U.N. Secretary General Ban Ki-moon on Wednesday called on world leaders to work together to fight climate change, rid the world of nuclear weapons, and find solutions to global conflicts. In his address to the opening of the United Nations' 64th General Assembly, Ban called climate change “the greatest challenge we face as a human family” and said nations will have to bridge their differences in order to reach a global climate pact in Copenhagen in December. “If ever there were a time to act in a spirit of renewed multilateralism - a moment to create a United Nations of genuine collective action - it is now,” Ban said. The U.N. chief also said the next 12 months should be a time when “nations unite to free our world of nuclear weapons,” adding that nuclear disarmament was an issue that had “lain dormant” for too long. On Thursday, U.S. President Barack Obama will chair a U.N. Security Council meeting on disarmament. Both the United States and Russia announced earlier this year that they plan on reducing their nuclear stockpiles, a move that Ban said signals “the climate is changing.” The recent elections in Afghanistan “revealed serious defects,” Ban said. He added, however, that “we should not forget the progress that has been made progress we can build on.” On Gaza, Ban said the U.N. worked to end the Israeli offensive “yet people continue to suffer.” He reiterated his call for revived negotiations towards a two-state solution to the Israeli-Palestinian conflict and a comprehensive peace in the Middle East. In conclusion, Ban noted that 132 U.N. staffers lost their lives in service last year and called for strong support from member states. “So much is possible if we work together,” he said. “Because we are the United Nations. We are the best hope for humankind.”