BEIJING — When the presidents of China and the United States pledged on Wednesday to reduce or limit carbon dioxide emissions, analysts and policy advisers said, the two leaders sent an important signal: that the world’s largest economies were willing to work together on climate change.
“This is a very serious international commitment between the two heavy hitters,” said Li Shuo, who researches climate and coal policy for Greenpeace East Asia.
Still, many questions surround China’s plans, which President Xi Jinping announced in Beijing alongside President Obama after months of negotiations. In essence, experts asked, do the pledges go far enough, and how will China achieve them?
Mr. Xi said China would brake the rapid rise in its carbon dioxide emissions, so that they peak “around 2030” and then remain steady or begin to decline. And by then, he promised, 20 percent of China’s energy will be renewable. Analysts said that achieving those goals would require sustained efforts by Beijing to curb the country’s addiction to coal and greatly increase its commitment to energy sources that do not depend on fossil fuels.
Many scientists have said that 2030 may be too long to wait for China’s greenhouse gas emissions to stop growing, if the world is to keep the average global temperature from rising more than 3.6 degrees Fahrenheit (2 degrees Celsius) above the preindustrial average. That goal was adopted by governments from around the world at talks in Copenhagen in 2009.
Almost no country has done enough yet to reach that goal, but because of its size and industrial development, China is crucial to any effort to even come close. (So is the United States, which promised on Wednesday to emit 26 percent to 28 percent less carbon dioxide in 2025 than it did in 2005.)
Some experts said that China should try to halt the growth of its emissions much sooner than it has pledged, by 2025 rather than 2030.
“Based on China’s current coal consumption numbers, they can do much more,” Mr. Li said on Wednesday. He said of the pledges made on Wednesday that “this should be the floor on which they work, rather than a ceiling.”
People involved in the internal Chinese debates said the seeds of Mr. Xi’s announcement could be found in public anger over rising levels of toxic smog in China. Over the past two years, Chinese cities have recorded some of the worst air pollution readings in the world.
To address the problem, Chinese leaders have turned their attention to cutting back the country’s reliance on coal, a main pillar of the economy but also a major source of pollution. That led to discussions about how weaning Chinese industries off coal would not just clean the air, but would also permit China to make global commitments in the battle against climate change, the insiders said.
Last month, the departing European Union climate commissioner, Connie Hedegaard, said that halting the growth in Chinese carbon dioxide emissions much sooner than 2030 would “be a very important gift from China to the whole world,” according to a report by Agence France-Presse.
Policy makers and climate experts inside and outside China face the task of assessing the trajectory that China’s emissions are on now, and whether China must do more to change course.
Internally, Chinese scientists and officials have been crunching data to try to pinpoint when carbon emissions will peak and how high that peak will be, given current economic growth projections and energy policies, but their estimates have varied. Foreign scientists and policy makers are also trying to judge whether Mr. Xi’s 2030 pledge represents a genuine campaign by the Chinese government to fight climate change, or just a business-as-usual date when emissions would probably have leveled off anyway.
A 2011 study by the Lawrence Berkeley National Laboratory suggests that it is not far from business as usual. Economic trends and government policies in China, the study said, had already put the nation on course to reach a peak sometime between 2030 and 2035, with an annual output of 12 billion metric tons of carbon dioxide in 2033. More aggressive measures, it said, might limit the peak to about 9.7 billion metric tons and advance the date to about 2027.
A study released last month by the Massachusetts Institute of Technology had somewhat different conclusions, with Chinese emissions peaking at 10 billion metric tons sometime between 2025 and 2035 if aggressive measures like higher carbon and coal taxes are imposed.
Wang Tao, an expert on climate and energy policy at the Carnegie-Tsinghua Center for Global Policy in Beijing, said that meeting President Xi’s 2030 pledge “would still demand quite a lot of change from China, in terms of energy structure and pushing for nonfossil-fuel renewables to reduce the reliance on coal.”
He called the 2030 goal a “reasonable target,” but he added, “Certainly the government could do more than that, and should be encouraged to.”
Wang Yi, a professor at the Chinese Academy of Sciences in Beijing, said that a consensus had grown recently among experts in China that the 2030 date was achievable, and that 2025 would be a more ambitious goal. But as recently as last week, researchers from the Chinese Academy of Social Sciences and the China Meteorological Administration issued a report saying that even with “stringent environmental planning,” the country’s emissions were not likely to peak until 2035.
As for renewable energy, Chinese officials have been trying in recent years to encourage development of alternatives to coal, including hydroelectric power, wind and solar energy and nuclear power.
Mr. Li, the Greenpeace researcher, said Mr. Xi’s 20-percent goal was ambitious. He said the country would need to add 800 to 1,000 gigawatts of power generation capacity from renewable sources over the next 15 years to meet the goal — a remarkable figure, given that the country now has a total of just 1,250 gigawatts of capacity from all sources, most of it coal-fueled.
At the end of 2013, China got 9.8 percent of its energy from sources not linked to fossil fuels, and the government intends to reach 15 percent by 2020.
“Twenty percent does sound fairly robust,” said Jake Schmidt, director of the international program at the Natural Resources Defense Council, an advocacy group in New York. “You’re talking about 20 percent of a huge economy being based on noncarbon-dioxide-emissions sources. That’s significant.”
