Chinese cities have the highest carbon emissions per capita and energy consumption per unit of GDP in Asia, while Singapore is the region's greenest metropolis, a study released by the German engineering company Siemens and the Economist Intelligence Unit showed on Monday.

Shanghai, Guangzhou and Beijing have the highest levels of CO2 emissions per capita among the 22 cities measured by the Asian Green City Index, which evaluates the environmental performance of major Asian cities in eight categories - CO2 emissions, energy, buildings, transport, water, waste and land use, air quality and environmental governance.

Shanghai emitted CO2 of 9.7 tons for each person annually, well above the average of 4.6 tons. The city also has the highest energy consumption per unit of GDP at 14.8 megajoules (MJ) for each dollar, whereas the average is 6.0 MJ a dollar.

Singapore came out on top of the index as the only city that was ranked "well above average" across all eight categories. Meanwhile, Karachi in Pakistan was ranked the worst in overall environmental performance among the 22 Asian cities.

The study also showed that Chinese cities are strong on government policies that focus on massive investments to improve air quality, landscaping and public transportation, indicating that environmental awareness and climate protection guidelines are playing an increasingly important role.

According to the index, Nanjing, the capital of Jiangsu province, generates the third-lowest amount of waste for each person while Guangzhou, Beijing and Wuhan, the capital of Hubei province, are in the top five for green spaces.

Barbara Kux, chief sustainability officer at Siemens, said that the rapid pace of urbanization has posed unprecedented environmental challenges for Asian cities, and that these areas are key in achieving sustainable development globally.

Kux said that Asia is currently the fastest-growing market for Siemens' green- technology products and the company plans to further grow its environmental business in the region to tap the rising demand for green products and services.

The company aims to increase its sales of energy-saving environmental products to 40 billion euros ($55 billion) by 2014, according to Kux.

Siemens' revenue from green-technology products and services totaled about 28 billion euros in the 2010 fiscal year, accounting for more than a third of its total revenues.

The company has supplied the high-voltage direct-current (HVDC) transmission system, which transmits 6,400 megawatts of hydropower from southwestern China to Shanghai over a distance of about 2,100 kilometers.

It is by far the longest low-loss electricity transmission system of its type and is scheduled to start commercial operations in 2012.