In 1972, in the Stockholm Declaration, the international community recognized that the apparent contradiction between environmental protection and the economic development of developing countries must be solved. Expressly formulated in 1992 in the Rio Declaration, the principle of common but differentiated responsibilities became the central feature of the Kyoto Protocol, which the American Senate refused to ratify, alleging that it placed emerging nations in a privileged position. After a period of “arm wrestling” in the negotiations following the Kyoto Protocol, the Agreement of Copenhagen seems to undermine its legitimacy. Will such agreement enable the international community to face its responsibility to protect the climate in the common concern of humankind?