Issued Tuesday by the U.N. Environment Program, the report warns of potentially catastrophic warming if governments don't step up climate protection efforts.
Even if governments implement what they have pledged as of now, temperatures could rise by up 7 degrees F before the end of the century, UNEP says.
"There is a gap between the science and current ambition levels," Achim Steiner, head of UNEP, said in a statement.
Politicians could, however, contain warming much better if they boost commitments beyond what was pledged at a summit in Copenhagen in 2009. The non-binding pledges from Copenhagen would deliver almost 60 percent of the carbon dioxide reductions needed.
U.N. Secretary-General Ban Ki-moon in a statement urged governments to "make good on their national mitigation pledges, and to further progress within the negotiations as well as through strengthened efforts on the ground to curb emissions."
The report comes before a Nov. 29-Dec.10 U.N. climate conference in Cancun, Mexico. Observers don't expect the meeting deliver a binding climate protection agreement, to succeed the Kyoto Protocol, which runs out in 2012.
The United Nations' top climate official in charge of the negotiations, Christiana Figueres, said the more than 180 governments meeting in Cancun "need to both anchor the pledges they made in Copenhagen in the U.N. context and to work swiftly to agree ways to reduce emission so that the world has a chance of staying below" a 3.6 degrees F temperature rise."
Predicting temperature rises from the level of CO2 in the atmosphere is difficult. Most nations aim for a rise of no more than 3.6 degrees F, a threshold needed to avoid an increase in potentially catastrophic storms, draughts and floods, scientists say.
Global climate negotiations have been deadlocked since the climate meeting in Copenhagen late last year, when leaders jetting to Denmark couldn't agree on concrete emissions reduction targets or a way to measure them.
Developing nations have resisted a legally binding treaty to succeed the Kyoto Protocol, which runs out in 2012, because they claim rich nations that have benefited from emitting during the past decades should shoulder more of the burden. Industrialized countries argue the developing nations need to commit to concrete reduction targets to enable a global effort.
The European Commission this year backtracked on a plan to unilaterally boost the bloc's greenhouse gas emissions reduction target from 20 percent to 30 percent -- likely also because other nations, including China and the United States, the world's top two emitters, haven't committed to similarly ambitious targets.
Officials from climate heavyweights, including the United States and the European Union, have in the past weeks suggested they might seek channels outside the U.N. process to tackle climate change.