World leaders should focus on reducing greenhouse gas emissions as much as possible over the next 40 years to avoid perilous warming conditions, researchers said.

In the first study of its kind, analysts used a detailed energy system model to analyze the relationship between emissions levels in 2050 and chances of achieving end-of-century targets of two or three degrees Celsius above the pre-industrial average.

UN climate talks in Copenhagen ended last month with a non-binding agreement to limit warming to two degrees Celsius but did not set binding targets to reduce the emissions of gases that scientists say are heating up the world's atmosphere to dangerous levels.

The study identified critical 2050 reductions that, if not met, could seriously complicate end-of-cen-tury targets with current energy sources.

Under one scenario, global emissions would need to be reduced by around 20 per cent below 2000 levels by 2050 in order to meet the target.

A second case accounting for a more rapid increase in demand for land and energy would require far steeper 2050 reductions: 50 per cent.

But the authors concluded that achieving these reductions was "barely feasible" with known energy sources.

"Even if we do everything possible to reduce emissions between now and 2050, we'd only have even odds of hitting the two-degree target - and then only if we also did everything possible over the second half of the century too," said co-author Keywan Riahi, a researcher at the International Institute for Applied Systems Analysis (IIASA) in Austria.

Lead co-author Brian O'Neill of the National Centre for Atmospheric Research noted that so long as an effective long-term strategy is adopted, emissions could be higher in 2050 than levels included in some proposals but still achieve the two-degree Celsius goal in the long term.

"Setting mid-century targets can help preserve long-term policy options while managing the risks and costs that come with long-term goals," he said.