Greenwire: Climate scientist James Hansen -- now retired and free from the constraints of his role at NASA -- last night promoted a carbon tax as the most effective way to curb greenhouse gas emissions in a keynote address before Colorado conservationists. In one of his first U.S. appearances since his April retirement, Hansen spoke at Conservation Colorado's annual Rebel With a Cause Gala, where he was honored with the event's namesake award. "I'm glad, by the way, that you called me a rebel, because in...