How We Measure

Watching the sea from the shore and from space

No single instrument can see everything that makes sea level change, so scientists combine several. Each one measures something different, and together they let us measure the total rise and explain its causes.

Tide gauges

Tide gauges have measured sea level at harbors for more than 200 years in a few places, such as Amsterdam and Stockholm, and since the 1800s in many others. They record sea level relative to the land they sit on, which is exactly what matters for coastal flooding. Their weakness is coverage: they are only at the coast, mostly in the Northern Hemisphere, and they move with the land.

Satellite altimeters

Since 1992, radar altimeters on satellites have measured the height of nearly the entire ice free ocean every ten days. The satellite sends a radar pulse to the sea surface and times its return. Knowing the satellite's exact position from GPS, laser ranging and the DORIS system, scientists convert that time into the height of the sea, accurate to about a centimeter for a single measurement and to a fraction of a millimeter per year for the global average. The current reference mission is Sentinel-6 Michael Freilich, and the SWOT mission now maps the sea surface in two dimensions at much finer resolution.

Gravity satellites

The GRACE and GRACE-FO missions measure month to month changes in Earth's gravity. These reveal how much ice the ice sheets and glaciers are losing and how much water the ocean is gaining.

Argo floats

Nearly 4,000 robotic floats measure the temperature and salinity of the upper 2000 m of the ocean. From these we calculate how much the ocean has expanded as it warms.

From raw measurements to a climate record

Making an altimeter measurement

The basic altimeter measurement is a range, the distance from the satellite to the sea surface. Turning that range into sea surface height requires a long list of corrections. Water vapor and free electrons in the atmosphere slow the radar pulse. Waves bias the return toward the troughs. Tides, atmospheric pressure and the solid Earth tide all move the surface and are removed. Each correction is measured by an instrument on the satellite or computed from models.

Stitching missions together

Each altimeter mission lasts roughly a decade, so a climate record needs several. Successive satellites fly in tandem along the same track for months so that any offset between them can be measured and removed. Tide gauges with GPS provide an independent check on instrument drift. This careful calibration is what makes it possible to detect an acceleration of less than a tenth of a millimeter per year per year.

Weighing the ocean

GRACE-FO consists of two satellites about 220 km apart. When the leading satellite passes over extra mass, such as a mountain range or a heavy ice sheet, it is pulled ahead slightly, and the distance between the two changes by a few micrometers. A laser and microwave ranging system measures those changes, and from them scientists build monthly maps of Earth's gravity field.

Reconstructing the past

Before satellites, the global average has to be estimated from a sparse network of tide gauges. Reconstructions combine these records with the spatial patterns learned from the satellite era and with corrections for land motion. Further back, salt marsh sediments, coral reefs and archaeological sites extend the record over thousands of years.

New ways of seeing

The Surface Water and Ocean Topography (SWOT) mission measures a wide swath of the sea surface at once, resolving features ten times smaller than earlier altimeters and reaching right up to the coast. Combined with GPS reflections, radar interferometry for land motion and a growing fleet of small satellites, these tools are bringing coastal sea level into much sharper focus.