WASHINGTON — Five months before a United Nations summit meeting aimed at forging a historic global accord to cut climate-warming emissions, significant signs of progress toward an agreement are emerging.

China, the world’s largest greenhouse gas polluter, submitted a 16-page plan to the United Nations on Tuesday detailing how it plans to shift its economy to reduce fossil fuel emissions by 2030. On the same day, President Dilma Rousseff of Brazil, which is among the top 10 carbon emitters, and President Obama announced in Washington that their nations had agreed to sharply expand electricity generation from renewable sources.

But it is increasingly evident that the policy actions by these countries and others will not be enough to stave off a rise in the atmospheric temperature of 2 degrees Celsius, or 3.6 degrees Fahrenheit. At that point, scientists say, the planet will be locked into a future of extreme storms, droughts, food and water shortages, and rising sea levels.

Under a United Nations accord reached in Peru last December, every nation is to submit a plan for cutting carbon emissions well ahead of the December summit meeting in Paris. The plans by the individual nations will form the core of the new agreement.

South Korea, Serbia and Iceland also submitted their plans for cutting emissions on Tuesday, joining the 40 or so countries that had already done so, including Canada, Mexico, Russia and the United States, along with the European Union.

And in the joint announcement by Brazil and the United States, the two nations committed to increasing the use of wind, solar and geothermal energy to make up 20 percent of each country’s electricity production by 2030, which would double power generation from renewable sources in Brazil and triple it in the United States. Brazil also pledged to restore about 30 million acres of Amazon rain forest, an area about the size of Pennsylvania.

“Following progress during my trips to China and India, this shows that the world’s major economies can begin to transcend some of the old divides and work together to confront the common challenge that we face — something that we have to work on for future generations,” said Mr. Obama, who has pushed other countries to sign on to a climate change deal.

Christiana Figueres, the top United Nations climate change official, said, “Over the past 24 hours, we’ve seen a very nice example of the diversity of countries engaging on the climate solution.” But she added, “The sum total of these does not get us to 2 degrees.”

Climate policy experts pointed to the significance of China’s plan in particular. The United States and China, the world’s top greenhouse gas polluters, have long been viewed as the biggest obstacles to reaching a meaningful global warming deal. That changed last November when Mr. Obama and President Xi Jinping jointly announced that the United States would lower its emissions up to 28 percent from 2005 levels by 2025, while China’s emissions would peak and then decline no later than 2030.

In March, Mr. Obama submitted a plan to the United Nations detailing how the United States would meet its target. It said it would do so chiefly through enactment of Environmental Protection Agency regulations on emissions from cars, trucks and power plants.

China’s plan included a broad commitment to decouple economic growth from the use of fossil fuels, and a move to lower its carbon intensity, or the amount of the pollutant generated by each point of economic growth, by 60 to 65 percent from 2005 levels by 2030. It also laid out plans to develop a national cap-and-trade system, a market-based program for reducing emissions in which companies must pay for permits to pollute, and can buy and sell those permits among themselves.

“It demonstrates the depth of climate policy architecture,” said David Sandalow, a fellow at the Center on Global Energy Policy at Columbia University and a former top Energy Department official in the Obama administration. “It’s very detailed and suggests the seriousness with which China takes its climate policy goals.”

In his first term, Mr. Obama tried unsuccessfully to push a similar cap-and-trade system through Congress.

“There is a lot of irony that the U.S., which invented free-market approaches to stopping climate change, is using command-and-control regulations while Communist China will use a carbon market,” said Paul Bledsoe, a top climate change official in the administration of former President Bill Clinton.

But Brazil, despite its announcement on Tuesday, has not yet submitted its climate plan to the United Nations, and several other major polluting economies, including India and Japan, also have not. In any event, climate policy experts say it is almost certain that their plans will not add up to strong enough action to curb the harmful effects of climate change.

Ms. Figueres and other United Nations negotiators say they now see a Paris accord as a first step in a multiyear process in which countries will put forth domestic climate policies and reconvene regularly to make their plans more stringent.

“It’s not a one-shot deal,” she said. “It’s like a highway, with each country going in the lane with the speed that they can go. Some will start out slowly and speed up later. It will be a progressive process over time.”

One hurdle to a pact is Republican lawmakers’ longstanding opposition to participation in a global climate change deal. They say that such a plan could hurt the American economy, while allowing other countries to continue to pollute at higher levels. Senator James M. Inhofe, Republican of Oklahoma, who is the chairman of the Senate Environment Committee, said he was deeply skeptical of China’s plan.

“China’s commitment to reduce carbon emissions is unattainable and unrealistic,” Mr. Inhofe said in a statement. “China’s commitment will allow the country to continue increasing its emissions for the next 15 years.”

Money is another major obstacle. In 2009, Secretary of State Hillary Rodham Clinton pledged that by 2020, developed economies would send $100 billion annually, from both public and private sources, to developing economies to help them adapt to the ravages of climate change. This year, the United Nations has sought to establish a $10 billion “Green Climate Fund” to help begin that fund-raising effort.

Although Mr. Obama has pledged $3 billion — more than any other nation has offered — Republicans in Congress have blocked efforts to appropriate the money.

Climate policy experts say that without the money from rich countries, developing economies will not be able to follow through on their pledges.