Global warming issue 'on par with slavery' Dealing with climate change is a moral issue on a par with ending slavery, the world's most celebrated climate scientist, James Hansen, of Nasa, believes. Dr Hansen, who heads Nasa's Goddard Institute of Space Studies, will be making the slavery comparison in his acceptance speech for the Edinburgh Medal next Tuesday, when he will also be calling for a global tax on all carbon emissions. Nothing less will do, he will argue, so urgent is the challenge which climate change presents for future generations. The Edinburgh Medal is awarded each year to scientists and technological experts judged to have made a significant contribution to the understanding and well-being of humanity. Widely thought of as "the father of global warming" - his dramatic alert about climate change in US Senate hearings in July 1988 put the issue on the world agenda - Dr Hansen is now one of the most outspoken advocates of drastic climate action. He said last year he thought climate sceptics were winning the global warming argument with the public. In his acceptance speech, he will argue that an immediate worldwide carbon tax is needed to force cuts in fossil fuel use, and that current generations have an over-riding moral duty to their children and grandchildren to act now.