The carrying capacity of an environment is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, water, and other resources available. The carrying capacity is defined as the environment's maximal load, which in population ecology corresponds to the population equilibrium, when the number of deaths in a population equals the number of births (as well as immigration and emigration). The effect of carrying capacity on population dynamics is modelled with a logistic function. Carrying capacity is applied to the maximum population an environment can support in ecology, agriculture and fisheries. The term carrying capacity has been applied to a few different processes in the past before finally being applied to population limits in the 1950s. The notion of carrying capacity for humans is covered by the notion of sustainable population.
At the global scale, scientific data indicates that humans are living beyond the carrying capacity of planet Earth and that this cannot continue indefinitely. This scientific evidence comes from many sources worldwide. It was presented in detail in the Millennium Ecosystem Assessment of 2005, a collaborative effort involving more than 1,360 experts worldwide. More recent, detailed accounts are provided by ecological footprint accounting, and interdisciplinary research on planetary boundaries to safe human use of the biosphere. The Sixth Assessment Report on Climate Change from the IPCC and the First Assessment Report on Biodiversity and Ecosystem Services by the IPBES, large international summaries of the state of scientific knowledge regarding climate disruption and biodiversity loss, also support this view.
An early detailed examination of global limits was published in the 1972 book Limits to Growth, which has prompted follow-up commentary and analysis. A 2012 review in Nature by 22 international researchers expressed concerns that the Earth may be "approaching a state shift" in which the biosphere may become less hospitable to human life and in which human carrying capacity may diminish. This concern that humanity may be passing beyond "tipping points" for safe use of the biosphere has increased in subsequent years. Recent estimates of Earth's carrying capacity run between two billion and four billion people, depending on how optimistic researchers are about international cooperation to solve collective action problems. These estimates affirm that the more people we seek to sustain, the more modest their average standard of living needs to be.
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