(not to say that China is the lone source of pollution, but it stands to surpass even the US in terms of detrimental effects on the environment)
It surpasses the US because it produces more than the US. After WW2 the US started outsourcing its production, starting with textiles, then radios and tv's, automobiles etc., to Asia (starting with Japan). The service proportion of the US's economy is getting bigger every year.
Japan started doing the same; when new technology came along, they outsourced the older industries to Korea and Thailand. These countries did the same by outsourcing to the ASEAN countries, and since Deng Xiaoping opened up the economy in 1991 it's China's turn.
So China now produces for a lot of countries, and because production is more pollutive than service-oriented industries, it pollutes more than USA or Japan. It would be very strange though to be angry with China and "forcing" them to clean up their industries; it is pretty much the same kind of growth England went through during the industrial revolution, or Japan after WW2. China can enjoy the benefits of the opening up of the economy and now all other countries (which are profiting from the cheap production) are complaining that China pollutes too much. Maybe these countries should just import less..
There is a bright side to the story though. China's energy economy relies mainly on coal. This has forced scientist and researchers to find more efficient and environment-friendly ways of turning coal into energy. There are some promising results, but many disagree on the profitability and thus sustainability of the methods. However, as there is more coal than there are oil reserves in the world (oil reserves are estimated to last for only 80 years more), a clean way of using coal could mean a very good alternative source of energy.
Also, there are Chinese firms that are concerned with the environment. Lenovo, a computer manufaturer, for example has been recognized several times for its environment friendly production. (I like to find articles to support this but I have to go now)