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Sea Level Budget

Do ocean warming and added water explain the rise satellites measure?

Does it add up?

Global mean sea level since 2005 against the sum of ocean warming and added water. Credit: NASA (altimetry, GRACE and GRACE-FO), NOAA NCEI (Argo)
Observed
…mm/yr
Steric + mass
…mm/yr
All records are set to zero over 2005 to 2007 and shown through the latest month where they overlap. Steric height is from Argo floats in the upper 2000 m, three month averages. Ocean mass is from GRACE (2002 to 2017) and GRACE-FO (since 2018). The gap is the eleven months between the two missions.

Where the added water comes from

Land mass change from GRACE and GRACE-FO, in global sea level equivalent. Credit: NASA JPL mascons, computed by slr.earth
Land sources
…mm/yr
Ocean mass
…mm/yr
Mass lost from land is mass gained by the ocean, so the sum of the land sources (dashed) should track the ocean mass record (faint blue) measured directly over the ocean. Record: GRACE and GRACE-FO.

Rates of change

Linear trend with the seasonal cycle removed, in millimetres of global sea level per year

Key takeaways

  • Since 2005, global mean sea level measured by satellites has risen about … mm per year. Ocean warming in the upper 2000 m and water added to the ocean together account for about … mm per year.
  • Added water now contributes more than warming: about … percent of the sum comes from ocean mass and … percent from expansion.
  • Most of the added water comes from melting land ice in Greenland, Antarctica and the world's glaciers. Changes in water stored on land add large swings from year to year.

What is the sea level budget?

Global mean sea level rises for two reasons. Seawater expands as it warms (the steric part), and water is added to the ocean from melting ice and from land (the mass or barystatic part). The sea level budget checks whether these independently measured pieces add up to the total rise that satellites observe.

Three observing systems make this possible. Satellite altimeters measure the height of the sea surface. Argo floats profile the temperature and salinity of the upper 2000 m. The GRACE and GRACE-FO satellites weigh how mass moves around the planet, both the water arriving in the ocean and the ice and water leaving the land.

What are we looking at?

The first chart compares the observed rise with the sum of steric height and ocean mass. When the two lines agree, we understand what is driving sea level rise. The second chart splits the added water by its source: the Antarctic and Greenland ice sheets, mountain glaciers, and land water storage, the water held in soils, aquifers, lakes, reservoirs and snow. The table gives the rate of each piece since 2005, when Argo first covered the global ocean, and over the most recent ten years.

Why doesn't it add up exactly?

Argo floats only reach 2000 m, so warming of the deep ocean is missing from the steric record. Each observing system also has its own errors, and GRACE ocean mass depends on corrections for the slow rebound of the solid Earth since the last ice age. The difference between the observed rise and the sum is usually a few tenths of a millimetre per year, within the combined uncertainty.

How the mass sources are computed

slr.earth sums the monthly JPL RL06.3 mascon solution over land each time a new GRACE-FO month is released. Greenland and Antarctica include the glaciers around their edges, which GRACE cannot separate from the ice sheets. Glaciers are thirteen boxes around the main glacier regions, including Alaska, Arctic Canada, Iceland, Svalbard, the Russian Arctic, High Mountain Asia and Patagonia. These boxes also contain some changes in land water, so the split between glaciers and land water storage is approximate. All remaining land, including the Caspian Sea, is land water storage. Mass is converted to sea level at 362 gigatonnes per millimetre.

Data: global mean sea level from NASA satellite altimetry (Global Mean Sea Level), steric height from NOAA NCEI (Steric Height), ocean mass and land mass from GRACE and GRACE-FO JPL RL06.3 mascons, NASA PO.DAAC (Ocean Mass). Updated as each record is updated.