How we measure sea level — and why the rate matters more than the number

The dashboard's data, the choices behind it, and the limits of what it can tell you.

The question this tool answers

Sea level is rising, and the more useful question is not "by how much?" but "how fast, and is it speeding up?" A few inches over thirty years sounds harmless. What turns it into a coastal problem is acceleration — the rate of rise itself increasing — because that is what compounds, decade after decade, into the difference between an occasional nuisance flood and a routine one. This dashboard tracks the global ocean from a satellite ruler, the local picture from tide gauges, the ice doing the melting, and the scenarios for where this century could end up.

How we know

The global curve comes from satellite radar altimetry. Since 1992, a line of satellites — TOPEX/Poseidon, then Jason-1, -2 and -3, and now Sentinel-6 Michael Freilich — has flown the same orbit, bouncing radar pulses off the sea surface and timing the echo to measure the satellite's height above the water to within a couple of centimetres. Average that over the whole ice-free ocean every ten days and you get global mean sea level. NOAA's STAR Laboratory for Satellite Altimetry stitches the missions into one continuous record beginning in 1993, which is the reference year all the millimetre figures on this page count from.

That is a genuinely different measurement from a tide gauge. A satellite measures absolute sea surface height against a fixed reference frame; a tide gauge bolted to a harbour wall measures relative sea level — the water plus whatever the land itself is doing. In places where the ground is sinking, like Jakarta or parts of the US Gulf Coast, the relative rise a resident experiences can far outrun the open-ocean satellite number. The ice-loss figures come from IMBIE, the Ice Sheet Mass Balance Inter-comparison Exercise, which reconciles satellite gravimetry, altimetry, and modelling into agreed mass-loss numbers for Greenland and Antarctica.

The choices we made

Lead with acceleration. The hero does not open with "the ocean is X millimetres higher." It opens with how much faster the rise is now than in the 1990s — a figure we compute directly from the satellite record by fitting a straight line to its first half and another to its second half, and reporting the difference in slope. Those same two lines are drawn on the chart, labelled with their mm/yr rates, so the headline claim and the picture are the same arithmetic. The absolute rise and the Empire-State-Buildings-of-ice analogy move to the footnote.

Absolute and relative, kept apart. We show the satellite global curve and the tide-gauge city table as separate things and say which is which, because conflating them is the most common error in sea-level reporting. The city projection tiles use an abstract range gauge — a fill for the mid estimate, a cap for the high end — rather than a cartoon skyline, because a generic skyline implies a city-specific shape the data does not contain. The gauge is honest about being a scale.

What this tool cannot tell you

It cannot tell you the flood risk at a specific street address. Global mean sea level is a planetary benchmark; what reaches your door depends on local land motion, the shape of the coastline, tides, and storms — which is what our Coastal Flood & Storm Surge dashboard is for. The city trends and 2050 ranges here are illustrative reference values, not live per-click projections, and the IPCC scenario table is static reference aligned with the AR6 assessment, not a forecast for one location.

The satellite record also needs corrections most readers never see — for the slow rebound of the solid Earth after the last ice age (glacial isostatic adjustment), for drifts between successive missions, and for the small sinking of the ocean basins themselves. These are handled upstream by NOAA and NASA; the residual uncertainty is why the rate, not the last decimal place, is the honest headline.

What's coming next

The SWOT mission (Surface Water and Ocean Topography), launched in 2022, measures sea-surface height in two dimensions rather than along a single track, resolving coastal and small-scale features the older altimeters smeared out. As its record lengthens it will sharpen the picture near coasts — exactly where the global average is least useful. On our side, the natural additions are a live tide-gauge feed (PSMSL) and per-city projection-tool integration.

Further reading

• Nerem et al. (2018), PNAS — detects the acceleration of sea-level rise in the altimeter era. The paper behind the "rising faster" headline. doi.org/10.1073/pnas.1717312115
• WCRP Global Sea Level Budget Group (2018), Earth System Science Data — accounts for every contributor to the rise: expansion, ice sheets, glaciers, land water. doi.org/10.5194/essd-10-1551-2018
• IMBIE Team (2018), Nature — the reconciled mass balance of the Antarctic ice sheet. doi.org/10.1038/s41586-018-0179-y
• IMBIE Team (2020), Nature — the companion reconciliation for the Greenland ice sheet. doi.org/10.1038/s41586-019-1855-2
• Hamlington et al. (2020), Reviews of Geophysics — why regional sea level differs from the global mean. Essential for the absolute-vs-relative distinction. doi.org/10.1029/2019RG000672

Credits

The sea-level record exists because of a decades-long partnership between NASA and France's CNES, who built and flew the TOPEX/Poseidon and Jason altimeter series, the ESA and EUMETSAT teams now operating Sentinel-6, the NOAA STAR scientists who maintain the merged climate-quality record, and the international IMBIE community who reconcile the ice-sheet numbers. The longest perspective comes from humbler instruments: tide gauges like the one at The Battery in New York, measuring continuously since the nineteenth century. This dashboard fetches from their public files. We are downstream of their work.