Robert Steven Nerem

• University of Colorado Boulder

The rise in globally averaged sea level—or global mean sea level—is one of the most unambiguous indicators of climate change. Over the past three decades, satellites have provided continuous, accurate measurements of sea level on near-global scales. Here, we show that since satellites began observing sea surface heights in 1993 until the end of 202...

The Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) provided twenty years of data on Earth's time-varying gravity field. Due to their design, GRACE and GRACE-FO are inherently limited in their spatio-temporal coverage, limiting their resolution to a few hundred kilometers and temporally to roughly monthly solutions. T...

Plain Language Summary The satellite altimetry record of global mean sea level (GMSL) change (1993–2020) is now of sufficient length that we can begin to average through some of the natural variability and detect the climate‐driven changes. If we model these changes using simple rate (mm/year) and acceleration (mm/year²) terms, we can extrapolate t...

Regional sea-level changes are caused by several physical processes that vary both in space and time. As a result of these processes, large regional departures from the long-term rate of global mean sea-level rise can occur. Identifying and understanding these processes at particular locations is the first step toward generating reliable projection...

Plain Language Summary Sea levels are rising globally due to human induced climate change. However, local sea levels that impact coastal communities often differ from the global increase, sometimes by a factor of 2 or more. Unfortunately, measuring sea level along the coast can be challenging, as radar satellites are primarily designed for the open...

Satellite altimeters have provided near‐global coverage of the ocean with a continuous record now approaching 3 decades. These observations have led to definitive evidence of an increase in global mean sea‐level, while offering a depiction of the spatial variability in sea‐level change. As a result of the increasing length of the altimeter record,...

Design of Future Smallsat Mass Change Mission Architectures

Simulations of gravity mission architectures using smallsats

The existing multi-mission altimeter observation system meets most of the requirements for monitoring of the absolute sea level and its change over time. However, with the increasing number of the studies on the impact of the sea level rise in coastal areas, a special attention should be paid on reevaluating and improving the absolute sea level mod...

A rise of a global mean sea level recorded during the 20th and 21st century may have significant influence onto the densely populated low-lying coastal regions. The global sea level rise rates at around 3.3 mm a year. Recent studies detected the acceleration of the sea level rise during the last two centuries both from the satellite altimeter and t...

Significance Satellite altimetry has shown that global mean sea level has been rising at a rate of ∼3 ± 0.4 mm/y since 1993. Using the altimeter record coupled with careful consideration of interannual and decadal variability as well as potential instrument errors, we show that this rate is accelerating at 0.084 ± 0.025 mm/y ² , which agrees well w...

Radarska satelitska altimetrija metoda je prikupljanja globalnih visoko-preciznih podataka o razini mora u odnosu na odabrani geocentrični referentni okvir. Opažanja satelitskom altimetrijom omogućuju određivanje oblika i veličine Zemlje te računanje Zemljina polja ubrzanja sile teže što su i osnovne geodetske zadaće zadane definicijom geodezije. M...

Radar satellite altimetry is a method that enables obtaining the global high-precision sea level data related to desired geocentric reference frame. Satellite altimeter observations enable efficient solving of the main geodetic tasks addressed by the definition of geodesy, which include measuring the Earth’s size and shape as well as the determinat...

A rise of a mean global sea level recorded during the 20th and 21st century is one of the most important indicators of climate change, which induces the thermal expansion of the oceans (steric effect) and ice melting. The mean rates of global sea level rise are estimated at around 3.1 mm a year. Those rates are routinely calculated from tide gauge...

Changes in the height of the ocean can be described through the relative and absolute sea level changes depending on the geodetic reference the sea level records are related to. Satellite altimetry provides absolute sea level (ASL) measurements related to the global geodetic reference, whereas tide gauges provide relative sea level (RSL) measuremen...

The mean rate of global sea level rise is estimated to be ~3.1 mm yr−1. Those rates are routinely calculated from tide gauge measurements and satellite altimetry. However, regional rates that are driven by local geophysical processes can vary significantly. Thus, the impact of sea level change in the areas that adjoin the oceans should be evaluated...

Based on the satellite altimeter data, sea level off the west coast of the United States has increased over the past 5 years, while sea level in the western tropical Pacific has declined. Understanding whether this is a short-term shift or the beginning of a longer-term change in sea level has important implications for coastal planning efforts in...

Although much of the focus on future sea level rise concerns the long-term trend associated with anthropogenic warming, on shorter time scales, internal climate variability can contribute significantly to regional sea level. Such sea level variability should be taken into consideration when planning efforts to mitigate the effects of future sea lev...

Internal climate variability across a range of scales is known to contribute to regional sea-level trends, which can be much larger than the global mean sea-level trend in many parts of the globe. Over decadal timescales, this internal variability obscures the long-term sea-level change, making it difficult to assess the effect of anthropogenic war...

In his News and Analysis piece reporting on the newly released fifth assessment report (AR5) by Working Group I (WGI) of the Intergovernmental Panel on Climate Change (IPCC) (“A Stronger IPCC Report,” 4 October, p. [23][1]), R. A. Kerr highlights three fundamental conclusions about climate

ISI Document Delivery No.: 274ZI Times Cited: 0 Cited Reference Count: 0 Church, John A. Clark, Peter U. Cazenave, Anny Gregory, Jonathan M. Jevrejeva, Svetlana Levermann, Anders Merrifield, Mark A. Milne, Glenn A. Nerem, R. Steven Nunn, Patrick D. Payne, Antony J. Pfeffer, W. Tad Stammer, Detlef Unnikrishnan, Alakkat S. payne, antony/A-8916-2008;...

1] Understanding and explaining the trend in global mean sea level (GMSL) have important implications for future projections of sea level rise. While measurements from satellite altimetry have provided accurate estimates of GMSL, the modern altimetry record has only now reached 20 years in length, making it difficult to assess the contribution of d...

In 2011, a significant drop in global sea level occurred that was unprecedented in the altimeter era and concurrent with an exceptionally strong La Nina. This analysis examines multiple data sets in exploring the physical basis for the drop's exceptional intensity and persistence. Australia's hydrologic surface mass anomaly is shown to have been a...

In the interest of improving the sea level climate data record, we compare the 19-year global mean sea level (GMSL) time series derived from TOPEX-Poseidon, Jason-1, and Jason-2 and produced by different research institutions. The GMSL time series are each produced using varying techniques and different applied corrections, and subsequently, the ti...

1] Global mean sea level (GMSL) dropped by 5 mm between the beginning of 2010 and mid 2011. This drop occurred despite the background rate of rise, 3 mm per year, which dominates most of the 18-year record observed by satellite altimeters. Using a combination of satellite and in situ data, we show that the decline in ocean mass, which explains the...

[1] We examine long tide gauge records in every ocean basin to examine whether a quasi 60-year oscillation observed in global mean sea level (GMSL) reconstructions reflects a true global oscillation, or an artifact associated with a small number of gauges. We find that there is a significant oscillation with a period around 60-years in the majority...

The influence of the El Nino/Southern Oscillation (ENSO) on terrestrial water storage is analyzed for the time period 2003-2010 using monthly estimates of continental water storage from the Gravity Recovery and Climate Experiment (GRACE). Peak correlation between NOAA's Multivariate ENSO Index (MEI) and the measured mass anomaly timeseries shows an...

Gravity Recovery and Climate Experiment (GRACE) monthly time variable gravity data for the period January 2003 to December 2010 were used to study the influence of the El Nino Southern Oscillation (ENSO) on global water storage variations. The datasets were detrended and seasonal variations removed. We corrected for Glacial Isostatic Adjustment [Pa...

The determination of the rate of change of mean sea level (MSL) has undeniable societal significance. The science value of satellite altimeter observations has grown dramatically over time as improved models and technologies have increased the value of data acquired on both past and present missions enabling credible MSL estimates. With the prospec...

Cyclostationary empirical orthogonal functions, derived from satellite altimetry, are combined with historical sea level measurements from tide gauges to reconstruct sea level fields from 1950 through 2009. Previous sea level reconstructions have utilized empirical orthogonal functions as basis functions, but by using cyclostationary empirical orth...

We use temporal gravity variations from GRACE to investigate changes in a 34-year time series of Earth's oblateness (J2) observed by satellite laser ranging (SLR). We use 2002-2010 GRACE data to compute the effects of Greenland and Antarctic ice mass variations on J2 (2.0 and 1.7 x 10-11/year respectively). Their combined effect on the J2 trend dur...

Besides a long-term trend identified in sea level records observed by satellite altimetry since the early 1990s, global mean sea level (GMSL) variability is dominated by interannual fluctuations of up to 5-7 mm in a single year. These variations are connected to net precipitation changes over land and oceans, related to the El Nino Southern Oscilla...

The Gravity Recovery and Climate Experiment mission has demonstrated the ability to quantify global mass variations at large spatial scales with monthly to sub-monthly temporal resolution. Future missions of this type taking advantage of improved measurement technologies will be limited by temporal aliasing errors. We suggest the addition of a seco...

The Gravity Recovery and Climate Experiment mission has demonstrated the ability to quantify global mass variations at large spatial scales with monthly to sub-monthly temporal resolution. Future missions of this type taking advantage of improved measurement technologies will be limited by temporal aliasing errors. We suggest the addition of a seco...

The Gravity Recovery and Climate Experiment (GRACE) satellite mission has been estimating temporal changes in the Earth's gravitational field since its launch in 2002. While it is not yet fully resolved what the limiting source of error is for GRACE, studies on future missions have shown that temporal aliasing errors due to undersampling signals of...

We use temporal gravity variations from GRACE to investigate changes in a 34-year time series of Earth's oblateness (J2) observed by satellite laser ranging (SLR). We use 2002-2010 GRACE data to compute the effects of Greenland and Antarctic ice mass variations on J2 (2.0 and 1.7 × 10-11/year respectively). Their combined effect on the J 2 trend du...

The gravity recovery and climate experiment (GRACE) has been providing monthly estimates of the Earth’s time-variable gravity field since its launch in March 2002. The GRACE gravity estimates are used to study temporal mass variations on global and regional scales, which are largely caused by a redistribution of water mass in the Earth system. The...

Extracting secular sea level trends from the background ocean variability is limited by how well one can correct for the time-varying and oscillating signals in the record. Many geophysical processes contribute time-dependent signals to the data, making the sea level trend difficult to detect. In this paper, cyclostationary empirical orthogonal fun...

Future satellite missions dedicated to measuring time-variable gravity will need to address the concern of temporal aliasing errors; i.e., errors due to high-frequency mass variations. These errors have been shown to be a limiting error source for future missions with improved sensors. One method of reducing them is to fly multiple satellite pairs,...

The GRACE mission has provided a wealth of new observations of the Earth system since it's launch in 2002. However, as the GRACE mission comes to the end of its lifetime, there will almost certainly be a gap between GRACE and a follow-on mission (currently planned for 2016). We describe a technique to bridge this gap between the missions using a co...

In this presentation we make the case that satellite and other climate data records are beginning to reveal a holistic picture of water cycle acceleration. Three lines of evidence are described. First, global, satellite-based oceanic precipitation, evaporation and sea level rise datasets are used to estimate a 13-year global discharge time series....

With the most optimistic scenarios predicting the GRACE mission to end around 2015, there is a strong interest in having a follow-on mission to continue the time series of temporal gravity measurements. This study examines the potential benefits of having two pairs of dedicated satellites sampling the gravity field of the Earth. An optimized archit...

Measurement of global water variation provides information critical to climate change and water resource monitoring. The Gravity Recovery and Climate Experiment II (GRACE II) was chosen as a Tier III mission by National Research Council's decadal survey because of its unique ability to measure the global mass distributions and variations in the mas...

1] We examine geoid rates and ocean mass corrections from two published global glacial isostatic adjustment (GIA) models, both of which have been used in previous studies to estimate ocean mass trends from Gravity Recovery and Climate Experiment (GRACE) satellite gravity data. These two models are different implementations of the same ice loading h...

Currently, in most sea-level rise projections computed using semi-empirical models, the well-established Church and White sea-level reconstruction (data 1880-2009) is used. Numerous papers have focused on the variation of the projections for sea level rise by 2100, which ranges between 20 cm (the lower limit of the Intergovernmental Panel on Climat...

The Jason-2 satellite altimeter mission was launched in June 2008, extending the record of precision sea level measurements that was initiated with the launch of TOPEX/Poseidon in 1992 and continued with the launch of Jason-1 in December 2001. We have used the measurements from these three missions to construct a seamless record of global mean sea...

We have used GPS carrier phase integer ambiguity resolution to investigate improvements in the orbit determination for the Jason-1 satellite altimeter mission. The technique has been implemented in the GIPSY orbit determination software developed by JPL. The radial accuracy of the Jason-1 orbits is already near 1 cm, and thus it is difficult to det...

IntroductionEstimates from Proxy Sea-Level RecordsEstimates of Global Sea-Level Change from Tide GaugesEstimates of Global Sea-Level Change from Satellite AltimetryRecommendationsAcknowledgmentsReferences

Cyclostationary empirical orthogonal functions, derived from satellite altimetry, are combined with historical sea level measurements from tide gauges to estimate nearly-global monthly sea level fields from 1900-2010. The reconstructed global mean sea level is shown to compare favorably with the satellite altimetry over the period from 1993-2010. T...

Climate is continually changing on numerous time scales, driven by a range of factors. In general, longer-lived changes are somewhat larger, but much slower to occur, than shorter-lived changes. Processes linked with continental drift have affected atmospheric circulation, oceanic currents, and the composition of the atmosphere over tens of million...

Since its launch in 2002, the GRACE (Gravity Recovery and Climate Experiment) mission has provided new information on water storage variations on the continents, as well as in the ocean and on the ice sheets. As the mission approaches its eighth year of operation, interannual variations in water storage are becoming more clear, and important trends...

The science value of satellite altimeter data has grown dramatically over time as enabling models and technologies have increased the value of data acquired on both current and earlier missions. With the prospect of an observational time series extending into several decades with Jason-1 and the Ocean Surface Topography Mission (OSTM), and later an...

While the expected end-of-lifetime date for the GRACE mission draws nearer, more attention is being given to the design of a follow-on mission for GRACE. The primary purpose of the follow-on mission should be to continue the time series of global mass flux measurements, with a secondary focus on improving the spatial and/or temporal resolution of t...

Advancements in satellite tracking systems, Earth-gravity field models, and the definition of the terrestrial reference frame have led to significant improvements in the orbit determination for low Earth orbit satellites over the last few decades. These improvements have allowed climate-driven variations in the hydrosphere to be detected for the fi...

Don P. Chambers received the 2008 Geodesy Section Award at the 2008 AGU Fall Meeting Honors Ceremony, held 17 December in San Francisco, Calif. The award is given in recognition of major advances in geodesy.

Water is exchanged globally among the ice sheets, continents, and ocean. On seasonal time-scales, we know that the largest exchange is between the oceans and the continents. The magnitude of exchange on longer-periods has not been well quantified, although it is generally believed that melting of ice sheets and continental glaciers will lead to a s...

Mountain glacier ice melt is the leading contributor to global ocean mass increase (Meier et al., 2007; IPCC, 2007), and may continue to dominate for much of this century (Meier et al., 2007). We present an improved estimate of glacial mass loss that combines ground-based observations of mountain glacier mass trends with estimates of surface mass v...

The launch of TOPEX/Poseidon (T/P) in 1992 ushered in a new era in sea level change studies, one that has been continued by the Jason-1 (2001) and OSTM/Jason-2 (2008) missions. The launch of GRACE in 2002 provided another important tool for studying sea level change via the monitoring of water mass movement on the Earth's surface. These missions re...

Thermosphere densities at 400 km altitude from accelerometer measurements on the CHAMP satellite are used to investigate oscillations at periods of less than 13 days during the declining phase of solar cycle 23 (2002-2007). The periodic oscillations around 7 and 9 days in neutral density tend to occur during the latter part of the declining solar c...

We report on an interferometer designed to provide 1 – 10 nm / √ Hz displacement measurement resolution, in the range 0.01 Hz to 1 Hz , while in low Earth orbit. The interferometer comprises two units, each with its own laser and in separate satellites, which would be in the same orbit separated by approximately 50 km . We discuss the requirements...

*† ‡ § ** ¶ # Orbit-mean densities derived from the accelerometer experiment on the CHAMP satellite are used to investigate the global-scale response of the thermosphere at 400 km altitude to variations in geomagnetic activity. Residuals from 5-day running mean densities are used to isolate density variability primarily attributable to changes in s...

1] In this paper we investigate the intra-annual variation in thermosphere neutral density near 400 km using 4 years (2002–2005) of CHAMP measurements. The intra-annual variation, commonly referred to as the ''semiannual variation,'' is characterized by significant latitude structure, hemispheric asymmetries, and interannual variability. The magnit...

1] We report on periodic oscillations in thermosphere density, measured by the accelerometer on the CHAMP satellite during 2006, and relate these periodicities to oscillations observed in solar wind speed and Kp index. Common periodic oscillations at 4–5, 6–7, and 9–11 day periods are observed in the neutral density at 400 km in the 2006 data set,...

Analysis of ocean temperature and salinity data from profiling fl oats along with satellite measurements of sea surface height and the time variable gravity field are used to investigate the causes of global mean sea level rise b etween mid-2003 and mid-2007. The observed interannual and seasonal fluctuations in sea level can be explained as the su...

Since its launch in 2002, the Gravity Recovery and Climate Experiment (GRACE) mission has been providing measurements of the time-varying Earth gravity field. The GRACE mission architecture includes two satellites in near-circular, near-polar orbits separated in the along-track direction by approximately 220km (e.g. collinear). A microwave ranging...

We report discovery of a solar-terrestrial connection between rotating solar coronal holes and density variations in Earth's thermosphere. Specifically, during 2005, a 9-day recurrence of fast streams in the solar wind exists due to solar coronal holes distributed roughly 120 degrees apart in longitude; this periodicity is transmitted to the geospa...

The thermospheric effects of magnetic storm activity occurring in October and November of 2003 are studied using a combination of in situ and remote imaging instruments. The CHAMP (Challenging Minisatellite Payload) satellite near 400 km can detect the slight satellite drag force from which the in situ neutral density is then determined. Global ima...

Satellite altimetry has provided precise globally-distributed sea level measurements since the early 1990s. With 15 years of measurements now in hand, we can start to examine the regional patterns of the trend in sea level change over this interval. In situ ocean termperature measurements (XBTs, ARGO) show that the geographic variations in sea leve...

Analysis of ocean temperature and salinity data from profiling floats along with satellite measurements of sea surface height and the time variable gravity field are used to investigate the causes of global mean sea level rise between mid-2003 and 2007. Between 2004 and 2006, the observed fluctuations in sea level can be explained as the sum of a m...

Sea level change is a sensitive indicator of climate change because it responds both to changes in ocean temperature (where most of the excess heat from climate change is being absorbed) as well as to exchanges of water mass between the continents and the oceans (which is dominated over long time periods by the melting of ice in glaciers and ice sh...

With the emergence and increased use of highly accurate accelerometers for geodetic satellite missions, a new opportunity has arisen to study nonconservative forces acting on a number of satellites with high temporal resolution. As the number of these satellite missions increases, so does our ability to determine the spatial characteristics and tim...

1] Thermosphere densities near 410 km altitude inferred from accelerometer measurements on the Challenging Minisatellite Payload (CHAMP) satellite are analyzed for solar irradiance variability effects during the period 2002–2004. Correlations between the densities and the solar irradiances for different spectral lines and wavelength ranges reveal s...

R. Steven Nerem received the Geodesy Section Award at the 2006 AGU Fall Meeting in San Francisco, Calif. The award is given in recognition of major advances in geodesy. Steve Nerem has been at the forefront of geodetic and oceanographic research since receiving his Ph.D. from the University of Texas at Austin, in 1989. He is currently professor of...

The Gravity Recovery and Climate Experiment (GRACE) was designed to measure variations in the Earth's gravity field from space at monthly intervals. Researchers have used these data to measure changes in water mass over various regions, including the global oceans and continental ice sheets covering Greenland and Antarctica. However, GRACE data mus...

A comment made on the article on estimating future sea level change from past records by Nils-Axel Mörner is presented, where the major errors are pointed out. It is believed that there are so many errors in the article that it completely misinterprets the record of sea level change from the TOPEX/Poseidon satellite altimeter mission. Any satellite...

ARGO hydrographic profiles have vastly improved the sampling of temperature and salinity in the global oceans, in particular by providing relatively frequent observations in regions (e.g. the Southern Ocean) with a sparse climate record. When compared with satellite altimetry and ocean mass estimates from GRACE, these observations, in principle, ca...

The Gravity Recovery and Climate Experiment (GRACE) mission has been providing unprecedented levels of accuracy in measuring the Earth's gravity field since its launch in 2002. GRACE employs a mission architecture consisting of a collinear satellite pair, constraining the observable to solely the along-track direction. This constraint leads to long...

The GRACE mission has been producing monthly estimates of changes in the Earth's gravity field since April 2002. Converting the raw GRACE range, accelerometer, and GPS measurements into estimates of the gravity field is a complex process, and therefore different analysis groups use various "recipes" resulting in different models of the time-varying...

The Gravity Recovery and Climate Experiment (GRACE) has ushered in a new era for satellite measurements of the Earth system. GRACE provides monthly estimates of the time-varying gravity field, which are largely due to the redistribution of water mass in the Earth system, with a spatial resolution of ~500 km and an accuracy of 1 cm equivalent water....

Mass changes of the Greenland Ice Sheet resolved by drainage system regions were derived from a local mass concentration analysis of NASA–Deutsches Zentrum für Luftund Raumfahrt Gravity Recovery and Climate Experiment (GRACE mission) observations. From 2003 to 2005, the ice sheet lost 101 ± 16 gigaton/year, with a gain of 54 gigaton/year above 2000...

1] In this paper we report the first measurements of the thermosphere neutral density response to a solar EUV flare. Two X17 flares are considered: The first on 28 October, 2003, and the second on 4 November, 2003. The density measurements are provided by accelerometers on the GRACE and CHAMP satellites near 490 km and 400 km, respectively. X-ray f...

Understanding of sea-level change has improved considerably over the last decade. Present-day knowledge of sea-level change is derived from tide gauge observations and satellite altimetry measurements. The average rate of sea-level change obtained from tide gauges over the last 50 years is +1.8 ± 0.3 mm yr −1 . In comparison, altimeter measurements...

NASA's Earth Science Technology office has funded an effort to advance the technology of interferometric ranging in Low Earth Orbit. The goal of this work is to reduce risk for a follow-on gravity mission to the gravity recovery and climate experiment (GRACE) by demonstrating performance of a heterodyne interferometric laser ranging instrument appr...

With the emergence and increased use of highly-accurate accelerometers for geodetic satellite missions, a new opportunity has arisen to study non-conservative forces acting on an array of satellites with high temporal resolution. As the number of these satellite missions increases, so does our ability to determine spatial characteristics and time r...

Our understanding of sea level change has improved consider ably over the last d ecade. Present- day knowledge of sea- level change is der ived from tide gauge observations and satellite altimetry measurements. The aver age rate of sea lev el change obtain ed from tide gauges ov er the last century is +1.8 mm/y ear. In comp arison, altimeter measur...

1] Densities in the 400–500 km height region inferred from accelerometer measurements on the CHAMP and GRACE satellites are utilized to study the response to the isolated and severe geomagnetic storm of 20–21 November 2003. The CHAMP and GRACE satellites provide data at approximate local times of 1110/2310 and 1430/ 0230 hours, respectively. In a g...

We report on interferometric range transceiver technology demonstration. The goal of this work is to reduce risk for a follow-on gravity mission to the Gravity Recovery and Climate Experiment (GRACE) by demonstrating performance of a heterodyne interferometric laser ranging instrument appropriate for a follow-on gravity mission to GRACE at the NASA...

On multi-decadal time scales or longer, the most important processes affecting sea level are those associated with the mass balance over the Earth's ice sheets. The vulnerability of the cryosphere to climate change along with the difficulty in acquiring uniform in situ observations in these inhospitable regions, makes the problem of understanding i...

The Gravity Recovery and Climate Experiment (GRACE) has been providing monthly estimates of the Earth's time variable gravity field since its launch in March 2002. The GRACE gravity estimates are used to study temporal mass variations on global and regional scales, which are largely caused by a redistribution of water mass in the Earth system. The...

Tri-axial accelerometers on the CHAMP and GRACE satellites offer a unique opportunity to observe the global-scale response of the thermosphere densities and winds to solar flares and coronal mass ejections. Since March, 2002, these instruments have provided contemporaneous measurements between about 400 and 550 km, nearly pole-to-pole, and at four...