President Barack Obama is calling on all nations to accept responsibility for fighting climate change.
But he says he's not expecting that it will be easy to reach an agreement at a climate change summit next month in Copenhagen.
Speaking in Tokyo, Obama said nations that are the biggest emitters must set clear targets for reducing those emissions. And he said developing countries will need to take substantial actions of their own.
Obama said that since he took office, the United States has done more than it had before to fight climate change, by investing in new energy, raising efficiency standards and taking advantage of the latest scientific advances.
But he says America knows there's more work to do.