There's countless threats to earth but just to put a different spin on it, I'll leave you with a quote from Robert Swan who said:

"The greatest danger to our planet is the belief that someone else will save it."

It shouldn't be just environmentalists saving our planet, every person should do more. When we finally wake up and realize that war is not the answer, then instead of recruiting troops for a mandatory service of 1year +, every country could recruit young people to go on environmental missions. I've always believed traveling is one of the most awarding things we can do on this planet. If each and every young person were given the opportunity to learn and travel the world, while saving our planet, we could actually make a difference.