NEWS: Around the Arctic April 05, 2016 - 1:10 pm

Scientists use gravity to measure Arctic glacier loss

"We’re losing more ice as time goes on”


Tell someone the earth’s glaciers are shrinking and sea levels are slowly rising and many people will tell you they already knew that.

But because glaciers grow and shrink with the seasons — and grow and shrink in different places throughout the ice field — you might wonder how scientists measure that kind of thing.

In a word: gravity.

Christopher Harig, a Geosciences professor at Princeton University, explained April 1, that, thanks to two satellites that orbit the earth — dubbed GRACE for Gravity Recovery and Climate Experiment — scientists can measure gravity changes above large ice fields and estimate ice mass growth and loss.

And the latest measurements show glaciers in the northern hemisphere are shrinking, and fast.

“The important thing I would say is that it’s accelerating over time so we’re losing more ice as time goes on,” Harig said.

Gravity is not uniform around the planet, Harig explained. And it changes with the seasons and with climactic events. If an ice field loses mass, the Earth’s gravity pull in that area dips slightly in a measurable way. Likewise, if an ice sheet grows, and gets heavier, the gravity pull downward increases.

Harig says they used data from the GRACE satellites from 2003 to 2014 to look at northern glaciers and discovered a worrying trend in ice mass loss in Greenland and Canada.

The findings, from an article co-authored by Frederik Simons, were published March 10 in the academic journal Geophysical Research Letters.

Greenland has the largest ice field, it has the most surface area touching the ocean and understandably, has the largest average annual loss for that time frame: 244 gigatonnes per year.

The scientists estimate Ellesmere Island has been losing, on average, about 38 gigatonnes of ice mass per year during that time frame and that Baffin Island has been losing about 22 gigatonnes per year.

Alaska’s glaciers, while still losing ice mass, don’t seem to be losing it at the same accelerated speed as in Greenland and Canada, Harig said.

So what does that much ice look like? It’s really big.

Harig made a presentation on the topic in Tucson, Arizona, recently, a city equivalent in area to the size of Edmonton.

He told his audience 100 gigatonnes of ice would take up the entire footprint of Tucson to the depth of about 170 metres — that’s roughly the height of a 60-story building.

So despite growth of Greenland’s ice field at the centre, the edges have been calving and melting at more than twice that amount annually, on average, since 2003.

The more those glaciers come into contact with a warming ocean, the more melting and cracking and calving occurs, Harig said. That’s one theory as to why melting is accelerating.

NASA just started a new study called Oceans Melting Greenland which is measuring ocean temperature, salinity and depth in and around Greenland to find out if the ocean is the main culprit in Greenland’s glacier loss.

While Harig and Simons’ data may be cause for concern, Harig admits there are limitations.

Firstly, GRACE measures 300-square kilometre chunks every month so with the proximity of Baffin Island and Greenland, it’s difficult to know for certain exactly what’s melting where.

To hone those figures, they also used something called laser altimetry. That’s when other satellites orbiting Earth shoot lasers to the Earth’s surface and measure the altitude of geographic features, such as ice fields.

Harig and Simons co-related altitude and gravitational figures to verify physical growth and shrinkage of glaciers throughout the year.

Suffice to say there’s a lot of math involved but they’re confident the two measurements give a fairly accurate picture of ice mass loss, give or take a few gigatonnes.

There are also limitations with short-term data, Harig said. While the trend in ice loss for northern hemisphere glaciers seems clear from 2003 to 2014, it is only a snapshot, not a prediction.

And 2013, for instance, was an anomaly; while 2012 showed record ice mass loss, 2013 showed very little loss. Then 2014 showed a lot of ice loss again, following the previous trend.

“We’re measuring something today but you should be cautious about projecting that into the future,” Harig said.

“We’re not saying that’s a prediction of what will happen in the next 10 years. The only way to fix that is to increase the observational record.”

The GRACE satellites are reaching the end of their lifespan — they’re running out of power and capacity — so GRACE II is expected to be launched in 2017, Harig said.

That should allow scientists to get at least another decade’s worth of data by 2027.

Meanwhile, Harig and Simons just completed looking at the numbers for Antarctica and have noted that during the same time frame, 2003 to 2014, Antarctica has lost, on average, an estimated 92 gigatonnes per year of ice mass.