After her speech in parliament, Merkel met British Prime Minister Theresa May for a 30-minute meeting in preparation for the summit.
"These will not be easy talks", Merkel said.
"The differences are obvious and it would be dishonest to try to cover that up".
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".
While Trump needs to be told that Europe disagrees with his stance on climate change, "it's always preferable to come up with joint statements", Macron said.
This time, the German Presidency is organising the summit on the three pillars of stability, sustainability and responsibility with the theme of "Shaping an Interconnected World".
The German hosts face a hard challenge.
Interstate 45 Shut Down, Pigs on Road After Semi Crash in Wilmer
Animal control, police and others had to corral several loose pigs on an interstate Thursday, following a tractor-trailer crash. Both sides of I-45 in the area were shut down as the pigs scattered on the road.
Also on the G20 guest list are Russian President Vladimir Putin, Turkey's Recep Tayyip Erdogan and Saudi Arabia's King Salman - meaning other minefield issues include the Syrian war, Ukraine conflict and the diplomatic shut-out of Qatar. All have strained relations with Merkel and other European leaders.
Protesters have voiced anger about Hamburg being turned into a police "fortress", including a ban on demonstrations during the summit in most of the city centre and along roads to the airport, an area of 38 square kilometres (15 square miles).
She said the European Union was "more determined than ever" to make the Paris accord a success following Trump's decision to pull Washington out of it. "That I won't do", she said.
"Merkel said, adding that she was" convinced that isolation policy" was not a solution.
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.
"As your biggest customer, we hope to obtain a larger share of your market", he said. "I believe we must continue an intense dialogue with the United States because of these ties which are historic and enduring". She added that this deal will not be negotiated at the G20 Summit next week in Hamburg.
"Only by working together can we overcome these problems".