As of January 2025, melting ice sheets and glaciers are the primary contributors to the fastest sea level rise our planet has seen in over 3,000 years, with the potential to raise ocean levels by a significant amount in the future. However, because sea level is so closely connected with complex dynamics in our climate, it’s difficult to forecast sea level rise precisely down to exact amounts in a single year.
Sea level rise, like global warming, is measured in small-sounding numbers, but a rise of a few inches can have big impacts on people, land, and infrastructure in coastal communities. 10% of the global population (around 770 million people) live in coastal areas less than five meters above the high tide line. For these millions of people, sea level rise contributes to hazards like flooding, inundation, and erosion that can impact infrastructure, agriculture, and daily life.
What is sea level rise?
The amount of water in the oceans is directly related to how much ice there is on land: The more ice on land, the less water there is in the oceans, and vice versa. This has always been true over the course of Earth’s history:
In each of these periods in our planet’s history, the amount of ice coverage on Earth directly correlates with the amount of water in the oceans.
Sea level rise describes the change in global sea level relative to land as compared to the pre-industrial era. Higher global average temperatures due to greenhouse gas emissions cause two important contributors to sea level rise: thermal expansion and ice melt. Today, these effects are happening simultaneously: Seawater is expanding, and melting ice sheets and glaciers are adding more water to the oceans.
Oceans absorb over 90% of the heat trapped by atmospheric greenhouse gasses. As the oceans warm, the molecules in seawater move faster and further apart, causing seawater to expand in a process called thermal expansion. Thermal expansion increases the volume of the water and pushes overall ocean levels higher.
There is a fixed amount of water in Earth’s water cycle —the amount of water in the oceans is directly related to the amount of ice on the planet. When there is less ice, there is more water in the oceans. Ice melt takes place both from above and below: Warmer air temperatures melt ice sheets and glaciers from above, while warmer ocean temperatures melt away the underside of coastal ice sheets. From the 1970s through the 2010s, approximately half of global sea level rise was due to thermal expansion and half was due to glacier and ice sheet melt. From 2005 to 2013, melting contributed nearly twice as much to sea level rise as thermal expansion.
How is sea level rise measured?
Although scientists average and refer to a global sea level, variations in topography, currents, and gravitational pull mean some places on Earth will experience more sea level rise locally than others. Scientists estimate global average sea level with three approaches.
Tidal stations and satellite sensors measure sea level today. Tidal stations measure local high and low tides, while satellites measure sea levels from space using a radar pulse. Scientists average these measures to find a global sea level and compare that averages to historical sea level to identify rise over time. Scientists use water samples, buoys, satellites, and other instruments to detect the temperature of Earth’s oceans. This temperature provides an estimate of thermal expansion. Since the amount of water in the oceans is directly related to the amount of ice on land, scientists use measures of ice quantity and melt rate to infer rising sea level.
Why does sea level rise matter?
Sea level rise impacts any land that is low lying or has coastline, places that are very densely populated with over 10% of the global population. For populations living in coastal areas, even small-sounding increases in sea level worsen many existing climate hazards. Those living outside of coastal areas don’t experience exposure risks (first order risk) of sea level rise but can experience systemic response risks (third order risk) over time.
More water in the oceans means more severe surges and coastal flooding from hurricanes, storms, high tides, and tsunamis.
Higher sea level also pushes water further into land. Already, high-tide flooding in coastal areas is 300% to 900% more frequent than it was 50 years ago. Over time, higher ocean levels and worsened storm surges can engulf and erode coastline or islands, leading to loss of arable land. As the ocean increasingly intrudes into land, the saltwater can salinate soil, making it harder for plants and crops to grow and affecting critical habitats like marshlands and estuaries.
Where is sea level rise headed?
Sea level rise is correlated to the amount of carbon dioxide in the atmosphere because that carbon dioxide, along with other greenhouse gasses, causes the additional atmospheric warming that contributes to sea level rise. Although we understand the relationship between CO2 and sea level and have effective ways to measure present day sea level rise, it is impossible to precisely project future sea level rise on a year-by-year basis. The pace of glacial melting and the dynamics of thermal expansion are too complex for scientists to model in detail, but the direction is clear. In 2023, sea level rise reached a record high of 3.99 inches (10.1 cm) above 1993 measurements. The future global sea level depends on greenhouse gas emission pathways and the melting of ice.
Emission pathways describe future greenhouse gas emission scenarios and the projected warming created by those emissions.
This graphic shows multiple pathways representing different future scenarios for sea level rise depending on emissions.
Depending on the rate and quantity of greenhouse gas emissions human activity contributes to the atmosphere, sea level rise may vary between 1 to 6 feet within this century. Scientists cannot predict exactly how thermal expansion, ice sheet collapse, changes in land elevation, and shifts in the gravitational pull of the Greenland ice sheet will impact sea level on a year-by-year basis. The West Antarctic Ice Sheet and some of Greenland’s glaciers have the potential to raise sea levels by more than 200 feet if they collapse or melt, but we don’t know when or if this will happen. We do know that, already, large sections of ice on Greenland and Antarctica have begun to melt.
Sea level rise is also a delayed climate impact: Even if we were to stop emitting greenhouse gasses today, oceans would continue to rise for centuries because greenhouse gasses remain in the atmosphere for long periods of time while continuing to warm the planet. We know that, eventually, global sea levels will “catch up” to atmospheric carbon concentrations, which are currently higher than at any point in at least 800,000 years.