GLASGOW — The United States and China announced a joint agreement Wednesday to “enhance ambition” on climate change, saying they would work together to do more to cut emissions this decade while China committed for the first time to reduce methane, a potent greenhouse gas.

The pact between the world’s two biggest polluters came as a surprise to the thousands of attendees gathered here for a United Nations climate summit. China and the United States, rivals that face growing tensions over trade, human rights and other issues, spoke as allies in the fight to keep global warming to relatively safe levels.

“We both see the challenge of climate change is existential and a severe one,” said Xie Zhenhua, China’s climate change envoy. “As two major powers in the world, China and the United States, we need to take our due responsibility and work together and work with others in the spirit of cooperation to address climate change.”

John Kerry, the U.S. special envoy for climate, followed the remarks from Mr. Xie with an assessment of his own. “The U.S. and China have no shortage of differences,” said Mr. Kerry, a former secretary of state with a long history of negotiating with the Chinese. “But on climate, cooperation is the only way to get this job done.”

Still, the joint agreement was short on specifics. It did not extract a new timetable from China under which the country would ratchet down emissions, nor did China set a ceiling for how high its carbon dioxide and other greenhouse gases would reach before they started to fall. China agreed to “phase down” coal, the dirtiest fossil fuel, starting in 2026, but did not specify by how much or over what period of time.

The announcement from China and the United States came on the same day that summit organizers issued an initial draft of a new global agreement to fight climate change that called on countries to “revisit and strengthen” by the end of 2022 plans for cutting greenhouse gas emissions and to “accelerate the phasing-out of coal and subsidies for fossil fuels.”

The language on coal and government fossil fuel subsidies would be a first for a U.N. climate agreement if it stays in the final version.

Yet many countries and environmentalists said the rest of the document was still too vague on crucial details like what sorts of financial aid richer nations should provide poorer ones struggling with the costs of climate disasters and adaptation.

The draft “is not the decisive language that this moment calls for,” said Aubrey Webson, chairman of the Alliance of Small Island States, a group of countries that are among those most threatened by climate change.

Scientists have said that nations need to cut global emissions from fossil fuels roughly in half this decade to keep average global temperatures from rising beyond 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, compared with preindustrial levels. Beyond that threshold, the risks of deadly heat waves, droughts, wildfires, floods and species extinction grow considerably. The planet has already warmed 1.1 degrees Celsius.

Negotiators here from nearly 200 countries are likely to demand significant changes to the draft as the talks enter their last, most difficult stretch. By tradition, a new global agreement requires every party to sign on. If any one nation objects, talks can deadlock.

The British prime minister, Boris Johnson, returned to Glasgow on Wednesday to urge countries to set aside differences and strike a deal. “The world has heard leaders from every country stand here and acknowledge the need for action,” he said. “And the world will find it absolutely incomprehensible if we fail to deliver that.”

But persuading nations around the world, many of which depend on fossil fuels for energy and have their own internal politics and vested interests, to move in a new direction is a herculean challenge.

Mr. Kerry said countries had no choice but to work together. “This is not a discretionary thing,” he said. “This is science, it’s math and physics that dictate the road we have to travel. And we cannot reach our goal unless everyone works together.”

Several experts said the joint pact between China and the United States fell short of a 2014 deal between the United States and China to jointly curb emissions, which helped spur the Paris climate agreement among nearly 200 nations a year later.

“While this is not a game changer in the way the 2014 U.S.-China climate deal was, in many ways it’s just as much of a step forward given the geopolitical state of the relationship,” said Thom Woodroofe, a former climate diplomat and a fellow at the Asia Society Policy Institute working on United States-China climate cooperation. “It means the intense level of U.S.-China dialogue on climate can now begin to translate into cooperation.”

The agreement won praise among leaders and diplomats at the climate summit, who said they hoped it would inject fresh energy into the global negotiations aimed at keeping global temperatures from rising to dangerous levels. With just days remaining before the summit ends, negotiators are working late into the night to try to hammer out a global accord that, they hope, can satisfy every country — no easy task.

Small island states like the Maldives, which has been inhabited for thousands of years but is projected to be swamped by rising seas within generations, want all countries to slash emissions as fast as possible. Oil and coal producers like Russia and Australia aren’t as eager to rapidly phase out fossil fuels. And large developing countries like India are holding out for financial help to shift to cleaner energy.

There are four major areas of contention as negotiators try to reach a deal before the summit ends on Nov. 12.

Speeding Up Emission Cuts

Under the landmark Paris climate agreement of 2015, every nation agreed that humanity should limit global warming to “well below” 2 degrees Celsius (3.6 degrees Fahrenheit) above preindustrial levels while “pursuing efforts” to hold warming to just 1.5 degrees Celsius (2.7 degrees Celsius).

To reach that target, each nation agreed to submit its own plan to shift away from fossil fuels and to curb deforestation, and to update those plans every five years. While everyone agreed that the initial pledges put forward in Paris were insufficient, the hope was that, over time, nations would ratchet up action and get closer to the goal.

But there are a few big problems.

First, the ratcheting process has been slow and uneven. Ahead of the Glasgow summit, most countries submitted new pledges to curb their emissions between now and 2030. Some, like the United States and European Union, vowed to make deeper cuts this decade. But others, such as Australia, Brazil and Russia, barely strengthened their short-term plans.

When analysts added up the short-term pledges, they found that the world was likely on track to heat up around 2.4 to 2.7 degrees Celsius this century. That’s an improvement over Paris, but it would still increase the likelihood of climate catastrophes that could exacerbate hunger, disease and conflict.

On top of that, many of the countries most vulnerable to climate change, such as Ethiopia and Bhutan, want the world to keep to the stricter target of 1.5 degrees Celsius, or else they will face unmanageable disasters.

“1.5 degrees is what we need to survive,” Mia Mottley, prime minister of Barbados, said last week. “Two degrees is a death sentence for the people of Antigua and Barbuda, Maldives, Domenica, Fiji, Kenya, Mozambique, Samoa and Barbados.”

How to speed up climate efforts remains a source of debate.

Many vulnerable countries want countries to come back to the United Nations annually with stronger plans until the world is on track for 1.5 degrees Celsius. Currently, countries aren’t expected to update their plans until 2025, which some fear could be too late.

But that proposal faces opposition from fossil fuel producers like Saudi Arabia and Russia. And there’s not even consensus that 1.5 degrees should be the official goal: The United States and the European Union have supported focusing on that stricter target, but some major emitters like China have balked.

Where’s the Money?

Money has long been a big sticking point in the global fight against climate change, and tensions over the topic have flared again in Glasgow.

President Biden and European leaders have insisted that developing countries such as India, Indonesia or South Africa need to accelerate their shift away from coal power and other fossil fuels. But those countries counter that they lack the financial resources to do so, and that rich countries have been stingy with aid.

A decade ago, the world’s wealthiest economies pledged to mobilize $100 billion per year in climate finance for poorer countries by 2020. But they have fallen short by tens of billions of dollars annually.

At the same time, little climate aid to date has gone to help poorer countries cope with the hazards of a hotter planet, such as sea walls or early-warning systems for floods and droughts.

Vulnerable nations are warning that they need far more help to survive. A group of African nations, along with China, India and Indonesia, has asked for as much as $1.3 trillion a year after 2025, That dwarfs anything that wealthy countries have been willing to propose so far.

‘Loss and Damage’

Even as vulnerable countries plead for more climate aid, they have asked for separate compensation for climate damages that they can’t adapt to. And they argue that wealthy nations like the United States and the European Union, which are historically responsible for most of the extra greenhouse gases now heating the atmosphere, should pay. This issue is known as “loss and damage.”

“Lots of people are losing their lives, they are losing their future, and someone has to be responsible, and those people need to be compensated,” said A.K. Abdul Momen, the foreign minister of Bangladesh.

Richer countries have, however, historically resisted calls for a specific funding mechanism for loss and damage, fearing that it could open the door to a flood of liability claims. Only the government of Scotland has been willing to offer specific dollar amounts, pledging $1.4 million last week for victims of climate disasters.

Regulating Carbon Markets

One of the thorniest issues is how to regulate the fast-growing global market for carbon offsets. The Paris agreement urged clearer rules on this topic back in 2015, but negotiators have been unable to agree on the extremely dense and technical subject.

Carbon offsets allow countries or businesses to compensate for their own emissions by paying for mitigation elsewhere. But it raises tricky questions about accounting, transparency and verification.

Some climate advocates said they would prefer negotiators to leave Glasgow without a resolution on these issues rather than with weak rules.

“No point in accepting phony carbon credits into the system, which would directly increase warming,” wrote Mohamed Adow, director of Power Shift Africa, a research institute in Kenya.

Developing countries have also called for a percentage of proceeds from all carbon credit trades to be set aside for an adaptation fund. But the European Union has criticized this idea, calling it a “mandatory international tax.”