TDAI affiliate Lei Wang, assistant professor of civil, environmental, and geodetic engineering, uses satellite data to understand phenomena that is at once too great and too subtle to accurately study from on-the-ground measurements. His work looks at changes caused by earthquakes, water use, and melting ice masses, and he recently received an NASA Early Career Award in Earth Science that will allow him to focus on a particularly urgent and difficult problem for the next three years.
“My overall interest is in geodesy, and my hire was in translational data analytics. I specialize in acquiring, analyzing and interpreting space-based geodetic measurements,” says Wang. He uses GPS data, satellite altimetry, Interferometric Synthetic Aperture Radar (InSAR) data, and Gravity Recovery & Climate Experiment (GRACE) data to determine how the world is changed by naturally occurring and human-caused phenomena. GRACE is a collaboration between NASA and the German Space Agency, DLR. The data is collected using two satellites that have been in use since 2002.
One use of GRACE data is to assess the strength of earthquakes, particularly very large ones, such as the 2004 earthquake in the Indian Ocean that caused a series of tsunamis affecting countries all along that body of water. “What’s the total amount of seismic energy that has been relayed, and what still remains? We can use GRACE to estimate the total seismic energy release of such an earthquake,” Wang says.
With drought, such as the recent water shortages in California, GRACE data can be used to determine the water storage change after years of depletion. As the drought persisted over years, farmers in the Central Valley drew water from the aquifer to maintain their crops. “GRACE data can tell us how much water remains,” Wang says. “It’s very hard to quantify underground water change. Using this data can give us a better picture of that.” However, this is not measurement as we normally think of it. “GRACE is not measuring water; it’s measuring gravity.” The amount of water underground very subtly affects the gravitational pull of the Earth, which Wang and his colleagues can calculate when analyzing the data.
The geodetic data can also be used to look at how climate change is affecting snowfall. “We can see the crust has seasonal variations. In the winter it subsides, in the summer it uplifts,” Wang explains. “During the winter, there’s a lot of snowfall in the mountains, such as Sierra Nevada. Due to the accumulated snow, there’s more weight on the bedrock, and this will press the crust down. During the summer, the snow melts, the crust comes up, and GPS can measure these subsidence and uplift of the ground.” However, in recent years, there has been rapid uplift over Sierra Nevada mountain range, indicating the amount of snowfall accumulation is gradually decreasing. “We can see there’s fewer snow added during the years,” he says.
Wang’s NASA Early Career Award in Earth Science will allow him to focus for three years on the Greenland ice sheet.
While many such projects use one form of data–GPS, Altimetry, InSAR, or GRACE–Wang plans to integrate them for a more complete picture. “Each of the techniques has a temporal and spatial resolution, and their strengths and limitations. My work is how to integrate these multiple types of data so we can make the most of the diverse geodetic data,” he says.
This project is particularly urgent. “The Greenland ice sheet is the biggest contributor to the present sea level change because it is melting most rapidly. I propose to quantify the Greenland ice mass change. If the Greenland ice sheet completely melted, the global sea level would rise by seven meters. Miami would disappear.
“To predict future sea level change, we need to accurately quantify the current change of Greenland ice sheet.” However, changes that took place on Earth 16,000 years ago add a layer of complexity. ”It’s a complicated system. There’s many other geophysical processes that are going on,” says Wang. “One is present-day elastic deformation of the crust associated with the ice mass change. The other is glacial isostatic adjustment.” During the last ice age, portions of the Earth’s crust covered by ice sank, while places around them rose, just as a couch cushion becomes indented where you sit and rises on either side of you. Similarly, as the couch cushion still has an indentation after you stand and takes time to resettle, the Earth’s crust is still shifting in response to the ice that melted thousands of years ago. This is glacial isostatic adjustment (GIA).
“GIA is contaminating the ice mass changes estimated by geodetic measurements. We need to separate the GIA signal from the ice mass signal in the space-based geodetic observations,” says Wang. “One option is to adopt an external GIA prediction. However, the problem for the GIA prediction is people need to adopt a lot of simplifications and assumptions. The GIA model is not perfect, there are a lot of uncertainties….This is the great limitation of the quantification of the ice mass change over Greenland…It’s not easy to separate them out.” Wang proposed that successfully integrating GRACE, GPS, satellite altimetry and InSAR data will provide the most comprehensive assessment possible of massive climate-driven geophyscial changes.