Bouncing lasers off satellites backs faster Greenland melt

NASA’s Goddard Geophysical and Astronomical Observatory in Greenbelt, Maryland, routinely bounces lasers off satellites, to perform Satellite Laser Ranging (SLR). This facility works with other SLR telescopes around the world. Credit: NASA

Japanese and Taiwanese researchers have used old satellite technology to add a decade onto modern gravity-based measurements of Greenland’s ice loss. They used measurements gained by tracking satellites with lasers, getting ‘rough data’ compared to more modern technology, but still confirming an acceleration already seen between the 1990s and 2000s. “There was no mass loss in the 1990s, but there was an acceleration after the year 2000,” says Kosuke Heki from Hokkaido University in Japan.

Though we don’t notice it, Earth’s gravity changes slightly across its surface, because mass isn’t shared equally over it. The link Isaac Newton famously discovered between mass and gravity means that where there’s more mass – for example, where ice sheets sit – gravity is stronger. Since their launch in 2002, twin satellites called Tom and Jerry have been chasing each other around the planet in a game of orbital cat-and-mouse to track these changes. They bounce microwaves from one to the other that scientists can use, among other methods, to track hair’s-breadth changes in the distance between them caused by changes in gravity.

Using Tom and Jerry, also known as the GRACE experiment, scientists have already shown the severity of global warming’s impact on Greenland. They found evidence that Greenland’s ice mass loss is now five times faster than it was in 1992. But for the data from the 1990s there were no gravity measurements. Instead, the researchers worked out the ice mass using laser or radar signals that had been bounced off the glaciers to record their shape, which is known as altimetry .

Down to Earth approach

The LAGEOS I, Laser Geodynamics Satellite, was launched on May 4, 1976. The two-foot diameter, 900-pound satellite still orbits the Earth from pole to pole, providing a way to measure variations in gravity. The mirrored surface of the satellite precisely reflects laser beams from ground stations for accurate Satellite Laser Ranging (SLR) measurements. Credit: NASA/MSFC

Kosuke’s team realised that if they could dig up gravity data, it would avoid possible problems with these approaches. “Laser altimetry is a geometric method, so you can tell the change in the height,” he explained. “But it could be, for example, vertical movement of Earth’s crust underneath the ice or just compaction of the ice without any change in the mass. Combining the gravity and geometric methods is important to compare the results.”

When he was Kosuke’s student Koji Matsuo, who is now at Kyoto University, started trying to get this gravity data from Satellite Laser Ranging (SLR) measurements. This approach bounces lasers off reflectors on satellites to track their movements, such as slight changes in orbit as gravity varies.  Though it dates back to 1964, it has only been good enough for Koji and Kosuke’s purposes since the 1980s. But even then, it is much less accurate than GRACE, so much so that many scientists were sceptical SLR could be used to track ice loss from glaciers. “The idea is very simple, but because it has very bad spatial resolution, not many people have tried it,” Kosuke said. “Other people tried but gave up.”

Gravity data precision is reflected in what’s called their “degree and order”. GRACE itself can reach degree and order 150 and with other data, including from SLR, can surpass 1000. Kosuke points out that a degree and order of two is needed to measure the fact the Earth is a slightly misshapen sphere. From the historic SLR data alone Koji and Kosuke’s team could reproduce gravity variations to degree and order four.

Mind the gap

Koji Matsuo (left) and Kosuke Heki (right) and their teammates have shown that SLR provides another line of evidence that Greenland’s ice loss is speeding up. Credit: Koji Matsuo

“Four is enough to see the bump at Antarctica and the North Pole,” Kosuke said. “As Greenland is so huge, it’s enough to give us some evidence on its ice sheet. We cannot, for example, specify that the ice mass is decreasing in southern Greenland, but if we talk about the whole of Greenland or the whole of Antarctica, then SLR is not so bad.”

Reaching back to 1991, the scientists find that Greenland’s gravity, and therefore its ice mass was almost steady until 2002. In a paper published on Tuesday in Geophysical Research Letters they suggest this could be a result of loss in some areas balancing gain in others. “[A]ltimetry in the last two decades suggested that ice thinning occurred especially in the coastal margins of the southeastern and northwestern Greenland, and ice thickening in the inland plateaus of the southern Greenland,” they write. But from 2003 onwards, the SLR data shows decreasing ice mass. Koji and Kosuke’s measurements match the faster ice loss shown by GRACE and also GPS stations that record the Earth’s crust lifting as the weight of ice decreases.

Koji is now hoping to apply SLR to measure changes in ice in Antarctica. And with appropriate scientific caution, Kosuke would like more data to check the Greenland gravity changes aren’t part of an unknown cycle. But the scientists stress that as well as reaching back into the past, SLR could have an important part to play in such future measurements. That’s because Tom and Jerry are only expected to stay in orbit until 2016 – and a replacement isn’t due to be launched until after 2017.

The changing rate of gravity disturbance around Greenland measured by SLR, from ice loss but not movements in the Earth’s crust. Image copyright Wiley, used with permission, see reference below.

Journal reference:

Koji Matsuo, Benjamin F. Chao, Toshimichi Otsubo, Kosuke Heki (2013). Accelerated ice mass depletion revealed by low-degree gravity field from satellite laser ranging: Greenland, 1991-2011 Geophysical Research Letters DOI: 10.1002/grl.50900