COPENHAGEN, Denmark - The cuts in greenhouse gases offered at the 192-nation climate conference are "clearly not enough" to assure the world it will head off dangerous global warming, a key U.N.-affiliated scientist said yesterday.

Such projections, moreover, don't even account for the "potentially hugely important" threat of methane from the Arctic's thawing permafrost, other researchers said.

Midway through the two-week U.N. conference, richer nations are offering firm reductions in emissions of carbon dioxide and other greenhouse gases ranging from 3 percent to 4 percent for the United States to 20 percent for the European Union, in terms of 2020 emission levels compared with 1990.

One authoritative independent analysis finds the aggregate cuts amount to 8 percent to 12 percent. But the Intergovernmental Panel on Climate Changes (IPCC), the U.N.-sponsored science network, recommends that reductions average in the 25 percent to 40 percent range to keep global temperature increases below 2 degrees Celsius (3.6 F) above preindustrial levels and head off the worst of global warming.

"I think it is clearly not enough," the IPCC's Thomas Stocker said of the numbers discussed here. "We are by far short of having security that the 2-degree target will be met."

The Swiss physicist heads the IPCC's Working Group I, the climate science group that, among other things, assesses the impact that emissions - from fossil-fuel burning, deforestation, and other sources - have on concentrations of global-warming gases in the atmosphere and then on temperatures.

Stocker told reporters the IPCC-recommended target "may be too much to ask at this stage" - too politically daunting to achieve in the current annual conference.

Stocker acknowledged that IPCC projections do not include the potential "tipping point" addition of trapped methane, a powerful greenhouse gas that is released as permafrost thaws in the far north.

Plant and animal matter entombed in frozen Arctic soil for millennia decomposes as it thaws and is attacked by microbes, producing carbon dioxide and - if it is in water - methane, many times more powerful than CO2 in warming the atmosphere.