[classicman]

Well this may offer some insight.

Scientists agree that global temperatures are rising, and so are levels of carbon dioxide. But the immediate impact of human activity on natural climate cycles—from ice-sheet dynamics to wind and ocean currents—remains unclear.

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Much of the current controversy over climate change centers on efforts to reconstruct past temperatures using what is known as "proxy" data from tree rings, harvest records, sea beds and lake sediments. Unlike ice cores, which contain telltale gases and particles from ages ago, the proxy data offer only indirect or fragmentary evidence of climate trends.
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"Unfortunately many of our proxies have significant errors and are prone to be a slave to assumptions," says climatologist John Christy of the University of Alabama in Huntsville, who has often criticized the IPCC. His research, using temperature readings from NOAA and NASA satellites, has undermined arguments that the atmosphere is warming at an unusual rate.

The ice-core data from Antarctica is "terribly important," Dr. Christy says. "We really need to know what the climate did before we can answer why it did what it did. If it happened before, it will happen again, and probably worse."
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To ensure accuracy, 27 independent laboratories will analyze the ice cores during the next three years. They expect to analyze 40 different trace chemicals related to climate, some in levels down to parts per quadrillion.

At every stage, the scientists must be able to prove that the ice cores haven't been contaminated. They must also make sure the samples stay at minus-20 degrees Celsius or so throughout their 8,000-mile journey to Colorado. Otherwise, the key gases will dissipate.

"Its credibility is of crucial importance," said Thomas Stocker at the University of Bern in Switzerland, a co-chairman of the United Nations working group that assesses data for the IPCC.

Link
Really informative and interesting article.