More than 500 scientists huddled at the closed-door meeting of the Intergovernmental Panel on Climate Change (IPCC) in Paris, poring over the first review of the scientific evidence for global warming in six years ahead of the report's release Friday.

"It is going very slowly, things have to speed up a bit -- there are lots of difficult issues to resolve today," said an environmental scientist who has participated in all the IPCC climate assessments since 1999.

The Eiffel Tower nearby was to extinguish its lights for five minutes at 7:55 p.m. (1855 GMT) as part of a campaign to raise awareness about energy efficiency and fossil-fuel pollution.

Compared with previous IPCC gatherings, the source said, "there is a strikingly low degree of conflict and a high degree of agreement by governments" on the core conclusions that the planet is warming and mankind is overwhelmingly responsible for it.

"None of the 'usual suspects' -- the United States, the oil-producing countries and China -- have attempted to obstruct the discussions" or "corrupt the science in the report," said another participant.

But the line-by-line vetting was slowed by the sheer task of making the document intelligible for policymakers without sacrificing scientific accuracy.

There was also sharp debate, both sources said, about what should be included, or not, in the phonebook-sized report's all-important summary.

"The two main sticking points have been how to describe the temperature projections and the rise in sea levels," said the environmental scientist.

Leaked drafts of the assessment predict that Earth's surface temperature will rise by 4.5 C (8.1 F) or even higher if carbon dioxide levels in the atmosphere increase by half compared with today's concentrations. The figures are hedged with qualifications and different scenarios, though.

Government representatives complained that this yardstick was too fuzzy for the general public and decision-makers, which means the final document will simply project the expected temperature rise by 2100.

A forecast in the draft that sea levels will rise by 28 to 43 centimeters (11.2 to 17.2 inches) have also been contested as too conservative by some scientists, both sources said, as it does not factor in recently-observed melting of ice in Greenland and Antarctica.

"The paleontologists point out that during the last intergalacial period, sea levels rose at one meter (3.25 feet) or higher per century," a source said.

As the week-long meeting got underway, global warming initiatives announced around the world underscored the extent to which climate change is fast becoming a top priority for policy makers.

The United Nations Environment Programme (UNEP) joined with the UN Framework Convention on Climate Change (UNFCCC) -- the offshoot of the 1992 Rio Summit -- to call on new Secretary General Ban Ki-Moon to call a special summit on global warming.

US lawmakers called Tuesday for an end to American complacency over global warming as the new Democratic-controlled Congress weighed measures to reduce greenhouse gases.

Two more volumes of the IPCCC assessment are due out in April and early May. They will assess the environmental and social impacts of these changes and ways of mitigating climate shift.