Scientists say temperatures should be limited to a rise of no more than 2 degrees Celsius (3.6 F) above pre-industrial times if devastating climate change is to be avoided.
Yearly greenhouse gas emissions should not be more than 40 and 48.3 gigatonnes of CO2-equivalent in 2020 and should peak between 2015 and 2021, according to new modeling released on Tuesday by the United Nations Environment Programme (UNEP).
Keeping within that range and cutting global emissions by between 48 percent and 72 percent between 2020 and 2050 will give the planet a medium or 50-50 chance of staying within the 2 degree limit, said the report, which was based on modeling by nine research centres.
However, the same study found that the world is likely to go over those targets. The pledges were made by nations that signed up to the Copenhagen Accord.
The expected emissions for 2020 range between 48.8 to 51.2 gigatonnes of CO2-equivalent, based on whether high or low pledges will be fulfilled, the report said.
In other words, even in a best-case scenario where all countries implement their promised cuts, the total amount of emissions produced would still be between 0.5 and 8.8 gigatonnes over what scientists see as tolerable.
Greenhouse gas levels are rising, particularly for carbon dioxide, because more is remaining in the atmosphere than natural processes can deal with.
Carbon dioxide is naturally taken up and released by plants and the oceans but mankind's burning of fossil fuels such as coal for power and destruction of forests means the planet's annual carbon budget is being exceeded.
UNEP's executive director Achim Steiner said the bleak prediction should motivate countries to make more ambitious cuts.
The message is not to sit back and resign and say we will never make it, he told reporters in Nusa Dua on the Indonesian island of Bali, which is hosting a major U.N. environment meeting.
But it's not enough at the moment and there are other options that can be mobilised.
Steiner said one such option was more investment in a scheme called reduced emissions from deforestation and degradation (REDD), in which poor countries are paid to preserve and enhance their forests.
A state of the environment assessment released by UNEP on Tuesday, the UNEP Year Book 2010, also advocated more investment in REDD.
It has been estimated that putting $22 billion to $29 billion into REDD would cut global deforestation by 25 percent by 2015, the report said.
Forests soak up large amounts of planet-warming carbon dioxide. Cutting them down and burning the remains releases vast amounts of the gas, exacerbating global warming, scientists say.
REDD is not yet part of a broader climate pact that the U.N. hopes to seal by the end of year at major climate talks in Mexico.
Steiner told reporters a day earlier he expected talks this year to be a tough slog. The Copenhagen climate summit last December ended with a political accord that was not formally adopted and no clarity on the shape of a new climate pact to succeed the current Kyoto Protocol.
A deal has become more difficult than in Copenhagen. Let's be very frank. The world has moved away, rather than closer, to a deal, he told reporters. The politics of international negotiation and the economics, the momentum that built up toward Copenhagen will not be there for Mexico.
(Editing by David Fogarty)