It counts tennis star Maria Sharapova and Crown Prince Haakon Magnus of Norway among its other ambassadors, who support advocacy efforts through pro-bono public service announcements and other activities, doing these in their spare time and paying for the costs themselves.

Zhou received a letter of designation for her latest green role - for the token fee of US$1 a year.

"Climate change has become the defining issue of our time, set to transform the very way we live," said Khalid Malik, UN resident coordinator and UNDP resident representative in China.

"We must work with the government, the private sector, civil society and with individuals like Zhou Xun who have the talent and commitment to join us. As a multi-talented, award-winning performer, Zhou Xun has demonstrated her status as one of the best in China's entertainment industry.

"I have no doubt she will help make us the best in the battle against global warming, delivering the message that if we all take action and change our lifestyle, we can collectively overcome climate change," he added.

It is a daunting challenge. Official statistics show that more than one-quarter of the country's surface water supplies do not meet minimum quality standards, the air quality in one-third of Chinese cities remains poor and pollution is accelerating its spread to rural regions from urban areas. Environmental accidents have also been recorded at an alarming rate.

One of Zhou's first tasks as ambassador will see her fronting a two-year multimedia campaign, motivating individuals to take part in the fight against climate change by adopting simple lifestyle habits, such as using reusable chopsticks in restaurants or switching to more efficient, energy-saving light bulbs.

"To save water, I personally try not to take baths anymore (only showers)," she said. "Habits can be very hard to change but I want to let people know that we can all make a difference."