After years of dodging the issue and appearing defensive on global warming, diplomats and activists say China is playing a constructive and positive role at the Bali conference.

''I think China is taking climate change seriously and that's a good sign that there will be a good outcome here in Bali,'' said Artur Runge-Metzger, head of the European Commission delegation at the conference on the Indonesian island.

China has expressed support for a post-Kyoto agreement and used the conference to spread the message that it is doing plenty to address climate change — from boosting it use of renewable energy to 10 percent by 2010 and improving their energy efficiency by 20 percent. It also has launched a massive tree planting program and has rolled out a national climate change policy.

''China is acting. We will do what we should and what we can do,'' Su said, describing his country's climate policy as ambitious,'' Su said. ''All we care for is the well-being and the future of mankind.''

Even hard-to-please environmental groups who relish lambasting industrialized countries for pollution are praising Beijing, although it's clear China has more to do. It relies heavily on dirty, outdated coal burning techniques, has 20 of the world's 30 most polluted cities, and some of the most polluted rivers.

China's ranking on Germanwatch's 40th annual index, which ranks 56 industrialized and emerging countries, was an improvement of four places over last year, the group said Friday.

The United States and Saudi Arabia were the worst on the list, at 55th and 56th places, respectively.

Environmental watchdog Germanwatch noted the Chinese government has enacted policies promoting renewable energy, including mandates that solar, wind, hydroelectric and other forms of renewable energy provide 10 percent of the nation's power by 2010. It also has ordered key industries to reduce energy consumption by 20 percent.

Politicians and activists said China's defensive posture of the past has given way to constructive discussions at the Bali conference. Beijing has also taken the lead among developing countries in calling for wealthier nations to speed up the transfer of cleaner technologies to emerging economies to help shift away from fossil fuels.

Hans Verolme, director of WWF International's Global Climate Change Program, attributed China's position to concerns over energy security and a recognition that climate change is already having a severe impact on the country, with worsening drought, water shortages and floods.

''China has made up its mind about a year ago that it was going to get serious,'' Verolme said. ''They want to show to the world it understands and it wants to do what is necessary to stop dangerous climate change.''