"Scientists are telling us that 350 parts per million [of carbon] in the atmosphere is the upper limit. We're at 387 parts per million now, and we're up in that zone where the risk of going past irrevocable tipping points is elevated. It's no different than going to a doctor and learning your cholesterol is too high, and you're at risk for a heart attack. You have to work to lower your cholesterol and hope to get there before the heart attack comes."