Push

While Man and Whale is a Greenpeace commercial about saving whales, and Push is a social satire on consumerism and nuclear warfare, essentially both of them are shorts about preserving our planet that come from very talented directors - Koji Yamamura and Osamu Tezuka respectively. So, if you're interested in the topic, check them both. 

Although Nekojiru-sou is a little bit longer, both Neko and Osamu Tezuka's PUSH share deep reflections on human behaviour, and are meant to be watched with close attention and open mind. Nekojirou-sou may disturb some viewers.