Aquifers: underground water stored between rocks or sediment that is drawn upon by wells and streams. Deep-water aquifers are a non-renewable source of fresh water and are being depleted, although how much water remains is unknown. See for more information.

Carrying Capacity: the number of a species that a given environment can hold. This can change with time as an ecosystem changes.

Environmental Justice: The belief that environmental resources are allocated unfairly on the basis of wealth and power, while the poorest people tend to suffer the most from environmental stress. Environmental Justice can be considered on a planetary scale, as with the argument that the poor countries of the global south are the victims of resource extraction from which they receive minimal benefit and suffer long-term degradation. On a local level, environmental injustice may occur, for instance, if sewage is dumped into a river that flows into an impoverished area.

Ecosystem services: The multifold ways the natural environment contributes to the human economy. These include air and water purification, agricultural pollination, nutrient cycling, soil enrichment, climate stabilization, medicinal products and drought mitigation; collectively, their global value has been estimated at some $33 trillion a year. See for more information.

Eutrophication: A naturally occurring process that human intervention has greatly sped up. As bodies of water age plants become more productive and create nutrients. Agricultural runoff is one way in which humans spur nutrient pollution, leading to excessive algal blooms and lowered oxygen supply in lakes and rivers. (see and for further discussion.)

Externalities: According to one source, "a technological externality is the indirect effect of a consumption activity or a production activity on the consumption or production possibilities available to some other consumer or producer" ( For environmental purposes, an externality is a cost inflicted on the environment--and indirectly on those who use or depend on the environment--not borne by those who inflict the cost.

Hierarchy of Needs: A ranking of basic needs such as food and shelter, which all people need for mere survival, to satisfactions, to higher level wants such as intellectual stimulation. These form a pyramid, with satisfaction of lower level needs occurring prior to higher level.

Keystone Species: Species crucial to the interactions, and overall health, of a given ecosystem.

Limiting Factor: that factor that limits a species or other phenomenon. For instance, a given area might have more than enough air, water, habitat, and space for a given number of grizzly bears, but not enough prey for the species to continue to increase. Prey is thus the limiting factor for grizzly bears in this instance.

Malthusian: A perspective that human population increases exponentially, beyond the capacity of the environment to support it, leading to conflict, starvation, and other forms of misery.

Overshoot (or Ecological Overshoot): From a planetary perspective the condition of putting more stress on our ecosystems than they are able to repair or recover from. Resource use is one factor subject to overshoot, although substitutability allows humans to recover from this state. Overshoot can also remain hidden until a resource runs out or an ecosystem suffers sudden decline, at which time the consequences become suddenly obvious.

Primary Production: Production by plants resulting in biomass. Such production relies largely on energy from the sun. Most human consumption draws upon primary production, either direct—using materials derived directly from primary production, such as vegetables or wooden furniture—or indirect, for instance in gasoline, which draws upon millions of years of heat and pressure that turned prehistoric plants and animals into crude oil.

Substitutability: The ability of one resource to replace another, as used by humans. For instance, as a source of protein fish may substitute for steak, and tofu may substitute for fish. Given infinite substitutability, many of the problems of sustainability would be solved. Some resources, such as water, have no substitute, however, while the transition from one resource to another can be expensive and even dangerous if not planned for.

Unintended Consequences: Originally used by the neoconservatives in the 1970s to describe the dangers of government intervention, in environmental parlance the term means unexpected or unplanned events that occur due to human action. Global climate change, for instance, is an unintended consequence of human use of fossil fuels and other energy sources. Causing one ecosystem change may have unintended consequences, as with the rabbits introduced to Australia for hunting that have devastated large areas of countryside, causing numerous species extinctions.

Threshold Effects: Consequences due to altering nature's processes that occur when a slow accumulation leads to a sudden change, as when overfishing leads to a sudden collapse of fishery stock.