A draft of the report, seen by AFP, suggests there is a 15-year window for feasible and affordable action to safely reach the UN-targeted global warming limit.
But deep, swift curbs in carbon emissions will be needed, with a revolution in energy use, it says.
The report is the third chapter in the Fifth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC).
Six years in the making, it provides policymakers with the latest science on global warming, to feed into national planning and the struggling effort to forge a worldwide pact by next year on curbing Earth-warming greenhouse gas emissions.
"I hope that this report will help to revitalise... climate politics and a sense of urgency from governments," Bill Hare of the think tank Climate Analytics told AFP.
The meeting comes eight days after the second volume of the report, on the likely impacts of climate change, was unveiled in Yokohama, Japan.
It issued unprecedented warnings that the risk of conflict, hunger, floods and mass displacement increased with every upward creep of the mercury.
The draft of the new 29-page summary, which will be hammered out in Berlin over five days before it is publicly released on Sunday, expresses no preferences for how to tame the problem, nor does it say what a safe level of warming would be.
2030 window
But it says the UN target, to limit average warming to 2.0 degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels, remains feasible.
There is a "likely" chance of meeting it if greenhouse-gas concentrations by 2100 are 430-480 particles per million of carbon dioxide equivalent (ppm CO2eq), according to the draft.
But this meant "all countries" will have act quickly to mitigate, or ease, carbon emissions.
"Delaying mitigation through 2030 will increase the challenges," cautions the document.
In raw terms, global carbon emissions of 49 billion tonnes of CO2eq in 2010 will have to be pegged to 30-50 billion tonnes in 2030.
Emissions have maintained an upward curve despite a small dip after the 2007-08 global economic crisis, and will be hard to contain as developing economies consume ever more fossil fuel and the global population expands.
Greenhouse-gas levels are on track to bust 450 ppm CO2eq by 2030 and reach 750-1,300 ppm CO2eq by 2100 if nothing is done, the document points out.
The summary points to a wide range of policy options, but notes that the most successful scenarios assume a stable, global carbon price.
Mitigation options include capturing and storing fossil-fuel emissions, switching to low-carbon alternatives, halting the thinning of carbon-capturing forests and boosting low-emission public transport schemes.
The draft turns the spotlight on energy -- the need for low-carbon sources and ending waste.
Energy production emitted 14.4 billion tonnes of CO2, the chief greenhouse gas, in 2010. That figure could double or even triple by 2050, according to the report.
On the consumption side, buildings spewed 8.8 billion tonnes in 2010 and industry 13 billion, both figures set to increase by 50-150 percent by mid-century.
Transport's 6.7 billion tonnes of CO2 could double over the same period.
Most scenarios that meet the 2 C target will entail a "tripling to nearly a quadrupling" in the share of energy from renewable and nuclear sources or fossil-fuel plants whose carbon gas is captured and stored.
This would require investment in conventional fossil fuels to fall by $30 billion (22 billion euros) annually by 2029, and in low-carbon power supply to rise by $147 billion per year.
"Climate change is already clearly happening and is likely to become much more severe in future without strong mitigation actions to keep temperatures below 2 C," Saleemul Huq of the International Institute for Environment and Development, commented ahead of the meeting.
"The costs of inaction will be many more times the cost of action."
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