What if the oceans became warmer?

When things get warm, many of them get a tiny bit bigger. Water does too!

What if the whole ocean got warmer? Would the sea get higher?

Most of the extra heat trapped by greenhouse gases does not stay in the air. It goes into the ocean, which covers more than 70 percent of Earth's surface and can soak up huge amounts of heat.

When water warms, it expands: the same amount of water takes up a little more space. So what happens if the ocean warms? Can the sea rise even without any ice melting? In this lab you can warm the ocean's upper layer in a model and measure the rise.

Global mean sea level changes through ocean mass (added water) and ocean density. The density part driven by temperature is called thermosteric change, or thermal expansion (IPCC AR6 WG1, Chapter 9).

This lab isolates it. With no ice melting at all, how much does the sea rise if the upper 700 m of the ocean warms by 2 °C, and why is such a small expansion of water enough to matter?

The ocean is very big and very deep. It soaks up warmth from the Sun.

It is like a giant warm bath for the whole planet.

The ocean is Earth's biggest heat store. NOAA says it is holding about 91 percent of the extra heat trapped by greenhouse gases, and it has been warmer than average since the mid-1990s.

Scientists measure the ocean's temperature with ships, buoys and a fleet of robot floats that dive and rise again. Satellites help too: because warm water expands, the height of the sea surface tells them about the heat below.

The IPCC (AR6 WG1 SPM, 2021) finds that ocean warming accounted for 91 percent of the heating of the climate system, with land, ice and atmosphere taking the rest. NOAA reports 1993–2024 upper-ocean (0–700 m) heat gain of about 0.39 to 0.46 W/m² averaged over Earth's surface.

Measurements combine ships, moored and drifting buoys, an array of profiling floats in the upper ocean, and satellite altimetry, which infers heat content from sea-surface height.

In our pretend lab, Olly warms the top of the ocean by 2 degrees.

The water spreads out a tiny bit. The sea goes up, even with no ice melting!

Set 'Ocean warming' to 2 °C. The model warms the top 700 meters of the ocean evenly. Each meter of water grows by only a tiny amount, about 0.4 millimeters, but there are 700 meters of it.

Add it all up, and the sea rises about 28 centimeters with no ice melting at all. On a gently sloping coast, where the land rises 1 meter for every 100 meters inland, the water's edge moves about 28 meters.

Δh = α·ΔT·H = 2 × 10⁻⁴ °C⁻¹ × 2 °C × 700 m = 0.28 m; 1 °C gives 0.14 m. At a steepness of 100:1 the bathtub shoreline moves 28 m.

The 2 °C is a chosen value, not a projection, and the model warms one uniform layer. In reality warming is concentrated near the surface and spreads downward slowly, and the expansion coefficient itself varies.

Water is made of tiny bits that wiggle. Warm bits wiggle more.

They push a little farther apart. So the water takes up more room.

Water is made of countless tiny molecules that are always jiggling. When water warms, they jiggle harder and push their neighbors a little farther apart, so the water expands.

The stretch is very small: a bathtub warmed by 2 degrees would rise by much less than a millimeter. But the ocean is kilometers deep and covers most of the planet, so a tiny stretch in every meter adds up to centimeters of sea level.

OpenStax lists fresh water's volume expansion coefficient at 20 °C as 210 × 10⁻⁶ per °C; the model uses a single value of 2 × 10⁻⁴ per °C. Because Δh scales with the depth of the warmed layer, the ocean's great depth converts a fractional expansion of 0.04 percent into tens of centimeters.

The model's limits: one coefficient for all temperatures, salinities and depths; an evenly warmed 700 m layer; no deep-ocean warming; and a uniform global rise. Real expansion depends on temperature: fresh water below 4 °C even contracts as it warms (OpenStax).

Scientists watch the ocean every day, with floating robots and satellites.

That helps people plan for the future and look after the sea.

Warming water is already raising the sea. The IPCC found that from 1971 to 2018, about half of the rise in sea level came from water expanding as it warmed. Melting glaciers and ice sheets supplied most of the rest.

Knowing this helps. Ocean measurements show how much heat Earth is gaining. Cutting the greenhouse gases that trap extra heat, which NASA calls mitigation, is how people work to reduce further warming.

IPCC AR6 (WG1 SPM, A.4.3) attributes 50 percent of 1971–2018 sea-level rise to thermal expansion, 22 percent to glaciers, 20 percent to ice sheets and 8 percent to land-water storage. NOAA notes that over 2005–2013 melting contributed nearly twice as much as expansion, as ice loss accelerated.

NOAA also notes that heat stored in the ocean will eventually be released, committing Earth to some further warming; the latest Earth system models it cites find that once emissions reach zero this is at most a few tenths of a degree Celsius, and may be closer to zero. NASA describes cutting emissions as mitigation.