But now China is struggling to limit emissions even to the “business as usual” levels that climate models assume if the world does little to address global warming.

“We really have an arduous task” even to reach China’s existing energy-efficiency goals, said Gao Shixian, an energy official at the National Development and Reform Commission, in a speech at the Clean Energy Expo China in late June in Beijing.

China’s goal has been to reduce energy consumption per unit of economic output by 20 percent this year compared with 2005, and to reduce emissions of greenhouse gases per unit of economic output by 40 to 45 percent in 2020 compared with 2005.

But even if China can make the promised improvements, the International Energy Agency now projects that China’s emissions of energy-related greenhouse gases will grow more than the rest of the world’s combined increase by 2020. China, with one-fifth of the world’s population, is now on track to represent more than a quarter of humanity’s energy-related greenhouse-gas emissions.

Industry by industry, energy demand in China is increasing so fast that the broader efficiency targets are becoming harder to hit.

¶Although China has passed the United States in the average efficiency of its coal-fired power plants, demand for electricity is so voracious that China last year built new coal-fired plants with a total capacity greater than all existing power plants in New York State.

¶While China has imposed lighting efficiency standards on new buildings and is drafting similar standards for household appliances, construction of apartment and office buildings proceeds at a frenzied pace. And rural sales of refrigerators, washing machines and other large household appliances more than doubled in the past year in response to government subsidies aimed at helping 700 million peasants afford modern amenities.