HONG KONG—China posted its slowest growth in a quarter-century in 2015, official data showed Tuesday, confirming a loss of momentum in the world's second-largest economy that stands to worry economists worldwide.

But some see a silver lining to China's cooling economy. The country's carbon dioxide emissions likely fell 3 percent last year thanks to its shrinking manufacturing production, declining coal consumption and booming renewable energy installation, Greenpeace said in a newly published analysis.

The environmental organization made their calculation by analyzing official economic and industrial data released Tuesday. Given the country's status of being the largest greenhouse gas emitter in the world, Greenpeace says, a tiny drop in China's carbon emissions could translate into a big change in the international stage.

For instance, fossil-fuel-related emissions in China decreased 3 to 4 percent last year, according to Greenpeace's analysis. The amount of carbon emissions avoided is roughly equal to Poland's total emissions.

Greenpeace's claim was echoed by others.

"We believe China's carbon emissions did decrease in 2015," said Jiang Kejun, a scholar at the Energy Research Institute of the National Development and Reform Commission, the country's top economic planner.

While Jiang and his team are still waiting for the government to release more data in order to calculate the exact decrease, he said that the falling trend in carbon emissions is certain because "China's coal consumption has scaled back significantly."

In 2015, China's coal output fell 3.5 percent, coal imports plummeted 30 percent and coal-fired electricity generation decreased 2.8 percent. There were similar decreases in coal-intensive heavy industry such as steel, cement and glass.

"Our previous expectation was that China would peak its emissions before 2025. But now we are confident that the emissions peak would arrive somewhere between 2020 and 2022," Jiang said.

$110B for clean energy

Jiang cited China's ongoing economic restructuring—a shift from power-hungry heavy industries to an economy that runs on service and high-end manufacturing—as a key driving force behind the acceleration. He said that a rapid expansion in the country's low-carbon power generation will also weigh in, helping the world's largest carbon polluter to reach its emissions peak faster.

According to the government plan, China should peak its carbon emissions around 2030.

Yang Fuqiang, a senior adviser on energy, environment and climate change at the Natural Resources Defense Council, agrees that in 2015, China's carbon dioxide emissions dropped for the first time, signaling that the country's emissions peak may come earlier than previously thought. But he holds a cautious tone on exactly how early.

"It depends on how much China's oil and gas consumption would grow," Yang said. He explained that although China's appetite for coal has been winding down, the domestic consumption of oil and natural gas may continue to grow.

"If the increase in emissions generated from burning oil and natural gas outpaces the emissions reduction associated with falling coal demand, then China's total emissions would still go up. Right now, we have several scenario studies to predict what the future trend would be. It depends on which scenario will be realistic," Yang said.

Regardless of when China's emissions peak might come—the year of 2020, 2022 or 2025—Yang said that falling Chinese emissions have already sent out a positive signal to the international campaign against global warming.

"China's progress in emissions reduction will pressure other countries to do more," Yang said. "For now, many countries still have mitigation targets, which aren't ambitious enough."

China has pledged to increase the share of non-fossil fuel in its energy mix to 15 percent by 2020 and 20 percent by 2030.

The country's investment in clean energy hit historic highs in 2015 at $110.5 billion. Already the largest wind and solar market in the world, China plans to install another 20 gigawatts of wind turbines and 15 GW of solar panels this year.

On the other hand, the golden era of the Chinese coal industry has ended. The country's total coal use dropped almost 4 percent last year compared with 2014 levels, according to Greenpeace's estimates, and policymakers in China issued a ban on new coal mines for three years.

Are the numbers right?

There's just one question: Can the outside world trust China's official data?

Earlier last year, Bloomberg New Energy Finance estimated that total carbon emissions in the world's second-largest economy dropped 2 percent in 2014 compared with a year ago, the first drop since 2001. The market research firm made its estimates based on preliminary energy demand data from the National Bureau of Statistics of China.

But that estimate was being questioned a few months later when the National Bureau of Statistics of China revised the energy data. The government agency said that the country's coal use has been underestimated since 2000 and particularly in recent years.

Then the quality of coal complicated the matters. Jiang of the Energy Research Institute said that in 2014, for example, China burned less coal in total. But since the actual consumption of high-grade coal—the fuel that contains more carbon content and produces more emissions once burned—was higher than the initial record, the revisions in the government's energy data led to a reverse in the country's emissions trend in 2014. It went from a year-over-year decrease to a year-over-year increase.

Jiang and others say that the adjustments on China's energy data will not fade away anytime soon. Neither will the confusion over the country's emissions profile.

"It is just too tricky to get the figures right in one time. Governments around the world face the same challenge," Jiang said.

"Data quality is no doubt a big issue here. It will take much more efforts before we can get perfectly accurate data in China," added Li Shuo, a senior energy and climate policy analyst with Greenpeace.

"However, we think the trend is rather reliable if you compare figures across multiple years," Li continued. "In this regard, our resolution may not be perfect if what we are trying to settle is whether consumption declined by 3 percent or 2.9 percent, but if the data points you to a decline, it is pretty safe to conclude there will be no increase."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500