The talks collapsed in Copenhagen last year after the world's two biggest emitters, China and the US, refused to sign up to legally binding targets to cut greenhouse gases.
Mr Ban said the world cannot afford to get it wrong again.
"I am deeply concerned that our efforts so far have been insufficient," he said. "Nature will not wait while we negotiate. Science warns that the window of opportunity to prevent uncontrolled climate change will soon close."
The UN Secretary-General accepted that a legally binding treaty, that will commit all countries in the world to cutting emissions, will not be possible this year.
However he said this year's meeting in Cancun, Mexico, must make progress in key areas. At the moment it is likely that a multi billion new "green fund" will be set up to help poor countries cope with climate change. Progress should also be made on halting deforestation and the world will agree to share information on new green technology.
Ultimately the talks in Cancun must move towards a binding deal that limits the growth of carbon emissions in order to keep global temperature rise below 3.6F (2C).
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