"If we fail to act, climate change will intensify droughts, floods and other natural disasters," Ban said at a forum near Seoul that came weeks ahead of his own conference on climate change in September.
"Water shortages will affect hundreds of millions of people. Malnutrition will engulf large parts of the developing world. Tensions will worsen. Social unrest even violence could follow," Ban said according to a prepared text of his remarks to a global environment forum in Incheon, west of Seoul.
Greenhouse gas emissions are widely considered to be one of the main causes of global warming. Nations will gather in Copenhagen in December to work out a new agreement on reducing emissions to succeed the current Kyoto Protocol, whose first phase ends in 2012.
Ban, calling climate change a fundamental threat to mankind, called on world leaders to act quickly so that a deal can be reached at Copenhagen.
In Bonn, delegates from about 180 nations meet for U.N. climate talks amid warnings that time was running out for them to reach agreement on a hugely complex pact, due for completion at the end of the year.
About 2,400 delegates at the August 10-14 negotiations in Bonn will try to shorten a draft text, outlining options for combating global warming, that has swollen to about 200 pages from 50 just a few months ago.
The Bonn meeting, the third in Germany this year, was added because of scant progress with the deadline looming. After Bonn, talks before Copenhagen are in Bangkok from September 28-October 9 and in Barcelona, Spain, from November 2-6.