Steve Nerem News

Seminar - Satellite Altimeter and Gravity Measurements: What they are telling us about how the Earth is changing - Feb 10

Anonymous — Mon, 10 Feb 2020 16:15:47 +0000

Seminar - Satellite Altimeter and Gravity Measurements: What they are telling us about how the Earth is changing - Feb 10 Anonymous (not verified) Mon, 02/10/2020 - 09:15

Steve Nerem
Professor, Smead Aerospace
Monday, Feb. 10 | 12:30 P.M. | AERO 111

Abstract: We now have more than 27 years of satellite altimeter measurements of sea level change (TOPEX/Poseidon, Jason-1, 2, and 3) and 17 years of satellite gravity measurements of mass change (GRACE and GRACE Follow-On). These measurements have helped us determine how much sea level is rising and perhaps more importantly, why sea level is rising. These measurements and the science they have facilitated will be reviewed. In addition, prospects for improving these measurement systems in the future using new instruments and satellites will be discussed.

Bio: Steve Nerem is a professor in Aerospace Engineering Sciences at the University of Colorado. He is currently Associate Director of CU’s Colorado Center for Astrodynamics Research, and a Fellow of CU’s Cooperative Institute for Research in Environmental Sciences. Prior to joining the university, he spent 6 years at NASA/Goddard Space Flight Center as a geophysicist, and over 4 years at the University of Texas at Austin as an assistant and associate professor. He is a specialist in satellite remote sensing and geodesy, with a main research focus on measuring sea level change using satellite altimetry and satellite gravity data. He became a Fellow of the American Geophysical Union in 2008. He is a member of the National Academies Committee on Earth Science and Applications from Space (CESAS).

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Uneven Rates of Sea Level Rise Tied to Climate Change

Anonymous — Tue, 04 Dec 2018 16:52:10 +0000

Uneven Rates of Sea Level Rise Tied to Climate Change Anonymous (not verified) Tue, 12/04/2018 - 09:52

The pattern of uneven sea level rise over the last quarter century has been driven in part by human-caused climate change, not just natural variability, according to a new study.

The findings suggest that regions of the world where seas have risen at higher than average rates — including the Eastern Seaboard of the United States and the Gulf of Mexico — can expect the trend to continue as the climate warms.

The study, published today in the Proceedings of the National Academy of Sciences, was authored by scientists John Fasullo at the National Center for Atmospheric Research (NCAR) and Steve Nerem at the University of Colorado Boulder.

“By knowing that climate change is playing a role in creating these regional patterns, we can be more confident that these same patterns may linger or even intensify in the future if climate change continues unabated,” Fasullo said. “With sea levels projected to rise a couple of feet or more this century on average, information about expected regional differences could be critical for coastal communities as they prepare.”

The research was funded by the National Science Foundation, which is NCAR's sponsor, the NASA Sea Level Change Team, and the U.S. Department of Energy.

Finding the signal of climate change

For the study, Fasullo and Nerem, both members of the NASA Sea Level Change Team, analyzed the satellite altimetry sea level record, which includes measurements of sea surface heights stretching back to 1993. They mapped global average sea level rise as well as how particular regions deviated from the average.

For example, the oceans surrounding Antarctica and the U.S. West Coast have had lower-than-average sea level rise, while the U.S. East Coast and Southeast Asia, including the Philippines and Indonesia, have experienced the opposite. In some parts of the world, the rate of local sea level rise has been as much as twice the average.

Regional differences in sea level rise are influenced by where heat is stored in the ocean (since warm water expands to fill more space than cold water) and how that heat is transported around the globe by currents and wind. Uneven sea level rise is also influenced by ice sheets, which lose mass as they melt and shift the gravitational forces affecting regional sea surface height.

Regional sea level trends in millimeters per year from 1993 through mid-2018 with the global average rate removed. Red colors indicate that the local rate of sea level rise was greater than average, and blue colors indicate the opposite. (Image: PNAS)

Natural shifts in ocean cycles — including the Pacific Decadal Oscillation, a pattern of sea surface temperatures similar to El Niño but longer lasting — are therefore known to affect sea levels. So scientists were not surprised to find that as the ocean rises, it rises unevenly. But it's been difficult to say whether these natural cycles were the dominant influence on regional differences

To investigate the role of climate change, the scientists turned to two sets of climate model runs, known as “large ensembles”: one created using the NCAR-based Community Earth System Model and one created using the Earth System Model at the National Oceanic and Atmospheric Administration. These large ensembles — many model simulations by the same model, describing the same time period — allow researchers to disentangle natural variability from the impacts of climate change. With enough runs, these impacts can be isolated even when they are relatively small compared to the impacts from natural variability.

The climate models suggest that in regions that have seen more or less sea level rise than average, as much as half of that variation may be attributed to climate change. The scientists also found that the impacts from climate change on regional sea level rise sometimes mimic the impacts from natural cycles.

"It turns out the sea level rise response to climate change in the Pacific resembles what happens during a particular phase of the Pacific Decadal Oscillation," Fasullo said. "This explains why it's been so difficult to determine how much of the pattern was natural or not, until now."

Improving forecasts

The research findings have implications for local officials, who are interested in improved forecasts of sea level rise for the areas they oversee. In the past, forecasters have had to rely on the global rate of change — about 3 millimeters a year and accelerating — and knowledge of the uneven regional impacts associated with continued melting of the ice sheets covering Greenland and Antarctica.

The findings add the possibility that the regional patterns of sea level rise tied to climate change can also be included, because the models predict that the regional patterns observed in the satellite measurements will continue into the future.

"We now have a new tool — long-term satellite altimeter measurements — that we can use to help stakeholders who need information for specific locations," said Nerem, a fellow of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder and a professor of aerospace engineering.

About the article

Title: Altimeter-Era Emergence of the Patterns of Forced Sea Level Rise in Climate Models and Implications for the Future

Authors: John T. Fasullo and R. Steven Nerem

Journal: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1813233115

About UCAR/NCAR: The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation.

CIRES is a partnership of NOAA and CU Boulder.

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Sea Level Rise Accelerating

Anonymous — Tue, 13 Feb 2018 18:28:13 +0000

Sea Level Rise Accelerating Anonymous (not verified) Tue, 02/13/2018 - 11:28

A research team led by aerospace professor Steve Nerem detects an acceleration in the 25-year satellite sea level record.

Global sea level rise is not cruising along at a steady 3 mm per year, it’s accelerating a little every year, like a driver merging onto a highway, according to a powerful new assessment led by CIRES Fellow Steve Nerem. He and his colleagues harnessed 25 years of satellite data to calculate that the rate is increasing by about 0.08 mm/year every year—which could mean an annual rate of sea level rise of 10 mm/year, or even more, by 2100.

“This acceleration, driven mainly by accelerated melting in Greenland and Antarctica, has the potential to double the total sea level rise by 2100 as compared to projections that assume a constant rate—to more than 60 cm instead of about 30.” said Nerem, who is also a professor of Aerospace Engineering Sciences at the University of Colorado Boulder. "And this is almost certainly a conservative estimate," he added. "Our extrapolation assumes that sea level continues to change in the future as it has over the last 25 years. Given the large changes we are seeing in the ice sheets today, that's not likely."

If the oceans continue to change at this pace, sea level will rise 65cm (26 inches) by 2100—enough to cause significant problems for coastal cities, according to the new assessment by Nerem and several colleagues from CU Boulder, the University of South Florida, NASA Goddard Space Flight Center, Old Dominion University, and the National Center for Atmospheric Research. The team, driven to understand and better predict Earth’s response to a warming world, published their work today in the journal Proceedings of the National Academy of Sciences.

Rising concentrations of greenhouse gases in Earth’s atmosphere increase the temperature of air and water, which causes sea level to rise in two ways. First, warmer water expands, and this "thermal expansion" of the oceans has contributed about half of the 7 cm of global mean sea level rise we've seen over the last 25 years, Nerem said. Second, melting land ice flows into the ocean, also increasing sea level across the globe.

These increases were measured using satellite altimeter measurements since 1992, including the U.S./European TOPEX/Poseidon, Jason-1, Jason-2, and Jason-3 satellite missions. But detecting acceleration is challenging, even in such a long record. Episodes like volcanic eruptions can create variability: the eruption of Mount Pinatubo in 1991 decreased global mean sea level just before the Topex/Poseidon satellite launch, for example. In addition, global sea level can fluctuate due to climate patterns such as El Niños and La Niñas (the opposing phases of the El Niño Southern Oscillation, or ENSO) which influence ocean temperature and global precipitation patterns.

So Nerem and his team used climate models to account for the volcanic effects and other datasets to determine the ENSO effects, ultimately uncovering the underlying sea-level rate and acceleration over the last quarter century. They also used data from the GRACE satellite gravity mission to determine that the acceleration is largely being driven by melting ice in Greenland and Antarctica.

The team also used tide gauge data to assess potential errors in the altimeter estimate. “The tide gauge measurements are essential for determining the uncertainty in the GMSL (global mean sea level) acceleration estimate,” said co-author Gary Mitchum, USF College of Marine Science. “They provide the only assessments of the satellite instruments from the ground.” Others have used tide gauge data to measure GMSL acceleration, but scientists have struggled to pull out other important details from tide-gauge data, such as changes in the last couple of decades due to more active ice sheet melt.

“This study highlights the important role that can be played by satellite records in validating climate model projections,” said co-author John Fasullo, a climate scientist at the National Center for Atmospheric Research. “It also demonstrates the importance of climate models in interpreting satellite records, such as in our work where they allow us to estimate the background effects of the 1991 eruption of Mount Pinatubo on global sea level.”

Although this research is impactful, the authors consider their findings to be just a first step. The 25-year record is just long enough to provide an initial detection of acceleration—the results will become more robust as the Jason-3 and subsequent altimetry satellites lengthen the time series.

Ultimately, the research is important because it provides a data-driven assessment of how sea level has been changing, and this assessment largely agrees with projections using independent methods. Future research will focus on refining the results in this study with longer time series, and extending the results to regional sea level, so they can better predict what will happen in your backyard.

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