Is Antarctica gaining or losing weight?
Scientists have come up with vastly different measures of Antarctica’s ice sheets. Belinda Smith reports.
Antarctica keeps throwing up paradoxes when it comes to climate change. Fringing ice shelves crumble along the finger-like Antarctic Peninsula below South America while the sea ice around the continent grows thicker and more extensive. Nevertheless, climate scientists generally agreed that on the whole the continent was losing ice.
Until now. New research suggests Antarctica is putting on weight. NASA glaciologist Jay Zwally employed a different method to calculate the continent’s ice mass. In an October paper published in the Journal of Glaciology he and colleagues reported that the frozen continent added 82 billion tonnes of ice to its stores each year from 2003 and 2008.
“It’s one paper that flies in the face of all the other studies,” says Matt King, an earth scientist at the University of Tasmania. That includes his own paper in Science in 2012 where he reported that between 2002 to 2010, Antarctica lost ice at the rate of 69 billion tonnes per year.
The 2013 Intergovernmental Panel on Climate Change assessment report arrived at double that figure: 147 billion tonnes per year lost from 2002 to 2011. Most recently, an April 2015 study in Earth and Planetary Science Letters split the difference with 92 billion tonnes lost from the continent annually from 2003 to 2014.
Zwally’s no stranger to Antarctic controversy. He was lead author on a 2002 paper that showed, for the first time, that Antarctic sea ice had steadily increased from 1979 to 1998, when most people thought it was shrinking.
At least we understand the reason for the discrepancy, he says. It comes down to different ways of calculating the shifty movement of the bedrock beneath Antarctica’s ice – the so-called glacial isostatic adjustment.
As ice sheets gain weight, the underlying rock squashes down – which means that the height of the ice sheet alone doesn’t reflect its growth.
Similarly, when ice melts, the rock rebounds disguising the level of ice loss.
The potential for error is greatest for the East Antarctic ice sheet, the largest and thickest ice sheet on Earth.
So how is glacial isostatic adjustment measured? Not easily. The terrain beneath the Antarctic ice sheets is uneven; mountains and valleys lie buried underneath. Different methods have been used to model that terrain.
Zwally’s team used ice cores to reveal a potted history of the ice sheet’s weight changes. They showed that 10,000 years ago, East Antarctica started stacking on ice. It depressed the underlying bedrock – more so than others have taken into account. This meant other researchers consistently underestimated how much
ice was being laid down.
To calculate the ice mass, Zwally combined glacial isostatic adjustment with ice sheet thickness, which is measured by a satellite.
Other teams, such as King’s, bolt GPS antennae on to rocks protruding through the ice, or exposed rock on the continent’s fringe to get a handle on how it is moving. They found that while coastal areas lift a couple of millimetres per year, that it can rise as much as eight millimetres per year in the centre of the continent, and maybe more.
Given the size of Antarctica, slight differences in the glacial isostatic adjustment can lead to differences of billions of tonnes of ice in the final calculations.
But the potential for error is greatest for the East Antarctic ice sheet, the largest and thickest ice sheet on Earth. It is about the size of the US and an average 2,226 metres deep.
If it melted, sea levels would rise 53 metres. A difference of a mere 1.6 millimetres in the glacial isostatic adjustment accounts for the difference between Zwally’s calculations and the other 2015 study.
The area of the West Antarctic ice sheet and the Antarctic Peninsula, on the other hand, are a quarter that of East Antarctica with average ice thickness of only 1,300 metres. This explains why there is far less discrepancy of the measurement of the ice mass here. Zwally agrees with others that their ice is shrinking. However this is offset by the greater gains in the east.
If Zwally’s sums are right, does all this extra ice mean global warming is over?
No way, Zwally says: “In fact, it means the planet is getting hotter. A warmer atmosphere carries more water vapour from the oceans, which condenses and falls as snow.”
By his calculations, East Antarctica’s current weight gain is slowing and will be overtaken by ice losses in the west in around 20 years.
Will Zwally be proved right? We’ll find out soon enough – he, King and other climate scientists will soon have more data to crunch. In 2017, two new sets of ice-measuring satellites will be launched.
But by the time we get the next tranche of data, Zwally’s net gains might have already disappeared, and Antarctica will be losing ice – no matter how it’s measured.