Can we consider the world to be a closed system?  The universe surrounding the Earth is largely a vacuum so that only the occasional meteorite enters the system and gravity prevents anything leaving, besides a rocket now and then.  Thus, it is not unreasonable to approximate the Earth as a closed system but it is not an insulated system because there is a continuous transfer in and out of energy derived from the sun.  There is a nice summary diagram of this energy transfer in a paper by Kiehl and Trenberth (1997) [http://www.cgd.ucar.edu/cas/abstracts/files/kevin1997_1.html].  The premise of the movement towards sustainability is that our world is closed system which thus contains finite material resources although there is a continuous supply of energy into the system from the sun, which provides us with the concept of ‘renewable energy’.  At the beginning of the industrial revolution our consumption of the world’s resources seemed miniscule compared to the available resources, which appeared effectively infinite and led to an ‘open-world’ mindset, i.e. an approach to living that assumed a continuous supply of energy and material into an open system.  Today, our consumption of resources has grown enormously and we close to exhausting some supplies, which is typified by the fact that the average person on Earth uses 5.7 acres of biologically productive land to supply resources and absorb waste, but only 4.7 acres/person are available for the current global population.  To achieve sustainability we need to switch from an open-world or open-system mind-set to a closed-world or closed-system way of thinking.  A report entitled ‘The New Sustainable Frontier’ published in 2009 by the US GSA Office of Governmentwide Policy [http://www.gsa.gov/graphics/ogp/2009_New_Sustainable_Frontier_Complete_Guide.pdf] is much more eloquent than me on this subject.  See also my ‘Two Earths’ post on August 13th, 2012.