Launching the latest report from the Intergovernmental Panel on Climate Change, he said real and affordable ways to deal with the problem existed.
The IPCC states that climate change is "unequivocal" and may bring "abrupt and irreversible" impacts.
Mr Ban urged politicians to respond at a UN climate change conference in Bali.
"Today the world's scientists have spoken clearly and with one voice," he said. "In Bali I expect the world's policymakers to do the same."
"We are all in this together - we must work together"