SHANGHAI, Dec. 8 -- Smog these days has covered most of China with visibility in some regions lower than 50 meters, and its the hot topic at home and abroad.

Shanghai mayor Yang Xiong told a two-day forum called International Dialogue on the Chinese Dream on Saturday that Shanghai had the worst air pollution, "but it will be fine in the following days."

The Air Quality Index (AQI) topped 500 in certain areas of Shanghai on Friday. AQI of over 300 is defined as "serious pollution".

More than 100 entries to expressways in east China's Shandong Province were temporarily closed due to haze on Saturday, and some flights were canceled or delayed at Jinan and Qingdao airports.

Hundreds of experts gathered in Shanghai to discuss on the Chinese Dream. Proposed by President Xi Jinping last November, the Chinese Dream is "realizing the nation's great rejuvenation."

For Kenneth Lieberthal, senior researcher at the Brookings Institution, the meaning of Chinese Dream also includes "beautiful China", that is to build an ecological society.

Kenneth listed challenges as the staggering resource scarcity and the tension between economic development and environmental protection. One such extreme shortage is usable water in the North China Plain.

"I raise the above issues not to suggest that pessimism is warranted but rather to indicate the types of objective, major obstacles that must be handled in order to satisfy national aspirations to achieve the Chinese Dream," he said.

According to Mohamed Noman Galal, former Egyptian Ambassador to China and Member of the Egyptian Council for Foreign Relations, environmental pollution is the second most serious weakness in a rapid growing society like China.

China has recognized that the condition of the environment has a close tie with the wellbeing of ordinary people.

In a plan unveiled in September, the government will clean up the air by cutting coal use, closing polluters and promoting cleaner production. The plan aims to cut the density of inhalable particulate matter by at least 10 percent in major cities nationwide by 2017.

Also at the Third Plenary Session of the 18th CPC Central Committee, the Decision on Major Issues Concerning Comprehensively Deepening the Reform was adopted to respond to people's concerns and requests.

Pan Jiahua, director of the Institute of Urban and Environmental Studies, the Chinese Academy of Social Sciences, said that low or zero carbon could help speed up the process of transformation from an industrial into an ecological society.

"However, we must face up to the realities," said Qiang Ying, a researcher at the Shanghai Academy of Social Sciences, adding that the mayor's reference to the haze and difficulties in haze control shows the city's frank attitude, which will help to reach a global consensus.

The treatment of smog will be a long battle. It is rather difficult for one single department or single nation to tackle air pollution, said Hu Angang, a professor with the School of Public Policy and Management of Tsinghua University.

"Chinese Dream is now merging into the dream of the world, as Chinese aim for a green and sustainable world but not a world with natural and environmental disasters, that global cooperation is required," Hu said.