LONDON—The U.K. called on countries including India and China to update their plans to reduce greenhouse-gas emissions, ratcheting up pressure on a handful of states as preparations for a United Nations climate summit in Glasgow accelerate.

With less than three weeks to go before the start of the high-profile conference—and still-deep uncertainty about how committed several key participants are to a meaningful deal—the stakes are growing for London, Washington and Brussels.

A...

LONDON—The U.K. called on countries including India and China to update their plans to reduce greenhouse-gas emissions, ratcheting up pressure on a handful of states as preparations for a United Nations climate summit in Glasgow accelerate.

With less than three weeks to go before the start of the high-profile conference—and still-deep uncertainty about how committed several key participants are to a meaningful deal—the stakes are growing for London, Washington and Brussels.

A key foundation of the talks, aimed at limiting global warming, is individual countries’ own technical plans to cut emissions. Several major economies including the U.K., the U.S. and the EU have recently updated their plans to pivot from fossil fuels. Some major polluters, including China and India, haven’t yet made public their own fresh commitments ahead of the meeting.

Other countries, such as Australia, Indonesia and South Africa, have updated climate targets or are considering revisions, though some environmental groups are concerned policy makers aren’t moving fast enough. In Mexico, President Andrés Manuel López Obrador has moved to give priority to fossil fuel on the national grid over renewable energy sources, which environmentalists say will hurt Mexico’s emission-reduction goals.

Alok Sharma, the U.K. climate envoy who is organizing the summit that is known as COP26, has in recent months tried to publicly cajole participants to work to bridge still-large gaps in positions ahead of the two-week meeting.

On many issues the parties “are still some way off consensus,” Mr. Sharma said in a speech Tuesday. “I want to be frank, it will be a challenging task to get us over the line.”

Last year, China unveiled plans to bring forward a number of targets to reduce emissions, but it hasn’t officially presented them to the U.N.

Photo: Mark Schiefelbein/Associated Press

Mr. Sharma said responses from the world’s biggest economies “will quite simply be make or break” for the success of the summit, citing specifically the Group of 20 industrialized countries, which includes China and India. Those still holding out “must deliver,” he said.

China and India are among those who have yet to set out their latest plans. In China, the world’s biggest greenhouse-gas emitter but also the world’s most populous country, President Xi Jinping pledged to bring the country’s carbon peak from “around” to “before” 2030 and for China to achieve net zero emissions before 2060. China hasn’t officially presented plans to the U.N. but has promised to do so this month.

A spokesman for India’s Ministry of Environment, Forest and Climate Change declined to comment.

Separately, a senior Indian government official, who declined to be identified, said India was on track to meet its climate goals, and has an ambitious renewable energy goals.

“We will be part of the solution and not problem creators. Transparency on climate finance and technology transfer is what we have asked for ahead of COP26 because that will be key in determining actions of developing countries,” said the official with direct knowledge of the matter.

The Glasgow summit is aimed at sustaining momentum for the Paris agreement, an accord struck in 2015 that calls for governments to limit the rise in global temperatures to close to 1.5 degrees Celsius compared with preindustrial days. A recent U.N. report said the Earth is expected to be around 2.7 degrees Celsius warmer by the end of this century, thanks in part, most scientists agree, to so-called greenhouse gas emissions from industrial activities like burning fossil fuels.

“The world has not done enough,” said Mr. Sharma. “The emissions continue to rise.”

Related Video

A Swiss startup has created a giant vacuum cleaner to capture carbon dioxide from the air, helping companies offset their emissions. WSJ visits the facility to see how it traps the gas for sale to clients like Coca-Cola, which uses it in fizzy drinks. Composite: Clément Bürge The Wall Street Journal Interactive Edition

British Prime Minister

Boris Johnson, as host of the conference, stands to burnish the U.K.’s post-Brexit diplomatic standing with a successful summit. The Biden administration and the European Union, meanwhile, have both put climate at the heart of ambitious, but costly, plans to restructure their respective economies. A deal among the rest of the world’s governments to move in the same direction could bolster domestic support, especially in the U.S., where Mr. Biden must push his plan through a divided Congress.

The G-20, which meets in Rome at the end of the month, will be a key moment for diplomats to try to forge consensus among big economies before heading into the climate summit talks. Ahead of the event in Glasgow, nations are expected to submit climate plans, called “nationally determined contributions,” which spell out specific targets and methods to lower carbon emissions. These are supposed to be submitted every five years.

An overarching aim of the Glasgow meeting is to find ways to ensure there is continued pressure on nations to accelerate existing climate plans in the coming decade. Another key aim is helping emerging economies with their fight against climate change. Wealthy countries were supposed to give at least $100 billion annually starting in 2020 to help the developing world shift away from fossil fuels and protect themselves against the effects of global warming.

Mr. Sharma is pressing richer nations to come good on that pledge, which he said is now “in touching distance.” The U.S., for instance, recently said it would double its level of financing, subject to approval by Congress.

South Africa says that to decarbonize its economy in line with the climate targets set out in the 2016 Paris agreement, and fund a transition away from coal as its primary energy source, it will need around $60 billion in international climate financing over the next decade.

Indonesia, a large greenhouse-gas emitter and home to some of the world’s largest remaining rainforests, told U.N. officials in July it would aim to peak greenhouse gases by 2030 and could achieve net zero by 2060, but would need “domestic and international sources of finance to do so.”

Smoke rises from a fertilizer plant in India, one of the world’s major polluters.

Photo: R S Iyer/Associated Press

Wealthier countries, too, face challenges. In Australia, a challenge for center-right Prime Minister Scott Morrison has been getting his governing coalition—which includes pro-climate action moderates from cities and more conservative members from regional Australia—to agree on climate goals. There are concerns from rural lawmakers that climate action could lead to job losses in their constituencies, particularly in the coal and gas industries.

On Wednesday, the cabinet is expected to sign off on achieving net zero emissions by 2050, according to local media and a person familiar with the matter. Lawmakers from Mr. Morrison’s governing coalition will then discuss the plan. Mr. Morrison has previously said that exactly when Australia would reach that goal would be determined by advancements in technology.

Australia’s current 2030 target is to reduce emissions by 26% to 28%, and it is already on track to exceed those targets, so the 2030 target could be increased slightly without requiring additional action, some activists said. A recent report from the Climate Council, a nonprofit, recently recommended that Australia aim to reduce emissions by 75% below 2005 levels by 2030 and reach net zero by 2035.

In Mexico, the country’s leftist president has focused less on renewable energy, and more on giving priority on the national electric grid to fossil-fuel electricity produced by the state-run utility, even though it is more expensive and polluting than electricity produced by private companies. Mr. López Obrador has also scrapped all new auctions for renewable energies, moves that environmentalists say will hinder emissions reduction in the country.

—Feliz Solomon and Sha Hua contributed to this article.

Write to Max Colchester at max.colchester@wsj.com and Mike Cherney at mike.cherney@wsj.com