"The need for urgency is obvious," said Frank Bainimarama.
"Our world is in distress from extreme weather events caused by climate change -- destructive hurricanes, fires, floods, droughts, melting ice and changes to agriculture that threaten our food security," he told the opening plenary of the 12-day talks, over which he will preside.
"Our collective plea for the world is to maintain the course we set in Paris," he added, referring to the 196-nation treaty inked in 2015.
The Paris treaty calls for capping global warming at "well under" two degrees Celsius, and 1.5 C if possible.
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So far, Earth's average temperature has gone up 1 C compared to pre-industrial levels -- enough to wreak havoc in many parts of the world.
Voluntary national pledges to reduce carbon pollution would still see the world heat up by a blistering 3 C, leaving a critical "emissions gap," and very little time to fill it.
That daunting task has been made all the more difficult by the US pullout, diplomats and experts said.
The problem extends beyond the likely shortfall in the reduction of US emissions, despite Trump's vow to protect carbon-intensive, coal-fired power plants from closure.
State-level governments led by California, along with major US-based companies, will likely pick up much of the slack.
The concern is more whether other leaders who were already reluctant to foreswear fossil fuels as the main engine of economic growth for their countries will lose resolve.
"The Paris Agreement boosted climate action, but momentum is clearly faltering," said Costa Rica's environment minister Edgar Gutierrez-Espeleta, recalling a smattering of applause when Trump told the UN General Assembly in September that the climate pact was "not fair" for the United States.
The Bonn meeting, she noted, was supposed to be mostly technical, a chance to complete a complicated "rule book" for implementing the treaty's provisions.
These include ensuring transparency and compliance, reporting of emissions, procedures for dispersing climate funds, and half-a-dozen other key areas.
"But with the US decision, it has in fact become an important 'political moment'," Tubiana told AFP.
Washington has kept a low profile going into the Bonn talks.
"The United States will participate in the 23rd meeting of the Conference of the Parties," a State Department official told AFP by email.
Leaders from a score of nations are expected to take part in the 12-day talks running through November 17, including French President Emmanuel Macron and German Chancellor Angela Merkel.
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