GLASGOW—More than 190 nations reached a deal at the United Nations summit here that aims to accelerate greenhouse-gas-emissions cuts across the world, but leaves big questions over how governments will follow through in the coming decade to try to avert the worst effects of global warming.

Supporters say the deal—struck Saturday evening after two weeks of negotiations—signals new determination among the world’s governments to shift away from burning fossil fuels, the main source of greenhouse gases that scientists say are...

GLASGOW—More than 190 nations reached a deal at the United Nations summit here that aims to accelerate greenhouse-gas-emissions cuts across the world, but leaves big questions over how governments will follow through in the coming decade to try to avert the worst effects of global warming.

Supporters say the deal—struck Saturday evening after two weeks of negotiations—signals new determination among the world’s governments to shift away from burning fossil fuels, the main source of greenhouse gases that scientists say are causing the earth to warm. The agreement, though, features weaknesses that have hamstrung U.N. climate talks over the decades.

It has no enforcement mechanism, relying instead on the good faith of the world’s governments to adhere to its rules as best they can. In key areas, it doesn’t require nations to act, but merely urges or requests them to do so, reflecting wiggle room that was needed to achieve consensus among all governments.

U.S. Secretary-General António Guterres reflected the disappointment of many delegates in not getting more concrete commitments through a process that required signoff from almost all of the world’s governments.

“The approved texts are a compromise,” he said. “They reflect the interests, the conditions, the contradictions and the state of political will in the world today.”

He said the deal makes important steps forward “but unfortunately the collective political will was not enough to overcome some deep contradictions.”

U.S. climate envoy John Kerry said the agreement accomplished several U.S. goals and brought the world “closer than ever before to avoiding climate chaos.”

The deal asks governments to strengthen their emissions-reduction targets by the end of next year to keep them in line with what scientists think is necessary to meet the climate target of the 2015 Paris accord: keeping global warming well under 2 degrees Celsius, and close to 1.5 degrees, by the end of the century compared with preindustrial-era temperatures.

Governments had failed to agree to cuts sufficient to hit that target ahead of the summit here, called COP26. The review next year was seen by delegates who pushed it as a way of wringing fresh cuts from some countries in the near future.

Diplomats agreed to insert language calling for a “phase down” of coal and “inefficient” fossil fuel subsidies. The language isn’t binding on any countries, and was watered down from earlier drafts. It was seized upon by many supporters, though, as a symbolic pivot from fossil fuels. The language also marked the first time that coal or fossil fuels were mentioned—and implicitly blamed—in a U.N. deal to fight climate change.

In a coup for a bloc of developed countries, delegates agreed to insert language calling for a “phase down” of coal and “inefficient” fossil-fuel subsidies. The language isn’t binding on any countries, and was watered down in several draft texts. It was seized upon by many supporters, though, as a symbolic pivot from fossil fuels.

Friday’s agreement also sets new rules for trading carbon credits between countries, allowing governments to achieve their emissions goals by funding greenhouse-gas-reduction projects in another country. Officials expect the rules will lay the foundation for an international carbon market.

Many businesses have looked to carbon trading as a way to lower their own net emissions, amid pressure from regulators, investors and consumers. While some jurisdictions, like the European Union and California, already trade carbon, executives hope a U.N. deal might speed the creation of a global marketplace.

The deal leaves unresolved how wealthy nations will provide a big increase in funding in the coming years for poorer nations to adopt renewable energy and to protect themselves from the effects of global warming. Without those funds, developing nations say they will suffer catastrophic damage from climate change and won’t be able to implement the emissions reductions they have promised.

Delegates at the U.N.’s climate change conference, where countries agreed to new rules for trading carbon credits.

Photo: PHIL NOBLE/REUTERS

The U.S., the EU and other rich nations hope the deal will nonetheless encourage private investors to shift trillions of dollars of capital into low-carbon technologies. Those funds will be crucial to cutting emissions quickly, developed countries say, dwarfing the money that they could feasibly provide from their government budgets.

Zhao Yiming, head of the Chinese delegation and vice minister at the Chinese Ministry of Ecology and Environment, said the developing world “needs more financial support in their fight against climate change. This is definitely something that we need to continue working on for the future.”

The world is far off track from its goal of keeping rising temperatures in check. The U.N. says current plans would allow emissions to rise 13% by 2030 compared with 2010. Scientists say they must fall 45% to hit the 1.5 degree target.

The big question following the deal struck Saturday is how to get the U.S., China and other big emitters to back even sharper emissions cuts when they have just updated their climate plans within the past year.

After the deal, Mr. Kerry said the U.S. wouldn’t update its climate plan next year, saying the target—a 50% reduction in greenhouse gases by 2030—is already ambitious. “That’s stretching the limits right now,” he said.

Meanwhile, China, the biggest emitter of greenhouse gases, has said it won’t update its emission plan next year.

The best hope for limiting warming to 1.5 degrees may come from China overperforming its targets, negotiators said during late stages of the Glasgow talks. The plan that Beijing submitted to the U.N. pledges to begin reducing emissions before 2030.

Climate talks were scheduled to wrap up on Friday but continued into the weekend.

Photo: robert perry/Shutterstock

But many analysts expect that China’s emissions will likely fall sooner, particularly if China’s coal use begins to decline in 2026, as President Xi Jinping said in a speech this April. Analysts say that other countries could overperform their targets if the cost of solar and wind power continues to fall and if battery technologies to store renewable energy for when it is most needed become widely available.

Some governments that are seen as most vulnerable to climate change voiced frustration with the U.N.’s process for the climate talks, which require unanimity among governments for a deal. They were looking for more decisive measures to limit warming to 1.5 degrees, but saw those efforts blocked by major emitters such as China and big fossil-fuel exporters such as Saudi Arabia.

“It’s always the lowest common denominator,” said Saber Hossain Chowdhury, a senior negotiator for Bangladesh. “Although consensus is the essence of multilateralism, it can be abused.”

Money is a sticking point in climate-change negotiations around the world. As economists warn that limiting global warming to 1.5 degrees Celsius will cost many more trillions than anticipated, WSJ looks at how the funds could be spent, and who would pay. Illustration: Preston Jessee/WSJ The Wall Street Journal Interactive Edition

Write to Matthew Dalton at Matthew.Dalton@wsj.com, Sha Hua at sha.hua@wsj.com, Timothy Puko at tim.puko@wsj.com and Sarah McFarlane at sarah.mcfarlane@wsj.com