German Chancellor Angela Merkel says it's up to Iran to convince the world it is NOT working on a nuclear weapon -- and that if it doesn't, she will support tougher sanctions. Speaking at the U.N. General Assembly on its opening day, she said the consequences of Iran having a nuclear bomb would be disastrous. Merkel also addressed the need for reform of the UN itself, stressing that Germany is ready to take more responsibility by assuming a seat on the Security Council. But she devoted the bulk of her speech to the urgency of fighting climate change.