The UN's former climate chief on Tuesday said the global warming pledge he helped set at the Copenhagen Summit little more than two years ago was already unattainable.

"I think two degrees is out of reach," Yvo de Boer, former executive secretary of the UN's Framework Convention on Climate Change (UNFCCC), said on the sidelines of a conference here on June's Rio+20 summit.

The UNFCCC's 195 parties have pledged to limit the rise in global average temperatures to two degrees Celsius (3.6 degrees Fahrenheit).

The target was set by a core group of countries in the final stormy hours at the Copenhagen Summit in December 2009 and became enshrined by the forum at Cancun, Mexico a year later.

But more and more scientists are warning that the objective is slipping away without radical, early cuts in greenhouse-gas emissions. Some consider the goal to be a dangerous political mirage, for Earth is now on track for 3C (5.4 F) of warming or more.

"The two degrees is lost but that doesn't mean for me we should forget about it," de Boer said in the interview with AFP.

"It is a very significant target, it's not just a target that was plucked out of the air, it refers to trying to limit a number of impacts."

He added: "You shouldn't forget about it, in the sense that you are ignoring the fact that you've gone through the trouble of formulating a goal and then not met it because of lack of policy action.

"The process therefore should be all about how can we get as close to 2C as possible, not to say 'start all over again and formulate a new goal,' having forgotten that we've been through this very recently."

Copenhagen marked a high-water line in the global climate forum.

Its disappointments, together with the financial and fiscal crunch that have hit western countries, have made many advanced economies mark time or even retrench their action against carbon emissions.

And the high price of oil and gas has prompted emerging economies to power their growth with coal, the dirtiest of the major fossil fuels, driving up atmospheric levels of carbon dioxide (CO2).

At the UNFCCC's annual get-together in Durban, South Africa last year, countries agreed to wrap up a new climate agreement in 2015 that would take effect in 2020, placing rich and poor for the first time under common legal constraints.

De Boer said he hoped the Fifth Assessment Report by the UN's Intergovernmental Panel on Climate Change (IPCC), due in 2014, would spur momentum for the 2015 deadline.

The four-day London conference, Planet Under Pressure, aims at delivering a snapshot of the world's environment before the June 20-22 20-year followup to the Rio Earth Summit.

On Sunday, 20 winners of the Blue Planet Prize, one of the world's most prestigious green awards, said there was only a "50-50" chance of limiting warming to 3 C (5.4 F).

There were "serious risks" of a 5 C (9.0 F) rise, a temperature last seen on the planet 30 million years ago.

"We have to be honest with each other that we will not reach the two degrees target," former IPCC head Bob Watson, now a scientific advisor to the British government, said on behalf of the laureates.

The UNFCCC's 2 C target has been widely criticised as inadequate, given that it fails to identify a date for achieving this goal or the stepping stones towards it.

The UNFCCC -- officially at least -- even holds to the possibility of limiting warming to 1.5 C (2.7 F), which is the demand of the world's poorest economies and small-island states.

They have most to lose from drought, floods, storms and rising seas driven by climate change.

The 2 C goal will be subject to a review in 2015, to see whether it should be brought down to 1.5 C.