This doesn't happen very often (like not in 600 years) that a Pope steps down instead of dying on the job, but Benedict XVI has done it. Karl Burkart at MNN wrote about the Pope's remarks at the failure of the Copenhagen conference in 2010:
"The Pope admonished world leaders for their failure to reach an agreement in Copenhagen. He also stated that the self-centered and materialistic" motives which caused the economic meltdown could result in an environmental meltdown:
Was it not easy to see the great harm which an economic system lacking any reference to the truth about man had done not only to the dignity and freedom of individuals and peoples, but to nature itself, by polluting soil, water and air?"