The correct answer is: "human activity".
The unlimited growth of industries which generate harmful emisions both in developed and developing countries has led to the current very dangerous environmental situation.
The concerns related to global warming have led to meetings of representatives of the most important countries in the world, in order to decide a common strategy to deal with the problem and try to save the planet from a fatal fate. Good intentions have been expressed in agreeements such as the Kyoto Protocol, but any program designed has been effective enough so far to face the huge magnitude of the problem.