With 15 years of monitoring how water and ice are distributed on our planet, the GRACE mission proved its value for monitoring and studying the global water cycle so thoroughly that the recent Earth Science Decadal Survey report released by the National Academy of Sciences in December 2017 listed measurement of mass changes as one of five top priorities in Earth observations for the next decade. The same recognition of GRACE's value led to the recommendation in NASA’s 2010 Climate Architecture report to develop GRACE-FO as a directed NASA mission.
WHY WE NEED
"Everything flows." These famous words by the ancient Greek philosopher Heraclitus express the truth that our complex world is ever-changing and evolving. Even Earth’s gravity is continually changing. The pull of gravity varies naturally from place to place on Earth, depending on the mass at the surface; greater mass exerts a stronger gravitational pull. But this geographically varying gravity itself also changes constantly as mass moves in time. The land and ocean move and change relatively slowly, but water mass on the land -- or above or below it -- changes daily.
On Earth, the processes that cause water to move and change are fundamental to today's most pressing climate science challenges. But keeping track of Earth’s evolving water cycle is a formidable task. Water constantly changes its shape, form and state as it moves between oceans, atmosphere and land. It can be in plain sight in a lake or hidden in an aquifer underground. It can evaporate from Earth's surface in moments after a rain shower or be stored for hundreds or thousands of years as ice in glaciers and ice sheets.
But regardless of whether water is solid, liquid or vapor, visible or invisible, it has one attribute that does not change: its mass. And since mass has a gravitational pull associated with it, engineers and scientists have come up with a unique twin-satellite observing system that uses the changing forces of gravity to track and follow Earth’s water masses from the top of the Himalaya to the deepest ocean depths and deep underground. The record of changing gravity and changing mass, combined with sea surface height observations, is directly linked with the heat trapped on Earth by greenhouse gases and stored in the ocean.
The concept of using two satellites flying in formation to measure gravity and its changes can be traced back to the late 1960s. But it wasn’t until the 1990s that advances in technology, notably GPS, enabled the development of a viable flight mission. Researchers and engineers led by Byron Tapley of the University of Texas at Austin and Michael Watkins, now director of NASA's Jet Propulsion Laboratory in Pasadena, California, came together and realized the unique potential for new Earth observations and data that could be gleaned from measuring variations in gravity. They proposed the GRACE mission to NASA and won the competition under NASA’s Earth System Science Pathfinder program.
Ocean currents distribute heat across the planet, and thus any changes in ocean currents are an important indicator of how our planet is responding and evolving in a warming climate. At the bottom of the atmosphere -- on Earth's surface -- changes in air pressure (a measure of air’s mass) tell us about flowing air, or wind. At the bottom of the ocean, changes in pressure tell us about flowing water, or currents. Felix Landerer and colleagues at JPL developed a way to isolate the signal in GRACE data that indicates tiny pressure variations at the ocean bottom caused by changes in deep ocean currents. The measurements showed a significant, temporary weakening of the Atlantic meridional overturning circulation, a crucial global climate regulator that transports vast amounts of heat from lower to higher latitudes. A network of ocean buoys that spans the Atlantic near 26 degrees north latitude also recorded this drop in the winter of 2009-10, but GRACE enabled the detection of this signal several thousand miles north and south of the buoys' latitude, providing new insights into large-scale ocean current processes. The measurements from GRACE agreed well with estimates from the buoy network, giving the researchers confirmation that the technique can be expanded to provide estimates throughout the Atlantic and beyond.
Although most of GRACE's discoveries have involved water and ice, its data are useful in other areas of Earth science. A recent research project suggested that eventually GRACE soil moisture data might help in forecasting regional wildfire risks. Thus, the satellites' data have now been applied to all four elements of the ancient cosmos -- earth, water, fire and air -- an appropriate indicator of how their use is expanding.