Saying the health of the planet is at stake, U.N. Secretary-General Ban Ki-moon urged 190 nations meeting in Mexico on Tuesday to agree to steps to fight climate change that fall short of a perfect deal.

“We cannot let the perfect be the enemy of the good,” Ban told a first session of environment ministers at the November 29 to December 10 talks in the Caribbean resort of Cancun where rich and poor nations are split over cutting greenhouse gas emissions.

After U.S. President Barack Obama and other leaders failed to work out a U.N. climate treaty at a 2009 summit in Copenhagen, Ban repeatedly stressed lower ambitions for the Cancun talks despite calls by some nations for radical action.