Water has multiple uses such as: water for drinking, growing crops and livestock, industrial or manufacturing purposes, personal cleaning and waste disposal, producing hydroelectric energy and cooling for other types of power plants that produce thermal pollution, mining coal, extracting petroleum and growing biofuel crops, and water as a medium for transportation.

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Water and food overlap at all times since globally, expansion of agriculture affects the hydrological cycle, reducing forest & grassland cover which affects water quality. Further, expansion of the human population increases the cutting of forests and ocean phytoplankton or ozone depletion and acidification. Thus, the interrelated expansions of both agriculture and the human population affect future climate and the hydrological cycle. Even though food is the ultimate limiting resource, and depends on water, the population growth rate of countries has very little correlation with food or water (Abrams, 2011).

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Using water requires energy for heating, transport, purification, and the like, however, using energy affects the global climate, which alters the hydrological cycle as evaporation prevents return and pollution and heating make returned water less usable (Abrams, 2011). The Intergovernmental Panel on Climate Change (IPCC) noted in their 2008 report on water that dry land has doubled since the 70’s and that water storage in mountain glaciers and the Northern Hemisphere snow cover has significantly decreased.

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Reduced food due to reduced water is most likely to have the largest effects on population growth. Agriculture in poorer countries cannot depend on energy intensive irrigation or expensive fertilization and soils and infrastructure may be lacking in areas that will have appropriate climates.

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Food is essential since starvation is a major contributing factor to most premature deaths and even short periods without food can be lethal as malnutrition harms the immune system and suppresses female fertility. Currently, 1.53 billion hectares (12% of ice free land) has crops; 3.38 billion hectares has pastures (26%); amount has gone up 3% in the last 20 years, mainly in the tropics. However, producing more food causes significant problems since farming has already destroyed or fragmented huge regions of natural habitat and is responsible for 30% of greenhouse gas emissions. Today, 1 billion people are already undernourished and in order to be able to feed an estimated 9 billion people in 2050, 1 billion hectares of wild land may have to be converted and large additional water withdrawals will be needed (Abrams, 2011).

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Pimentel et al. (2010) suggested that only a smaller number of the world’s population will have access to proper nourishment and in roughly 100 years when the planet will run out of fossil fuels only about 2 billion people may be sustainable if it relies on renewable energy technologies and reduces per capita use of the planet’s natural resources.

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Cohen (1997) noted a number of time constraints in meeting the human resource needs such as: developing commercial fusion energy that was mentioned as a possibility by 2030, but now is much further than 35 yrs from commercial development, breeding new crop plants (many generations required), low quality mineral reserves are not discovered and/or exploited until richer reserves are used up, but it takes time to develop mining techniques and infrastructure, population momentum keeps populations increasing well beyond the year when replacement fertility characterizes the population (2.1 children/family), and an organizational breakdown in high-complexity systems. Cohen (1997) further offered two possible future scenarios: (1) increasing food prices, because of competing demands for energy and decreasing fossil fuel supplies, cuts in aid from developed to less‐developed countries due to economic problems in former, and large decrease in population in poorer parts of the world due to starvation and war, (2) population growth will likely make continued economic improvement more difficult/less likely and will probably add to conflict.

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Abrams (2011) defined resources as a set of entities with respect to a set of consumers if: (1) the entities can be consumed by a consumer individual, where consumption means that they are at least temporarily transformed to a state that is unavailable to other consumer individuals, (2) consumption has a positive effect on the fitness of the consumer individual, and (3) sufficient consumption has the potential to decrease resource abundance enough to decrease consumer per capita population growth.

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Consumer‐resource interactions (CRI) automatically account for one major reason for the delay in the impact of higher densities on birth/death, as it applies to everything in ecology and evolution. All life requires the acquisition of substances and energy, thus, CRI interactions include: uptake of water, light, and nutrients by plants, attraction of pollinators by plants and uptake of nectar by bees, transmission of disease organisms and their use of the energy and materials inside their host, predation, space occupation by plants and sessile invertebrates, and interactions that affect all of the above (Abrams, 2011).

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The current human resources, or limiting factors, which also affect population growth are food, water, shelter, energy and materials for technology in support of all other resources, disease, and homicide and war. However, positive social interactions, cooperation, division of labor, and cultural knowledge that allows more efficient exploitation of more basic resources would significantly offset resource limits (Abrams, 2011).

human resources