What are the other threats to sustainability?
Agricultural production, which uses an enormous amount of water, becomes limited as aquifers deplete. According to Brown, "policymakers are beginning to look at water as the limiting factor for food production." Soil erosion and impoverishment also limit crop production, while energy prices affect intensive farming. Still crops are a far more efficient means of feeding large numbers of people than meat, since an enormous amount of productivity is lost as animals eat crops and are themselves converted into products for human consumption.
Pollution is another driving factor that threatens not only human health, but also resources on which we depend, not only water but also biodiversity and renewable resources. Industry, energy, and automobiles are key sources of emissions that harm our air. Common air pollutants include ozone, nitrogen oxides, sulfur dioxide, carbon monoxide, lead, particles, and organic pollutants. Among the numerous affects on human health are headaches, skin rashes, asthma, bronchitis, blood poisoning, neurological impairment, birth defects, and cancer.
Sunrise reveals air pollution across Brisbane city Source: http://www.epa.qld.gov.au/environmental_management/air/air_quality_monitoring/ air_pollutants/
Sulfur dioxide emissions produce acid rain, which ends up in our water, as do atmospheric depositions of numerous substances, such as nitrogen. Sewage runoff and factory wastes are direct sources of water pollution. In the U.S. Great Lakes, for instance, pollutants include polychlorinated biphenyl, dichlorodiphenyl trichloroethane, dieldrin, toxaphene, mirex, methylmercury, benzopyrene, hexachlorobenzene, furans, dioxins, and aklylated lead.18 Beyond their impact on human health, these substances threaten the food chain and impact ecosystem health. Radioactive wastes are the key drawback to an energy source that might otherwise be a magic bullet for many of our problems: nuclear energy. Nuclear wastes remain hazardous for centuries or millennia (depending on the grade of uranium or plutonium). Current storage methods include reprocessing, storage, and eventual permanent disposal in deep geological repositories. Transportation and the possibility of terrorist hijacking are other controversial issues regarding nuclear wastes.
Air and water pollution are key ecosystem stress factors. Even more obvious direct threats to ecosystem health include the cutting down of forests and destruction of habitat. Alteration of the physical environment is multifold. Non-permeable surfaces, such as pavement and buildings, increase runoff of silt, sewage, garbage, and other harmful materials into lakes and rivers while preventing water from replenishing aquifers. Invasive species, displaced from their original environment, alter ecosystem functions; invasive vines, for instance, strangle and kill trees. Overall biodiversity is lowered, leading to a multitude of effects, many as yet unknown. Ecosystem services refers to the multifold ways the natural environment contributes to the human economy. These services include air and water purification, agricultural pollination, nutrient cycling, soil enrichment, climate stabilization, medicinal products and drought mitigation. Nature magazine has estimated the value of these global services at an average of US $33 trillion a year.