President Obama and Chinese President Xi Jinping in Beijing on Wednesday announced a plan for curbing pollution. Combined, the two countries make up 45 percent of the world’s greenhouse-gas emissions. (Reuters)

For many countries reluctant to undertake deep cuts in greenhouse gases, China has been the ultimate excuse: a fast-growing threat to the climate so enormous as to render all other efforts meaningless.

But with President Xi Jinping declaring Wednesday that China’s greenhouse-gas emissions will peak in 2030 and that China will make non-fossil fuels 20 percent of its energy mix, the excuse is gone.

“If China and the United States say, ‘We’re both doing it,’ what does India say?” said Jennifer Turner, director of the China Environment Forum at the Woodrow Wilson International Center for Scholars. “No one is hiding behind China now.”

In a country of superlatives, China’s energy sector and greenhouse-gas emissions rank among the most daunting. And while Xi’s pledge to set a limit on carbon emissions was hailed by environmental groups and climate experts, a look at the sheer volume of China’s climate-busting gases underlines the importance of Xi’s pledge — and its limitations.

China is the world’s largest emitter of greenhouse gases, outstripping the United States by a substantial margin. By 2030, China’s emissions will have grown anywhere from 17 percent to 30 percent, depending on the forecast. The difference between those two estimates equals about a quarter of total emissions in the United States today.

“The nuances in the Chinese numbers can be as important as the headlines about the U.S. ones,” said Michael Levi, a senior fellow at the Council on Foreign Relations.

So even if China meets its new commitments, its greenhouse-gas emissions could still make it difficult to meet international targets designed to limit global temperature increases to 2 degrees Celsius.

But it’s a start, say many climate experts. “The peak could be Himalayan, but the fact that they’re even talking about it is amazing,” Turner said. “Five years ago, you couldn’t imagine someone would talk about peak emission levels.”

Moreover, Xi’s pledge to ratchet up the share of non-fossil fuels from the current 8 percent to 20 percent of the country’s energy consumption could turn out to be more important than the 2030 pledge. To reach that carbon-free energy goal will require immediate and massive action.

“They will have to start building the capacity for that now,” said Ken Lieberthal, a former National Security Council adviser on Asia and a senior fellow at the Brookings Institution. “That isn’t back-end-loaded. You can judge their commitment to this and progress over time.”

Consider the solar sector alone, which makes up less than 1 percent of Chinese energy use. China last year installed 12 gigawatts’ worth of solar panels — more than the entire installed solar power base in the United States and 50 percent more than any other country has installed in a single year. This year China will install even more, finishing 2014 just short of Germany’s world-leading total.

Other forms of non-fossil-fuel plants will also get new attention, including hydropower (now 6 percent of total energy), wind turbines and nuclear power plants, which contribute only 1 percent of total energy consumption, according to the U.S. Energy Information Administration. China is already home to the biggest nuclear reactor construction program.

The symbolism of Xi’s announcement was as important as the nuts and bolts. In past rounds of climate negotiations, countries have haggled over cuts in ­greenhouse-gas emissions right up to the deadline. This announcement comes a year ahead of the December 2015 climate talks in Paris. Lieberthal said that it was a way of saying that “this is not a game-playing exercise. They’re stepping up to a big issue.”

Xi, who has been trying to burnish his stature in state-controlled news media, also appeared statesmanlike, unveiling the plan in Beijing at the end of a summit of Asia-Pacific leaders and a bilateral session with President Obama. It marked a departure, Levi said, because Chinese leaders are usually eager to look as though China has acted on its own for its own interests.

Yet how Xi arrived at the point of making his pledge alongside Obama is also a story about change in modern China — a slow, bureaucratic tectonic shift pushed forward by a domestic public increasingly exasperated about air pollution that is causing hundreds of thousands of premature deaths a year.

As China has grown wealthier, more Chinese citizens have ­middle-class concerns — food safety, affordable and decent hospital care, the quality of education, and air pollution. Many Chinese use smartphone apps that give both U.S. and Chinese estimates of air quality in major Chinese cities, with the U.S. estimates usually showing higher levels of pollution. (During the Asia-Pacific Economic Cooperation summit, the government blocked data flowing to such apps.)

But even before the “airpocalypse” — a stretch of particularly bad air pollution in early 2013 that aroused new ire among Chinese — the government had begun a campaign to rein in the runaway growth in greenhouse-gas emissions. It has imposed regional bans on new coal plants, launched cap-and-trade pilot projects in seven major cities and provided massive subsidies for wind and solar power.

In 2011, in adopting its current five-year plan, China took an earlier program designed to conserve energy at 1,000 of the biggest ­energy-consuming enterprises and expanded it to cover about 17,000 companies. The plan also set a goal for non-fossil-fuel energy to provide 15 percent of the energy mix by 2020 — although it appears unlikely that the country will meet that target.

To some extent, the choice of 2030 as a peak year for Chinese greenhouse-gas emissions is a logical one. By that time, China’s economy will look very different. The country is shifting away from manufacturing and toward the service sector, which is less energy-intensive.

Moreover, by 2030, the massive migration of Chinese citizens from the countryside to cities probably will have waned, reducing the need for housing construction and the consumption of energy-intensive cement and steel. China consumes half of the world’s cement, Turner said. In September, the country’s key climate planner and negotiator, Xie Zhenhua, told reporters in Beijing that the government aims to cap 2020 emissions from “high carbon sectors” such as cement and steel at 2011-2015 levels. Steel output is expected to peak by 2020.

But until Wednesday, China had not set a peak year for emissions for the entire economy.

Meanwhile, emissions — and the visible pollution that comes along with coal burning — remains a pressing problem in China’s cities. That’s why China closed industries within a 400-mile radius of Beijing to clear the air for this week’s APEC summit. Qi Ye, director of the Brookings-Tsinghua Center for Public Policy in Beijing, said friends of his said that they had finally thought of an effective anti-pollution strategy: making Beijing the permanent site for future summits. And extending Obama’s visit — indefinitely.