CANCÚN, MEXICO — At the recent opening ceremony of the United Nations climate talks in Cancún, Mexico’s president, Felipe Calderón Hinojosa spoke of climate change as a challenge that humanity must heed as a call to action. “Putting a stop to climate change is a true challenge,” he said, “and there is only one power to rise to this challenge: The power of humanity itself.” During a two-week summit, which ends December 10, negotiators and world leaders will once again attempt to hammer out agreements on climate change, carbon emission reductions, adaptation and mitigation.