salinity Amount of various salts dissolved in a given volume of water.

salinization Accumulation of salts in soil that can eventually make the soil unable to support plant growth.

saltwater intrusion Movement of salt water into freshwater aquifers in coastal and inland areas as groundwater is withdrawn faster than it is recharged by precipitation.

sanitary landfill Waste disposal site on land in which waste is spread in thin layers, compacted, and covered with a fresh layer of clay or plastic foam each day.

scavenger Organism that feeds on dead organisms that were killed by other organisms or died naturally. Examples are vultures, flies, and crows. Compare detritivore.

scientific law Description of what scientists find happening in nature repeatedly in the same way, without known exception. See first law of thermodynamics, law of conservation of matter, second law of thermodynamics. Compare scientific data, scientific hypothesis, scientific methods, scientific model, scientific theory.

scientific methods Ways that scientists gather data and formulate and test scientific hypotheses, models, theories, and laws. See scientific data, scientific hypothesis, scientific law, scientific model, scientific theory.

scientific model Simulation of complex processes and systems. Many are mathematical models that are run and tested using computers.

scientific theory Well-tested and widely accepted scientific hypothesis. Compare scientific data, scientific hypothesis, scientific law, scientific methods, scientific model.

second law of thermodynamics In any conversion of heat energy to useful work, some of the initial energy input is always degraded to a lower-quality, more dispersed, less useful energy, usually low-temperature heat that flows into the environment; you cannot break even in terms of energy quality. See first law of thermodynamics.

secondary consumer Organism that feeds only on primary consumers. Compare detritivore, omnivore, primary consumer.

secondary pollutant Harmful chemical formed in the atmosphere when a primary air pollutant reacts with normal air components or other air pollutants. Compare primary pollutant.

secondary sewage treatment Second step in most waste treatment systems in which aerobic bacteria break down up to 90% of degradable, oxygen-demanding organic wastes in wastewater. This usually involves bringing sewage and bacteria together in trickling filters or in the activated sludge process. Compare advanced sewage treatment, primary sewage treatment.

secondary succession Ecological succession in an area in which natural vegetation has been removed or destroyed but the soil is not destroyed. See ecological succession. Compare primary succession.

second-growth forest Stands of trees resulting from secondary ecological succession. Compare oldgrowth forest, tree plantation.

sedimentary rock Rock that forms from the accumulated products of erosion and in some cases from the compacted shells, skeletons, and other remains of dead organisms. See rock cycle. Compare igneous rock, metamorphic rock

seed-tree cutting Removal of nearly all trees on a site in one cutting, with a few seed-producing trees left uniformly distributed to regenerate the forest. Compare clear-cutting, selective cutting, shelterwood cutting, strip cutting.

selective cutting Cutting of intermediate-aged, mature, or diseased trees in an uneven-aged forest stand, either singly or in small groups. This encourages the growth of younger trees and maintains an uneven-aged stand. Compare clear-cutting, seed-tree cutting, shelterwood cutting, strip cutting.

septic tank Underground tank for treating wastewater from a home in rural and suburban areas. Bacteria in the tank decompose organic wastes, and the sludge settles to the bottom of the tank. The effluent flows out of the tank into the ground through a field of drainpipes.

shale oil (also called oil shale) kerogen Slow-flowing, dark brown, heavy oil obtained when kerogen in oil shale is vaporized at high temperatures and then condensed. Shale oil can be refined to yield gasoline, heating oil, and other petroleum products. Kerogen is the solid, waxy mixture of hydrocarbon compounds.

shelterwood cutting Removal of mature, marketable trees in an area in a series of partial cuttings to allow regeneration of a new stand under the partial shade of older trees, which are later removed. Typically, this is done by making two or three cuts over a decade. Compare clear-cutting, seed-tree cutting, selective cutting, strip cutting.

shifting cultivation Clearing a plot of ground in a forest, especially in tropical areas, and planting crops on it for a few years (typically 2-5 years) until the soil is depleted of nutrients or the plot has been invaded by a dense growth of vegetation from the surrounding forest. Then a new plot is cleared and the process is repeated. The abandoned plot cannot successfully grow crops for 10-30 years. See also slash-and-burn cultivation.

slash-and-burn cultivation Cutting down trees and other vegetation in a patch of forest, leaving the cut vegetation on the ground to dry, and then burning it. The ashes that are left add nutrients to the nutrient-poor soils found in most tropical forest areas. Crops are planted between tree stumps. Plots must be abandoned after a few years (typically 2-5 years) because of loss of soil fertility or invasion of vegetation from the surrounding forest. See also shifting cultivation.

slowly degradable pollutant Material that is slowly broken down into simpler chemicals or reduced to acceptable levels by natural physical, chemical, and biological processes. Also used persistent pollutant. Compare biodegradable pollutant, degradable pollutant, nondegradable pollutant.

sludge Gooey mixture of toxic chemicals, infectious agents, and settled solids removed from wastewater at a sewage treatment plant.

smart growth Form of urban planning that recognizes urban growth will occur but uses zoning laws and an array of other tools to (1) prevent sprawl, (2) direct growth to certain areas, (3) protect ecologically sensitive and important lands and waterways, and (4) develop urban areas that are more environmentally sustainable and more enjoyable places to live.

smelting Process in which a desired metal is separated from the other elements in an ore mineral.

Smog Originally a combination of smoke and fog but now used to describe other mixtures of pollutants in the atmosphere. See industrial smog, photochemical smog.

soil Complex mixture of inorganic minerals (clay, silt, pebbles, and sand), decaying organic matter, water, air, and living organisms.

soil conservation Methods used to reduce soil erosion, prevent depletion of soil nutrients, and restore nutrients already lost by erosion, leaching, and excessive crop harvesting.

soil erosion Movement of soil components, especially topsoil, from one place to another, usually by wind, flowing water, or both. This natural process can be greatly accelerated by human activities that remove vegetation from soil.

soil horizons Horizontal zones that make up a particular mature soil. Each horizon has a distinct texture and composition that vary with different types of soils. See soil profile.

soil permeability Rate at which water and air move from upper to lower soil layers. Compare porosity.

soil profile Cross-sectional view of the horizons in a soil. See soil horizon.

soil structure How the particles that make up a soil are organized and clumped together. See also soil permeability, soil texture.

soil texture Relative amounts of the different types and sizes of mineral particles in a sample of soil.

solar capital Solar energy from the sun reaching the earth. Compare natural resources.

solar energy Direct radiant energy from the sun and a number of indirect forms of energy produced by the direct input. Principal indirect forms of solar energy include wind, falling and flowing water (hydropower), and biomass (solar energy converted into chemical energy stored in the chemical bonds of organic compounds in trees and other plants).

solid waste Any unwanted or discarded material that is not a liquid or a gas. See municipal solid waste.

spaceship-earth worldview View of the earth as a spaceship: a machine we can understand, control, and change at will by using advanced technology. See planetary management worldview. Compare environmental wisdom worldview.

specialist species Species with a narrow ecological niche. They may be able to (1) live in only one type of habitat, (2) tolerate only a narrow range of climatic and other environmental conditions, or (3) use only one type or a few types of food. Compare generalist species.

Speciation Formation of two species from one species because of divergent natural selection in response to changes in environmental conditions; usually takes thousands of years. Compare extinction.

species Group of organisms that resemble one another in appearance, behavior, chemical makeup and processes, and genetic structure. Organisms that reproduce sexually are classified as members of the same species only if they can actually or potentially interbreed with one another and produce fertile offspring.

species diversity Number of different species and their relative abundances in a given area. See biodiversity. Compare ecological diversity, genetic diversity.

Spoils Unwanted rock and other waste materials produced when a material is removed from the earth's surface or subsurface by mining, dredging, quarrying, and excavation.

S-shaped curve Leveling off of an exponential, J-shaped curve when a rapidly growing population exceeds the carrying capacity of its environment and ceases to grow.

stability Ability of a living system to withstand or recover from externally imposed changes or stresses. See constancy, inertia, resilience.

Stewardship View that because of our superior intellect and power or because of our religious beliefs, we have an ethical responsibility to manage and care for domesticated plants and animals and the rest of nature. Compare environmental wisdom worldview, planetary management worldview.

storage area Place within a system where energy, matter, or information can accumulate for various lengths of time before being released. Compare input, output, throughput.

Stratosphere Second layer of the atmosphere, extending about 17-48 kilometers (11-30 miles) above the earth's surface. It contains small amounts of gaseous ozone (O3), which filters out about 95% of the incoming harmful ultraviolet (UV) radiation emitted by the sun. Compare troposphere.

stream Flowing body of surface water. Examples are creeks and rivers.

strip cropping Planting regular crops and close-growing plants, such as hay or nitrogen-fixing legumes, in alternating rows or bands to help reduce depletion of soil nutrients.

strip cutting Variation of clear-cutting in which a strip of trees is clear-cut along the contour of the land, with the corridor narrow enough to allow natural regeneration within a few years. After regeneration, another strip is cut above the first, and so on. Compare clear-cutting, seed-tree cutting, selective cutting, shelterwood cutting.

strip mining Form of surface mining in which bulldozers, power shovels, or stripping wheels remove large chunks of the earth's surface in strips. See area strip mining, contour strip mining, surface mining. Compare subsurface mining.

subatomic particles Extremely small particles--electrons, protons, and neutrons--that make up the internal structure of atoms.

subduction zone Area in which oceanic lithosphere is carried downward (subducted) under the island arc or continent at a convergent plate boundary. A trench ordinarily forms at the boundary between the two converging plates. See convergent plate boundary.

subsidence temperature inversion Inversion of normal air temperature layers when a large mass of warm air moves into a region at a high altitude and floats over a mass of colder air near the ground. This keeps the air over a city stagnant and prevents vertical mixing and dispersion of air pollutants. See temperature inversion. Compare radiation temperature inversion.

subsistence farming Supplementing solar energy with energy from human labor and draft animals to produce enough food to feed oneself and family members; in good years there may be enough food left over to sell or put aside for hard times. Compare industrialized agriculture.

subsurface mining Extraction of a metal ore or fuel resource such as coal from a deep underground deposit. Compare surface mining.

succulent plants Plants, such as desert cacti, that survive in dry climates by having no leaves, thus reducing the loss of scarce water. They store water and use sunlight to produce the food they need in the thick, fleshy tissue of their green stems and branches. Compare deciduous plants, evergreen plants.

sulfur cycle Cyclic movement of sulfur in different chemical forms from the environment to organisms and then back to the environment.

superinsulated house House that is heavily insulated and extremely airtight. Typically, active or passive solar collectors are used to heat water, and an air-to-air heat exchanger is used to prevent buildup of excessive moisture and indoor air pollutants.

surface mining Removing soil, subsoil, and other strata and then extracting a mineral deposit found fairly close to the earth's surface. See area strip mining, contour strip mining, mountaintop removal, open-pit mining. Compare subsurface mining.

surface runoff Water flowing off the land into bodies of surface water. See reliable runoff.

surface water Precipitation that does not infiltrate the ground or return to the atmosphere by evaporation or transpiration. See runoff. Compare groundwater.

survivorship curve Graph showing the number of survivors in different age groups for a particular species.

sustainability Ability of a system to survive for some specified (finite) time.

sustainable agriculture Method of growing crops and raising livestock based on organic fertilizers, soil conservation, water conservation, biological pest control, and minimal use of nonrenewable fossil fuel energy.

sustainable living Taking no more potentially renewable resources from the natural world than can be replenished naturally and not overloading the capacity of the environment to cleanse and renew itself by natural processes.

sustainable society A society that manages its economy and population size without doing irreparable environmental harm by overloading the planet's ability to absorb environmental insults, replenish its resources, and sustain human and other forms of life over a specified period, usually hundreds to thousands of years. During this period, it satisfies the needs of its people without depleting natural resources and thereby jeopardizing the prospects of current and future generations of humans and other species.

sustainable yield (sustained yield) Highest rate at which a potentially renewable resource can be used without reducing its available supply throughout the world or in a particular area. See also environmental degradation.

symbiosis Any intimate relationship or association between members of two or more species. See symbiotic relationship.

symbiotic relationship Species interaction in which two kinds of organisms live together in an intimate association. Members of the participating species may be harmed by, benefit from, or be unaffected by the interaction. See commensalism, interspecific competition, mutualism, parasitism, predation.

synergistic interaction Interaction of two or more factors or processes so the combined effect is greater than the sum of their separate effects.

synfuels Synthetic gaseous and liquid fuels produced from solid coal or sources other than natural gas or crude oil.

synthetic natural gas (SNG) Gaseous fuel containing mostly methane produced from solid coal.