Global warming is likely to surpass the previously recognised danger threshold of a 2C average increase in temperature, according to the world-leading climate scientists meeting in Sweden this week.
On the eve of the 2013 report by the Intergovernmental Panel on Climate Change, the scientists involved are understood to have reached agreement in principle on key topics as the week of negotiations draws to a close.
By 2100, the average projection for how much warming will occur is expected to be slightly above the 2C threshold, considered to be the temperature above which it is considered that climate change will damage the global environment.
The scientists also agreed they were certain that global warming was caused by human emissions of greenhouse gases, but on a range of other topics the discussions – fundamental to inform the policy response of governments around the world – were expected to continue up to the 8am Friday morning deadline.
The Stockholm meeting is the first time since 2007 that the IPCC has produced such an assessment of the risks of global warming, and the report will draw on hundreds of new papers from more than 800 scientists.
It is a crucial forum, not only because it is peopled by the most distinguished scientists in their field, but it was set up and works under the auspices of every one of the world's governments, who all have a say in its construction.
Sources involved in the talks also indicated that important open questions remained. These included the effects of climate change on the deep oceans and sea currents, and the distribution of heat in the atmosphere.
People involved in the process said progress on Thursday was slow, with some delegates accused of nonsensical interventions" and the chairs of being too lenient in letting them do so, as they debated the precise wording of the 50-plus page summary intended for use by policymakers.
The scientists have been locked into an old brewery turned conference centre in Stockholm since Monday. The IPCC and the scientists were forbidden to comment and media were excluded.
Stephan Singer, of the World Wildlife Fund, said he was confident the report would provide strong scientific backing for urgent action on climate change, and would be accepted by governments in the runup to crunch negotiations on a new global agreement on emissions that will culminate at a conference in Paris in 2015.
"The governments own this process, that's very important," he said. "If you did not have success here, the prospects for Paris would be rather low."
The report will contain more detail on the effects of climate change at a regional level than was previously possible, as models have improved.
The areas most vulnerable will be highlighted, including swathes of Africa, Asia, and Australia. But though it is not yet possible to produce reliable forecasts for individual countries. The UK is not likely to be among the worst hit.
There is an increased likelihood of warmer winters, but the effects on the British summer are still too uncertain to be included. One factor that could slow temperature rises in Europe is the weakening of a major ocean current system called the Atlantic Meridional Overturning Circulation.
Researchers have suggested for several years this change could mask some of the effects of global warming, by moderating Europe's temperatures.
Meanwhile, on the fringes of the IPCC meeting, a group of risk experts and businesses released a separate study showing that the risks of high levels of warming are on a scale that would not be tolerated in other fields.
The risk of the Earth warming by as much as 6C – which would cause catastrophic changes around the globe – is likely to be put at about 1%, but the Global Challenges Foundation used risk modelling developed for industries and investors to compare this with areas such as transport and construction. The experts calculated that an equivalent level of risk in aviation would equate to more than 500,000 fatal plane crashes per year.
At such levels, public outrage would be immense, but we are prepared to tolerate such risks from the climate because the prospects of catastrophic levels of warming are still regarded by many as remote, the study suggested, and we are poor at calculating risk.
One of the reasons for the long-running debates in Stockholm about sometimes arcane details, apart from the sheer complexity of the task, is that the scientists are acutely conscious that they must make their report watertight.
In 2009, two years after the last assessment report on the science, which ran to more than 1,000 pages, a handful of the IPCC's projections in were found to be inaccurate. Most of the mistakes were trivial, but one stood out as a glaring and embarrassing error – a claim that the glaciers of the Himalayas could almost disappear by 2035.
