Searching for a Cause
Dickey said that she and her colleagues sought an explanation for the planet's shape-change much like a treasure hunt, systematically looking at recent data on the oceans, glaciers, atmosphere, and groundwater. The team suspected that oceans were a culprit because the start of Earth's shift toward a more oblate shape coincided with the strongest El Niño event of the century in 1997 and 1998. The researchers also observed a marked shift in a fluctuating ocean current temperature pattern known as the Pacific Decadal Oscillation (PDO), which is similar to El Niño.
Both El Niño and PDO involve a periodic warming of ocean currents. El Niño effects are concentrated primarily in the southern oceans, while PDO is observed in the northern.
Dickey's team created a computer model of the pressure exerted by the oceans on the Earth in the years before and after the El Niño and PDO events and found a pronounced shift in oceanic mass towards the Equator, said Dickey.
"It wipes out the contribution from post glacial rebound," said Dickey.
The team also found a significant contribution from the world's glaciers. According to data from the National Snow and Ice Data Center in Boulder, Colorado, the world's glacial melt rate increased 5.4 percent between 1980 and 1998.
In the 1980s and most of the 1990s, the average melt rate was 24 cubic miles (100 cubic kilometers) per year. In 1997 that figure jumped to 77 cubic miles (320 cubic kilometers). In 1998 the rate increased yet again to 130 cubic miles (540 cubic kilometers).
Dickey said that all of this excess water released from glaciers in regions such as Alaska, Russia, and Patagonia accounts for the bulk of the observed change in the Earth's gravity field.
"I thought the ocean would be the lion's share, but it is the glaciers," she said.
While Cox agrees that glacial melting is certainly a contributing factor, he does not believe that the melting is significant enough to account for the bulk of the observed changes. Nor is he sure where all the melt water is going.
"From what we have seen looking at the data we have, the majority is explained by the ocean model," said Cox. "However, I do not find it surprising that melting may be playing at least a contributing role."
Scientists hope that the discussions over the cause of the changes in Earth's gravity field will be aided in the coming months from data collected by a pair of satellites launched by NASA in March.
The mission, known as the Gravity Recovery and Climate Experiment, or GRACE, will map variations in the Earth's gravity field with unprecedented accuracy, allowing scientists to pinpoint where the changes in mass redistribution are occurring.
Cox said that scientific investigations into the Earth's oblateness are just beginning and will likely evolve. His most recent data shows that the Earth is once again becoming less oblate.
"Any explanation put forth will have to explain that," he said. "I suspect there will be several more papers, from different authors, on this subject before it is all settled."
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