for National Geographic News
Scientists have identified a few suspects behind the Earth's sudden weight gain around the Equator: glacial melt and shifting ocean mass.
"It is quite striking that we are able to explain the [change] with oceans and glaciers," said Jean Dickey, a research scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.
Dickey and colleagues Steven Marcus and Ichiro Fukimori, together with Olivier de Viron of the Royal Observatory of Belgium in Brussels, report their findings in the December 6 issue of the journal Science.
The research helps resolve the conundrum of why, after decades of doing just the opposite, five years ago Earth's gravity field started getting fatter at the Equator and flatter at the poles.
Prior to 1997, the Earth grew increasingly rounder as it recovered from thousands of years of being squished at the poles by the weight of Ice Age glaciers, an effect scientists refer to as post glacial rebound.
"The Earth is not perfectly elastic. It takes a period of time to come back up," said Dickey, who, like most scientists, was perplexed as to what is causing the rebound effect to be cancelled out.
Christopher Cox and Benjamin Chao, research scientists at the Goddard Space Flight Center in Greenbelt, Maryland, reported in the August 2 issue of Science that the Earth's girth is bulging at a rate of one millimeter (.04 inch) per year, a subtle rate of change.
Limited data left scientists scrambling to explain the cause of this shift. Cox and Chao suggested that the most probable cause lay in the oceans. But they also left glacial melt and movement within the Earth's core open as possible factors.
Commenting on the latest findings of Dickey and her colleagues, Cox said, "[They] have basically taken a few of our recommendationsreviewing the more recent glacier data and looking at the ocean data, as we have been doingand gone the next step with them."
Dickey's team concluded that rapid melting of the Earth's glaciers coupled with a dramatic redistribution of ocean mass is causing Earth's bulging girth.
Cox said the team's research conclusions are reasonable. "But it is only somewhat more conclusive than our identification of the cause, for the same reason," he said. "They too are hampered by lack of recent data."
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