WASHINGTON — After more than two decades of trying but failing to forge a global pact to halt climate change, United Nations negotiators gathering in South America this week are expressing a new optimism that they may finally achieve the elusive deal. Even with a deal to stop the current rate of greenhouse gas emissions, scientists warn, the world will become increasingly unpleasant. Without a deal, they say, the world could eventually become uninhabitable for humans. For the next two weeks, thousands of diplomats from around the globe will gather in Lima, Peru, for a United Nations summit meeting to draft an agreement intended to stop the global rise of planet-warming greenhouse gases. The meeting comes just weeks after a landmark announcement by President Obama and President Xi Jinping of China committing the world’s two largest carbon polluters to cuts in their emissions. United Nations negotiators say they believe […]