The world now has a rough deadline for action on climate change. To forestall the worst effects of global warming, nations needs to take aggressive action in the next 15 years to cut carbon emissions, says the Intergovernmental Panel on Climate Change. Expect a certain part of our political class to insist that man-made climate change is not settled science, and that until it is, nothing should be done. But the idea that every prominent skeptic must be satisfied before the country moves forward is wrong-headed. Senator John McCain had it right back in 2008, when the science wasn’t as strong as it is today. McCain, who favored a cap-and-trade system for greenhouse gases, noted that if climate-change believers were wrong and we moved forward, we’d still have a cleaner planet, but if the skeptics were wrong and we did nothing, we would have done a profound disservice to future generations.