In environmental science, the concept of overshoot means demand in excess of regeneration. It can apply to animal populations and people. Environmental science studies to what extent human populations through their resource consumption have risen above the sustainable use of resources. For people, "overshoot" is that portion of their demand or ecological footprint which must be eliminated to be sustainable. Excessive demand leading to overshoot is driven by both consumption and population.
A decline in population as a consequence of overshoot has been termed 'collapse'. The trajectory undergone by such a population has been called 'overshoot-and-collapse'. A collapse, as much as overshoot, can be the result of different conditions, a particular but not synonymous case of collapse is the Malthusian catastrophe.
Overshoot can occur due to lag effects. Reproduction rates may remain high relative to the death rate. Entire ecosystems may be severely affected and sometimes reduced to less-complex states due to prolonged overshoot. The eradication of disease can trigger overshoot when a population suddenly exceeds the land's carrying capacity. An example of this occurred on the Horn of Africa when smallpox was eliminated. A region that had supported around 1 million pastoralists for centuries was suddenly expected to support 14 million people. The result was overgrazing, which led to soil erosion.
The 1972 book The Limits to Growth discussed the limits to growth of society as a whole. This book included a computer-based model which predicted that the Earth would reach a carrying capacity of ten to fourteen billion people after some two hundred years, after which the human population would collapse. The model was based on five variables: "population, food production, industrialization, pollution, and consumption of non-renewable natural resources".:25 This simulation modelled human populations after the overshoot and collapse seen in yeast cells in a petri dish. It was highly controversial and generally dismissed by economists.
William R. Catton, Jr., a sociologist, wrote about relationships between human societies and their environment, as well as his beliefs that there were too many people and more needed to die, in his 1980 book Overshoot: The Ecological Basis of Revolutionary Change. He wrote that humanity would overshoot Earth's carrying capacity, due to both overpopulation and overconsumption.
The Global Footprint Network purports to be able to measure how much the human economy demands against what the Earth can renew. The Optimum Population Trust (now called Population Matters) has listed what they believe is the overshoot (overpopulation) of a number of countries, based on the above.
A widely discussed study published in January 2021 in Frontiers in Conservation Science, emphasizes the significance of overshoot stating that "simultaneous with population growth, humanity's consumption as a fraction of Earth's regenerative capacity has grown from ~ 73% in 1960 to 170% in 2016, with substantially greater per-person consumption in countries with highest income." These numbers are based on recent Ecological Footprint studies. The Frontiers in Conservation Science publication explains that "[t]his massive ecological overshoot is largely enabled by the increasing use of fossil fuels. These convenient fuels have allowed us to decouple human demand from biological regeneration: 85% of commercial energy, 65% of fibers, and most plastics are now produced from fossil fuels."
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