A NEW study has shown that global warming has had an even greater impact on the polar ice caps than was previously reckoned. A study of sonar data gathered by naval submarines showed that the Arctic ice cap has shrunk by almost 40 percent in the last few decades. Up to now satellite pictures have been used to measure the shrinkage. But the sonar soundings show that melting UNDER the ice caps is happening. The melting ice caps mean that sea levels will rise.