Ice loss in Antarctica and Greenland has contributed nearly half an inch to the rise in sea levels in the past 20 years, according to an assessment of polar ice sheet melting that researchers are calling the most reliable yet. What's more, ice loss is rapidly speeding up in the north, while the rate in Antarctica has been fairly constant, the researchers report Friday (Nov. 29) in the journal Science.. "Greenland is really taking off," said National Snow and Ice Data Center scientist Ted Scambos, a co-author of the paper released Thursday by the journal Science. Study lead author Andrew Shepherd of the University of Leeds in England, said their results provide a message for negotiators in Doha, Qatar, who are working on an international agreement to fight global warming: "It's very clear now that Greenland is a problem." Scientists blame man-made global warming for the melting. Burning fossil fuels, such as coal and oil, emits carbon dioxide and other greenhouse gases that trap heat, warming the atmosphere and oceans. Bit-by-bit, that erodes the ice sheets from above and below. Snowfall replenishes the ice sheets, but hasn't kept pace with the rate of melting.