John Wahr
Professor, Physics
E-mail: wahr@lemond.colorado.edu
Office: Duane Physics F727
Phone: 303-492-8349
Web: Department of Physics
Research Interests
Theoretical geophysics; earth rotation; earth and ocean tides; earth/ocean and earth/atmosphere interactions; terrestrial normal modes; crustal deformation.
Current Research: Applications of Time-Variable Gravity Measurements from GRACE
Funding: NASA, JPL, NSF
Mass variability summed over the entire Greenland Ice Sheet, monthly Gravity Recovery and Climate Experiment (GRACE) results (black line; the orange line is a smoothed version) April 2002 and December 2009.
Distribution of mass loss rate across Greenland, as determined from
the GRACE solutions.
The GRACE (Gravity Recovery and Climate Experiment) satellite mission, launched by NASA and the German Space Agency in March 2002, is providing global maps of the Earth’s gravity field to astonishing accuracy every month. Because the Earth’s gravity field is caused by its mass distribution, time-variations in gravity as determined from GRACE data can be used to estimate month-to-month changes in the Earth’s mass distribution. GRACE can recover mass variability at scales of about 250-300 km and larger.
Accomplishments
We have been using these data to look at a number of geophysical signals, particularly those that involve the storage of water (including snow and ice) on continents and in the polar ice sheets.
For example, because of its large effective footprint and its sensitivity to mass, GRACE offers the best available method for measuring the total mass balance of the polar ice sheets. The top figure at right shows monthly GRACE results (black line; the orange line is a smoothed version) for the mass variability summed over the entire Greenland ice sheet, between April 2002 and November 2013. The trend of the best fitting straight line is about 290 km3/yr of ice volume lost per year, which generates enough melt water each year to cover all of Colorado to a depth of 1 meter. There is a notable downward curvature to the results, indicating that the mass loss rate has been increasing during this time period. The lower figure shows how this mass loss rate is distributed across Greenland, as determined from the GRACE solutions. By far the largest rates occur in the southeast (note the break in the color scale), where dramatic acceleration of outlet glaciers and accompanying ice thinning has been observed over the last decade. But mass loss has also been occurring up along the western ice sheet margin, and even up in the northeast, particularly within the last few years. There appears to have been a modest mass gain in the northern interior, presumably associated with increased accumulation rates there.
Publications
Click here for a complete list of published works »
Honors and Awards
- Elected Member of the National Academy of Sciences (2012)
- Charles A. Whitten Medal of the American Geophysical Union (2006)
- Vening Meinesz Medal of the European Geosciences Union (2004)
- Vening Meinesz Medal from Utrecht and Delft Universities in The Netherlands (1998)
- James B. Macelwane Award of the American Geophysical Union (1985)
- Fellow of the American Geophysical Union (1985)
- Guy Bomford Prize for Geodetic Researchfrom the International Association of Geodesy and London’s Royal Society (1983)
Professor Wahr is a professor at the University of Colorado at Boulder.
