What Is Changing
Global mean sea level has been rising for well over a century, and the rate has increased in recent decades. Estimates from tide gauges and, since the early 1990s, satellite measurements suggest the ocean has risen on the order of 20 centimeters (roughly 8 inches) since 1900. That may not sound like much, but water spread across the entire ocean surface represents an enormous volume, and even small increases raise the baseline from which storms and tides flood the land.
It helps to be precise about what “sea level” means. Local sea level, measured relative to the land at a particular coast, can differ from the global average, because land itself moves and ocean water is not distributed evenly. The global figure is an average of many measurements and corrections.
Two Main Causes
The warming of the planet raises sea level in two main ways.
Thermal expansion. Water expands slightly as it warms. The oceans have absorbed the large majority of the extra heat trapped by greenhouse gases, so the water has expanded, raising the level even without adding any water. Because the deep ocean warms slowly, this effect continues long after the surface has warmed, which is part of why sea level keeps rising for a long time even if emissions are reduced. It is closely tied to the properties of water as a substance.
Added water from land ice. Melting glaciers and ice sheets add water that was previously stored on land. This comes from:
- Mountain glaciers and ice caps, which have been shrinking in most regions as part of wider glacier and ice loss.
- The Greenland ice sheet, which has been losing mass through surface melting and the discharge of ice into the sea.
- The Antarctic ice sheet, which is so large that its behavior is the biggest source of uncertainty about future sea level. Parts of West Antarctica rest on land below sea level and are vulnerable to warm water reaching beneath floating ice shelves.
A smaller contribution comes from changes in groundwater storage on land, including water pumped from aquifers that ends up in the ocean.
Sea ice and ice shelves that are already floating do not raise the sea level much when they melt, much as a melting ice cube barely changes the level of water in a glass. Their importance is different: ice shelves hold back land ice behind them, and their loss can speed up the flow of glaciers into the ocean.
Why It Is Not Uniform
Sea level does not rise evenly around the world. Several factors cause regional differences:
- Land movement. In some places, land is sinking, due to natural settling, the removal of groundwater or oil and gas, or the delayed rebound of land that was once covered by heavy ice sheets. A sinking coast experiences faster relative sea level rise. Elsewhere, land is rising and local sea level may appear to fall.
- Ocean circulation and winds. Changes in ocean currents and winds can pile water against some coastlines and draw it away from others, so regional rates vary from year to year.
- Gravity. When a large ice sheet melts, its gravitational pull on nearby water weakens, so sea level near the ice sheet may fall slightly while it rises more than average far away.
What Projections Say
Future sea level depends on how much greenhouse gas is emitted and how ice sheets respond. Assessments generally project further rise this century, with higher emissions leading to higher outcomes, and the range includes a rise of several tenths of a meter to around a meter or more by 2100, depending on scenario. The upper end carries large uncertainty because of ice-sheet dynamics that are hard to model. Beyond 2100, sea level is expected to keep rising for centuries to millennia, since oceans and ice respond slowly. The long-term amount depends strongly on how much warming is limited.
These are projections, not predictions of a certain outcome, and researchers continue to refine them with better observations, including climate satellites that measure the height of the sea surface and the mass of the ice sheets.
What Higher Seas Mean for Coasts
The consequences go beyond a gradual creeping of the shoreline.
More frequent flooding. A higher baseline means high tides alone can flood streets in some low-lying places, a pattern sometimes called nuisance or sunny-day flooding. Storm surges ride on top of higher seas, so a storm of a given strength can push water farther inland.
Erosion. Beaches, dunes and cliffs retreat, and coastal property and infrastructure are put at risk.
Saltwater intrusion. Salt water can move into coastal rivers, estuaries and groundwater, threatening drinking water and making farmland less productive. Deltas, which are often densely populated and highly productive, are especially exposed, particularly where they are also sinking.
Ecosystems. Wetlands, mangroves and marshes can keep pace with rising water by building up sediment, if the rise is slow enough and they have room to move inland. Where seawalls or development block them, they can be squeezed out.
Infrastructure and people. Ports, airports, roads, sewage systems and power plants near the coast face higher risks, and many large cities are located on coasts. Small island states and low-lying communities face particularly high risks, including the possibility that some places become uninhabitable over long periods. Poor communities with fewer resources to protect themselves are often the most exposed.
Responses
Options include limiting emissions to slow the rise, building defenses such as sea walls and barriers, restoring natural buffers, raising buildings and infrastructure, changing land-use rules to discourage building in the most exposed areas, and, in some cases, relocating communities. Planners increasingly design for a range of possible outcomes and update them as new data arrives. Taken together, these choices make up much of what is meant by climate adaptation.
Sea level rise is one of the more certain consequences of a warming climate, since the physics are well understood and the observed trend is clear. The main uncertainties concern how fast it will accelerate and how large the contribution from ice sheets will be, which is why monitoring and flexibility in planning both matter.