We have already crossed four “planetary boundaries,” including carbon dioxide levels and extinction rates, deforestation and the flow of nitrogen and phosphorous into the oceans, according to a new paper published in “Science Magazine” by 18 researchers trying to gauge the breaking points of the natural world.
http://www.sciencemag.org/content/early/2015/01/14/science.1259855
“2011 Horn of Africa famine Oxfam 01” by Oxfam East Africa – http://www.flickr.com/photos/oxfameastafrica/5758386784/. Licensed under CC BY 2.0 via Wikimedia Commons –