Melting Glaciers Muck Up Earth’s Gravitational Field
Clever use of gravity data shows that glacial retreat really is a global phenomena.
By George Musser

SINGAPORE—Photographs never quite capture the sparkling blue tint of glacial ice, so when I visited the Perito Moreno glacier in Patagonia on a backpacking trip through South America some years ago, I was happy to get this camera angle: the blue of the Argentine flag gives you a sense of what the blue of the ice looks like in person. Perito Moreno is still rolling on, but the vast majority of the world’s rivers of ice are retreating. Forget the models, the abstractions, the rhetoric: visit a glacier at regular intervals and you will see climate change right before your eyes.

But how can we be sure that glacial melting is truly a global phenomena rather than a string of unrepresentative anecdotes? Individual glaciers have always receded and advanced as local and global conditions change. To see the regional and planetary situation, climate scientists used to have to extrapolate from field studies of individual glaciers. Only about a decade ago did they begin to monitor glaciers on a global scale by using measurements of Earth’s gravitational field.

I recently met one of the scientists mining this data, geophysicist Emma Hill, on a visit to the Earth Observatory of Singapore, located at Nanyang Technological University. She and her colleagues had a paper last month on the effects of glacial melting in Alaska, and they’re rather more intricate than I’d expected.

The data comes from the joint NASA-German GRACE satellite launched in 2002. It actually consists of two satellites that use a microwave link to monitor the distance between them for slight deviations. The satellites move together or spread apart as they pass over areas of stronger and weaker gravity, demonstrating that our planet is not a perfectly spherical ball of uniform density. Mathematically, scientists represent the gravitational field in terms of spherical harmonics, which quantify undulations on different angular scales. GRACE has a resolution of a fraction of a degree and the data set consists of millions of numerical coefficients.

What I found engrossing was the richness of the data. Glaciers have multiple effects on Earth’s gravitational field. First, they are large lumps of mass that exert a direct gravitational pull on the satellites. Second, they also tug on nearby ocean water, causing a localized rise in sea level, which in turn acts on the satellite, amplifying the direct effect of the ice. Third, glaciers weigh down the land, pushing down on Earth’s crust. On the timescales we’re talking about, the depression of the land surface is elastic, like bending a plank of wood, as opposed to a fluidlike flow of rock.

The latter two effects complicate what glacier melting does to the local sea level, the height of the sea relative to the shoreline at a given location. Although the global average sea level has been rising several centimeters per decade as glaciers melt and ocean water warms and expands, the local sea level in polar regions can decrease, as water is no longer attracted gravitationally to glaciers and land freed of its icy yoke pops back up.

In Alaska, Hill’s team estimates that the sea level has been decreasing by nearly 1 centimeter per year–which is good news for Alaska, but only worsens the sea level rise in the tropics. Elsewhere, the first GRACE results, announced five years ago, showed that Greenland is shedding ice much faster than expected. More recent findings suggest the melting both there and in Antarctica has been accelerating.

Considering that melting ice caps are probably the single biggest climate threat, you’d think that governments would put a high priority on monitoring them. Alas, like many other Earth-observing missions, GRACE may well die before its replacement is good to go. The onboard batteries are no longer able to hold a full charge, so the instruments have to be cycled on and off, causing loss of data. “The satellites are showing their age,” says GRACE’s science operations manager, Srinivas Bettadpur of the University of Texas.

Relief, in the form of the unimaginatively named GRACE Follow-On, won’t come for another five or six years, meaning that there will be no overlap of the missions to help with calibration of the new instruments. Other instruments can provide partial coverage, notably the European GOCE mission, but GOCE was designed for high spatial resolution rather than tracking changes over time. While we’re not looking, a lot of that beautiful blue ice will have turned to beautiful blue sea.

Photo by George Musser. Diagrams courtesy of Emma Hill, Earth Observatory of Singapore.

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  1. George, you are talented in taking a complex topic and making it interesting and understandable to us mortals.