Google quiety supporting fight against global warming?

Google frequently changes its logo during holidays, special events, or what have you. Today I noticed the regular old Google logo at their landing page:


You can see all of their previous logos online.

earthday07res.gifToday, on their search results page, where the logo is found in a smaller version at the upper-left corner, they have a customized logo. The ALT text reads simply “Earth Day”. I’m taking this as, “Wake up – the ice is melting.” This is great – the normal Earth Day images that come to mind for me are the Earth, green plants, green lettering, etc. This blue ice melting image is awesome – puts more of a “Hurry up and do something!” spin on it.

Just one question – why not put this on their landing page with the big logo?