In 2006, Isabella Velicogna and John Wahr, both at the University of Colorado at Boulder, published two studies that interpreted the first few years of Grace data; their initial paper was about Antarctica’s loss of ice, while the second was about Greenland’s, which appeared to be losing at least 100 billion tons per year. Some scientists were awe-struck. “I remember reading their first paper, and I literally couldn’t believe it,” said Berrien Moore, a dean at the University of Oklahoma who has worked on NASA missions on and off for several decades. “A quantitative measure of a mass change from year to year? It was just unheard-of.” Velicogna, now a professor at the University of California at Irvine and a J.P.L. scientist, told me that the Grace measurements didn’t suddenly make field studies of individual glaciers less important — her data couldn’t tell scientists why the ice sheet was losing mass. But they allowed her to systematically account for drastic losses in places so far-flung that they were almost impossible for human beings to reach, like parts of Greenland and West Antarctica. What’s more, the measurements enabled glaciologists to look at the decline of massive mountain glaciers, like those in Central Asia, which are a critical resource for regional water supplies. As Velicogna noted, those glaciers “could mean the difference between life and death in those places.” They can also lead to profound geopolitical conflicts. Grace soon indicated that many were shrinking.
Similar changes began to be revealed in the world’s hidden aquifers. Jay Famiglietti, a hydrologist at J.P.L. who focuses on tracking changes in groundwater — water stored in underground aquifers around the world — worked as a professor at the University of Texas at Austin in the late 1990s. Back then, a typical way to study aquifers was to monitor wells in the field. When Famiglietti was invited to meetings in Austin to hear about what Watkins and Tapley were planning, he told me: “I didn’t believe it would work. They were all talking about how we’re going to be able to see groundwater. I thought, these guys are nuts.” As the data began coming in, however, Famiglietti found that Grace could measure groundwater with astounding effectiveness. He came up with a nickname for Grace — “the scale in the sky” — and began tracking California’s water supplies during what eventually became a decade of unrelenting drought.
One of Grace’s shortcomings is its limited resolution: It can map increases and losses in only large aquifers. Still, Famiglietti told me that from 2011 to 2015, California lost so much water every year — trillions of gallons — that it showed up clearly in the Grace measurements. About two-thirds of the losses appeared to be groundwater. He also noted that the satellites were able to capture a freshwater predicament bigger than California’s. Before Grace, Famiglietti said, most of our knowledge about underground water reserves was a hodgepodge. “What Grace added was a regional, global understanding — like, holy crap, this is happening all over the world.” In 2015, Famiglietti’s team used Grace to determine that more than half of the world’s largest aquifers were “past sustainability tipping points.” They were being depleted significantly faster than they were being replenished. The Arabian Aquifer System, on which 60 million people rely, appeared severely overstressed; so did water reserves in northwestern India and northern Africa.
Beyond showing declines in the world’s ice sheets and aquifers, Grace clarified the factors influencing the rise in sea levels. In the early 1990s, NASA began putting a succession of Earth satellites into orbit, the most recent being NOAA’s Jason-3, that use a tool called a radar altimeter to measure the ocean’s surface, which has been rising by an eighth of an inch per year since 1993. But in a warming world, sea levels increase for two distinct reasons. The first is that the ocean expands as it gains heat: It just gets bigger. The second is that ice sheets and glaciers melt and break into the ocean. “With an altimetric measure like Jason,” Felix Landerer, a J.P.L. Earth scientist, told me, “you know the height change of the ocean, but you don’t know really what’s causing it.” How much is from heat expansion, in other words, and how much is because of the displacement caused by more ice? Grace, however, constantly weighs the oceans, which makes it possible to determine how much water is pouring into them from melting ice and other sources.
Landerer showed me a video he had just made from Grace data. It showed losses on the Greenland ice sheet from 2002 to 2016. A map of Greenland was white, and the areas with the biggest declines grew yellow and then dark brown and then black with each passing year. Since about 2008, the ice loss has totaled nearly 300 billion tons annually. To watch the progression was to see the entire southeast and southwest coasts of Greenland become increasingly dark over the course of a decade. The ice sheet looked like a piece of paper burning from the edges.
The goal of remote sensing, as Landerer’s map demonstrates, is not merely to measure unmeasured aspects of the planet. It’s to measure them nonstop, ideally for decades on end, so that long-term trends can be identified in a notoriously chaotic and variable natural world. This is why recent calls in Washington to defund some satellite missions rattled so many of the NASA scientists I spoke with. The end of a mission means the data stream will stop or pause, and a gap in the data makes it more difficult to infer whether the melting of an ice sheet or the loss of water from an aquifer is accelerating. Just as crucially, a gap can interrupt a long series of observations that allow researchers to understand future events. “From the perspective of Earth scientists,” Watkins told me, “you need to understand the system so that you can then model that system physically and make forecasts with some skill.” In the case of Grace, such observations could help predict the future of Greenland’s ice, as well as California’s water.
The administration’s exit from the Paris accords this year represents a public retreat from diplomatic engagement on climate change. But its budget priorities — seeking to minimize the need, as well as the means, for gathering climate information — suggest the start of a quieter but arguably more consequential shift: undercutting the very data and evidence that has helped bring urgency to the issue. This strategy has hardly been covert (“We’re not spending money on that anymore,” Mick Mulvaney, President Trump’s budget director, said about climate change), yet the implications are almost certainly more significant than they appear, especially if they become law in a new federal budget. While there have always been policy debates about allocating taxpayer dollars to environmental research, according to Holdren, the fundamental reason we collect the data is because it has long been considered an apolitical “public good,” with a variety of benefits for the nation’s economy, public health and safety. Even the curiosity-driven forays at NASA — undertaken more in pursuit of scientific understanding than a desire to improve, say, storm or drought forecasting — seem to be gaining value in an era of disruptive climate-related events. “About a decade ago, NASA could have gone out and counted easily the users of all its data,” Bill Gail, an executive at the Global Weather Corporation, a forecasting-services company based in Boulder, Colo., told me. But now it’s used by a multitude of local planners, fishermen, agribusiness companies and shippers who want longer-term insights — even if they avoid calling it climate-change forecasting. Sometimes, Gail said, they prefer to call it “long-range weather prediction.”
In the event of a drastic scaling back of our Earth-sciences efforts, is it possible that other countries — China, Japan, India, the nations of Europe — would step up their satellite investments? “They’re a valuable complement,” Waleed Abdalati, a professor at the University of Colorado at Boulder and former chief scientist at NASA, told me. “But to really understand these processes requires more than any one nation can do, and the U.S. has really been the leader. And to say that other nations can pick up the slack is not really accurate.” Stopgap solutions don’t look so promising, either. While Gov. Jerry Brown of California has vowed that his state would send up its own climate-research satellites, the logistics and financial costs — climate satellites now usually require half a billion dollars and a decade to plan, build and launch — would prove formidable, even if backed by California voters. Private-sector satellite companies have in recent years been expanding the business of collecting and selling Earth observational data, but it’s very unlikely that such firms (or a group of tech philanthropists) could adequately replace NASA’s work. “These are projects that are too expensive or require a large and diverse group of collaborators that can only be assembled as an international project,” said Rush Holt, a former Democratic congressman who is now the head of the American Association for the Advancement of Science. “Or this is work that has to be sustained for a longer period of time than any board of directors from a private company would consider, because it’s not clear enough that it would produce a return on investment in anyone’s lifetime.” That explains why the government’s involvement in basic research, going back at least to the late 1940s, was premised on the idea that it fills a role that would not be filled otherwise. As one scientist put it to me, If the government thinks a project is too long-term to make it worth funding, no other organization is going to pay for it either.
Grace, as one of those long missions, has enjoyed some good fortune: Its durability has exceeded expectations, and thanks to international partnerships it has almost certainly dodged contemporary political disruptions. What’s less fortunate, though, is Grace’s current death spiral. Over the past year, a soft-spoken engineer at J.P.L. named Rob Gaston has been focused on extending Grace’s life as long as possible. When I visited him at his office in the spring, he explained that the batteries were already failing and that the fuel, which made adjustments in orientation possible, was nearly exhausted. Another problem was its distance from Earth. “You don’t think about it,” he said, “but there’s actually atmosphere at that altitude, and even though it’s very thin, it does create some friction on a satellite. And it does cause the orbit to degrade.” Even if Grace’s depleted batteries allow it to function for the next few months, its physical demise is approaching; it will begin when the satellites fall to 186 miles in altitude. At the beginning of September, Gaston updated me: Grace was at 201 miles and dropping about 250 feet per day.
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