Jean-Claude Juncker said that "the Americans can't just get out of the agreement," adding that "it takes three to four years" to pull out.
Juncker went on to say that the Group of Seven leaders "tried to explain this in clear simple sentences to Mr Trump" at a recent summit in Italy. He said that even though "it looks like that attempt failed" ... The "law is the law".
Juncker said: "If the US president pulls out of the Paris agreement, and he will in the next days or hours, then it is Europe's duty to say that that is not how it works."
A White House official said earlier in the day that Trump was planning to pull out of the Paris deal, although a final decision hadn't been made.
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Trump yesterday declared that abandoning the Paris climate agreement would be a victory for the American economy.
The official told reporters that the EU and China will also "spell out" how they plan to meet their commitments to the landmark international accord to fight global warming at talks in Brussels tomorrow.
The official is involved in preparing the meeting between EU Council President Donald Tusk, Juncker and Chinese Premier Li Keqiang, but can't speak on the record because their meeting statement wasn't finalized. Li and a major Chinese delegation are due to arrive in Brussels today following talks in Berlin.
A White House official said yesterday that there could be "caveats in the language" announcing a withdrawal, leaving open the possibility that his decision isn't final.
That possibility was met with derisive howls from EU lawmakers when a session of the European Parliament was informed about it.
"Climate change is not a fairy tale. It is a tough reality which affects peoples' daily lives," European Parliament President Antonio Tajani said in a statement.
Tajani suggested that Washington's withdrawal should be a signal for Europe to step up its efforts - and reap the benefits.
"Our climate action strategy represents an opportunity to attract investment, innovation and develop new green technologies," he said. "We have got the talent and the will to make this possible in all sectors."
Tajani said earlier he would confer with Tusk and Juncker about "joint initiatives to be adopted together as a European Union" to offset the decision.
In Madrid, the leaders of India and Spain expressed their commitment to fighting climate change and reiterated their support for implanting the Kyoto and Paris accords.
On Tuesday, Modi said in Berlin that it would be a "crime" to spoil the environment for future generations as the world awaits a decision on US climate policy.
Rajoy and Modi agreed to boost bilateral cooperation in the field of combating climate change.
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