Reuters: Three British polar adventurers will this month begin a 620-mile trek to the North Pole with an experimental portable radar set to gauge exactly how fast Arctic ice sheets are melting, they said on Thursday. The 9-pound (4-kilogram) radar has been designed to give much more accurate read-outs of ice thickness than the current method of using submarines or satellites. Arctic ice cover in 2008 dropped to its second lowest extent during the melt season since satellite measuring …