German Chancellor Angela Merkel has said this year's G20 summit will focus on furthering the aims of the Paris climate deal - putting her on a direct collision course with the US president.
Mrs Merkel said she knew the topic may be difficult following Donald Trump's decision to pull the US out the accord.
However, she said, tackling climate change remained a priority for Europe.
She was backed by other European leaders who said they would speak with "one voice" in Hamburg.
The summit of the world's largest economies takes place next week. As host, it is up to Mrs Merkel to set out the priorities of the annual meeting.
This year, the Paris climate agreement, established to limit the impact of carbon emissions on the environment, with countries committed to keeping the rise in global temperatures "well below" 2C, will be among them.
Mrs Merkel laid out her views in a speech to the German parliament on Thursday, hours before a meeting of her fellow European leaders.
"The differences are obvious and it would be dishonest to try to cover that up. That I won't do," she said.
"The European Union unconditionally stands by its agreement in Paris and will implement it speedily and with determination.
"More than that: since the decision of the United States to leave the Paris climate agreement, we are more determined than ever to make it a success."
Taking what appeared to be a second swipe at Mr Trump, Mrs Merkel went on to say: "Those who think that the problems of this world can be solved with isolationism or protectionism are terribly wrong."
Mrs Merkel does not stand alone.
At a news conference which followed the meeting of Europe's leaders, European Council President Donald Tusk said: "We will speak with one voice at the G20 summit".
The group had met in Berlin to discuss their priorities - including climate change and people smuggling - ahead of next week, when they will be joined by Mr Trump.
However, they made clear they were not trying to isolate Mr Trump, and were hoping it would give them the opportunity to change his mind.
French President Emmanuel Macron - who is a staunch opponent of Mr Trump's climate change policies - emphasised that "the relationship [with the US] is a long term relationship".
But he said Europe would "clearly reaffirm our very strong commitment to the Paris accords", adding: "I hope that the others can be brought back to their senses."
Mr Trump pulled the US out of the deal on 1 June, saying the Paris agreement was a deal that aimed to hobble, disadvantage and impoverish the US.
He claimed the agreement would cost the US $3tn in lost GDP and 6.5 million jobs - while rival economies like China and India were treated more favourably.
Mr Trump said he wanted to negotiate a "fair" deal for the US.
The decision places the US as one of just three countries which has not signed up to the deal. Nicaragua - which felt the agreement did not go far enough to tackle climate change - and Syria are the other two.