‘Faster and faster’: Why did global sea levels rise more than expected in 2024?

Global sea levels rose faster than expected in 2024 as the world experienced its hottest year on record, a new study has found.

This was due to an unusual amount of ocean warming combined with meltwater from land-based ice such as glaciers, according to the NASA-led analysis.

“Every year is a little bit different, but what’s clear is that the ocean continues to rise, and the rate of rise is getting faster and faster,” said Josh Willis, a sea level researcher at NASA’s Jet Propulsion Laboratory in Southern California.

Global sea levels are reaching their highest in three decades

Since the satellite recording of ocean height began in 1993, the rate of annual sea level rise has more than doubled. In total, the global sea level has gone up by 10 centimetres since 1993.

This long-term record is made possible by an uninterrupted series of ocean-observing satellites starting with TOPEX/Poseidon in 1992.

According to the NASA-led study of the information sourced via the Sentinel-6 Michael Freilich satellite, 2024 saw a rate of sea level rise at 0.59 centimetres per year, compared to the expected rate of 0.43 centimetres per year.

The upcoming Sentinel-6B satellite will continue to measure sea surface height down to a few centimetres for about 90 per cent of the world’s oceans.

Human-induced warming is a key factor in sea level rise

In recent years, about two-thirds of sea level rise was from the addition of water from land into the ocean by melting ice sheets and glaciers. About a third came from thermal expansion of seawater.

But in 2024, those contributions flipped, with two-thirds of sea level rise coming from thermal expansion.

“With 2024 as the warmest year on record, Earth’s expanding oceans are following suit, reaching their highest levels in three decades,” said Nadya Vinogradova Shiffer, head of physical oceanography programs at NASA Headquarters in Washington.

What is the thermal expansion of water?

There are several ways in which heat makes its way into the ocean, resulting in the thermal expansion of water.

Normally, seawater arranges itself into layers determined by water temperature and density.

Warmer water is lighter and floats on top of cooler water, which is denser. In most places, heat from the surface moves very slowly through these layers down into the deep ocean.

But extremely windy areas of the ocean can agitate the layers enough to result in vertical mixing.

Very large currents, like those found in the Southern Ocean, can tilt ocean layers, allowing surface waters to slip down deep more easily.

The massive movement of water during El Niño - in which a large pool of warm water normally located in the western Pacific Ocean sloshes over to the central and eastern Pacific - can also result in the vertical movement of heat within the ocean.

The UN has warned that rising sea levels are endangering vast numbers of people living on islands or along coastlines.

Particularly vulnerable areas include low-lying coastal communities in India, Bangladesh, China and the Netherlands, as well as island nations in the Pacific and Indian Oceans.