Talks on a universal climate pact shifted to a higher gear Monday, with U.N. Secretary-General Ban Ki-moon urging governments to set off an "energy revolution" to rein in heat-trapping carbon emissions and avert disastrous global warming.

The European Union appeared to be softening its position on its demand that emissions targets in an eventual Paris climate accord need to be legally binding. And U.S. Secretary of State John Kerry said that even if it's not legally binding, a deal could still change the way world business thinks about energy.

Foreign and environment ministers joined the talks outside Paris after lower-level negotiators who met last week delivered a draft agreement with all crunch issues left unresolved.

Warning that "the clock is ticking toward climate catastrophe," Ban told ministers the world expects more from them than "half-measures."

"Your work here this week can help eradicate poverty, spark a clean energy revolution and provide jobs, opportunities and hope for tomorrow," he said.

The Paris conference is the 21st time world governments have met to seek a joint solution to climate change - and is aiming at the most ambitious, long-lasting accord yet. The talks are focused on reducing emissions of carbon dioxide and other greenhouse gases, primarily by shifting from oil, coal and gas to cleaner sources of energy.

Kerry, after arriving in Paris to join the talks, said that if the more than 190 countries in attendance agree to a plan, the private sector will then take the reins and create sustainable power technologies that will ease climate change.

"Even without a fixed number and a legal shell, we are going to see an enormous amount of movement without creating political obstacles that prevent us from being able to send that signal," Kerry told a gathering on the sidelines of the climate conference in the French capital.

"I have absolute confidence in the ability of capital to move where the signal of the marketplace says 'go' after Paris," he said.

The EU has been among the most outspoken advocates of binding targets.

However, EU Climate Commissioner Miguel Arias Canete told reporters Monday that he understands "the political situation in the United States," where Republicans in Congress would be unlikely to approve binding targets for carbon dioxide emissions. Many U.S. Republicans question whether climate change is happening and oppose emissions limits out of concern that it would hurt U.S. industry and jobs.

In a statement, Canete said the EU still favors internationally binding targets, but "at the same time, we have signaled our readiness to discuss alternative approaches which would ensure that the Paris agreement can provide a robust legal framework and maximum certainty in parties delivering on their targets."

The envisioned Paris agreement is supposed to be the first deal to ask all countries to rein in their emissions; earlier pacts only required wealthy nations to do so.

"Developed countries must agree to lead, and developing countries need to assume increasing responsibility in line with their capabilities," Ban said.

How to define those responsibilities is the biggest challenge in the Paris talks. India and other major developing countries insist on their right to use some fossil fuels to advance their economies - just like Western nations have done since the Industrial Revolution. They argue the West therefore is historically responsible for raising levels of carbon dioxide in the atmosphere.

"India is here to ensure that rich countries pay back their debt for overdraft that they have drawn on the carbon space," Indian Environment Minister Prakash Javadekar said.

Meanwhile, in China, Beijing issued its first ever red alert for smog, urging schools to close and invoking restrictions on factories and traffic. While that's different from greenhouse gas emissions, much of the air pollution is blamed on coal-fired power plants and vehicle emissions which also are key sources of carbon emissions.

Another major issue is helping poor countries cope with dangerous warming effects, from rising seas to intensifying droughts and heat waves.

More than 180 countries have already presented national pledges for reining in carbon emissions. But scientific analyses show that won't be enough to meet the international goal of limiting warming to 2 degrees C (3.6 degrees F), compared to pre-industrial times.

Many countries have called for a review of all targets within five years to see if there are ways of ramping them up. The draft, however, sets 2024 as the earliest date of such a reappraisal.

Tuvalu Prime Minister Enela warned that his island nation and others face potential extinction if temperatures continue to rise.

"Let's achieve a legally binding agreement," he said. "Let's do it for Tuvalu. If we save Tuvalu, we save the world."

U.N. climate chief Christiana Figueres said she's kept up at night by a vision of "the eyes of seven generations beyond me asking me, 'What did you do?'

"The same question will be asked of each of you," Figueres told the ministers. "May we all be able to stand tall and clearly say we did everything that was necessary."

Kerry said he was hopeful that the negotiations would reach an agreement by a Friday deadline, but would not be surprised if the talks continued into the weekend.

"I think the stage is set, I think the attitude is currently there," he said.