The UN process for tackling global warming is now captainless as well as rudderless after climate chief Yvo de Boer suddenly announced his resignation two months after the ill-starred Copenhagen summit.

De Boer, 55, announced on Thursday he would step down as executive secretary of the UN Framework Convention on Climate Change (UNFCCC) to pursue a career in the private sector.

His abrupt departure -- and the search for a successor able to satisfy 194 nations -- adds another thick layer of problems for the UN as it grapples with what many experts describe as the greatest challenge of this century.

"There is drift and hesitation, and the resignation will add to the mood," said a senior European diplomat, speaking on condition of anonymity.

The December 7-19 conference in Copenhagen bitterly disappointed those clamouring for a new offensive against climate change.

It was supposed to unlock a historic pact that, from 2012, would unite all nations in rolling back greenhouse gases and defending poor countries from the damaged weather systems.

But it ended in near-fiasco.

Desperate for a face-saving result, the leaders of the major carbon emitters cobbled together an outcome of the lowest common denominator.

Warming should be limited to two degrees Celsius (3.6 degrees Fahrenheit) and 30 billion dollars would be mustered in aid by 2012, with potentially hundreds of billions more by 2020, they declared.

But the agreement set down no roadmap for reaching the goal and in any case, the pledges made by nations are simply voluntary.

Barely was the ink dry on the accord than countries began to tiptoe away from it. China and India have so far failed to endorse the document, which entails declaring that they wish to be "associated" with it.

Today, momentum and direction seem to have drained out of the climate process, say negotiators.

The Copenhagen Accord lacks traction, for it raises suspicions of a rival forum, driven by an elite group, to the global format.

And hopes that the United States, the world's second biggest polluter, will give a dynamic push are fading as President Barack Obama's domestic political problems worsen.

"There is a post-Copenhagen daze. Everyone is trying to take stock of what's going on," said Laurence Tubiana, director of the Institute of Sustainable Development and International Relations (IDDRI) in Paris.

After initially talking up the outcome of Copenhagen as a useful consensus, some governments are now openly admitting it was a setback.

"We need to admit that it was a failure," Russian President Dmitry Medvedev said on Thursday. "But at the same time it was a lesson. Such events require a different kind of preparation in the future."

European Commission chief, Jose Manuel Barroso, in a letter to the 27 leaders of the European Union (EU), described Copenhagen as "a reality check" for the EU's ambitious -- but unilateral -- climate strategy.

But these are only the first signs of what could be a long, messy rethink about how to put the UNFCCC process back together, a business that will be further complicated by the choice of the right individual to take the helm.

Only two meetings have been scheduled this year -- one in Bonn, at the relatively low level of technocrats, the other in Mexico, climaxing at ministerial level.

"The real problem is a lack of political guidance. Who is giving the lead?" complained a European climate negotiator.