Posted on behalf of Nicola Jones.
The Earth is becoming more squashed – thicker across the equator than it is from pole to pole – thanks to the melting of Greenland and the Antarctic, according to a new paper in Geophysical Research Letters.
The Earth isn’t exactly round – it’s 42 km fatter around the Equator than it is North to South, mainly thanks to the planet’s rotation. This ‘oblateness’ wobbles a bit from season to season, mostly because of the redistribution of water around the planet, and from year to year and decade to decade, thanks to an 18.6-year ‘solid Earth tide’ and effects like El Nino. On longer timescales, geologists have assumed that our planet should be ‘unsquashing’ or getting rounder over time, because the disappearance of giant ice caps from North America and Scandinavia after the last ice age has left large portions of the northern hemisphere rebounding upwards, redistributing planetary mass to higher latitudes.
Laser-based satellite measures of the Earth’s gravity field, taken since the mid-1970s, show that the planet was indeed getting rounder, with the difference between the equatorial and polar diameters shrinking at about 1.4 mm / year – until the 1990s or so, when the trend flattened out. The cause for this has been a mystery, though people have speculated it might have to do with melting mountain glaciers or long-term changes to El Nino. “This has been a long-standing issue,” says John Wahr of the University of Colorado.
Wahr and his colleague Steven Nerem, also of the University of Colorado, wondered if the disappearance of ice from Greenland and the Antarctic might be to blame. Although this ice loss might contribute a little to land rebound, more importantly it spreads water mass around the planet, adding to the planet’s already-thickened waistline. They used the detailed gravity measures available from the GRACE satellite since 2002 to check, and found that a model of the ice melt’s impacts on the planet’s oblateness explained all of the observed change in trend. “It is speculative. But it makes sense: you can tie the information together,” says Wahr.
Interestingly, their work suggests that this ice mass loss probably started in the mid-1990s, or even as early as the mid-1980s. In a roundabout way, looking at the changes in Earth’s gravity might help to pin a date on the start of a phenomenon that glaciologists are struggling to model and predict (see ‘Losing Greenland‘).
Image: Factors that affect the Earth’s shape / NASA