Agence France-Presse: Most people worldwide believe climate change is a very serious problem that their governments must tackle, a poll said Monday, as the landmark Copenhagen summit on the issue was set to get under way. However the poll of more than 24,000 people also showed concern about rising global temperatures from man-made emissions has dropped in the United States and China — the world’s two biggest polluters. Sixty-four percent of people think climate change is a "very serious" problem, …