The planetary boundaries concept refers to the idea that humans are substantially altering natural systems, and that beyond a certain level of change this may become irreversible and self sustaining. The potential result is a planet with environmental conditions that differ substantially from those in which human civilisation developed and to which many species and ecosystems are adapted. Planetary boundaries have so far been proposed for climate change, biodiversity loss, biogeochemical cycles, ocean acidification, land use, freshwater, and ozone depletion.