Yvo de Boer, who announced last week he would resign July 1, said there was not enough time to recover from the disappointing summit in December in Copenhagen, where world leaders failed to agree on a legally binding climate pact.
Bickering between rich and poor countries over emissions cuts and financial assistance undermined the talks and forced them to settle on a voluntary plan.
"I think Copenhagen demonstrated that sometimes if you try and go too quickly, you actually achieve less progress," de Boer told The Associated Press in an interview.
More than 190 nations will reconvene in Cancun, Mexico, later this year for another attempt to reach a binding agreement to keep the Earth's average temperature from rising more than 3.6 degrees Fahrenheit (2 degrees Celsius) above levels that existed before nations began industrializing in the late 18th century.
U.N. scientists have said any temperature rise above that figure could lead to catastrophic sea level rises threatening islands and coastal cities, the killing off of many species of animals and plants, and the alteration of agricultural economies of many countries.
De Boer said more time was needed to establish a framework of mitigation steps, along with financial and climate change aid that can convince developing countries to support a new deal.
He said the focus should shift toward reaching an agreement at a summit next year in South Africa before the Kyoto Protocol expires in 2012.
"It's very close to the deadline and that's a problem," de Boer said.
"The first priority is to rebuild confidence and trust in the process," he said, adding that developing countries need to be convinced that "there are incentives that will allow them to act on climate change but also meet national economic development goals."
"If you can't show that there are real advantages, then it will never happen," he said.
De Boer spoke on the sidelines of an annual U.N. conference of environmental ministers. The conference is being held on the Indonesia island of Bali, where de Boer oversaw a historic agreement to start climate talks in 2007.
De Boer said his resignation last week had nothing to do with the outcome of the Copenhagen meeting, saying he was encouraged that countries pledged to provide tens of billions of dollars in financial aid that will help poor countries adapt to climate change.
He urged the immediate dispersal of those funds, which start at $30 billion over the next three years and will be scaled up to $100 billion a year by 2020.
"I think my message would be that you can achieve much more in life with carrots than with sticks," de Boer said.
But it is unclear when that will happen.
Karl Falkenberg, the E.U. Commission's director-general of environment, said Wednesday that such aid to poor countries could not be disbursed in the absence of any binding agreement on greenhouse gas emission cuts.
"If the international community cannot find any kind of agreement, then it's going to be very difficult to put this into effect," Falkenberg said.