How Big Is Our Ecological Footprint: The carrying capacity of Earth depends on the quality of service systems that exist.   Here is a nice website and quiz to help build awareness of our ecological footpprint http://www.myfootprint.org/

Of course, “Our Ecological Footprint” is a classic book by Mathis Wackernagel (search Google books). There is a great deal of work required to model humanities ecological footprint http://www.footprintnetwork.org/en/index.php/GFN/page/footprint_basics_overview/ as well as to understand nature’s service http://en.wikipedia.org/wiki/Nature’s_services and ecological service http://en.wikipedia.org/wiki/Ecosystem_services