rises, unless there are obstructions along the retreat path. If they cannot move, these natural protections are washed over or drowned.

Many shorelines have sea walls, jetties, and other artificial defenses to protect roads, buildings, and other vital coastal resources. In these areas, sea-level rise increases erosion of stranded beaches, wetlands, and engineered structures

Saltwater intrusion. Sea-level rise can mean that saltwater intrudes into groundwater drinking supplies, contaminates irrigation supplies, or overruns agricultural fields. Low-lying, gently sloping coastal areas are particularly vulnerable to contamination of freshwater supplies.

Scientific research indicates sea levels worldwide have been rising at a rate of 0.14 inches (3.5 millimeters) per year since the early 1990s. The trend, linked to global warming, puts thousands of coastal cities, like Venice, Italy, (seen here during a historic flood in 2008), and even whole islands at risk of being claimed by the ocean.

Core samples, tide gauge readings, and, most recently, satellite measurements tell us that over the past century, the Global Mean Sea Level (GMSL) has risen by 4 to 8 inches (10 to 20 centimeters). However, the annual rate of rise over the past 20 years has been 0.13 inches (3.2 millimeters) a year, roughly twice the average speed of the preceding 80 years.

Over the past century, the burning of fossil fuels and other human and natural activities has released enormous amounts of heat-trapping gases into the atmosphere. These emissions have caused the Earth's surface temperature to rise, and the oceans absorb about 80 percent of this additional heat.

The rise in sea levels is linked to three primary factors, all induced by this ongoing global climate change:

Thermal expansion: When water heats up, it expands. About half of the past century's rise in sea level is attributable to warmer oceans simply occupying more space.

Melting of glaciers and polar ice caps: Large ice formations, like glaciers and the polar ice caps, naturally melt back a bit each summer. But in the winter, snows, made primarily from evaporated seawater, are generally sufficient to balance out the melting. Recently, though, persistently higher temperatures caused by global warming have led to greater-than-average summer melting as well as diminished snowfall due to later winters and earlier springs. This imbalance results in a significant net gain in runoff versus evaporation for the ocean, causing sea levels to rise.