WASHINGTON  The United States, long a laggard in international efforts to reduce global warming pollution, will arrive at the meeting of the world’s major powers in Italy this week carrying a newly assertive message on the dangers of climate change and the steps needed to address it.

The European hosts of the Group of 8 summit meeting welcome the shift. But the new stance also worries them, in part because they fear that the United States is working toward an independent deal with China outside the global negotiating framework.

President Obama has stated a commitment to addressing climate change. That has been followed by the recent passage by the House of a landmark bill that, if also approved by the Senate, would begin to regulate heat-trapping gases. Those moves have given the Europeans, as well as climate scientists and some environmental groups, hope that the United States will take a leadership role in global talks toward a new climate-change treaty.

Angela Merkel, the German chancellor, said during a visit to Washington in late June that she had detected a “sea change” in Washington’s approach to climate change since Mr. Obama took office, raising the odds for a successful outcome to the treaty negotiations, to be held in Copenhagen in December.