Sea Levels Are Rising, But How Fast?

Orbit of Topex-Poseidon, Jason-1, and Jason-2 satellites and CNES control center in Toulouse — *The orbit of the Topex-Poseidon (1992), Jason-1 (2001), and Jason-2 (2008) satellites. CNES control center in Toulouse. Image source: NASA/JPL (new window).*

Sea Levels Are Rising: A Measurable Acceleration

Rising sea levels are one of the most tangible consequences of global warming. Since the beginning of the 20th century, the global mean sea level has risen measurably, and this rise has accelerated over the past few decades. What do recent scientific data tell us about the crucial question: how fast are sea levels rising?

Historical and Contemporary Measurements

Tide gauges, instruments that measure sea level from the coast, provide data dating back to the 18th century in some regions. Analysis of these records reveals that the global mean sea level rose by about 1.5 mm per year between 1900 and 2000. However, this average rate hides a significant acceleration over the century.

Since 1993, altimetry satellites (TOPEX/Poseidon, Jason-1, Jason-2, Jason-3, and Sentinel-6) have enabled precise and global measurements. These data show an average rise of 3.4 mm per year over the period 1993-2023, more than double the average rate of the previous century.

Causes of the Rise

The rise in sea levels is mainly due to two physical phenomena linked to global warming:

Thermal expansion: Water expands as it warms. This phenomenon accounts for about 40% of the observed rise.
Melting of land ice: The loss of mass from mountain glaciers and the Greenland and Antarctic ice sheets accounts for about 60%.

Future Projections

IPCC (Intergovernmental Panel on Climate Change) projections indicate that future rises will depend closely on our greenhouse gas emissions. According to scenarios:

For warming limited to 1.5°C (optimistic scenario), the rise could be 0.3 to 0.6 m by 2100.
For warming of 3-4°C (pessimistic scenario), the rise could reach 0.6 to 1.1 m by 2100.

Beyond 2100, the rise will continue for several centuries, even if emissions are stabilized, due to the inertia of the climate system.

Regional Variations

It is important to note that the rise is not uniform across the globe. Some regions experience a rise above the global average, while others record a lower rise, or even a relative drop. These variations are explained by:

Ocean currents and heat redistribution
Vertical movements of the Earth's crust (post-glacial rebound, subsidence)
Changes in the gravitational field due to the melting of ice sheets

Numerical Data and Trends

Evolution of the global mean sea level rise rate
Period	Rise rate (mm/year)	Cumulative rise (cm)	Uncertainty	Main method	Scenario
1901-1990	1.5	~13.5	± 0.2	Tide gauges	Observed data
1993-2023	3.4	~10.2	± 0.4	Satellite altimetry	Observed data
2006-2018	3.7	~4.8	± 0.5	Altimetry + tide gauges + Argo floats	Observed data
Projection 2030	4.5-5.5	+4-6 (vs 2020)	Average	Climate models	RCP4.5
Projection 2050	5.0-8.0	+15-25 (vs 2020)	Moderate to high	Climate models	RCP4.5 - RCP8.5
Projection 2080	6.0-12.0	+35-60 (vs 2020)	High	Climate models	RCP4.5 - RCP8.5
Projection 2100 (RCP8.5 scenario)	8-16	+60-110 (vs 2020)	Very high	Climate models	RCP8.5

Source: IPCC, 6th Assessment Report (2023) (new window), NASA Sea Level Change (new window) and Nature Climate Change (2021) (new window).

Scientific data converge to confirm that sea levels are rising at an accelerating rate, directly linked to human activities and global warming. The current rate of about 3.4 mm per year could increase significantly during the 21st century, with major consequences for coastal populations and ecosystems.