You don't have to like Al Gore to appreciate An Inconvenient Truth, a compelling documentary about global warming. Combining data, graphs, catastrophic images and Gore's passionate presentation, the film - directed by Davis Guggenheim - delivers a convincing and disturbing message: if we don't start reducing carbon dioxide emission right now, it will be too late. In 10 years' time, human civilization will crumble under the rising heat. The film is like a lecture. But, while you have to concentrate in order to absorb the huge amount of information, it is never boring. If you live in a developed city like Hong Kong, you have a moral duty to watch this movie. City dwellers constantly use electricity, which is one of the main ways greenhouse gases are produced. We cannot afford to be ignorant of the unpardonable crimes we commit against the planet that we call home. Some have criticised the film, calling it propaganda. To a certain extent, this is true. There are some awkward scenes portraying the former US vice-president as the sole believer in this cause. But Gore's self-righteousness can be forgiven, because he is truly passionate about this problem. Few people - especially those who have been involved in politics - stay true to their beliefs. Even fewer can be bothered to spread the word to other people. An Inconvenient Truth is not the kind of film to brighten your day. But it is a film that could help save the world.