Climate change has displaced nuclear risk reduction as an intense focus of national leaders and activists across the globe. This sense of commitment was reflected in the Paris pact that sets targets to reduce harmful emissions to slow the pace of man-made climate change and the rise of sea levels. Arms control activists, an increasingly lonely and dispirited bunch, watched with a combination of gratitude and envy as environmental leaders applauded the compact struck in Paris. Now comes the hard part: even more than with arms control agreements, successful reductions in carbon emissions will be challenging.