With his 90th birthday in July, a trip into space scheduled for later in the year and a new book out next month, 2009 promises to be an exciting time for James Lovelock. But the originator of the Gaia theory, which describes Earth as a self-regulating planet, has a stark view of the future of humanity. He tells Gaia Vince we have one last chance to save ourselves - and it has nothing to do with nuclear power