Speaking at the opening of the annual week of talks, which are intended to lay the ground work for the UN's next major climate summit in Poland later this year, the head of the UN's climate change secretariat, Christiana Figueres, warned diplomats they "must do more and do it faster".

She warned governments had already used a third of the time between the 2011 Durban commitment to finalise a new international treaty and the 2015 deadline for agreeing that treaty.

The week-long talks follow the release last week of fresh data from the Scripps Institution of Oceanography in San Diego that suggests for the first time in human history concentrations of carbon dioxide in the atmosphere are set to rise above 400 parts per million (ppm) for sustained lengths of time from next month.

Climate scientists have long maintained that concentrations need to be kept below 350ppm if the world is to stand a reasonable chance of meeting international targets to keep average temperature increases below 2C, while concentrations of above 400ppm put the plant on track for levels of warming deemed 'dangerous' by the international community.

"I wish it weren't true, but it looks like the world is going to blow through the 400ppm level without losing a beat," said Scripps geochemist Ralph Keeling. "At this pace we'll hit 450ppm within a few decades."

Citing the imminent 400ppm threshold, Figueres said pressure was mounting on governments to accelerate efforts to curb global emissions.

However, the latest round of Bonn talks is convening with something of a renewed sense of optimism following an encouraging few months for the long-running negotiations.

The Least Developed Group of poorer nations recently signalled that they could be willing to accept binding greenhouse gas emission targets as part of a more ambitious 2015 deal, while Indian officials have indicated that they plan to assess the viability of more demanding emission reduction targets.

Observers have noted that President Obama's re-election and John Kerry's appointment as US Secretary of State have signalled a shift in the US stance towards the negotiations, with sources predicting the traditionally obstructive US delegation is willing to play a more constructive role in the talks.

A recent series of bilateral meetings between Kerry and his counterparts in China, Japan, South Korea and the EU have all addressed climate change issues and resulted in agreements promising more ambitious action to accelerate the roll out of clean technologies.

A senior British government source told BusinessGreen there had been a discernible shift in the approach and ambition adopted by the US and the new leadership in Beijing. "One of the main causes for optimism is the fact there is definitely more focus on upping the ambition than there has been in recent years," they said.

The week-long talks will operate on two tracks, with one part of the negotiations focusing on the "scope, structure and design" of the proposed 2015 climate agreement, and the second part, dubbed the 'Ad Hoc Working Group on the Durban Platform for Enhanced Action', focusing on exploring how action to tackle emissions can be accelerated before 2020.

No major breakthroughs are expected, but diplomats will be under mounting pressure to move from debating the key issues to drafting concrete proposals on a number of components of the proposed 2015 agreement, including how to set fair yet ambitious carbon targets, how to monitor and verify emission reductions, the role of carbon trading in any new deal, the legal form for any treaty, and how to ensure the agreement allows for emissions targets to 'ratchet' up over time.