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The president said his plans for a summit later this week in Pittsburgh with representatives from the world’s 20 largest economies include working with them to phase out fossil-fuel subsidies.

Mr. Obama pointed out he already has had several top-level meetings across the globe on the topic and said he has put climate “at the top of our diplomatic agenda” in meetings with China, Brazil, India, Mexico, Africa and Europe.

“We understand the gravity of the climate threat. We are determined to act. And we will meet our responsibility to future generations,” Mr. Obama said, adding that both industrialized and developing nations must do their part.

The president acknowledged that implementing change around the globe will be difficult but said “unease is no excuse.”

He also bemoaned “too many years of inaction and denial.”

Democrats said Mr. Obama’s tone was a sharp departure from the Bush administration’s.

The Republican National Committee planned a Tuesday afternoon conference call to respond to the speech.