Global Mean Sea Level
Global mean sea level provides an integrative measure of the state of the climate system, encompassing both the ocean and cryosphere (ice covered portions of Earth), and it can be viewed as an important indicator of what is happening to the climate in the present and what may happen in the future. Long-term changes in global mean sea level (GMSL) are predominantly driven by three processes:
Ice Melt: Due to the warming atmosphere and ocean, ice sheets and mountain glaciers are melting, resulting in the addition of fresh water into the ocean.
Thermal Expansion: Ocean water expands as it absorbs trapped heat, causing sea levels to rise.
Land Water Storage: Water that is either removed from land (through groundwater pumping, for example) or impounded on land (through dam building, for example) can cause a net change in the total water found in the ocean.
Observations and the GMSL "Budget"
Changes in GMSL have been measured by satellite altimeters for the past 27 years. With the launch of the Sentinel-6/Michael Freilich satellite in 2020, this record will soon surpass three decades. The rate of GMSL rise from 1993 to present has been measured at 3.4 millimeters per year, and there are indications that the rate of GMSL rise has increased during the satellite altimeter record. Other systems in the sea-level observation network provide estimates of the individual process contributing to GMSL rise. The GRACE and GRACE-FO satellites measure mass-driven changes in GMSL associated with ice melt and changes in land water storage. The Argo profiling floats measure temperature and salinity changes in the global ocean, and provide an estimate of thermosteric GMSL change. Combining the GMSL estimates from GRACE/GRACE-FO and Argo and then comparing to GMSL from satellite altimeters allows us to check our understanding of the contributors to GMSL and see if the "global sea level budget" can be closed.