The most obvious environmental problem in China is air pollution, as I have from time to time -- OK, maybe five million times -- mentioned in this space. But environmental experts consistently stress that the most consequential problems are the related issues of CO2 output, climate change, and water supply. (On Chinese environmental issues in general, here is one article by me and one very valuable blog site.)
The Asia Society's "China Green" project has just posted a riveting and sobering series of videos on how climate change is affecting the once-vast glacier fields of the Tibetan Plateau that are in turn the source of nearly all the major rivers of Asia: Yellow and Yangtze in China, Mekong and Salween in Southeast Asia, Brahmaputra and Ganges in India, Indus in Pakistan, and others. This is an introductory three-minute trailer:
There is a lot more, and a lot that's more dramatic, at the project's main site, here. I recommend spending a minute with the interactive opening-page splash shot, which allows you to run your mouse over a photo of Mt. Everest and watch how its surrounding glaciers have changed from 1921 to 2008.
This past August, during the Beijing Olympics, Michael Zhao of the Asia Society posted a wonderful series of daily shots of air-quality conditions in Beijing in the months leading up to the Games. They showed, among other things, the minimal correlation between what was officially a "blue sky day" and how the sky really looked. (The photo-chronicle is ongoing.) Zhao has also put together the Glacier project and really is demonstrating the potential of online video to dramatize public issues everyone "knows about" but has a hard time visualizing. Making these issues vivid is a necessary though not sufficient step to getting something done about them.
UPDATE: Have swapped a version with English subtitles for the previous Chinese-subtitled trailer. Ever considerate!