Nicholas Stern, the British economist known for a major report in which he declared that combating climate change would cost less than ignoring it, has announced that he was wrong — about how bad the problem is. “We badly underestimated the degree of damages and the risks of climate change” in the Oct. 2006 report, he said in a speech Wednesday. “All of the links in the chain are on average worse than we thought a couple of years ago.” Thawing permafrost is releasing methane, oceans are acidifying faster than expected, and carbon sinks are becoming less effective, said Stern. He urged nations to come up with a stringent global climate treaty taking food production into account, and reiterated that the world should aim to produce zero-carbon electricity by 2050 (he backs carbon sequestration, nuclear power, and renewable energy). “This is about buying down risk,” Stern said. “Starting now, that means it requires at least 1 percent of world GDP. That is small relative to a planetary catastrophe.”