China is now the world's second largest economy, and it is expected to overtake the United States to become the largest by 2020. What are the implications for the global environment and climate change if China surpasses the United States? There are major concerns with China's rapid rise because its economic and industrial structure is increasingly dependent on the consumption of energy, raw materials, and electricity. In 2010, China's GDP was approximately 40 percent of the United States' GDP, yet it was the world's largest polluter and the biggest consumer of energy and electricity. This implies that the energy efficiency of the Chinese economy measured by energy consumption per unit of GDP is about one-third of that of the United States and one-fourth that of the EU and Japan. If the Chinese economy continues to grow as fast as it has in the past, without changing its structure and improving energy efficiency, China's growth will cause severe damage to the global environment. This paper analyzes the evolution of energy efficiency in the Chinese economy and stresses the importance of transforming China's economic structure.