Sea level rise and global warming

A variety of human activities and Earth system processes affect sea level today, including:

• Human activities that cause global warming, primarily through the release of greenhouse gases via the burning of fossil fuels, agricultural activities, and deforestation. Warming increases sea level in two ways. First, increasing average global temperatures, and especially warming at the poles, causes glaciers and ice sheets on land to melt, which increases the amount of water in the oceans, thus increasing sea level. Second, the water in the oceans is expanding as it absorbs heat from the warming atmosphere, thus taking up more space, leading to sea level rise. This second phenomenon is called thermal expansion.
• Tectonic activity that causes the uplift or sinking of land at a particular location. This can be due to the major movement of land. For example, large earthquakes can sometimes cause meters of uplift. The uplift or sinking of land can also occur from more passive tectonic processes. For example, uplift is still happening where there were ice sheets during the last ice age. The weight of the ice caused the land to sink deeper into the mantle. As the ice melted, the weight of the continental crust was reduced, making it more buoyant. In these places the crust is still rising (called isostatic rebound), reducing the effective sea level rise.  To learn more about the structure of the Earth see plate tectonics.
• Ocean circulation patterns, which can alter how water is distributed across ocean basins, causing sea level to rise more or less in certain locations.

Sea level rise is due to water flowing into the ocean from melting land ice, and because of the overall warming of the world, which causes ocean water to expand (called thermal expansion).

In the past, sea levels have changed over thousands to millions of years due to a variety of Earth system processes and phenomena, including:

• Changes in Earth’s spin, tilt, and orbit, which increase or decrease the amount of sunlight absorbed by different areas of the Earth’s surface. This affects the climate, including regional temperatures and rain and snowfall, and thus the distribution and extent of snow and ice cover and the amount of water in the ocean.
• Increases in greenhouse gases that increase global temperatures, which reduce the extent of ice sheets, and cause thermal expansion of ocean water.
• The movement of tectonic plates, which determines the distribution, size, and depth of ocean basins, influencing how water is distributed around the continents.

Changes in sea level affect various Earth system processes and phenomena, including:

• Displacing human populations and other species due to flooding and also erosion.
• Increasing ground water salinity from coastal flooding, which reduces freshwater quality and availability and decreases soil quality. This, in turn, affects agricultural crops and livestock.
• The distribution of coastal habitats. For example, marshes and swamps protect coastlines by reducing coastal erosion and by promoting sedimentation. If there is space for these habitats migrate inland as sea level rises, this will decrease the vulnerability of coastal regions to sea level rise, especially storm surges.
• Species ranges, as sea level rise affects where organisms live, where they can move, and what other species they interact

Can you think of additional cause and effect relationships between sea level rise and other parts of the Earth system?

Visit the snow and ice cover, water cycle, and burning of fossil fuels pages to explore more connections between the hydrosphere and other global changes.

Investigate

Learn more in these real-world examples, and challenge yourself to construct a model that explains the Earth system relationships.

• Lessons for today in ancient mass extinctions

Links to Learn More

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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.) New research published on February 15, 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’s latest technical data, which updates 2017 projections with the most precise estimates yet.

Rick Spinrad, the NOAA administrator, called the findings “historic,” and warned that the projected rise will occur regardless, even if carbon emissions are drastically cut. 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.

Floods 101

No other kind of natural disaster in America has caused more death and destruction than floods.

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.

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.

Climate 101: Causes and Effects

The climate is certainly changing. But what is causing this change? And how does the rising temperature affect the environment, and our lives?

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.

The most recent 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.

Many people think of global warming and climate change as synonyms, but scientists prefer to use “climate change” when describing the complex shifts now affecting our planet’s weather and climate systems.

Editor's Note: This story was updated on February 1, 2022, with new sea level rise data from NOAA.