He says: "President Trump should join these leaders in protecting Americans from the mounting impacts of climate change and reaping the economic benefits of the clean energy revolution, rather than trying to shore up the flagging fortunes of the polluting coal and oil industries".
Trump's failure to come to a decision on the Paris agreement reflects the intense discussions that are going on inside the White House that have pitted the nationalists in the administration with the more mainstream staffers.
President Trump failed to commit to remaining within the Paris climate agreement during a two-day meeting with world leaders that ended here Saturday, but he tweeted that he was still considering it and would announce a final decision "next week".
As the G-7 summit came to a close yesterday, the six other members - Britain, France, Germany, Italy, Canada and Japan - renewed their commitment to the accord.
Merkel, by contrast, was attending her 12th such gathering, and clearly believed she had overcome climate change scepticism at a 2007 summit, when she convinced the then U.S. President George W. Bush to pursue substantial cuts in greenhouse gases.
German Chancellor Angela Merkel criticised what she called "a very hard, not to say very unsatisfactory" discussion with Trump on the issue. His Treasury secretary, Steven Mnuchin, has said the United States reserves the right to be protectionist if trade arrangements are unfair to USA companies and workers.
At Nato headquarters, he complained that many Nato member states were not spending enough on defence, expecting the USA to bear the burden.
Trump also understands that Germany is bound by the rules of the European Union and could not unilaterally change its trade policies, Cohn said.More news: Castroneves fastest on Carb Day as another Honda engine goes
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Speaking to USA servicemen and women at the end of the summit, Trump promised to defeat terrorism and said he had made "extraordinary gains to advance security".
"We're thrilled to be here right now and we're getting on that very big plane and we're heading back to Washington and the United States".
However, Trump did pledge to fight protectionism and commit to a rule- based global trade system.
After a first round of meetings Friday, Cohn, who favors retaining the agreement, had said Trump's position was "evolving".
"Even so, the world has made it clear that it will forge ahead".
"I'm not prepared to talk about it", McMaster said, adding that he and Cohn were prepared only to speak about Trump's trip.
Trump retained the option of pulling out of the treaty altogether or, more likely, scaling back on the specific commitments made by the Obama administration. He said that attacks would only steel the resolve of world leaders to fight terrorism. He is not expected to hold a news conference in Sicily before returning to Washington on Saturday.
At Sigonella, Trump said his appeals to North Atlantic Treaty Organisation allies to pay more was working.
Trump was warmly welcomed in the Middle East, but in Europe he's faced a far cooler reception. The president also raised eyebrows with his comment in a private meeting that Germans are "bad" for having a large trade surplus with the U.S.