Cooperative Institute for Research in Environmental Sciences

R. Steven Nerem

Research Interests

Satellite altimetry, sea level change, Earth gravity field determination, time variations of the Earth's gravity field, planetary geodesy, precision orbit determination, astrodynamics.

Current Research: Satellite Observations of Present Day Sea-Level Change

Observations of longterm sea-level change can provide important corroboration of climate variations predicted by models and can also help us prepare for the socioeconomic impacts of sea-level change. The Topography Experiment- Poseidon (TOPEX/ Poseidon, 1992), Jason-1 (2001), and Jason-2 (2008) satellites have observed a mean rate of sea-level rise of 3.4 mm/year since 1993 (Figure 1). My current research focuses on determining the causes of this change and relating the satellite record of sea-level change to the longer-term record from tide gauges.

A relatively new technique has been developed that allows the direct measurement of the continental water contributions from space. The Gravity Recovery and Climate Experiment (GRACE) satellite mission has precisely measured temporal variations in Earth’s gravitational field since 2002. As the melting of ice in mountain glaciers and ice sheets, in addition to other runoff, adds water mass to the oceans, GRACE has demonstrated the ability to directly measure this change in mass. GRACE can also determine the relative contributions of different areas on the continents. At seasonal frequencies, GRACE ocean-mass estimates have been shown to compare quite well with estimates from satellite altimetry corrected for thermal expansion using shipboard measurements. The seasonal variations in ocean-water mass are due to the seasonal exchange of water with the continents, and thus GRACE measurements are expected to make their greatest impact on studies of the global water cycle. However, eventually GRACE should help explain the differences we have seen between the altimetry and ocean-temperature measurements (Figure 2), since in theory, these are due to changes in global ocean mass.

The biggest advance we made in the past year was a better understanding of the rapid changes occurring in Greenland, and their contribution to sea-level rise. This is largely due to the GRACE mission, which shows Greenland and Antarctica contributing 0.6 and 0.5 mm/year to sea level rise, respectively. Of the observed 3.4 mm/year global averaged sea-level rise, approximately one-third is now thought to be due to the warming of the oceans (thermal expansion), one-third due to the melting of ice in mountain glaciers, and the rest due to other exchanges of freshwater with the continents, including ice melt from Greenland and Antarctica. The total rise is significantly greater than has been observed during the last half of the 20th century from tide gauges (about 1.8 mm/year). A substantial part of our future work will involve helping assemble the sea-level chapter of the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5) due in 2013.

Satellite altimeter and gravity measurements are expected to have major roles in the formulation of the IPCC AR5. Satellite altimetry has conclusively shown that sea-level rise has been greater during the last 17 years than during the last century. The record of ice-mass changes from the GRACE mission (eight years), while too short to definitively detect climate signals, has demonstrated the ability to measure changes in the mass of the oceans and the mass of the polar ice sheets. Thus, as this time-series lengthens, it is expected that satellite gravity missions will play a role equal to satellite altimetry in diagnosing the magnitude of sea-level change and its causes.

Publications

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Professor Nerem is a member of the CIRES Professor.