Desert Sun: Environmental writer and activist Bill McKibben begins his new book “Eaarth” with 100 pages of relentlessly grim facts and figures aimed at convincing readers that climate change is not something that might happen in the future. The greenhouse gases people have pumped into the atmosphere have already changed the Earth irreversibly — hence McKibben’s new spelling — and, he writes, our traditional answer to everything, economic growth, won’t save us. To survive, he says, a new model is needed,…