BEIJING – If China's economy continues to expand rapidly and rely heavily on coal and other fossil fuels until the middle of the century, its power consumption would be unsustainable, according to a study by government think tanks released Wednesday.

The two-year study, supported by the U.S.-based Energy Foundation and the international environmental group WWF, also said if China's energy usage structure remains unchanged, its emissions of greenhouse gases blamed for global warming would reach 17 billion tons a year by 2050. That would represent 60 percent of total global emissions and three times China's current production, it said.

"If the current mode of economic development drags on, the scale of China's fossil fuel consumption will be shocking," said the study, titled "China's Low Carbon Development Pathways by 2050."

The researchers said global warming will challenge China more than many other countries, with its developed east coast cities contending with rising sea levels, and already drought-prone agricultural areas suffering further water shortages.

While the study does not officially represent the government's views or policy, it is from a group of high-profile experts at government-backed institutes. It also follows comments by Premier Wen Jiabao, who has said the government will accelerate a shift away from fossil fuels that produce carbon dioxide, a leading greenhouse gas.

Global leaders hope to reach an agreement at a U.N. climate summit in Copenhagen in December on future cuts in emissions of carbon dioxide, but China has resisted making concrete commitments, saying rich countries have a heavy historical responsibility to cut emissions and that any deal should take into account countries' levels of development.

Using energy consumption growth trends from 2002 to 2008, the study said China's energy usage could exceed 100 billion tons of standard coal in 2050, more than the Earth's capacity to sustain and far more than the 16.1 billion tons of standard coal the entire planet consumed in 2008.

But the report says that with massive investment in low-carbon technology and large-scale use of renewable energy and nuclear power, China's carbon dioxide emissions could be reduced to 2005 levels by 2050. Under this scenario they are projected to peak at around 2030 to 2035, with consumption of fossil fuels also peaking before 2040.

The projections are based on an assumption that China's economy will grow at an annual rate of 8.8 percent from 2005 to 2020, then 6 percent from 2020 to 2035 and 4.4 percent from 2035 to 2050. They incorporate how Western countries have developed in the past, as well as U.N. predictions of carbon dioxide growth.

Doing that requires China to be a leader in energy-saving technology, build environmentally friendly homes, develop more public transportation and limit car usage, researchers involved in the study said at a news conference.

A reduction to 2005 levels of carbon dioxide will also require the widespread installation of costly carbon capture and storage projects by the middle of the century, the report said.

It will require an extra investment of 1 trillion yuan ($146 billion) every year from now until 2020, and 1.7 trillion yuan ($248 billion) every year from 2020 to 2030 in improving buildings and heavy industry and building transportation such as railways and subways, they said.

Yang Ailun, the climate and energy campaign manager at the environmental group Greenpeace China, who was not involved in the research, said it is one of the most credible studies of China's energy situation and provides a good foundation for the country's climate policy. But the reduction in emissions envisioned by the study would require large amounts of international financial and technical assistance and strong political will from the Chinese government, she said. The international community has so far been reluctant to give that kind of assistance.

"There are a lot of uncertainties in terms of reducing consumption," said Dai Yande, the lead author of the study and deputy director-general of the Energy Research Institute at the National Development and Reform Commission.

Changes in lifestyle will contribute most to a reduction in energy demand, but it is doubtful that once-impoverished Chinese will be willing to give up the chance to buy new cars and houses, he said.

But the researchers warned China had little choice.

"We should not only be concerned about the cost of transformation, but also the cost of inaction," said Feng Fei, director at the State Council's Development Research Center, who took part in the study.