Bloomberg: When David Letterman, the late-night TV host, needed a scientist to explain climate change on his show last April, he chose John Holdren, the Harvard University physicist who helped Al Gore earn an Academy Award. Holdren’s dire predictions about global warming, illustrated in riveting charts and graphs, helped ex-Vice President Gore win an Oscar for his 2006 documentary, “An Inconvenient Truth.” Holdren also helped persuade Ford Motor Co. and ConocoPhillips Co. executives to accept …