Unless, there is something in the Constitution that I am unaware of, it doesn't matter. The "Earth Charter" is a form of treaty, unless the Senate ratifies it, a signature by Clinton doesn't mean a thing.
What happens when states sign on though? There are a large amount of our states signing on with the United Nations Association. What does this mean?
http://www.unausa.org/site/pp.asp?c=fvKRI8MPJpF&b=260414