Meeting the target for global warming enshrined in the 2009 Copenhagen Accord will require carbon emissions to decline by more than eight percent by 2020 compared to 2010 and then continue their fall, a study said on Sunday.

World leaders at the ill-starred UN climate-change summit in Copenhagen in December 2009 set a goal of limiting warming to two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels.

The Copenhagen Accord does not set a timetable for reaching the objective and the national pledges of carbon cuts for achieving it are voluntary.

In a paper published online in the peer-reviewed journal Nature Climate Change, researchers led by Joeri Rogelj of the Institute for Atmospheric and Climate Science at ETH Zurich in Switzerland, revisited computer models in the light of 2 C (3.6 F) objective.

In scenarios that saw a "likely" -- higher than 66-percent -- chance of staying below 2 C (3.6 F), global emissions would have to peak between 2010 and 2020.

By 2020, annual emissions would have to be 44 billion tonnes, or gigatonnes, of carbon dioxide or its equivalent (CO2e) per year.

This would amount to an 8.5-percent cut compared with 2010, when global emissions were 48 gigatonnes.

Emissions would have to keep falling thereafter.

"If the mechanisms needed to enable an early peak in global emissions followed by steep reductions are not put in place, there is a significant risk that the 2 C (3.6 F) target will not be achieved," the paper warns.

CO2e is a measurement of all greenhouse gases, ranging from carbon dioxide -- emitted by the burning of fossil fuels -- to methane, which comes from deforestation and agricultural processes.

The International Energy Agency (IEA) says that CO2-only emissions in 2010 were the highest in history, rebounding sharply after a dip in 2009 caused by the global financial crisis.

Intended to usher in a new global treaty on climate change, the December 2009 summit in the Danish capital nearly ended in a fiasco.

The Copenhagen Accord was drafted at the last minute by a small group of leaders.

It failed to win the endorsement of a plenary of 194-party UN Framework Convention on Climate Change (UNFCCC), after it was attacked for being undemocratic, and green groups say its voluntary approach will not deliver the carbon cuts that are needed.

Scientists, meanwhile, are cautious about the 2 C (3.6 F) target, saying it is not a guarantee of safety.

Many say there have already been perceptible changes to snow and ice cover, habitat and reproductive patterns by migrating species as a result of existing warming, of roughly 1 C (1.8 F) since 1900.