Hong Kong (CNN Business) China's data centers produced 99 million metric tons of carbon dioxide last year, the equivalent of about 21 million cars on the road , according to a new report.

Data centers store electronic information like emails, photos and videos, and worldwide they consume between 3% and 5% of total global electricity , and rival the airline industry in terms of carbon emissions.

China's data center industry is among the world's largest and last year consumed just over 2% of the country's power, according to a report by Greenpeace and the North China Electric Power University.

China is outpacing the US in renewable energy , and has made huge progress in developing solar projects . In 2017, it set a goal for clean energy to meet 20% of its energy needs by 2030. However, despite this vast capacity for green power, most Chinese data centers don't use it, the report said.

"Nearly three quarters of that power comes from coal," said Ye Ruiqi, a Greenpeace climate expert, who said that centers' reliance on coal-fired power was mostly due to their geographic location.

"Most facilities are located on the east coast, near business hubs and away from China's renewable energy capabilities in the center and the west of the country. Out of the 44 data centers we surveyed, only five used clean energy in their mix," she said.

China's data center industry is dominated by domestic players like Alibaba and Tencent, though Apple is currently building a data center in the country's southern Guizhou Province . The joint venture with Guizhou-Cloud Big Data Company will allow the US company to improve the speed and reliability of its services, but also comply with new, tougher cybersecurity laws

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Wind and solar energy

With a population of almost 1.4 million people, demand in China for data storage is growing fast.

The report's authors predict that in five years' time, China's data centers will consume two-thirds more energy, putting its electricity consumption at 267 TWh in 2023, more than the total amount consumed by Australia in 2018.

Carbon dioxide emissions are expected to climb from 99 million to 163 million metric tons, the same as 35 million vehicles.

"To prevent this, China's data centers need to decouple their electricity consumption from their carbon footprint by relying more on wind and solar energy," said Ye.

"They can build their own renewable energy capacity, buy clean energy on the market or purchase green certificates to offset their emissions," she added.

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Currently, China's data centers rely on an energy mix of 73% coal, 23% renewable energy and 4% nuclear. If renewable energy increased to 30% by 2023, 16 million metric tons of CO2 emissions could be avoided, according to Greenpeace. That's equal to 10 million round-trip transatlantic flights.

But the Chinese government is aware it needs to act. In 2015, it launched a green data center pilot program. That same year, Alibaba, one of China's largest cloud computing providers, inaugurated a data center near Hangzhou, using solar energy and hydraulic power. It also relies on lake water to cool its servers.

In the United States, data centers have significantly reduced their carbon footprint. All of Apple's data centers are powered by clean energy, according to the company , and its new center in China will also use some renewable energy.