" In September 2009 the lead article in the journal Nature said that above 350 we “threaten the ecological life-support systems that have developed in the late Quaternary environment, and severely challenge the viability of contemporary human societies.”41 A month later, the journal Science offered new evidence of what the earth was like 20 million years ago, the last time we had carbon levels this high: sea levels rose one hundred feet or more, and temperatures rose as much as ten degrees. "

― Bill McKibben , Eaarth: Making a Life on a Tough New Planet