PARIS — President Barack Obama and more than 100 world leaders will convene with thousands of diplomats Monday on the outskirts of Paris to open two weeks of intense negotiations aimed at forging an accord that could begin to avert the most devastating effects of global warming and redefine the economy of the 21st century.

Here is a guide to what is at stake. If the talks fail — as they did in two previous attempts to achieve such a deal — then nations will continue on a trajectory that scientists say locks the planet into a future of rising sea levels, more frequent floods, worsening droughts, food and water shortages, destructive hurricanes and other catastrophic events.

Recent scientific reports have concluded that the first effects of human-caused climate change have already started to sweep across the Earth, from rising sea levels flooding Miami to savage heat waves in Australia. The National Oceanic and Atmospheric Administration projects that 2015 will be the hottest global year on record, beating the record set in 2014.

Still, veterans of two decades of United Nations climate change negotiations say there is reason to think that the Paris summit meeting could be different from previous efforts, and yield a deal that could stave off the more destructive effects of warming.

“The stage is set for the possibility of getting this right, but on the other hand there are all these wrinkles and hurdles at the last minute,” Secretary of State John Kerry said in an interview last week.

EARLIER ATTEMPTS

If you are thinking you have seen this before, you are right. Since 1992, UN negotiators have held annual summit meetings aimed at drafting a global climate change treaty. They came close to a deal twice.

In 1997, world leaders signed the Kyoto Protocol, which assigned the largest economies legally binding targets for cutting their greenhouse gas emissions. There were two big problems: Kyoto assigned targets to the developed world, but made no requirements of developing economies like India and China, which today are two of the largest greenhouse gas polluters. President Bill Clinton never sent the deal to the Senate for ratification because it would not have passed, thus exempting the world’s largest economy from action.

In 2009 in Copenhagen, Mr Obama and other world leaders drafted a new pact to replace the Kyoto deal. It would have committed developing countries like China to action, but failed to achieve the unanimous consent required for legal enforcement, and has served as little more than a voluntary agreement.

Mr Obama pledged that by 2025 the United States would cut emissions up to 28 per cent from 2005 levels, largely through aggressive environmental rules on greenhouse gases from power plant smokestacks and vehicle tailpipes. President Xi Jinping announced that by 2017, China would begin a national cap-and-trade system placing limits on industrial emissions and requiring companies to pay for government-issued permits to pollute. Mr Xi also has pledged that China’s emissions will begin to drop no later than 2030.

Since then, more than 170 countries, representing more than 90 per cent of global carbon emissions, have put forth proposals to reduce emissions. Those plans, expected to form the core of a new agreement, represent a major change from the usual brinkmanship of climate negotiations.

‘IT IS NOT THE ENDPOINT’

Another difference between Paris and the Kyoto and Copenhagen deals: Negotiators have managed expectations. At the conferences in Japan and Denmark, world leaders promoted the expected pacts as treaties that would save the planet. No one is saying that about Paris. For one thing, a Paris deal would not take the legal form of a traditional treaty. And it would not save the planet, but only be a strong first step.

The objective of climate change policy is to curb greenhouse gas emissions enough to avoid a global average temperature increase of 3.6 degrees Fahrenheit. Beyond that threshold, scientists say, there will be devastating effects. Research shows the globe has already warmed about 1.5 degrees since the Industrial Revolution.

Together the more than 170 national plans for Paris would still allow the planet to warm by as much as 6 degrees, according to several independent and academic analyses. Scientists say that that level of warming is still likely to cause food shortages and widespread extinctions of plant and animal life.

Still, that would count as progress. Without the Paris climate policies, scientists say, the planet is headed toward a far more destructive temperature increase of more than 8 degrees Fahrenheit.

“The national plans will not put us on the path to lower temperatures enough, but they will bend the temperature curve,” said Mr Janos Pasztor, the UN assistant secretary-general for climate change. “While it is significant progress, it is not the endpoint. It is a turning point. It must mark the floor, not the ceiling.”

THE BILL FOR TOUGHER POLICIES

Who would pay is another big obstacle.

In Copenhagen, Ms Hillary Rodham Clinton, then the secretary of state, pledged that by 2020 the developed world would send developing countries US$100 billion (S$141.5 billion) annually to help them make the transition to cleaner energy and pay for the effects of climate change, including floods and droughts.

The idea, U.S. negotiators said, was that the money would come from public appropriations and private investments. A report this year by the Organisation for Economic Cooperation and Development found that about US$62 billion of so-called climate finance money, from public and private sources, went from developed to developing countries in 2014. But in Paris, many developing countries want to lock down an agreement binding wealthier countries to spend far more of their own money to help them.

“The cost of action is not US$100 billion,” said Mr Prakash Javadekar, the Indian environment minister. “It is trillions; US$100 billion is just a reparation.”

That argument, and the word “reparations,” are likely to be heard until the talks end.

WHAT COULD GO WRONG

The United Nations is a unique institution. It passes new pacts only by unanimous consent. So in a system designed to ensure that even the smallest, poorest nations have equal voices, a single country can upend a deal. That is what happened when fewer than a half-dozen countries opposed the Copenhagen accord.

After Paris, a single wild-card nation — eyes are on Malaysia, Saudi Arabia, Sudan or Venezuela — could bring the process down. NEW YORK TIMES