"It is, simply, the greatest collective challenge we face as a human family," Ban said in a keynote speech at a gathering in Seoul of the World Federation of U.N. Associations.
He said, however, there was cause for hope. Referring to a December meeting in Copenhagen, he said "we have a chance to put in place a climate change agreement that all nations can embrace, which will be equitable, balanced, comprehensible."
The Copenhagen meeting is meant to negotiate a new U.N.-brokered climate treaty to succeed the 1997 Kyoto Protocol that expires in 2012. Ban called it a "once-in-a-generation opportunity."
He also called on governments to "seal the deal in the name of humankind" through a "renewed multilateralism, a compassionate multilateralism."
Ban said he expects more than 100 heads of state and government to participate in a U.N. summit on climate change at the global body's New York headquarters on Sept. 22, and called for them to pressure their negotiators so a deal can be concluded in Copenhagen.
Though identifying climate change as the world's biggest problem, Ban said it faces other serious problems as well, including the proliferation of nuclear weapons and four problems that carry serious weight given their timing.
"We are living through an age of multiple crisis," Ban said. "Fuel, flu and food, and most seriously, financial. Each is something not seen for years, even for generations. But now they are hitting us all at once."
Ban, a former South Korea foreign minister, took over leadership of the U.N. in 2007.
