Carbon-curbing pledges under the Copenhagen Accord are likely to see Earth warm 3°C or more, compared to the deal's target of 2°C, German scientists report.
In an analysis published by the journal Nature, researchers at the Potsdam Institute for Climate Impact Research (PIK) near Berlin believe the promises fell very short of the headline-making mark.
"It's amazing how unambitious these pledges are," they write.
Born in the final hours of the UN climate summit last December, the Accord sets a goal of limiting warming to 2°C.
But it does not set a date for achieving this, nor stepping-stone targets for getting there, and the roster of pledges it sets up, gathering rich and poor countries alike, is voluntary.
'Hoping to stop'
If the promises are carried out, global yearly emissions of greenhouse gases will increase by 10% to 20% above current levels, reaching the equivalent of 47.9 to 53.6 gigatonnes (billion tonnes) of carbon dioxide (CO2) equivalent by 2020, says the study.
"This would result in a greater than 50% chance that warming will exceed 3°C by 2100," according to the researchers.
"To be on track for meeting the 'below 2°C' climate target, global emissions of no more than 40 to 44 gigatonnes (billion tonnes) of CO2 equivalent have to be achieved by 2020."
PIK researcher Malte Meinshausen says, "Forty-eight gigatonnes of CO2 emissions is not on track to meet the 2°C goal - it is like racing towards a cliff and hoping to stop just before it."
The Copenhagen Accord remains politically contested.
In their analysis, the PIK researchers say a big loophole was surplus allowances under the Kyoto Protocol, whose current provisions expire at the end of 2012.
These surplus allowances can be used by industrialised countries who undershoot their Kyoto targets for emissions reductions.
The United States, the world's number two carbon emitter, is not party to Kyoto, nor is China, the world's number one, because it is a developing country and does not have binding emissions targets.
The authors say that the Kyoto targets were weak, which means many countries will be banking their surpluses for use later - a tally that they estimate at a huge 11 gigatonnes.
Previous studies have suggested that warming of 3°C or more would have a huge effect on Earth's climate system, possibly leading to more frequent drought, flood, storms and rising seas affecting millions of people.
Since pre-industrial times, Earth's mean surface temperature has risen by about 0.8°C.