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. Read more.