Japanese artist Kawano Takeshi's 2007 rendition of global warming is simple, a little funny, and a lot sad.
For another version of the same theme - using a real child's toy - check out Ours (the Bear), a video by French artist Simon Dronet. I'll try to embed it, but the link's wonky, so you might have to click through to see it.
Takeshi's global warming images are really well done. I find the penguins especially poignant.
I liked the artist's poster for Les Yeux Ouverts, too.
I can't help but be reminded of this:
http://vimeo.com/5550600