What if sea levels rose?

At the beach, the sea comes up to the sand. Waves go in and out.

What if the sea were a little higher? Where would the water's edge be?

The line where the sea meets the land, the shoreline, moves a little every day with the tides. But over many years, the average height of the sea, called sea level, can change too.

If the sea rose by less than a meter, would the beach just get a bit narrower? Or could the water's edge move a long way? In this lab you can raise the sea in a model and see how far the shoreline moves on different kinds of coast.

Global mean sea level is the ocean's average height once tides and waves are averaged out. It changes when water is added to the ocean or when seawater expands as it warms.

This lab separates two questions: how much the sea rises for a given amount of melted land ice, and how far the shoreline then moves. The second depends strongly on the shape of the coast, which is the point of this experiment.

The sea has been rising very slowly. It is a tiny bit higher each year.

Scientists measure it with special tools and with satellites in space.

Sea level is already rising slowly. NOAA reports that the global average has gone up about 21 to 24 centimeters since 1880. In the 20th century it rose about 1.4 millimeters a year; from 2006 to 2015 it rose about 3.6 millimeters a year.

The rise comes mostly from two things: water from melting glaciers and ice sheets, and seawater taking up more room as it warms. Scientists measure it with tide gauges on coasts and with satellites that bounce signals off the sea surface.

IPCC AR6 (WG1 SPM, 2021) puts the rise at 0.20 [0.15–0.25] m between 1901 and 2018, accelerating from 1.3 mm/yr (1901–1971) to 3.7 mm/yr (2006–2018). NOAA reports 2023 global mean sea level 101.4 mm above 1993, the highest of the satellite record.

The causes are ice loss on land and thermal expansion, with a small contribution from changes in water stored on land. Local rise can exceed the global average where land is sinking.

In our pretend lab, Olly melts lots of ice on land. The sea rises.

On a flat beach, the water moves far inland. On a steep cliff, it hardly moves.

Melt 300 thousand cubic kilometers of land ice, about a tenth of Greenland's ice, and the model's sea rises about 76 centimeters. Now look at the shoreline.

On a very gentle coast, where the land rises just 1 meter for every 1,000 meters inland, the water's edge moves about 760 meters, three quarters of a kilometer. Change the coast to a steep one, 10 meters inland per meter up, and the same rise moves the edge only about 7.6 meters.

300 thousand km³ × 0.917 ÷ 361.8 million km² = 0.76 m. Shoreline retreat Δx = Δh × steepness: 760 m at 1,000:1, 76 m at 100:1 and 7.6 m at 10:1. The model fills a straight slope like a bathtub.

The ice volume is a chosen example, about 10 percent of Greenland's, not a forecast: the model says how much, not when. It ignores erosion, dunes, defenses and the uneven way real seas rise.

On a flat beach, the land goes up very slowly.

So a little more water spreads a long way. On a steep cliff, the water just climbs up a bit.

Think of pouring water into a tilted tray. If the tray is nearly flat, a tiny bit more water spreads a long way across it. If it is steep, the water edge hardly moves.

Coasts work the same way. Wide, flat places like marshes and low sandy beaches are the ones where a small rise moves the shoreline farthest.

For a planar slope with run-to-rise ratio s, a vertical rise Δh moves the waterline horizontally by s·Δh. Low-gradient coasts (s of hundreds to thousands) therefore turn centimeters of rise into tens to hundreds of meters of inundation.

The model's limits: a uniform ocean of fixed area, a straight slope, and no sediment movement. Real coasts erode and build, marshes can trap sediment and grow upward, and sea level rises unevenly because of gravity, land motion and currents.

People are getting ready. Some build walls to hold back the sea.

Others plant marsh grasses. The plants calm the waves and help hold the shore.

Coastal towns are planning for higher seas. Some build flood barriers. Others create 'living shorelines' using marsh plants, sand and oyster reefs. NOAA says just 15 feet (about 4.6 meters) of marsh can soak up half the energy of incoming waves, and marshes can trap mud and grow taller as the sea rises.

Scientists also help by measuring the sea and making careful projections, so communities know what to prepare for.

NOAA describes living shorelines, native vegetation alone or with low sills, as more resilient than bulkheads in hurricanes; 15 ft of marsh can absorb 50 percent of incoming wave energy, and marshes trap sediment, letting them gain elevation as sea level rises.

NOAA Climate.gov reports that U.S. high-tide flooding is now 300 to more than 900 percent more frequent than 50 years ago, and that global sea level is likely to rise at least 0.3 m above 2000 levels by 2100 even on a low-emissions pathway. NASA classes protecting coastlines as adaptation.