President Barack Obama has challenged world leaders to fight climate change telling a UN summit that a failure to tackle the problem could lead to “irreversible catastrophe”.The UN Secretary General Ban Ki-Moon set the tone when he warned that time was running out and a way had to be found to agree conditions for a new pact to combat global warming. China’s President Hu Jintao pledged to reduce the carbon intensity of his country’s economic growth. China, he said, would reduce the amount of greenhouse gases produced for each dollar of national economic output by a “notable margin” by 2020. And Barack Obama pulled no punches when he appealed for urgent action. “Our generation’s response to this challenge will be judged by history, for if we fail to meet it, boldly, swiftly, together, we risk consigning future generations to an irreversible catastrophe,” he said. “Each of us must do what we can when we can to grow our economies without endangering our planet, and we must all do it together. We must seize the opportunity to make Copenhagen a significant step forward in the global fight against climate change.” What remains to seen is if all the warnings will have been heeded by the time the Copenhagen UN climate summit begins on December 7.