"One thing that did not work well in Copenhagen is that a small circle met and the regional groups felt left out of the debate," she said as delegates from some 45 countries convened to breathe life into stalled climate talks.
"A preparatory job before Cancun will be to find a basis of trust for all countries that will be present in Cancun so that no one feels left out," Merkel told the assembled ministers and negotiators.
Many of the 194 nations in the UN Framework Convention on Climate Change (UNFCCC) have not backed the Copenhagen Accord, complaining that it was hammered out at the last minute behind closed doors by a handful of powerful economies led by China and the United States.
The contested accord calls for reducing greenhouse gas emissions enough to keep global temperatures from rising more than 2.0 degrees Celsius (3.6 degrees Fahrenheit), but does not share out responsibility for reaching that goal.
Merkel pointed out that voluntary pledges currently registered in the accord put Earth on track for a 3.5 C or even a 4.0 C jump by 2100, far above the widely held threshold for dangerous warming.
She also sought to allay fears that forums such as the so-called Petersberg Climate Dialogue - unfolding over the next two days outside Bonn - could clash with the UN talks.
"There is no alternative to the UN process ... In the end all of this has to go into one UN process," she said.
The two-and-a-half day meeting - the highest-level climate gathering since the December fiasco - was jointly launched by Mexican President Felipe Calderon, who will host the UN conference in Cancun starting in late November.
Calderon likewise emphasised the need for building trust, especially in the issue of finance for poor countries bracing for the ravages of global warming.
The Copenhagen Accord called for $30 billion up to the end of 2012, to be scaled up to $100 billion annually by 2020.
"This atmosphere of trust is something we really need to make use of for the 'fast track' financing," he told the ministers Sunday evening. "2010 is the year when we need to take action."
Just how hard that may be was laid bare in Bonn only weeks ago at the first meeting since Copenhagen of the UNFCCC, the main vehicle for global talks.
The only thing that the negotiators seemed to agree on was that the session was tense and full of suspicion.
Also on Sunday, German environment Norbert Röttgen said the European Union should unilaterally raise its target for reducing greenhouse gas emissions from 20 to 30 percent by 2020.
"The EU has to push to 30 percent to create confidence - it's one way to create credibility with the developed countries," said Switzerland's top climate negotiator, Jose Romero.
The Petersberg talks in Bonn appear designed to sidestep some of the biggest political landmines that derailed Copenhagen, focusing instead on narrower issues where some progress has been made such as technology cooperation, verification regimes and fighting deforestation.
"The politics of getting a full-blown treaty are still very divided," said Alden Meyer, a climate policy analyst at the Washington-based Union for Concerned Scientists.
Industrialised countries bound by the Kyoto Protocol to slash their carbon pollution say emerging giants such as China and India must take on binding, if lesser, commitments too.
Developing countries argue they are not historically responsible for climate change, and thus should be allowed to take purely voluntary steps to help fix it.
But Calderon called on rapidly developing nations - already among the world's top emitters of CO2 - to play a bigger role.
"Until now we spoke about different responsibilities," he said, alluding to a cardinal principle underlying the climate Convention. "Now we have to see that it is really a joint responsibility."