The slow-moving U.N. talks on combating global warming took a step forward Saturday with revised proposals for a $100 billion-a-year climate aid fund and other issues for debate by the world’s environment ministers this week.

Despite that advance, the chairwoman of key closed-door negotiations warned the open conference that obstacles remain to what delegates hope will be a package of decisions next Friday on financial and other side matters under the U.N. climate treaty.

“Progress has been made in some areas,” Zimbabwe’s Margaret Mukahanana-Sangarwe said. But she said the talks were “going backwards” on important issues. “We need to redouble our efforts.”

Environment ministers began flying in Saturday for the final days of the annual two-week climate conference, hoping to put new life in the U.N. talks.

Last week, under Mukahanana-Sangarwe’s leadership, a working group from among the 193 treaty nations sought to whittle down the contested texts of proposed decisions.

In one sign of the work facing them, only 170 words had been undisputed among the 1,300 on two pages of a key text on the “shared vision” of what the treaty nations want to accomplish. The disputed language was options proposed by various parties and placed within brackets.

Some parties, for example, want the world to reduce emissions of global warming gases so that temperatures don’t rise more than 2 degrees C (3.6 degrees F) above preindustrial levels, beyond which scientists say serious damage from climate change would set in. Others want to aim even lower, at 1.5 C (2.7 F) above preindustrial levels {mdash} a position favored by island states and others most threatened by warming’s impacts, such as sea-level rise.

The Zimbabwean’s revised text eliminated the 1.5-degree option, drawing an immediate protest from the Bolivian delegation at Saturday’s open meeting, a sign of the contentiousness sure to mark the coming days.

In many important areas, her text revisions retained multiple options {mdash} on the supervision of the proposed climate fund, for example {mdash} setting the stage for further sharp debate.

The issue of reducing carbon dioxide and other greenhouse gases emitted by industry, vehicles and agriculture is the core dispute of the long-running climate talks, and will not be fully resolved at Cancun.

For 13 years, the U.S. has refused to join the rest of the industrialized world in the Kyoto Protocol, a 1997 add-on to the climate treaty that mandates modest emissions reductions by richer nations. The U.S. complained that it would hurt its economy and that Kyoto should have mandated actions as well by such emerging economies as China and India.

For their part those poorer but growing nations have rejected calls that they submit to Kyoto-style legally binding commitments -- not to reduce emissions, but to cut back on emissions growth.

This impasse brought last year’s climate summit in Copenhagen, Denmark, to near-collapse. That conference ended with a nonbinding “Copenhagen Accord,” under which the U.S., China and other nations inscribed voluntary pledges to scale back emissions. The agreement has been endorsed by 140 nations, not the treaty’s full 193.

Two debates under way in Cancun stem from Copenhagen: how to “anchor” those voluntary pledges more officially under the treaty, and how to monitor and verify the pledges are being met.

Negotiators hope for agreements on some secondary issues as well: setting up a “green fund” to disburse aid to poorer countries to reduce emissions and adapt to climate change; making it cheaper for developing nations to obtain climate-friendly proprietary technology from more advanced countries; and pinning down more elements of a complex plan to pay developing countries for protecting their tropical forests.

Firmly establishing a green fund at Cancun is a priority for developing-world delegations. In the Copenhagen Accord, richer nations promised them $100 billion a year in such support by 2020.

“We need clarity and detail on the setting up of the global climate fund {mdash} clarity on the composition of its board and on its relationship to the United Nations,” Sol Oyuela of Christian Aid said Saturday, speaking on behalf of a coalition of climate and development advocacy groups.

Developing nations generally want a U.N. body overseeing disbursement of climate funds, rather than, for example, the World Bank, which is controlled by developed nations.

Echoing the sentiments of many here, veteran Brazilian climate negotiator Sergio Serra said Saturday the Cancun conference of parties, or “COP,” must produce such concrete advances, or undermine the U.N. negotiating process.

After Copenhagen, he said, “if we have two COPs in a row with the same kind of no results, we’re putting at risk the whole multilateral exercise of negotiation on climate change, and this is very bad, because I don’t see alternatives.”