Delegates from 191 countries and the European Union opened UN-sponsored talks Monday in Durban, South Africa, amid deeply divisive questions over when and how to replace the 1997 Kyoto Protocol with an agreement that is legally binding on all major greenhouse-gas emitters, not just developed countries.

The talks are taking place against a backdrop of grim news concerning Earth’s climate, including rising concentrations of heat-trapping carbon dioxide gases and a recent report by the Intergovernmental Panel on Climate Change (IPCC) that highlights the increased risks emerging from extreme-weather events.

Adding to pressure on the Durban delegates, a recent analysis from the UN Environment Program found that pledges some 80 countries made over the past two years to reduce emissions or slow emissions' growth rates fall far short of what's needed by 2020 to put the world on a path to holding the increase in global average temperatures to 2 degrees Celsius by the end of the century.

Veterans note that when the talks open, delegations tend to lay out their toughest bargaining positions. But after modest progress on several issues at last year's talks in Cancun, Mexico, positions that parties – particularly the US – are staking out heading into the next two weeks paint a picture that's "pretty discouraging," says Alden Meyer, director of strategy and policy for the Union of Concerned Scientists.

For countries that have ratified the 1997 Kyoto Protocol, the key issue to be resolved is whether or not to adopt a second five-year enforcement period. The current period expires at the end of next year. Japan, Russia, and Canada have formally stated they will not accept new emission-reduction targets. The US withdrew from the Kyoto process in 2001.

The European Union's delegation has said it is willing to commit to a second enforcement period, but only if countries not part of the agreement put together a road map at Durban that leads to a legally binding agreement in 2015 covering all major emitters, including China, India, and the US. The EU’s aim is to have the new pact ultimately replace the Kyoto Protocol and take force in 2020, when the voluntary commitments countries formally adopted in Cancun expire.

Small-island nations threatened by sea-level rise and least-developed countries also back the EU's approach, says Mr. Meyer.

Yet the US appears reluctant to support a road map toward a legally binding, post-2020 pact without knowing more about what it's likely to contain – provisions that presumably could be informed by the next set of periodic IPCC reports on climate change, due out in 2014.

"Our thinking is that putting the form of the action before the substance doesn't make a great deal of sense," said the lead US negotiator, Jonathan Pershing, at a briefing Monday in Durban.

In the end, the US has expressed a willingness to negotiate a legally binding treaty, but with preconditions that are nonstarters with developing countries, Meyer adds.

Among them: Some mechanisms for bumping developing countries into developed status as their economies progress, which would then require them to take on legally-binding commitments; and ensuring that when such commitments are made, they don't hinge on receiving aid either for adaptation to climate change or to buy the technology they would need to meet those commitments.

Such a quid-pro-quo was part of the agreement that came out of climate talks in Bali in 2007, Meyer explains.

Asked about the challenges facing negotiators over the next two weeks, the meeting's president Maite Nkoana-Mashabane, South Africa's minister of international relations and cooperation, acknowledged Monday that "we're under no illusion that this is an easy process.”