Working on nuclear issues on Capitol Hill in 1999 as an aide to Democratic lawmakers, the risks from human-caused global warming seemed to outweigh the dangers of nuclear power. By 2005 ... I was serving on the Nuclear Regulatory Commission, where I saw that nuclear power ... was a powerful business as well as an impressive feat of science. In 2009, President Barack Obama named me the agency's chairman. Two years into my term, an earthquake and tsunami destroyed four nuclear reactors in Japan. I spent months reassuring the American public that nuclear energy, and the U.S. nuclear industry in particular, was safe. But by then, I was starting to doubt those claims myself. I now believe that nuclear power's benefits are no longer enough to risk the welfare of people living near these plants. The current and potential costs - in lives and dollars - are just too high. For years, my concerns about nuclear energy's cost and safety were always tempered by a growing fear of climate catastrophe. But Fukushima provided a good test of just how important nuclear power was to slowing climate change: After the accident, all nuclear reactors in Japan were shuttered indefinitely, eliminating production of almost all of the country's carbon-free electricity and about 30 percent of its total electricity production. Fewer than 10 of Japan's 50 reactors have resumed operations, yet the country's carbon emissions have dropped below their levels before the accident. How? Energy efficiency and solar power.