PARIS — Parts of a global climate agreement being hammered out in Paris should be legally binding, President Obama said Tuesday. The declaration was a boost to climate negotiators seeking a tough accord and a challenge to Republican senators, many of whom don’t believe that global warming is real.

Whether or not to make the climate accord legally binding is a major sticking point at the two-week talks in Paris, which aim to get all countries to agree to cut emissions that scientists say are warming the Earth and are increasing extreme weather such as droughts and floods.

Obama has spent months prodding other countries to make ambitious carbon-cutting pledges to the agreement, which he hopes will become the framework for countries to tackle the climate issue long beyond the end of his presidency in early 2017.

In Paris, Obama said the specific emissions targets each country is setting to reduce greenhouse gas emissions may not have the force of treaties. But he says to hold each other accountable, it’s critical that “periodic reviews” of those commitments be legally binding. He’s referring to a mechanism sought by negotiators under which countries would ratchet up their commitments every five years.

“Although the targets themselves may not have the force of treaties, the process, the procedures that ensure transparency and periodic reviews, that needs to be legally binding. And that’s going to be critical in us having high ambitions and holding each other accountable,” he said.

Obama would have little chance of getting the Republican-run Congress to vote to approve a fully binding new climate treaty fighting global warming. So the White House has been searching for a compromise, in which parts of the deal are binding and others are not, sparing the need for a new vote in Congress.

Republicans have sowed uncertainty about whether the U.S. will make good on its promises made at the Paris conference. Senate Majority Leader Mitch McConnell, R-Ky., and other Republican leaders have warned other countries not to trust any deal Obama may strike. Other Republican politicians are working to nullify Obama’s emissions-cutting steps at home.

Leaders of poor nations most affected by climate change, meanwhile, shared their stories of global warming threats Tuesday with Obama and French President Francois Hollande, from deserts encroaching on African farmland to rising sea levels shrinking islands of the South Pacific.

The encounters highlighted another big debate in the effort to reach an international accord to fight global warming: how much aid rich countries should give poor ones to help them adapt to climate change and reduce their emissions.

Hollande heard from 12 African leaders who described the Sahara Desert encroaching on farmland, forests disappearing from Congo to Madagascar and rising sea levels swallowing homes in West African river deltas.

“When a young student is forced to go study under a street lamp at night, it clearly demonstrates the electricity issue,” Malian President Ibrahim Boubacar Keita said.

Hollande said France would invest billions of euros in the coming years for renewable energy in Africa and to increase Africans’ access to electricity.

“The world, and in particular the developed world, owes the African continent an environmental debt,” he said.

Obama also met with envoys from island nations hit hard by rising seas and increasingly violent storms, which scientists attribute to climate change prompted by man-made carbon emissions. The Hawaiian-born Obama said he understands the beauty and fragility of island life and called their populations “among the most vulnerable to the ravages of climate change.”

African leaders, meanwhile, stressed the need to address shrinking resources in the troubled Lake Chad region, where the Islamic extremist group Boko Haram carries out regular attacks.

The lake, surrounded by Chad, Cameroon, Niger and Nigeria, has shrunk as much as 90 percent compared to 1960, changing the lives of nearby farmers, fishermen and herders. Some also say the increasing desperation is driving people into the extremists’ ranks.

“There’s a close link between the drying-up of the lake and the terrorism of Boko Haram,” said Niger President Mahamadou Issoufou. “We must act quickly, before it’s too late” to help people in the region.

The Nigeria-based Boko Haram has expanded attacks this year into Cameroon, Chad and Niger in a 6-year-old uprising has killed thousands and driven up to 2.3 million from their homes.

The climate conference began Monday with an unprecedented gathering of world leaders outside Paris. Presidents, prime ministers and princes urged the delegates to build a better planet for future generations, hoping to avoid a repeat of the embarrassing collapse of a similar effort in Copenhagen in 2009.

On Tuesday, the negotiations began in earnest, with the key task of figuring out who will pay for everything the leaders say needs to be done.

“You have now started the fundamental work,” French Foreign Minister Laurent Fabius told the negotiators. “I implore you to advance on the substance in a way that allows us to respect the strong mandate given by the diverse heads of state and government yesterday.”

Developing countries say they need financial support and technology to relocate threatened populations and make their own transition to cleaner energy.

The talks, which run through Dec. 11, are aimed at a broader, tougher replacement to the 1997 Kyoto Protocol. That treaty required only rich countries to cut their emissions, while this time the goal is for everyone to pitch in.

One of the proposals involves saving the world’s forests, which absorb carbon dioxide released by burning oil, gas and coal.

Britain’s Prince Charles, indigenous leaders and other dignitaries met Tuesday to call attention to shrinking global forests from South America to Russia and Africa, in part because of illegal logging.