With China and the U.S. coming to an agreement in 2014 on limiting their respective greenhouse-gas emissions, the Peru talks suddenly gained new momentum toward a deal on a global scale. To be sure, even the U.S.-China agreement would not kick in for years, if not decades, and a global agreement would not even take effect until 2020 at the earliest. This drawback may pale in comparison to one of nature’s own contributions to greenhouse-gas emissions, and nature itself cannot agree to voluntarily restrict its own output. Accordingly, we should not assume that a global agreement will save the day, rendering the planet still inhabitable for humans in the next century.
The full essay is at "Beyond the Reach of Any Greenhouse-Gas Agreement."