Sea Level Rise Science

Thermal Expansion

Water expands as it warms, a property called thermal expansion. Since the ocean has absorbed more than 90 percent of the excess heat trapped by greenhouse gases, this warming causes a measurable increase in ocean volume. Thermal expansion contributed roughly 1.4 millimeters per year to sea level rise in recent decades. Because the ocean takes centuries to fully warm in response to surface forcing, thermal expansion will continue for hundreds of years even if atmospheric temperatures stabilize, representing a committed future rise.

The deep ocean below 2,000 meters is only beginning to warm and will continue absorbing heat for centuries. This deep ocean warming contributes relatively little to current sea level rise but represents a long-term commitment. Full equilibration of ocean temperatures to current atmospheric forcing would eventually produce roughly 0.5 to 1 meter of thermal expansion alone.

Glacier Mass Loss

Mountain glaciers worldwide contain enough water to raise sea level by roughly 0.4 meters. They are currently losing mass at approximately 270 billion tonnes per year, contributing about 0.7 millimeters annually to sea level. Glaciers respond relatively quickly to temperature changes, with most small glaciers projected to lose 50 to 80 percent of their mass by 2100 under moderate warming scenarios. Some regions, including the tropical Andes and parts of Central Asia, face near-complete glacier loss with severe consequences for water supply.

Ice Sheet Dynamics

The Greenland Ice Sheet contains enough water to raise global sea level by 7.4 meters if fully melted. It is currently losing mass at roughly 270 billion tonnes per year, contributing about 0.7 millimeters annually. Surface melting dominates Greenland's mass loss, with the melt zone expanding to higher elevations and further inland as temperatures rise. In summer 2012, surface melting was detected across 97 percent of the ice sheet's surface, an event unprecedented in the satellite record.

The Antarctic Ice Sheet is far larger, containing enough water for 58 meters of sea level rise. Its contribution is currently smaller (about 150 billion tonnes per year, roughly 0.4 millimeters annually) but is accelerating, particularly in West Antarctica where warm ocean water is melting ice shelves from below. Ice shelves are floating extensions of glaciers that buttress the grounded ice behind them. As they thin and retreat, the glaciers they restrain can flow faster toward the ocean.

The marine ice sheet instability hypothesis suggests that portions of the West Antarctic Ice Sheet resting on bedrock that slopes downward inland could undergo irreversible retreat once a tipping point is crossed. As the grounding line (where ice lifts off the bedrock and begins floating) retreats into deeper water, the ice becomes thicker at the grounding line, increasing discharge rates and causing further retreat. Several glaciers in the Amundsen Sea sector show signs of this process beginning.

Projections and Uncertainty

The IPCC Sixth Assessment Report projects global mean sea level rise of 0.28 to 0.55 meters under very low emissions (SSP1-1.9) and 0.63 to 1.01 meters under very high emissions (SSP5-8.5) by 2100. However, these projections do not fully account for potential ice sheet instabilities that could add substantially to the upper estimates. Low-confidence high-end scenarios suggest rises approaching 2 meters by 2100 cannot be ruled out.

Beyond 2100, sea level will continue rising for centuries regardless of emissions pathway, because ice sheets respond slowly to warming. At 2 degrees of warming sustained for millennia, Greenland will eventually lose most of its ice (7+ meters of rise), and portions of West Antarctica are also likely committed to significant loss. At 4 to 5 degrees, the East Antarctic Ice Sheet could contribute additional meters over thousands of years.

Coastal Impacts

Even modest sea level rise dramatically increases the frequency of coastal flooding. A 20-centimeter rise can increase the frequency of what was previously a 1-in-100-year flood event by a factor of 3 to 10, depending on local conditions. By mid-century, many coastal cities will experience annual flooding at levels that currently qualify as rare extreme events. Low-lying island nations and deltaic regions face existential threats from combined sea level rise and storm surge.