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End Also See: CDC's Climate Glossaries and Weather Tools

A - B- C - D- E- F- G- H- I- J- K- L- M- N- O - P - R- S- T-U-Z

abrupt climate change - a change in the climate (for example, temperature or precipitation) that takes place over a few decades or less, persists for at least a few decades, and causes substantial disruptions in human and natural systems.

absolute dating - the process of determining a specific, numerical age for an old material. This may be accomplished using one of a variety of methods, including radiocarbon dating.

accumulation - used in paleoclimatology to describe the deposition of sediment or other material over time. Accumulation rates are usually measured as mass/time, volume/time, or mass/length2/time.

aerosol - a suspension of solid or liquid particles in a gas, for example sulfate molecules (SO4-) suspended in the earth's atmosphere.

albedo - the reflectivity of a surface, defined mathematically as the fraction of radiation striking a surface that is reflected.

analog - something that is similar to, or may substitute for, a related object or situation.

anomaly - a deviation from the average or usual state.

anthropogenic - caused by the actions of humans.

arid - lacking moisture, especially having insufficient rainfall to support trees or woody plants.

atmosphere - the whole mass of gases surrounding the earth or other celestial bodies. The earth's atmosphere today is composed primarily of nitrogen (78%), free oxygen (21%) and greenhouse gases which can capture solar radiation: water vapor, which ranges from less than 1% in arid regions to over 3% in moist areas, carbon dioxide (0.035%) and methane (0.00018%). The composition of the Earth's atmosphere has varied in the past.

benthic foraminifera - single-celled animals that live near the sediment water interface and have calcium carbonate skeletons. The skeletons of benthic foraminifers are often preserved in ocean sediments, providing a rich fossil record of the environmental conditions of the water.

bipolar see-saw - the occurrence of warm temperatures in the Southern Hemisphere while temperatures in the Northern Hemisphere are colder-than-average, or vice versa. This pattern is thought to be caused by changes in the ocean's thermohaline circulation.

carbon dioxide - (CO2) A gas whose molecules consist of one carbon and two oxygen atoms. It is a greenhouse gas and a critical component in the global carbon cycle.

calcium carbonate (CaCO3) - A molecule consisting of calcium, carbon and oxygen that is secreted by corals, mollusks (clams, oysters, etc), foraminifera and other organisms to form a skeleton or protective shell.

calcite - the mineral name of calcium carbonate.

calving - separating or breaking so that a part becomes detached, as in icebergs separating from ice sheets.

chaotic - impossible to predict.

chronology - a general term for the age-depth relationship in ice, sediment, or another deposit. Ages are usually measured for discrete samples, and the ages of intermediate samples are interpolated.

climate - the "average weather" (or, more specifically, the mean and variability of variables such as temperature, precipitation and winds) over a time period ranging from months to thousands of years to millions of years.

climate model - computer programs based on physical laws that are used to calculate how climate has changed in the past and may change in the future.

cryosphere - the portion of the world's climate system which consists of snow and ice deposits

Dansgaard-Oeschger event - one of a series of climate changes during the last glacial period that occurred roughly every few thousand years. In Greenland ice core records, each event consisted of a rapid warming over the span of several decades and was followed by a gradual cooling. D-O events occurred in many other parts of the world, as well.

deepwater - an ocean water mass with relatively uniform temperature, salinity, and density, found near the bottom of the ocean.

density - the mass per unit volume of a substance under specified conditions of pressure and temperature.

deuterium - the stable isotope of hydrogen with one proton and one neutron. In comparison, hydrogen has one proton and no neutrons. Deuterium is abbreviated as either 2H or D.

diatom - single-celled phytoplankton that produce silica skeletons. Diatoms are one of the most abundant, widely distributed primary producers in the ocean. Different species of diatoms living in ocean and lakes have affinities for different environmental conditions such as alkalinity, available nutrients, salinity and acidity.

dolomite - a mineral composed of calcium magnesium carbonate (CaMg(CO3)2) that occurs in some sedimentary rocks.

eccentricity - the variation of an ellipse from a circle, where a circle has an eccentricity of 0. It is used to quantify the amount that the earth's revolution deviates from a circular path.

ecosystem - relationships between and among living organisms and their non-living environment.

evaporation - the process by which molecules of a liquid become a gas.

feedback - the process by which an initial change brings about additional change that either reinforces or counteracts the initial change. They may be either positive (the initial change is reinforced) or negative (the initial change is counteracted).

forcing - factors that drive or "force" a system to change. Forcings that affect our climate system include changes in solar radiation and in greenhouse gas concentrations.

freshwater - water with little or no salt content, such as in most lakes and streams, as opposed to salty ocean water.

flux - the amount that flows through a given area during a given time.

gastropod - a member of the class of mollusks that includes snails.

glacial - a time interval characterized by widespread glacial ice, see also "Ice Age".

glacier - river of ice that under pressure can deform and flow plastically.

GISP2 - Greenland Ice Sheet Project 2, the second phase of an effort by a group of scientists to produce long records of climate by drilling at the summit of the Greenland Ice Sheet from the surface to bedrock.

global warming - an increase in temperature that occurs globally such as during the transition from a glacial to an interglacial climate state, or that predicted to result from human increases in greenhouse gases.

greenhouse gases - any one of the gases found in the atmosphere (including CO2, H2O, CH4) that absorb and re-emit longwave radiation, contributing to the greenhouse effect.

greenhouse effect - the process by which the equilibrium temperature of the earth is increased due to presence of gases in the atmosphere that absorb and re-emit outgoing longwave radiation, slowing its loss to space.

half-life - the time required for a quantity to decay to half its original value.

haline - related to salt.

Heinrich Events - periods of increased discharge of icebergs from the margin of ice sheets into the North Atlantic Ocean during the last glacial. They are preserved in deep-sea cores as sediment layers rich in debris eroded from land areas.

Holocene epoch - an epoch of the Quaternary period dating from the end of the Pleistocene approximately 11,500 years ago until the present.

hydrologic cycle (or water cycle) - the combined fluxes of water occurring on, above, and below the surface of the earth.

hypothesis - a suggested explanation for a phenomena that requires further testing by scientific research. If a hypothesis is tested and supported by enough scientific studies, it becomes a theory, or commonly-accepted scientific principle.

Ice Age - period during which polar ice extends to much lower latitudes than normal and global temperatures are lowered substantially.

ice rafted detritus - continental material transported within a matrix of ice and deposited in marine sediments when the ice matrix melts.

ice sheet - a mass of glacier ice that covers a wider area than a glacier does.

ice sheet margin - edge of the ice sheet where melting and calving of icebergs occurs.

ice streams - region within an ice sheet where ice moves towards the ice sheet margin significantly faster than the surrounding ice does.

insolation - amount of solar radiation received on a given body or in a given area.

instrumental record - data measured using instruments, such as thermometers and rain gauges.

interglacial - a time interval characterized by lack of glacial ice in mid-latitudes, such as modern Holocene climate.

Intertropical Convergence Zone - a band of low pressure circling the Earth near the equator, where the trade winds of the Northern and Southern Hemispheres converge and warm moist air ascends to produce abundant precipitation.

ion - atoms that have either a positive or negative charge.

isotope - any of the different forms of an element having the same number of protons, but different numbers of neutrons.

jökulhlaup - a flood of meltwater from a glacial lake.

Laurentide ice sheet - the ice sheet that grew over a large part of North America during the last glacial period.

lithic - made of rock.

loess - a wind-deposited sediment consisting mostly of silt, the silt commonly derived from finely ground rock washed out of continental glaciers.

mechanism - the means by which an effect or change is produced.

methane (CH4) - An odorless, colorless, flammable gas that acts as a greenhouse gas in the atmosphere and is produced by a variety of natural sources.

mineral - a substance that occurs naturally due to geological or biological processes and that has a specific chemical composition and properties.

monsoon - a climate pattern with a pronounced rainy season and a pronounced dry season that are accompanied by changes in wind patterns.

negative feedback - the process by which an initial change brings about an additional change in the opposite direction. Negative feedbacks dampen variability and add stability to a system.

non-linear - a system that is not describable as the sum of its parts and, as a result, is difficult to predict. One example of non-linear behavior is a small perturbation to a system causing a large response.

obliquity - the tilt of the earth's axis of rotation, currently 23.4 degrees.

orbital cycles - variations in the Earth's orbit around the sun with periods of 23,000 (precession of the equinoxes), 41,000 (obliquity), and 100,000 years (eccentricity).

ostracod - a small crustacean that lives in either the ocean or in freshwater.

oxygen isotope ratio (δ18O) - an expression for the ratio of the two isotopes 18O to 16O in a sample relative to a standard, defined as: δ18O= (18O/16Osample - 18O/16Ostandard)/ 18O/16Ostandard

paleoclimate - past or ancient climates.

paleoclimatologist - one who studies ancient (paleo-) climate.

paleoenvironmental proxy - an environmental relict (pollen grains, tree rings, lake sediments, pack rat middens, ice cores, coral skeletons) that is used by scientists to infer past environmental conditions.

particulate - material made up of small pieces.

photosynthesis - the process in green plants and certain other organisms by which carbohydrates are synthesized from carbon dioxide and water using light as an energy source. Carbon dioxide (CO2) and water (H2O) are combined in the presence of sunlight and the green pigment chlorophyll, to produce food (C6 H12 O6 ) and oxygen (O2 ).

plankton - Small or microscopic organisms, including algae and protozoan, that float or drift in great numbers in fresh or salt water, especially at or near the surface, and serve as food for fish and other larger organisms.

planktonic foraminifera - Marine zooplankton that passively float or weakly swim, and have calcium carbonate skeletons that are present in large numbers on the surface of the ocean. The skeletons of planktonic foraminifers are preserved in large numbers in deep-sea sediments, providing a rich fossil record of the environmental conditions of the upper ocean. The size of the shells is typically from 50 to 100 microns.

Pleistocene epoch - An interval of the Quaternary period, from 1.8 million to about 11,500 years before present.

pollen - grains produced by seed plants.

positive feedback - the process in which an initial change will bring about an additional change in the same direction. With positive feedbacks, a small initial perturbation can yield a large change.

precession (of the equinoxes) - the slow clockwise motion of the equinoxes along the path of the earth's orbit due to the wobble of the earth's axis.

precipitation - 1. rain, snow, sleet and other forms of water falling from the sky. 2. the condensation of a solid from a solution during a chemical reaction.

proxy - a substitute. Paleoclimatologists use proxy evidence in place of direct climate measurements for times before instrumental measurements were made. Properties of ocean sediments, glacial ice, and tree rings can all be used as proxies for past climate conditions.

Quaternary period - the second period of the Cenozoic era containing the Pleistocene epoch and the Holocene epoch and lasting from 1.8 million years to the present.

radiocarbon (14C) - a radioactive isotope of carbon. Since it decays with a half-life of 5570 years, its concentration in organic materials relative to the stable (non-radioactive) isotope 12C can be used to determine the age of samples up to about 50,000 years old.

radiocarbon dating - the process of providing an absolute age for a material by using measurements of the amount of radiocarbon it still contains.

relative dating - the process of determining the order in which certain events occurred or certain materials were formed. This is accomplished, for example, by using stratigraphic position or a property such as magnetic polarity that varies through time.

Sahara - a large desert in northern Africa.

Sahel - a large savanna in northern Africa.

savanna - a semi-arid region of grasses and scattered trees.

sediment - any particulate that can be transported by fluid flow. Sediment may be composed of, for example, rock fragments, sand, shells or plants.

solar radiation - electromagnetic energy emitted by the sun. In additional to visible light, this includes ultraviolet and infrared radiation.

solar variability - changes in the amount of radiation emitted by the sun due to the sun's internal dynamics.

speleothem - a mineral deposit formed in caves. Two types of speleothems are stalagmites and stalactites.

stable isotope - different species of an element that have the same atomic number, but different atomic mass, that are not affected by radioactive decay.

stable state - a steady condition in which the recently observed behavior is expected to continue unchanged in the future.

steppe - a semiarid grassland without trees.

Sverdrup - a unit of measure of volume transport equal to 106 m3/s, named in honor of the oceanographer Harald Sverdrup.

terrigenous - derived from land areas. For example, wind-blown dust from desert areas can settle into oceans and form terrigenous sediment on the sea floor.

teleconnection - linkage between weather or climate changes occurring in widely separated parts of the globe. There is a teleconnection, for example, between tropical Pacific sea surface temperature changes (El Niño) and temperature in the southeastern United States.

tephra - material produced by a volcanic eruption that falls from the air to the land or ocean.

thermohaline circulation (THC) - density-driven circulation system in the world's oceans. In the Atlantic, warm water flows northward in the surface ocean, loses heat to the overlying colder atmosphere, and becomes more dense. This water sinks in the North Atlantic, forming deepwater. Eventually, the deepwater is returned to the surface over broad areas around the globe and joins near-surface currents to be returned to the areas of deepwater formation. This circuit is sometimes referred to as the "ocean conveyor belt."

threshold - the point a system must cross to enter a new state or condition.

trade winds - a system of low-level winds occurring in the tropics; the trade winds blow from the northeast to the equator in the Northern Hemisphere and from the southeast to the equator in the Southern Hemisphere.

transpiration - the evaporation of water from plants.

tree rings - a pattern observed in tree trunks, in which one band is usually equivalent to one year's growth.

upwelling - rising of cold, nutrient-rich water towards the surface.

varve - annual layer of sediment deposited in an ocean or lake.

Vostok - site in Antarctica (78°S, 106°E) where a 400,000 year long ice core was collected.

visual stratigraphy - process of identifying different layers in ice, rock, sediment, or other deposit, based on visually apparent distinctions such as color or other properties. The identified stratigraphic layers are usually assigned different ages or age intervals.

weather - the state of the atmosphere at a particular time, as defined by variables such as temperature, precipitation and winds.

Younger Dryas - interval of abrupt climate changes approximately 12,700-11,500 years ago. The name is derived from the Dryas plant, a cold-loving plant that pollen records indicate was widespread in Europe during this period.

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