They came to the City of Lights with no less a goal than saving the entire human species.

Time will tell if their work over these last two weeks paid off.

At the Paris climate talks Saturday, the world approved a new deal on climate change that aims to save humanity from the worst effects of global warming, although many critics say it doesn't go far enough.

The agreement was unanimously approved in Paris by the 21st Conference of the Parties, or COP21, Saturday afternoon. It took the conference almost a full day longer to approve a deal than initially anticipated amid late-stage haggling over exact language, but many cheered an agreement that attempts to compensate for the failure in Copenhagen six years ago.

"The most difficult part is over, it would seem," said Laurent Fabius, president of the conference and French foreign minister.

The pact between 196 countries allows each of them, voluntarily, to set emissions reductions targets that will be reviewed every five years starting in 2023. The United States has pledged to cut its greenhouse gas emissions between 26 and 28 percent over 2005 levels by 2030.

The agreement seeks to hold global temperature rise "well below" 2 degrees Celsius, about 3.6 degrees Fahrenheit, with an eye toward keeping it below 1.5 degrees Celsius, or 2.7 degrees Fahrenheit.

It's an ambitious goal, one that scientists wonder is even possible to achieve: The globe has already warmed 1 degree Celsius, or 1.8 degrees Fahrenheit, above pre-industrial levels. The commitments from countries at the conference would only limit global temperature rise to 2.7 degrees Celsius, or nearly 5 degrees Fahrenheit.

But, the hope from many at the conference is that Paris is just the start. The five-year reviews are meant to get countries to review their progress and then set more and more ambitious goals. Many have said Paris is not the solution to climate change, but the start of the road that will end in a solution.

Australian Foreign Minister Julie Bishop said the deal was not perfect, but was a good start. She said it's now time to go home and capitalize on the momentum from Paris.

"Each of us will progress our efforts to the best of our capacity, and no country will step back," Bishop said.

The agreement would see the countries agree to peak greenhouse gas emissions "as soon as possible," with rapid reductions coming after emissions peak. Although it is couched in diplomatic terms, the agreement also calls for countries to work toward 100 percent renewable energy.

The language in the deal states countries have "common but differentiated" responsibility to fight climate change, which will be seen as a win by developing countries who seek to blame developed nations for causing climate change with greenhouse gas emissions. Countries with low emissions will be required to do less under the agreement than high-emission nations.

"It should be recognized the Paris agreement represents a major leap forward for developing countries," said Edna Molewa, South Africa's minister of environmental affairs who was speaking on behalf of the G77 countries.

The agreement also requires developed countries to provide financial resources to developing countries under international law. This has been a major sticking point for Republicans in Congress who have rejected the notion of sending billions abroad to developing countries to help fight the effects of climate change.

The agreement, which is not a formally binding treaty, does not need congressional approval.

President Obama has pledged $3 billion to a so-called Green Climate Fund for developing countries, much to the chagrin of his opponents in Congress. The agreement requires developed countries set a goal of providing $100 billion annually to developing countries by 2020.

The agreement "strongly urges" countries to ramp up their contributions to developing countries by 2020.