A Salvador Daliesque graphic graces Geoff Blewitt’s PowerPoint presentation on the Global Positioning System (GPS). The research geophysicist with the Nevada Bureau of Mines and Geology used cover art from an obscure ’70s “Yes” album, with a title that unwittingly presaged his work, “Tales of Topographic Oceans.” Blewitt and a colleague have found a novel way to measure oceans and discovered that their topography, as well as the entire globe’s, fluctuates seasonally. In geophysicist terminology, the Earth deforms — it bulges and subsides —in response to factors such as icecap loading at the poles during winter.
“It’s almost as if the oceans have little hills and troughs in them in response to the overall shape of the Earth,” he says, adding, “Imagine the whole globe like it’s a balloon and you are squeezing it. It’s changing every way. Its height changes very slightly and the surface is stretched seasonally.”
Blewitt and Peter Clarke, a geoscientist at the University of Newcastle upon Tyne, England, applied quantum mechanics mathematics to satellite data that measures the shape of the Earth. “We calculated how much the sea level changes over the year without actually looking at the ocean at all, just looking at the shape of the Earth,” Blewitt explains, noting that the shape of the Earth defines the bottom of the ocean while the Earth’s gravity field defines the top of the ocean.
“It’s a difficult mathematical problem,” he understates. “It really is a new way of looking at things.”
Blewitt and Clarke, whose research was funded by the National Science Foundation, have been able for the first time to measure the Earth to the millimeter across its diameter — which was part of what attracted Blewitt to the research.
“It’s a challenge to measure something as big as the Earth to the millimeter, first of all,” he says, “and if you can do that, then there’s the problem of interpreting the data. There are dozens of reasons why the Earth might fluctuate. You’ve got to untangle the whole problem. It’s like trying to solve a puzzle.”
An important distinction the geoscientists focused on, according to Blewitt, was that the satellite data didn’t account for whether the sea level was rising due to thermal expansion or due to mass added from seasonal ice cap melting and continental runoff. Thermal expansion is caused by global warming and creates an increase in volume but not weight, while runoff adds weight and thus affects the shape of the Earth. “We can actually use our methodology as a tool to separate the thermal expansion effect from the effect of melting glaciers and ice sheets,” he says.
Taking these factors into account, Blewitt and Clarke found an average yearly sea level increase of about 2 millimeters. From the Earth’s shape, they also calculate that the seasonal runoff of water into the ocean causes sea level to rise and fall nearly 2 centimeters during the year, at its lowest level in March just before the snowpack starts to melt. “It doesn’t change very much,” Blewitt notes, adding that the researchers will continue observations over a decade, looking for other significant trends. They published their paper, “Inversion of Earth’s changing shape to weigh sea level in static equilibrium with surface mass redistribution,” in the Journal of Geophysical Research, Vol. 108, 2003, and presented their findings at the quadrennial International Union of Geodesy and Geophysics conference in Sapporo, Japan on July 8.
A pioneer in GPS research, Blewitt was in charge of the group that developed GIPSY-OASIS (GPS-Inferred Positioning SYstem and Orbit Analysis SImulation Software) software, which set the world standard. GIPSY-OASIS is used in GPS systems to turn measurements taken by satellite-tracking ground receivers into precise coordinates that can be interpreted scientifically. NASA has presented Blewitt with eight awards for GPS research.
Prior to his appointment at Nevada in 1999, he was a professor in the Department of Geomatics, University of Newcastle upon Tyne, England, starting in 1994. From 1985 to 1994, he was group supervisor in the Space Geodesy and Geodynamics Systems Group at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena. He has contributed to four books and co-authored some 60 papers in journals including Nature and Science, as well as given presentations and workshops at some 75 scientific conferences worldwide. In addition, his work has been featured in general-audience publications such as Discover Magazine.
Blewitt earned a bachelor’s of science degree in physics from Queen Mary College, University of London, 1981, and a Ph.D. in physics from the California Institute of Technology, 1986.
Blewitt and Clarke’s paper can be found on Blewitt’s home page: www.nbmg.unr.edu/staff/geoff.htm.
By Jennifer Sanzi
Geophysics doctoral student Emma Hill was the recipient of the “Outstanding Student Paper” award from the American Geophysical Union at its December 2002 conference.
Hill graduated with a degree in geomatics from the University of Newcastle upon Tyne, England, where she studied GPS, remote sensing and cartography. She moved to the United States four years ago at age 21 to continue learning from her adviser, Geoff Blewitt, who with his wife, Debra Vigil, assistant professor of speech pathology and audiology, moved to Reno to accept positions at Nevada.
Hill’s presentation was on the use of Global Positioning Systems (GPS) to study movement at the Yucca Mountain site due to earthquake faults.