Why Are Sea Levels Rising? Causes, Measurements and Coastal Impacts

Sea level rise is one of the clearest and longest-lasting effects of a warming planet. It happens slowly, a few millimeters a year, so it can be easy to overlook. But over decades those millimeters add up, and they change how often coastal communities flood. This explainer covers why the sea is rising, how scientists measure it, and why the rise is not the same everywhere.

The two main causes

1. Warming water expands

Like most materials, water takes up more space as it warms. The oceans have absorbed more than 90 percent of the extra heat trapped by greenhouse gases, and as that heat spreads into deeper water, the ocean swells. This effect, called thermal expansion, has been responsible for a large share of sea level rise over the past century.

Because heat takes a long time to mix down into the deep ocean, thermal expansion will continue for centuries even if global temperatures stabilize.

2. Melting ice on land

When ice that sits on land melts or slides into the sea, it adds water to the ocean. There are three main sources:

• Mountain glaciers around the world, from Alaska to the Alps to the Himalayas, are shrinking and sending meltwater to the sea.
• The Greenland ice sheet is losing ice through surface melting and through glaciers flowing faster into the ocean.
• The Antarctic ice sheet is losing ice mainly where warm ocean water melts glaciers from below, especially in West Antarctica.

In recent decades, melting land ice has overtaken thermal expansion as the largest contributor to sea level rise, according to NASA's sea level team.

How much has the sea risen?

The IPCC's Sixth Assessment Report estimates that global mean sea level rose by about 20 centimeters between 1901 and 2018. The pace has picked up. During much of the 20th century, the rise averaged under 2 millimeters per year. Since the 1990s, satellite measurements show it averaging more than 3 millimeters per year, and in recent years closer to 4.

That may sound small, but a faster rate compounds over time. A few millimeters a year becomes tens of centimeters over a lifetime.

How scientists measure sea level

Tide gauges

Tide gauges are instruments at harbors and piers that record water levels continuously. Some records go back more than 150 years. In the United States, NOAA's Tides and Currents program runs a national network of these stations. Tide gauges measure sea level relative to the land they sit on, which is exactly what matters to local communities, but it also means they mix together ocean changes and land movement.

Satellites

Since 1993, a series of satellites has used radar altimetry to measure the height of the sea surface across almost the entire globe. They bounce radar pulses off the ocean and time the echo, measuring sea surface height to within a few centimeters for each reading. Averaged over many readings, they can detect changes of a millimeter or so per year. Satellites give a global picture that tide gauges cannot.

Checking the pieces add up

Scientists also measure each contributor separately: Argo floats track ocean heat (and therefore expansion), and gravity satellites detect how much ice mass Greenland and Antarctica are losing. When the parts are added together, they largely match the total rise measured by altimeters. That agreement increases confidence in the whole picture.

Why sea level rise varies from place to place

The ocean surface is not flat like water in a bathtub. Local sea level rise can differ substantially from the global average for several reasons:

• Land sinking (subsidence): In some places the land itself is sinking, due to groundwater or oil extraction, sediment compaction or natural geological processes. Parts of the US Gulf Coast, including areas around New Orleans and Houston, experience some of the fastest relative sea level rise in the country for this reason.
• Land rising: In places once covered by heavy ice-age ice sheets, such as parts of Scandinavia and Alaska, the land is still slowly rebounding. Relative sea level there can even be falling.
• Ocean currents and winds: Changes in currents like the Gulf Stream can pile water up against some coastlines.
• Gravity effects: Large ice sheets pull ocean water toward them by gravity. As they lose mass, that pull weakens, so water near the ice sheet actually drops while sea level rises more far away.

This is why local planners use local projections rather than a single global number.

What it means for coastal flooding

The most noticeable effect of sea level rise is not permanent underwater land, at least not yet. It is more frequent flooding. Higher baseline sea levels mean that ordinary high tides reach farther inland and that storm surges from hurricanes and other storms start from a higher level.

In the United States, NOAA reports that so-called "high tide flooding," sometimes called nuisance or sunny-day flooding, has become several times more common in many coastal cities than it was a few decades ago. Streets flood on sunny days, storm drains back up, and salt water seeps into soil and freshwater supplies.

Rising seas also make hurricane storm surge more damaging, which is covered in Hurricanes and climate change.

What comes next

Sea level will keep rising for centuries, because the oceans and ice sheets respond slowly to warming that has already happened. How fast and how far depends heavily on future emissions.

The IPCC projects that by 2100, global sea level is likely to rise roughly 0.3 to 0.6 meters above the 1995 to 2014 average under a low-emissions path, and roughly 0.6 to 1.0 meters under a very high-emissions path. Higher outcomes cannot be ruled out, mainly because of uncertainty about how quickly the Antarctic ice sheet could lose ice. Scientists are open about that uncertainty: the lower end is fairly well understood, while the upper end depends on ice processes that are still being studied.

Cutting emissions slows the rise and buys time to adapt. Adaptation measures range from restoring wetlands and dunes, which buffer storm surge, to raising roads, updating building codes and, in some places, planning for managed retreat.

The bottom line

Sea levels are rising because the ocean is warming and expanding and because ice on land is melting. Tide gauges and satellites both confirm the rise and show it speeding up. Local factors make some coastlines more vulnerable than others. To understand where all that extra heat comes from in the first place, see What is the greenhouse effect?