The dramatic announcement came as a summit of G7 leaders in Sicily wrapped up in deadlock on the issue, with U.S. partners voicing frustration at the president's failure to commit to the deal aimed at stemming global warming.
While at the G7 summit, Trump was heavily lobbied to endorse the Paris Climate Agreement, but he declined to do so.
According to NPR, the document published by the G7 nations at the end of their meeting included a brief section on climate change.
"The entire discussion about climate was very hard, if not to say very dissatisfying", she told reporters.
Ms Merkel said that as the traditional western alliance is threatened by the new U.S. presidency and Brexit, "the times in which we can fully count on others are somewhat over, as I have experienced in the past few days".
This is despite U.S. defence secretary James Mattis saying in an interview to air on Sunday that the president is now "wide open" on the issue.
Mr Trump, who has previously called global warming a hoax, came under concerted pressure from the other leaders to honour the 2015 Paris Agreement on curbing carbon emissions.
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After all, the man who defeated Gianforte in the race for governor, Steve Bullock, is a Democrat. House candidate Greg Gianforte and his campaign have been silent on this special election day.
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".
"His views are evolving, and he came here to learn", Cohn said.
"Therefore, I can only say that we Europeans have to take our fate into our own hands, of course in friendship with the United States of America, in friendship with Great Britain, as good neighbors wherever possible, also with Russian Federation and also with other countries", she said. "The United States, however, is undergoing a review process".
President Donald Trump did not give a single press conference on the trip, breaking with tradition of speaking with reporters during major trips to promote the White House narrative. According to the Union of Concerned Scientists, the US has previously been ranked the world's second-highest emitter, behind China.
In the aftermath of Trump's election victory, the chairman of the Turnbull government's backbench committee on the environment and energy, the Liberal MP Craig Kelly, said on Facebook the Paris agreement was now "cactus".
"What the president means by free and open is, we will treat you the way you treat us, meaning if you don't have barriers to trade or you don't have tariffs, we won't have tariffs", he said, quoted by Reuters. The G7 includes Germany, Britain, Canada, France and Japan.
Greenpeace regretted the outcome of the summit but held out hope that Trump might change tack.
"Trump's attempts to derail global climate action won't make America great, it will only make America late".