The trap was sprung in February 2006. The White House ordered that Dr Jim Hansen was to be denied the oxygen of publicity forthwith. He was to be banned from appearing in newspapers and on TV and radio. He was effectively to disappear. It was the kind of treatment that might be reserved for terrorists, criminals or, in a totalitarian regime, for political dissidents. Hansen, however, was none of these things. The director of NASA's renowned Goddard space science laboratories was a dry, rather self-effacing climate change scientist with a worldwide reputation for accurate and high-quality research. Hansen's visit to London last week was partly inspired by the decision to approve construction of a new coal-fired power station at Kingsnorth in Kent. This, Hansen wants to warn us, is a recipe for global warming disaster. The recent warm winters that Britain has experienced are a clear sign that the climate is changing, he says. We are fast approaching a series of tipping points. Changes such as the melting of the Arctic ice cap, the acidification of the oceans and the global rises in temperature could be approaching the point of becoming irreversible. In the face of such threats it is madness to propose a new generation of power plants based on burning coal, which is the dirtiest and most polluting of all the fossil fuels. We need a moratorium on the construction of coal-fired power plants and we must phase out the existing ones within two decades."