Government envoys and scientists gathered under the UN banner in Berlin Monday to hammer out a list of options for curbing carbon emissions driving dangerous climate change.

Fresh from issuing its starkest-ever warning about the impacts of global warming on Earth's weather system, the Intergovernmental Panel on Climate Change (IPCC) will meet until Friday to vet the choices meant to inform policymakers.

A draft of the document, seen by AFP, suggests there is a 15-year window for affordable action to safely reach the UN's warming limit of two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial times.

The report, the product of four years' work by hundreds of experts, aims to provide governments with the latest science on climate change.

It is also designed to inform the struggling effort to forge a worldwide pact by next year to curb greenhouse gases and help poor countries cope with climate impacts.

"Preventing dangerous interference with the climate system entails mitigating climate change," said Ottmar Edenhofer, co-chairman of the panel's Working Group III, which compiled the report.

"On a transparent, scientific basis, our report provides an understanding of the available options to meet this challenge."

A Summary for Policymakers will be publicly released in the German capital on Sunday, and the full report -- authored by scientists but not submitted to the IPCC plenary for scrutiny -- will be released shortly afterwards.

A draft of the summary expresses no preferences for how to tame the problem, nor what a safe level of warming should be.

But it says the UN's 2 C target remains feasible if "all countries" 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 CO2 equivalent in 2010 will have to be pegged to 30-50 billion tonnes in 2030.

"To keep within 2 C, greenhouse-gas emissions would have to fall very quickly," French climatologist Herve Le Treut told AFP in the runup to the meeting.

"They would have to fall roughly by a third by 2050."

Most scenarios that meet the 2 C target entail a "tripling to nearly a quadrupling" in the share of energy from renewable and nuclear sources and fossil-fuel plants whose carbon gas emissions are captured and stored, according to the draft.

Other mitigation options include halting the thinning of carbon-capturing forests, and boosting low-emission public transport schemes.

Government representatives and scientists will go through the summary line by line over the next few days.

"In the plenary, all countries can voice their concerns and all of them are heard," said co-chairman Youba Sokona.

"In the end, it is scientific accuracy that decides."

The meeting comes eight days after the second volume of the report, on the likely impacts of climate change, was unveiled in Yokohama, Japan.

[City pollution. Photo:]