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: The impact of changes in terrestrial water storage on global mean sea level variations

During the satellite radar altimeter era (1993 to present), global mean sea level has been increasing at a rate of 3.2 millimeters per year. However, there is considerable year-to-year variation (Figure 1). Much of this variability is understood to be due to changes in land/ ocean precipitation patterns in response to climate variability (El Niño Southern Oscillation, ENSO, Pacific Decadal Oscillation, PDO, etc.). The launch of the Gravity Recovery and Climate Experiment (GRACE) satellite gravity mission in 2002 gave us an additional tool with which to study these variations. GRACE allows us to track changes in land/ ocean water storage, and determine where on the land the water is being stored or lost. In addition, GRACE can tell us how the mass of the water in the oceans is changing due to changes in land water storage (Figure 1).

In the last few years, we have examined these interannual sea level variations by using a combination of tide gauge measurements, satellite radar altimeter measurements, and satellite gravity measurements. In particular, we examined the drop of global mean sea level during the 2011 La Niña by using GRACE estimates of land water storage (Boening et al., 2012; see Figure 2) and found a large water storage signal, driven mainly by Australia (Fasullo et al., 2013). The Australia event was rather unique in the satellite record (Fasullo et al., 2013), but in general, there tends to be more precipitation over land during La Niña events and more precipitation over the oceans during El Niño events.
We also examined PDO’s influence on global mean sea level changes. Hamlington et al. (2013) found that the PDO may have contributed as much as 0.5 millimeters per year to the 3.2 millimeter- per-year trend in global mean sea level since 1993. This means the anthropogenic component of sea level rise may be somewhat smaller than previously thought.
It is important to understand the causes of changes in global mean sea level so we can better isolate the anthropogenic and non-anthropogenic climate signals. The former is likely accelerating, but detecting this acceleration is difficult with the short 21-year satellite record. ENSO and PDO also have significant impacts on regional sea level, which is an important consideration when studying the regional impacts of sea level change.
Boening, C, JK Willis, FW Landerer, RS Nerem, and J Fasullo. 2012. The 2011 La Niña: So strong, the oceans fell. Geophys. Res. Lett. 39:L18607.

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Professor Nerem is a CIRES Professor.