Measuring changes in freshwater availability from space: a brief and narrow history
During its fifteen years the NASA/German GRACE satellite mission revealed massive shifts in freshwater storage around the world.
Our paper titled “Emerging trends in global freshwater availability” can be found here: Rodell et al., 2018.
In 1997 the Gravity Recovery and Climate Experiment (GRACE) became the second satellite mission selected for NASA’s Earth System Science Pathfinder Program. The Principal Investigator was Byron Tapley of the University of Texas at Austin, where I was a graduate student studying hydrology at the time. Although I had recently proposed a different dissertation topic in passing my qualifying exam, my Ph.D. supervisor (and co-author) Jay Famiglietti recommended, and I agreed, that I instead focus on the potential to apply GRACE for studying large scale terrestrial water storage variability. It was a risky decision, considering that GRACE would not launch for another five years – if it launched – and some scientists doubted that it would ever achieve its ambitious goal to measure changes in Earth’s gravity field with unprecedented precision.
GRACE did launch on 17 March 2002 as joint mission of NASA and the German Aerospace Center. A few months later we saw the first GRACE derived maps of changes in Earth’s gravity field, and they were ugly. A set of north-south stripes obscured most of the signal, rendering the data almost unusable. I was glad that (1) my GRACE hydrology papers had already been published, (2) my Ph.D. was in hand, and (3) my research at the time focused on land surface modeling, not GRACE. But it wasn’t long before some clever geodesists figured out that a filter could be applied to “destripe” the GRACE data1. GRACE hydrology was back in business.
By 2004 the thinning of the ice sheets on Greenland and Antarctica was clear in the GRACE data2, 3. While the fact of their thinning was well known, never before had accurate estimates of their mass loss trends been achievable. It would be a few more years before persistent trends in liquid freshwater storage on land were recognized in the GRACE data. The largest of these was caused by groundwater depletion associated with irrigated agriculture in northern India4, 5.
Following publication of the India studies, which made a huge impact in the water resources community, the hunt was on for other terrestrial water storage trends. Jay and I began labeling features in the global, GRACE-based trend map with both established and speculative causes, and flagging any information that might assist our analysis. We distinguished trends that were natural and likely to fade over time from those caused by climate change or direct human impacts. We knew others would be thinking along the same lines, which motivated us to move quickly, yet explanations for many of the apparent trends were elusive and progress was slow.
The annotated, global, GRACE-derived terrestrial water storage trend map and associated discussion published we published in Nature are the products of nearly a decade of scientific sleuthing, debate, and refinement. It was stressful at times, particularly when we sensed competition, but it was mostly a labor of love. I have many more roles and responsibilities now than I had when I began my career, and in recent years I’ve cherished whatever time I had to scrutinize the map and follow clues to the origins of the apparent freshwater storage trends. Completion of the paper and the end of the GRACE mission in October 2017 have left a hole in my life. Happily, the GRACE Follow On mission launched on 22 May 2018. If we are lucky it will allow us to reevaluate our GRACE based conclusions and pursue new research for another 15 years.
1 Swenson, S., & Wahr, J. (2006). Post‐processing removal of correlated errors in GRACE data. Geophysical Research Letters, 33(8), doi:10.1029/2005GL025285.
2 Velicogna, I., & Wahr, J. (2005). Greenland mass balance from GRACE. Geophysical Research Letters, 32(18), doi:10.1029/2005GL023955.
3 Luthcke, S. B., Zwally, H. J., Abdalati, W., Rowlands, D. D., Ray, R. D., Nerem, R. S., ... & Chinn, D. S. (2006). Recent Greenland ice mass loss by drainage system from satellite gravity observations. Science, 314(5803), 1286-1289, doi:10.1126/science.1130776.
4 Rodell, M., Velicogna, I., & Famiglietti, J. S. (2009). Satellite-based estimates of groundwater depletion in India. Nature, 460(7258), 999, doi:10.1038/nature08238.
5 Tiwari, V. M., Wahr, J., & Swenson, S. (2009). Dwindling groundwater resources in northern India, from satellite gravity observations. Geophysical Research Letters, 36(18), doi:10.1029/2009GL039401.