Some had hoped for a breakthrough as global leaders convened for a global-warming summit at the United Nations headquarters in New York on Tuesday. Early indications suggest otherwise, but leaders of the major economies will get a second chance when the G20 meets in Pittsburgh on Thursday and Friday. Nature takes a closer look.

President Barack Obama and Chinese President Hu Jintao said little that was new.AP Photo/Charles Dharapak

Expectations were high for both US President Barack Obama and Chinese President Hu Jintao. Did either rise to the occasion?

Everybody is looking for signs of progress from the two biggest emitters, who together account roughly 40% of emissions, but neither president offered the kind of commitments needed to re-energize the talks. Obama was in the unenviable position of needing to make bold promises before the US Congress has weighed in on the issue. Nonetheless, he declined to acknowledge, let alone address head on, the challenges he is facing on the domestic front.

For his part, Hu largely underscored existing policies, promising to expand forests, produce 15% of the country's power using renewable energy and decrease energy intensity per unit of gross domestic product by a "notable margin" between now and 2020. All of these would substantially reduce Chinese emissions compared with baseline forecasts, and China is beginning to win some praise for its energy policies. Nonetheless, cumulative emissions are expected to continue rising, and Hu made no reference to any specific emissions targets or a date by which the country might try to stabilize its emissions.

Was there any sign of a convergence between the two countries?

Read more about the climate at the Road to Copenhagen special.

Both Hu and Obama offered generic language about the need for all nations to move together, but the basic stumbling blocks — who will move first, who will pay and what kind of commitments will be required — remained solidly in place.

Hu began his speech by underscoring the idea of "common but differentiated responsibilities", and then went on to demand that wealthy nations provide financial aid to support climate mitigation and sustainable development in the developing world. Obama said what so many industrialized leaders have been saying all along: developed nations must move first and provide financial support to those who need it, but developing countries will need to commit to some kind of verifiable action as well.

Both positions seem reasonable. What's the problem?

Although almost everybody agrees with these statements, these same phrases have been uttered countless times by officials all over the world. Finding a middle ground has proven elusive.

What kind of discussions are expected at the G20 summit this week?

Global warming made it on to the agenda, but discussions about global economic recovery are expected to dominate. Although leaders are planning discussions about a climate finance package that would help developing countries pay for adaptation and mitigation in the years to come, few think the heads of state are likely to produce any kind of conclusive statement.

What does all this mean for the United Nations Climate Change Conference in Copenhagen in December?

Such things are hard to predict, but it has become increasingly clear that getting a groundbreaking deal on greenhouse-gas emissions to replace the Kyoto Protocol will be difficult.

Speaking in Washington recently, Michael Zammit Cutajar, Malta's climate change envoy and chairman of one of the core negotiating groups in the UN climate process, said that this kind of diplomacy is like the Big Bang in reverse — painfully slow for a long time, "with a big bang at the end".

Jennifer Morgan, director of the climate and energy programme at the World Resources Institute in Washington DC, has laid out four possible outcomes. The 'Breakthrough' outcome, in which countries come to the table with strong proposals and then build them into an even stronger agreement. The 'foundation' scenario, which relies on a solid framework that spells out basic commitments and things such as funding, monitoring and verification, leaving the details for later. The third, 'greenwash', involves nothing but bottom-up commitments that nations would have made anyway, which means there is no value added by the international framework. And the last is self-explanatory: 'collapse'.

Right now, Morgan says she thinks the negotiations are hovering between foundation and greenwash.

Meanwhile, French President Nicolas Sarkozy has called for heads of the major industrialized nations to meet again after the G20 meeting but ahead of Copenhagen.