Sea levels are rising at an extraordinary pace. Here's what to know.

As humans continue to pour greenhouse gases into the atmosphere, oceans have tempered the effect. The world's seas have absorbed more than 90 percent of the heat from these gases, but it’s taking a toll on our oceans: 2021 set a new record for ocean heating.

Rising seas is one of those climate change effects. Average sea levels have swelled over 8 inches (about 23 cm) since 1880, with about three of those inches gained in the last 25 years. Every year, the sea rises another .13 inches (3.2 mm.) Research published in February 2022 shows that sea level rise is accelerating and projected to rise by a foot by 2050.

That translates into as much sea level rise in the next 30 years as occurred over the last century, according to the National Oceanic and Atmospheric Administration. Rick Spinrad, the NOAA administrator, called the findings “historic,” and warned that the projected rise will occur even if carbon emissions are drastically cut. 

(How carbon dioxide and other greenhouse gases contribute to climate change.)

In the United States, the most vulnerable populations live on the East and Gulf Coasts, where damaging flooding is predicted to occur 10 times more often in 2050 than it does today. The acceleration of sea level rise along these coasts is "unprecedented in at least 120 years," according to a study published in April 2023. 

Here's what you need to know about the causes—and consequences—of sea level rise.

Causes of sea level rise

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

• Thermal expansion: When water heats up, it expands. About half of the sea-level rise over the past 25 years is attributable to warmer oceans simply occupying more space. 
• Melting glaciers: Large ice formations such as mountain glaciers naturally melt a bit each summer. In the winter, snows, 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. That creates an imbalance between runoff and ocean evaporation, causing sea levels to rise.
• Loss of Greenland and Antarctica’s ice sheets: As with mountain glaciers, increased heat is causing the massive ice sheets that cover Greenland and Antarctica to melt more quickly. Scientists also believe that meltwater from above and seawater from below is seeping beneath Greenland's ice sheets, effectively lubricating ice streams and causing them to move more quickly into the sea. While melting in West Antarctica has drawn considerable focus from scientists, especially with the 2017 break in the Larsen C ice shelf, glaciers in East Antarctica are also showing signs of destabilizing.

Consequences

When sea levels rise as rapidly as they have been, even a small increase can have devastating effects on coastal habitats farther inland, it can cause destructive erosion, wetland flooding, aquifer and agricultural soil contamination with salt, and lost habitat for fish, birds, and plants.

(Here's how warming oceans affects wildlife and storm systems.)

Higher sea levels are coinciding with more dangerous hurricanes and typhoons that move more slowly and drop more rain, contributing to more powerful storm surges that can strip away everything in their path. One study found that between 1963 and 2012, almost half of all deaths from Atlantic hurricanes were caused by storm surges.

Already, flooding in low-lying coastal areas is forcing people to migrate to higher ground, and millions more are vulnerable from flood risk and other climate change effects. The prospect of higher coastal water levels threatens basic services such as Internet access, since much of the underlying communications infrastructure lies in the path of rising seas.

Adapting to the threat

As a result of these risks, many coastal cities are already planning adaptation measures to cope with the long-term prospects of higher sea levels, often at considerable cost. Building seawalls, rethinking roads, and planting mangroves or other vegetation to absorb water are all being undertaken.

In Jakarta, a $40 billion project will aim to protect the city with an 80-foot-high seawall. Rotterdam, home to the Global Center on Adaptation, has offered a model to other cities seeking to combat flooding and land loss. The Dutch city has built barriers, drainage, and innovative architectural features such as a “water square” with temporary ponds.

Of course, communities vulnerable to rising seas can only go so far in holding back the tide. In the Marshall Islands, where rising sea levels are forcing a choice between relocating or building up the land, residents will need help from other nations if they decide to undertake the expensive latter option.

How high will it go?

Most predictions say the warming of the planet will continue and is likely to accelerate, causing the oceans to keep rising. This means hundreds of coastal cities face flooding. But forecasting how much and how soon seas will rise remains an area of ongoing research.

A special report from the Intergovernmental Panel on Climate Change says we can expect the oceans to rise between 10 and 30 inches (26 to 77 centimeters) by 2100 with temperatures warming 1.5 °C. That’s enough to seriously affect many of the cities along the U.S. East Coast. Another analysis based on NASA and European data skewed toward the higher end of that range, predicting a rise of 26 inches (65 centimeters) by the end of this century if the current trajectory continues.

If all the ice that currently exists on Earth in glaciers and sheets melted it would raise sea level by 216 feet. That could cause entire states and even some countries to disappear under the waves, from Florida to Bangladesh. That’s not a scenario scientists think is likely, and it would probably take many centuries, but it could eventually happen if the world keeps burning fossil fuels indiscriminately.

In the meantime, scientists keep refining their models of sea-level changes. They also point out that the extent to which countries work together to limit release of more greenhouse gases may have a significant impact on how quickly seas rise, and how much.