But the hosts are yet to propose a headline ambition for the marathon November talks, raising concerns they could fall flat.
The build-up has been overshadowed by devastating conflicts on three continents and the US withdrawal from global cooperation on climate, trade and health.
Expectations have dimmed since Brazilian President Luiz Inacio Lula da Silva's pitch three years ago to host climate talks in the Amazon.
A warm-up UN climate event in Germany that concluded on Thursday saw disputes flare over a range of issues, including finance, adding to anxiety about how much headway COP30 can make.
Brazil is a deft climate negotiator, but the "international context has never been so bad", said Claudio Angelo, of the Brazilian organisation Climate Observatory.
Given the stakes, former UN climate chief Patricia Espinosa said Brazil may have to make do with "baby steps".
"One of the main messages that should be coming out of COP30 is the unity of everyone behind multilateralism and international cooperation. Not achieving that means everybody will suffer," she told AFP.
"Failure is not an option in this case."
- 'Survival' -
Previous COPs have been judged on the deals clinched between the nearly 200 nations that haggle over two weeks to advance global climate policy.
Recent summits have produced landmark outcomes, from a global pledge to transition away from fossil fuels, to the creation of a specialised fund to help countries hit by climate disaster.
COP30 CEO Ana Toni said that "most of the big flashy topics" born out of the 2015 Paris Agreement on climate change had been dealt with.
That leaves Brazil with an arguably harder challenge -- trying to ensure what has been agreed is put into practice.
Much of the action is set for the COP30 sidelines or before nations arrive in the Amazonian city of Belem.
National climate plans due before COP30 from all countries -- but most importantly major emitters China, the European Union and India -- will be more consequential than this year's negotiations, experts say.
It is expected this latest round of national commitments will fall well short of containing global warming at 1.5 degrees Celsius, and possibly even 2C, the less ambitious of the Paris accord's climate goals.
"I expect that the COP will need to react to that," said Ana Toni, although what form that reaction would take was "under question".
Uncertainty about how COP30 will help steer nations towards 1.5C has left the Alliance of Small Island States bloc "concerned", said lead negotiator Anne Rasmussen.
"Our survival depends on that," she told AFP.
- 'Threat to humanity' -
How countries will make good on their promise to transition away from fossil fuels may also become a point of contention.
Angelo said he hoped Brazil would champion the idea, included in the country's climate plan, of working towards "schedules" for that transition.
But he likened Brazil's auctioning of oil and gas extraction rights near the mouth of the Amazon river this month -- just as climate negotiators got down to business in Bonn -- to an act of "sabotage".
Another key priority for Brazil is forest protection, but otherwise COP30 leaders have mostly focused on unfinished business from previous meetings, including fleshing out a goal to build resilience to climate impacts.
According to the hosts of last year's hard-fought climate talks, global tensions might not leave room for much else.
"We need to focus more on preserving the legacy that we have established, rather than increasing ambition," said Yalchin Rafiyev, top climate negotiator for COP29 host Azerbaijan.
He fears that trying and failing to do more could risk undermining the whole UN process.
Those close to the climate talks concede they can move frustratingly slowly, but insist the annual negotiations remain crucial.
"I don't think there's any other way to address a threat to humanity as big as this is," Espinosa told AFP.
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