The great ice sheets on the continent are starting to melt. An airport hangar outside Paris shook with cheers, toasts and foot-stomping in December as diplomats ushered a landmark climate agreement into existence. On Friday morning, world leaders will gather again at the United Nations for a grand ceremony to sign the document. After a quarter-century of failed diplomatic efforts, signs are growing that nations have turned a corner in their political willingness to tackle climate change. Many leaders are pushing to make the Paris agreement legally binding years earlier than originally expected. The falling cost of clean energy is providing a powerful tailwind for their efforts. For all the signs of progress and political will, however, new challenges to implementing the accord have arisen just since December. Outside experts also say the countries’ bare-bones plans are still far from enough […]