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Glossary Absorption Term for the process in which a substance retains rather than reflects radiant energy; it can be thought of as the inverse of albedo. Absorbed energy is always converted to another form of energy, according to the medium doing the absorbing. Advection A form of convection, in which heat or water vapor is transported through a fluid (e.g. Earth’s atmosphere) by the motion of currents in the fluid. In meteorology, advection refers to horizontal movement in the atmosphere—for example, winds caused by differences in temperature. Aerosols Particles of matter—solid or liquid—larger than a molecule but small enough (up to 100 µm diameter) to remain suspended in the atmosphere. Aerosols affect climate by participating in chemical reactions in the atmosphere and by absorbing and scattering solar radiation, which usually makes them negative radiative forcing agents. Some, such as salt particles from sea spray and clay particles from the weathering of rocks, have natural origins. Aerosols originating from human activities, such as particulates in car exhaust, are often considered pollutants. Albedo The proportion of solar radiation reflected, rather than absorbed, by a surface that it strikes. On Earth, this can range from 90%, or .9, for a surface covered in fresh snow to 4%, or .04, for asphalt. Oceans and forests, especially coniferous forests with their darker leaves, also have low albedos, which means they absorb more radiation than light-colored areas such as deserts and grasslands. Clouds, whose albedo is usually higher than that of the surface below them, contribute powerfully to maintaining a relatively high global average albedo (30%). Ambient Surrounding, used, for instance, of the air or temperature of a surrounding environment. Anthropogenic Made or induced by humans; often used of emissions. Atmosphere The mixture of gases surrounding the Earth. Nitrogen accounts for 78% of its volume, followed by oxygen (20.9%), water vapor in varying amounts, carbon dioxide (0.03%), and trace amounts of other gases. This mixture is arranged in self-contained layers, each with its own name and thermal properties: Biogeochemical cycle The global process, with many components such as the nitrogen, carbon, and hydrologic cycles, in which chemical constituents essential to life move along circular paths through the Earth system. All elements and compounds on Earth participate in the biogeochemical cycle, even if they spend millions of years in one position within it (such as a coal deposit). Biomass For a given area, the mass of material of biological origin (plants, bacteria, etc.), either living or not yet fossilized. Water content does not "count" towards biomass, so it is usually expressed as dry weight per unit area. Plant biomass serves as a measure of primary production. Biosphere All parts of the earth—on land, in the oceans, and in the atmosphere—inhabited by living organisms. Boreal forest One of the largest terrestrial biomes in the world, extending through circumpolar Scandinavia, Russia, and Canada. (In Europe and Asia it is known as the taiga, a term which in Canada denotes a transitional zone between boreal forest and tundra.) A very few species, mostly conifers—pine, spruce, fir, larch—and some broadleaves, such as aspen and birch, tend to dominate, thanks to their tolerance for cold winters, poor soil, and short growing seasons. Boreal ecosystems fix so much carbon that worldwide levels of atmospheric carbon dioxide fall each year in the northern spring and rise in the autumn. The boreal forest also contains an estimated 13% of all the carbon stored in biomass and 43% of the carbon stored in soil worldwide. Carbon cycle The process, illustrated below, by which carbon (in various forms including carbon dioxide) moves through and among the Earth’s systems. For instance, a single molecule of CO2 might be expelled into the atmosphere by volcanic activity, taken up by photosynthesis to become part of the biomass of a tree, re-released into the atmosphere by a forest fire that kills the tree, taken up again by a phytoplankton in the ocean, and then consumed by a zooplankton, only to be released into the atmosphere once more by respiration. Both the hydrosphere and the terrestrial biosphere cycle through enormous amounts of carbon. The exchange of carbon between the terrestrial biosphere and the atmosphere amounts to more than ten times that produced by human burning of fossil fuels. 
Illustration: Wikipedia Carbon dioxide (CO2) The most significant anthropogenic greenhouse gas. It exists naturally in the atmosphere as a trace gas, measured in parts per million, but its concentrations have risen 35%—to around 385 ppm in 2007—since about 1750, mostly because of deforestation and the burning of fossil fuels. Much of the carbon dioxide we have added to the atmosphere appears to have been absorbed by natural carbon sinks such as oceans and forests, which may become carbon sources as atmospheric concentrations continue to rise, driving other changes in the climate system. Carbon sequestration This usually means the naturally occurring storage of carbon in carbon sinks. Artificial carbon sequestration, often called carbon capture and storage, may involve capturing carbon dioxide before it is emitted into the atmosphere, then injecting it into underground reservoirs or the deep ocean. Chlorofluorocarbons A group of compounds containing chlorine, fluorine, and carbon in varying amounts, used in industry, as coolants, and as propellants for gases and sprays. Emitted only by human activities, they not only act as greenhouse gases, but also persist for as long as 100 years thanks to their stability. Ultraviolet radiation (UVB and UVC) can break them up only in the stratosphere, and when this happens, the resulting free chlorine atoms deplete the stratospheric ozone layer; a single such atom can destroy as many as 10,000 O3 molecules in its lifetime. The ozone layer is expected to recover slowly, as the Montreal Protocol now restricts the use of CFCs. Climate The typical weather conditions—predominantly temperature, humidity, precipitation, wind, and sunshine—of a particular area. Because these elements are averaged over the long term (the World Meteorological Organization typically uses 30 years), climate data may tell us little about weather conditions. For instance, annually averaged temperature does not help in predicting the high temperature on a particular July day. The global climate system consists of the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere and their interactions with each other. Climate change Because climate is never static, scientists define this as a significant change in climate conditions that persists on a scale of (typically) decades or longer. The UN Framework Convention on Climate Change distinguishes between naturally occurring "climate variability" and "climate change" driven by human activity. In the media, "climate change" and "global warming" are often used interchangeably. Conduction Energy transfer by means only of molecular motion, as contrasted with convection and radiation. When you put your hand on a hot surface, heat is transferred to it by conduction. Convection The transfer of heat within a fluid (water or air) driven by currents in the fluid. Water heated in a pot will circulate as the water closest to the flame rises and the cooler water sinks ("free convection," driven by gravity and buoyancy), while a central-heating system in a house requires energy from a fan or pump to move heat through the rooms ("forced convection"). In meteorology, convection refers to the vertical movement within the atmosphere and oceans that serves, along with advection, as a major driver of climate. Cryosphere The portions of the Earth where water is in solid form, including ice and snow cover and frozen ground. Deforestation Long-term conversion of forest to nonforest. Deforestation ranks as a principal cause of the enhanced greenhouse effect, both because burning or decomposition of wood releases carbon dioxide and because the trees lost no longer remove carbon dioxide from the atmosphere through photosynthesis. Tropical deforestation, in particular, also drives changes in the hydrologic cycle (such as reduced rainfall) both regionally and globally. Denitrification Part of the nitrogen cycle in which bacteria convert nitrates and nitrites into nitrogen gas, which is biologically unavailable to plants. While denitrification takes place in the oceans, on land it happens fastest in wetland soils, which make an important contribution to removing the nitrates humans have added to the ecosystem in fertilizers. Disturbance A pronounced change in environmental conditions leading to a change in an ecosystem; for forests, this is defined as a significant reduction in leaf area index for a period of more than a year, and may include fire, harvest, insects, disease, and windthrow. Ecosystem Term for all the living and non-living parts of a natural system that functions as a unit of interdependent relationships. A scientist may define anything from a tiny area to the entire Earth as an ecosystem, depending on what he or she is investigating. Eddy A fluctuation, or turbulence, in a fluid (gas or liquid). Eddy covariance Also called eddy correlation, this is a technique used in micrometeorology to measure vertical fluxes (e.g. of CO2, water vapor, heat) within the atmospheric boundary layer. According to the micrometeorology theory, vertical fluxes of scalars (i.e. quantities) are proportional to the covariance between fluctuations in vertical wind velocity and scalars concentrations. Eddy covariance is the only technique that allows measuring fluxes at the scale of the ecosystem (typically ~1km2) at a half-hourly time scale over extended periods of time. A more complete description of the eddy covariance technique can be found here. Electromagnetic spectrum The range of wavelengths and frequencies of electromagnetic radiation: 
Illustration: Wikipedia In climate studies, shortwave radiation often refers to visible light, in contrast to longwave (infrared) radiation. Differences in the behavior of short-and longwave radiation in the atmosphere account for the greenhouse effect. Emission The release of a substance, such as a greenhouse gas, into the atmosphere. Energy The capacity to do work. Potential, kinetic, thermal, and radiant energy all play important roles in driving processes in the Earth system. Energy exchange For a given ecosystem, such as a research site, the total flux of energy between organisms and their environment, beginning with the incoming solar radiation that powers photosynthesis. Other important fluxes include convective heat transfer (wind) and evapotranspiration. The energy "budget" also needs to account for conduction, latent heat, and the radiation the system reflects back into the atmosphere. Enhanced greenhouse effect A term for the intensifying of the natural greenhouse effect because of humans’ increased emissions of greenhouse gases, with the result that since about 1750 Earth has warmed more, and more quickly, than it otherwise would have. Greenhouse gases responsible include carbon dioxide, methane, and nitrous oxide, all of which occur naturally as well as anthropogenically; chlorofluorocarbons, which are emitted only by human activities; and even water vapor, whose concentrations are largely regulated by the hydrologic cycle, but also increase in response to warming. Environment All of the external conditions—physical, chemical, and biological—in which an organism or community lives and with which it interacts. Evapotranspiration A term for the sum of two processes by which ecosystems return water to the atmosphere: evaporation, from soil, bodies of water, and the surfaces of plant tissue, plus transpiration, in which water is transported up through the bodies of plants and released through tiny holes in their leaves. These holes, called stomata, mean significant water loss for plants, but they must be open to allow photosynthesis to take place, and transpiration also helps to cool plants and transport nutrients from roots to stems and leaves. Forcing mechanism Any process, natural or anthropogenic, that changes the energy balance of the climate system and produces radiative forcing. Forest management Human interventions that affect forest ecosystems either for conservation or economic activities. Such interventions may include cutting, planting, thinning, preventing fires and insect outbreaks. Fossil fuel Fuel formed over millions of years from remains of dead plants and animals. Fossils fuels include natural gas (methane), petroleum and coal. Their combustion releases carbon dioxide to the atmosphere and constitutes the main source of greenhouse gases of anthropogenic origin. Fossil fuel burning emitted an estimated 8.5 billion tons of carbon (i.e. 31.2 billion tons of CO2) in 2007. Flux The measure of the flow of some quantity—heat, CO2, water vapor--per unit area per unit time. Geosphere See lithosphere. Global warming An increase in the near surface temperature of the Earth. The Intergovernmental Panel on Climate Change (IPCC) concluded that increased concentrations of greenhouse gases are causing an increase in the Earth's surface temperature. A warming of 0.8°C occurred since 1850 and climate models suggest a further increase of ~1 to 6°C during the 21st century. Periods of global warming also occurred in the distant past as the result of natural influences. Greenhouse effect A naturally occurring process in the Earth’s atmosphere, without which the planet’s surface would be about 33°C colder than it is. It arises because greenhouse gases in the atmosphere trap some of the longwave, or infrared, radiation that the Earth’s surface emits after it has been heated by shortwave radiation. The atmosphere then re-emits this thermal radiation in all directions, including back to the surface, which it warms further. This should not be confused with the anthropogenic enhanced greenhouse effect. Greenhouse gas One of several trace gases in the atmosphere that allow shortwave radiation to pass through them but absorb and re-emit longwave (infrared) radiation coming from Earth’s surface. In descending order of their concentrations in the atmosphere, the main greenhouse gases are water vapor, CO2, methane, nitrous oxide, and tropospheric ozone. But a particular gas’ role in producing the greenhouse effect depends not only on its abundance but on how effectively it traps and recycles thermal radiation and on how long it persists in the atmosphere. For instance, methane acts as a much more powerful greenhouse gas than CO2, but CO2 exists in greater abundance, remains in the atmosphere for much longer, and accounts for more than half of the enhanced greenhouse effect. Hydrocarbon One of a group of compounds consisting of carbon and hydrogen. Hydrocarbons in the form of fossil fuels—petroleum, natural gas, and coal—supply most of the world’s electric energy and heat; when burned, they release CO2 and contribute to the formation of tropospheric ozone. Hydrologic cycle The movement of water molecules within and between the atmosphere, hydrosphere, geosphere, and biosphere. This enormous system, driven by the sun’s energy and in turn releasing huge amounts of energy as water passes through changes of state between solid, liquid, and gas form, is schematically illustrated below. 
Illustration: Wikipedia Water molecules may follow myriad different paths through this system. A major such path, evaporation, supplies 90% of the atmosphere’s water vapor, most of it from the oceans, which evaporation keeps much cooler than they would otherwise be. The formation of clouds, and the precipitation that sustains life in the terrestrial biosphere, result from water vapor condensing again as it rises in the atmosphere. While water exists in all three states in the atmosphere, in vapor form it acts as a greenhouse gas more powerful than any other; the natural hydrologic cycle, however, regulates it, as an individual water molecule remains in the atmosphere for an average of only nine days. The same molecule may spend thousands of years as part of a polar ice sheet. Hydrosphere A term for all the water on Earth, which covers 71% of its surface, the majority of it found in the oceans, and sets it apart from all other planets in the solar system. Temperature and pressure conditions on Earth allow water to exist in all three of its states, helping to drive the hydrologic cycle. As well, the enormous volume of the oceans, which respond only very slowly to changes in temperature, keep the Earth’s climate much more stable and less extreme than it would otherwise be. Infrared radiation Electromagnetic radiation of wavelengths slightly longer than those of visible light. Thermal radiation released by the earth’s surface and atmosphere, after solar radiation heats them, falls mostly in this section of the electromagnetic spectrum. This means that by measuring infrared radiation, even at night, we can determine geological and biological properties of the Earth’s surface and even (based on waves of the lowest frequencies) its atmosphere. It also means that infrared radiation is sometimes used, mistakenly, as a synonym for heat. Kinetic energy Energy a body possesses when it is in motion; a classic example is a rolling billiard ball that transmits energy to another ball it strikes. Latent heat Energy absorbed by water (or any substance) as it goes through a change of state from solid to liquid or liquid to gas, and released when it goes through state changes in the opposite direction. We measure latent heat flux as part of the energy budget of a system (such as a research site). Leaf area index For a given ecosystem, the ratio of total upward-facing leaf surface area to the surface area of the land. LAI may range from 0 for bare ground to 6 for a dense forest, and is useful in predicting primary production. Lithosphere The solid outer part of the Earth composed of rock. It includes the crust and the uppermost mantle. Micrometeorology The study of small-scale, short-lived atmospheric conditions and processes, such as heat transfer and gas exchange between an ecosystem and the atmosphere, in the atmospheric boundary layer. The eddy covariance technique is the most widely used micrometeorological technique. Old-growth forest A somewhat hazy term for a forest of significant age, usually one that has not been disturbed for at least 180 years. Some definitions hinge on disturbance by humans, others on distinguishing traits of such forests: the presence of large older trees, mixed stand age and canopy height, plenty of decaying wood, and well-structured, undisturbed soil. They support species and ecological processes that younger forests do not. Ozone Oxygen in a less common, less stable form: three oxygen atoms bonded together (O3) rather than two (O2). In the stratosphere, ozone molecules are constantly formed and split apart again by the energy of ultraviolet radiation. This stratospheric ozone layer, which has recently been depleted by human-produced chlorofluorocarbons, plays a vital role in absorbing harmful UVB and UVC radiation. On the other hand, ozone acts as a pollutant and greenhouse gas when it occurs in the troposphere, where it is produced indirectly by industrial and other fossil-fuel emissions. Long-term exposure damages human health, and high levels of ozone downwind of large cities can make plants less productive. Peatland Ecosystems in which plants—typically sphagnum and other mosses—grow in waterlogged or even submerged soil and so are prevented, when they die, from decomposing fully by lack of oxygen and (often) acidic soil. This makes peatlands important carbon sinks, as their net primary productivity exceeds the carbon they release into the atmosphere (instead the carbon is stored as peat). They also help to regulate local hydrology by storing large amounts of water. Photosynthetically Active Radiation (PAR) Electromagnetic radiation of wavelengths plants can use for photosynthesis, usually corresponding with visible light. Plants have evolved to absorb radiation from this part of the spectrum very efficiently while scattering radiation with longer wavelengths, which lacks enough energy to drive photosynthesis. Potential energy Energy an object possesses that may be converted into kinetic energy. A book sitting on a table, for instance, has potential energy because of its position in the Earth's gravitational field. If it falls to the floor, the kinetic energy released will reduce its potential energy accordingly. Because energy can be neither created nor destroyed, the potential energy of the book on the table is equal to the amount of work required to move it there. Examples of other kinds of potential energy include the energy stored in—thus, the work that can be done by—a piece of coal or a charged battery. Production/productivity These terms both refer to the conversion of atmospheric (or, in the ocean, aquatic) CO2 into plant biomass through photosynthesis. Gross primary production (GPP) refers to the total amount of carbon taken in by a given ecosystem. Net primary production (NPP) refers to the net amount of carbon stored by plants (subtracting the carbon released in respiration or used to maintain existing tissue). The terms gross and net ecosystem productivity (GEP and NEP) refer to fluxes: the rate at which CO2 is converted, usually expressed as grams of carbon per square meter per year. Accurately calculating NPP and NEP is difficult, but techniques include eddy covariance and measuring new biomass in a given year and using the leaf area index of an ecosystem to predict production. Respiration A fundamental process in all living cells, whereby stored energy (in plants, in the form of sugars produced by photosynthesis) and, usually, oxygen are converted to carbon dioxide and the energy the cell needs. When plants decompose, their stored carbon is released by the respiration of the fungi and bacteria that break them down. Radiant energy The energy of any type of electromagnetic radiation. Radiation Energy transfer in the form of electromagnetic waves or particles, ranging from radio waves through visible light to gamma rays. The behavior of radiation depends on its frequency (wavelength) or position on the electromagnetic spectrum. Radiation of any frequency (not only infrared radiation) transfers energy to, and raises the temperature of, bodies that absorb it. "Radiation" as a means of heat transfer from a body to its environment means simply thermal radiation. Radiative forcing The result of any imbalance between the total amount of energy the Earth absorbs and the total amount it emits back into space. A positive forcing will warm the Earth’s surface, while a negative one will cool it. The enhanced greenhouse effect amounts to an anthropogenic radiative forcing; natural causes include variations in solar output and volcanic activity. Scattering The redirecting of electromagnetic radiation by molecules in the atmosphere, ocean, or land surface. Particles in the atmosphere scatter sunlight near the blue end of the spectrum much more than they do light near the red end, which explains the color of the sky. Sensible heat Heat gained or lost by a system that is not associated with state changes in water (as with latent heat). Sink A carbon sink is a reservoir of carbon that accumulates and stores carbon for an indefinite period of time. It is the opposite of carbon source. In natural ecosystem, plants act as carbon sinks by fixing carbon dioxide through photosynthesis. Ecosystem components can be sinks of carbon (e.g. a green leaf during daytime) or sources (e.g. the respiration of cells that constitute the same green leaf). The ecosystem is considered a net sink of carbon if the total amount of carbon fixed is larger than the total amount of carbon emitted. It is a net source when emissions are larger than sequestration. Source A carbon source is a reservoir of carbon that emits carbon to the atmosphere. It is the opposite of sink. In natural ecosystems, carbon emissions are mainly caused by the respiration of living organisms and the decomposition of dead organic matter. Ecosystem components can be sinks of carbon (e.g. a green leaf during daytime) or sources (e.g. the respiration of cells that constitute the same green leaf). The ecosystem as a whole is considered a net source of carbon if the total amount of carbon emitted is larger than the total amount of carbon fixed. It is a net sink when emissions are smaller than sequestration. Stock A carbon stock is a reservoir of carbon. It can either act as a source or a sink of carbon. Examples of carbon stocks include cells, trees, peat and coal deposits. Suspension A fluid containing solid particles that will eventually settle to the bottom of the fluid. Common examples are muddy water and aerosols in the atmosphere. Taiga Confusingly, this refers to the boreal forest in Europe and Asia, while in North America it is used for a transitional zone between boreal forest and tundra, with trees growing farther apart on swampy ground. Temperate rainforest Forests in regions with mild climate and heavy rainfall, often growing in narrow bands along western coasts. Coniferous forests, like the ones in the Pacific Northwest of North America, produce enormous trees and sustain the highest levels of biomass of any terrestrial ecosystem. Thermal energy Heat—that is, energy transferred from one body to another as a result of a difference in temperature. This transfer can take the form of conduction, convection, or radiation. Thermal radiation Heat transfer that arises only from the radiant energy of the molecules of a substance, without the need for any kind of medium (as with conduction and convection). An example is the heat you feel when you hold your hand above a hot stove without touching it. The wavelength of this radiation may fall into a wide range along the electromagnetic spectrum; as the temperature of a body increases, so does the main frequency of the radiation. This explains why a white-hot object feels hotter than a red-hot one. Total solar irradiance A measure of the amount of radiation of all wavelengths that reaches the Earth from the sun, taken from the top of the atmosphere (i.e. before it begins to be absorbed). The best estimate is about 1,368 watts per second per square meter, a figure also known as the solar constant, somewhat misleadingly as it varies seasonally and according to small changes in the sun’s output. Ultraviolet radiation Electromagnetic radiation of wavelengths just shorter than those at the violet end of the visible spectrum, amounting to about 5% of the total solar energy reaching the Earth, and classified as UVA, UVB, or UVC radiation (from longest to shortest wavelengths). UVB and UVC radiation are almost entirely blocked by the atmosphere, especially the stratospheric ozone layer, whose formation they also drive. UVB radiation that does reach Earth’s surface causes sunburn and skin cancer and can harm plant life. UVC radiation causes so much biological damage that scientists think life on land could not have evolved without the atmosphere’s protection from it. |
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