ASK the top brass of any oil or coal firm about global warming, and they are likely to dismiss it as a well-imagined theory cooked up by a handful of ardent environmentalists. In recent years these people have taken great pains to acquire satellite data that apparently show how the Earth, contrary to the environmentalists' doomsday prophecies, is cooling slightly. Environmentalists usually counter their argument by recounting measurements taken at Earth's surface which indicate that temperatures have risen by 0.3