This image come through to me on Facebook today & made me sit up straight & take notice. The small spheres on planet Earth depict the global water & air volume. Frankly I found it a scary image in light of the current & ongoing massive air & water pollution affecting the planet. It visually depicts exactly what we need to conserve & take care of to keep us alive. Trees help of course.
It is well worth reading the information from the Science Photo Library, which goes into detail about what the image means. – http://www.sciencephoto.com/media/159214/enlarge
The Smarter Earth Institute has done separate calculations to see if the image is correct & according to them it is. That also makes for interesting reading. http://www.smarterearth.org/MeFTWaterAndAir