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Earth Science

Ask Dr. Bryan Killett About Climate Change and GRACE 122

Bryan Killett is a physicist working on the Gravity Recovery And Climate Experiment (GRACE) for NASA's Jet Propulsion Laboratory. GRACE is a joint mission of NASA and the German Aerospace Center which collects satellite data to learn about Earth's changing gravity field, specifically the high frequency changes associated with ocean tides. As the high tide comes in, more water is present, so gravity in that location is temporarily strengthened. These changes are detected with GRACE and used to improve ocean tide models. Dr. Killett provides the open source (GPLv3) code used to process GRACE data on his home page. Bryan has agreed to take a break from measuring gravity fields and answer your questions about GRACE and the climate changes it has revealed. Feel free to ask as many as you like but please confine your questions to one per post.
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Ask Dr. Bryan Killett About Climate Change and GRACE

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  • What in the world was up with this letter from ex-employees [] (also discussed on Slashdot [])? Was that just totally out of left field? Was there an internal reaction to it? Did you respond?
    • Yes, it was totally out of left field. I responded [] in an internal JPL email, and copied the email to my (other) website.

      -Bryan Killett, aka khayman80, aka Dumb Scientist

  • Your work seems to be primarily centering on observing and simulating these changing gravitational fields. What benefit or detriment could result from controlling such tides? From a science fiction standpoint, do you see this as a possibility? What engineering feat would you propose to achieve it? Is there any reason at all why this might one day be a necessity?
    • One modest example is extracting energy from the ocean tides. I've explained [] that harnessing tidal power would actually move the moon farther away from the Earth, even faster than its current ~3.8cm/year recession rate. Tidal amplitudes are influenced by the coastlines and bathymetry, so in principle we might eventually be able to change the tidal amplitudes in some location (bigger for more tidal power, smaller for easier navigation) by carefully modifying the bathymetry.

      Just to clarify the summary, GRACE

  • by eldavojohn ( 898314 ) * <> on Thursday August 02, 2012 @02:07PM (#40858821) Journal
    If you had unlimited resources and unlimited materials (planet sized masses, black hole measuring devices, you name it), what hypotheses and tests would you construct to give us more information on what precisely gravity is?
    • Erm, I could point you to someone working on general relativity or quantum gravity. My analysis only uses Newtonian gravity, though special and general relativistic corrections are applied to the GRACE data before they get to me.
      • I'd be very interested in learning a little bit about the General Relativistic effects observed with GRACE. I hadn't thought about it until you brought it up, but you're right, it does seem to be sensitive enough, and to be observing over a long enough period of time that GR corrections may be observable. Can you observe Lense-Thirring effect? Or are you just correcting geodetic precession?

        • See here [] and here []. With respect to the Lense-Thirring effect, the first abstract suggests you need both GRACE and LAGEOS, but I don't know if they analyzed GRACE by itself.

          • Excellent links. However, note that the first link doesn't use GRACE to detect Lense-Thirring frame dragging. It's merely using improved gravity models from GRACE to eliminate measurement noise due to imperfections in our model of Earth's static gravity field that could be mistaken as frame dragging.
          • See here [] and here []. With respect to the Lense-Thirring effect, the first abstract suggests you need both GRACE and LAGEOS, but I don't know if they analyzed GRACE by itself.


        • No, frame dragging is only barely detectable by Gravity Probe B, which was designed specifically for that purpose. GRACE relies on highly accurate timing, which requires correcting for time dilation due to special and general relativity. Here's a reference [].
  • I always wondered if either of these have any play on the gravitational field and in turn the tides. For example could a very powerful solar storm, in the right conditions, cause a massive tidal shift?
    • No, tides are caused by gravity, which is caused by mass. Solar storms are violent, but the amount of mass involved is miniscule compared to that of the Sun. A very powerful solar storm wouldn't even cause a small tidal shift.
  • by Anonymous Coward

    What does this have to do with Climate Change? There's no reference to it in any of the pages linked. I assume mapping tides is important if sea level rises, but that's just a guess.

    • Melting ice (Score:4, Informative)

      by Ambitwistor ( 1041236 ) on Thursday August 02, 2012 @02:38PM (#40859215)

      GRACE's main use in climate change is to detect the loss of mass from melting glaciers (mostly in Greenland and Antarctica), which results in sea level rise. It can also help map surface currents in the ocean, and track the motion of water through the hydrological cycle.

    • GRACE measures the accelerating mass loss in Greenland [] and Antarctica [], and tracks global water storage [].
    • GRACE also measured the 2005 Amazon drought [], regarded as the worst in over a century. Just five years later, the 2010 Amazon drought [] might have been even more severe.

      GRACE also measured the 2010-2011 floods [] in Australian and Columbia, which dumped so much water on land that sea level temporarily dropped by ~6mm.

  • by Anonymous Coward

    We've seen a lot of surface melting on Greenland over the past few weeks. Does GRACE provide enough detail quickly enough so to quantify that melting, and shed any light on how well we understand ice sheet melting dynamics?

    • The GRACE gravity field data product has monthly time resolution.

    • Other satellites noticed Greenland's extensive surface melt [] because melting snow lowers the ice sheet's albedo []. However, water has the same mass as a liquid or solid, so GRACE can't tell the difference between ice and meltwater. GRACE can measure how much meltwater flows into the ocean, because in that case there would be less mass on Greenland.

      Also, Ambitwistor referred to the popular monthly GRACE fields, which are available as spherical harmonics [] and gridded fields []. In addition, CNES produces 10 day solu []

  • In your field of study is there any evidence to suggest the possibility the poles are shifting, or at least moving uncharacteristically?
    • I've tried to solve for the Earth's nearly-diurnal free wobble [], but it yielded a map that looked suspiciously like the mass trend. I think this happens because the NDFW's frequency is similar to that of the K1 tide, which experiences aliasing in the GRACE data as discussed in section 4.4 of my 2011 paper []. So... no.
  • Focus (Score:2, Interesting)

    by onyxruby ( 118189 )

    Should science drop the "Climate Change" mantra and get back to basics like pollution and sustainability? I believe climate change has become a political boogeyman and that science would be better off focusing on more clearly defined goals (making renewable energy usage more affordable etc).

    • by jo_ham ( 604554 )

      Should science drop the "Climate Change" mantra and get back to basics like pollution and sustainability? I believe climate change has become a political boogeyman and that science would be better off focusing on more clearly defined goals (making renewable energy usage more affordable etc).

      I think you're under the mistaken impression that "scientists" all do one thing at a time.

      We already *are* focussing on renewable energy, improved drugs, advances in medicine, the search for the Higgs field...

      It's only the media and various special interests with a financial stake in discrediting the inconvenient results of climate science that create such a stir. In the actual world of science and research, climate science is just a small part. It gets far more media attention in proportion to the work and

    • Measuring the melting of the Earth's ice sheets, such as the GRACE mission does, is not a clearly defined goal? And should we just stop doing basic geoscience simply because it's politically controversial?

  • by Bootsy Collins ( 549938 ) on Thursday August 02, 2012 @02:50PM (#40859357)
    How have GRACE GPS radio occultation results compared with TEC data from other observations at the same time and along nearby paths (from GPS ground sites, from other radio occultation observations from e.g. C/NOFS or COSMIC, etc.)? Is the GRACE GPS R/O data publicly available? If so, with how much delay? Thanks.
    • For those who may not be aware, GPS signals can be used to measure atmospheric properties. As GRACE (or any satellite with a modern GPS receiver) listens to a GPS satellite that's about to pass below the horizon, the GPS signal passes through [] the atmosphere. Thus the GPS signal is refracted and delayed in ways that can reveal the temperature, pressure, and refractivity of the atmosphere at different altitudes. These are known as GPS occultation measurements.

      I've never used GPS occultation measurements, so I

  • I would like to know if there is an appreciable change in sea surface evaporation rates due to the changes in the gravity at the tidal location. Can the gravitational changes ( though very minute), change weather patterns by channeling atmospheric molecules; or is the gravity force simply much weaker than molecular collisions, wind patterns, etc. ?
    • Roderick et al. 2007 [] shows that pan evaporation rates are dominated by changes in wind speed, with contributions from solar irradiance. You're right to guess that gravity is much weaker than these forces, so changes in gravity don't affect evaporation rates.
  • I recognize that this is a way out there question to the point of making me laugh, but nevertheless, it's a real physics question in the same general domain as GRACE's measurements. A general idea of the magnitudes involved would certainly be interesting.

    By how many orders of magnitude do orbital measurements of local gravity fall short of being able to detect human or human-generated movement on the planet's surface, for example the travel of a train across the country? Related, would the main difficul

    • Partial answer: GRACE has a horizontal spatial resolution of several hundred kilometers. I think its time resolution ends up being monthly, after a lot of post-processing of data from individual orbits (not realtime). So pretty far from what's required. There's talk of a GRACE follow-on mission with 1 angstrom inter-satellite distance resolution (compared to its current micrometer resolution). Not sure what that would translate into in terms of Earth's gravity field resolution.

    • GRACE can resolve nearly uncorrelated mascons that are blocks 400km on each side with a noise floor of ~1cm equivalent water height. (This is latitude dependent because GRACE's denser ground tracks near the poles allow for better resolution.) Each mascon has a mass of ~1.6 gigatons, and a fully-loaded coal train [] is ~10 kilotons, so GRACE falls short by about five orders of magnitude.

      The improved [] laser ranging on the GRACE follow-on will increase sensitivity, and David Wiese [] analyzes improvements due to lowe

  • how do you define 'climate' as opposed to 'weather'?
    • One definition of climate is the statistical accumulation of weather data over long time periods.

    • That's a good question; I described [] the difference between climate and weather at the beginning of my article. I later updated it with a better analogy [] from NOAA: One way to distinguish between weather and climate is that the climate of your hometown will determine how many sweaters you have in your closet. The weather will determine if you should be wearing a sweater right now.

      Many times the climate being discussed is global, so an average is taken over the entire Earth. For global temperatures, Santer et []

    • Also, Barton Paul Levenson [] explains the World Meteorological Organization (WMO) definition of climate, which uses a more scientifically conservative 30 year minimum timespan.
  • If the code and his research are federally funded by the US Government, shouldn't it be released into public-domain instead of GPLv3?

    • There's no requirement to do so. For instance, ask for the code to the Hubble or WMAP telescopes, or most of the other code produced at NASA. You'll probably be disappointed. I personally think that government funded science missions should release their code in some manner, but I don't want to start a license war. Personally, I chose a license rather than releasing my code to the public domain because I was nervous that a corporation might "invent" my code and sue me for using it. If you'd like different t
  • There is obviously a lot of valuable information from the GRACE satellites that continues to change over time. How much longer are the GRACE satellites expected to last? Are there any plans for replacements once they die?

    • GRACE is dying. Its batteries aren't reliable enough to keep the satellites' microwave ranging system running continuously now. The JPL engineering team has worked miracles to give us 10 years of GRACE data, especially since the mission was only supposed to last 5 years. The GRACE follow-on [] mission is scheduled to launch in 2017, and has nifty features like a laser ranging system which is ~1000 times more precise than the current microwave ranging system. Because GRACE almost certainly won't last until 2017
      • Thanks, it seems to me the information from GRACE is so valuable that it's sad it won't be continuous. Will the GRACE follow-on be built to have a longer lifetime or is it more practical just to keep replacing them periodically.

        • I've previously discussed [] some of the advantages of GRACE 2, which is the real successor to GRACE and has a vague launch date in the (hopefully early) 2020s. It'd be great if we could find some way to service and refuel these satellites to extend their lifespans, especially if the low-altitude drag-free option is chosen. But for now we're trying to do what we can with less funding.

Karl's version of Parkinson's Law: Work expands to exceed the time alloted it.