Sea-level rise

The principal climate-related contributors are the expansion of seawater as it warms and the transfer of water from melting land ice—mountain glaciers and the Greenland and Antarctic ice sheets—into the ocean. Changes in the storage of water on land, including groundwater depletion, also contribute, but are smaller than these main factors. Melting floating sea ice has little direct effect on sea level because it already displaces seawater; the important ice contribution comes from ice resting on land entering the ocean.

Tide gauges record the height of the sea relative to land at particular coastal locations. Satellite altimeters measure the height of the ocean surface across broad areas and support estimates of global mean sea level. The two records complement one another: tide-gauge observations extend back well before satellite measurements began in the early 1990s, while satellites provide wide and consistent coverage of the open ocean. Scientists account for natural fluctuations when estimating the long-term trend.

Global mean sea level has risen since modern records began in the nineteenth century, and the rate has increased in recent decades. NASA’s Sea Level Change portal reports that roughly 10 centimetres of global rise occurred between 1902 and 1992 and a similar amount between 1993 and 2023. NOAA Climate.gov reported that the 2023 annual mean was 101.4 millimetres above the 1993 level, the highest annual average in the satellite record available at the time of its account. These dated measurements describe a global average, not a prediction for an individual coast. Relative sea level—the sea height measured against nearby land—differs from place to place. Land subsidence or uplift, regional currents, winds, ocean heat distribution and local water storage can alter local trends. As a result, a local tide-gauge record can diverge from the global mean even while the ocean’s global average rises.

A higher baseline sea level can allow storm surges and high tides to reach farther inland, increase the frequency of damaging coastal floods, accelerate shoreline erosion and contribute to saltwater intrusion into freshwater systems. The consequences depend on local geography, ecosystems, infrastructure, land movement and preparedness. Sea-level rise is therefore both a global physical trend and a locally experienced hazard.