small liquid and solid particles, that remain suspended in the atmosphere. Their sources vary – some natural, like dust storms and volcanic activity, and some caused by human activities, like fossil fuel and biomass burning.

the fraction of solar energy reflected from the Earth back into space Albedo varies depending on factors such as cloudiness, snow, and land cover. Because of its whiteness, snow typically has a high albedo. Dull black substances like charcoal are more absorbent and therefore have a low albedo.

resulting from human activities.

a mixture of gases that surrounds the Earth. It consists of about 79.1 percent nitrogen by volume, 20.9 percent oxygen, 0.036 percent carbon dioxide, and trace amounts of other gases. In addition, the atmosphere contains water vapor in the form of clouds and aerosols.

Atmosphere-Ocean General Circulation Models (AOGCMs):
global climate models with coupled atmosphere and ocean components. They are highly complex, and require a lot of computing power to run.

portion of the Earth that supports living organisms. This can be in oceans (marine biosphere) land (the terrestrial biosphere) and in the atmosphere. It includes all ecosystems and living organisms, and also dead organic matter.

Carbon Dioxide (CO2):
a greenhouse gas that affects the Earth’s radiative balance. It occurs naturally and also from human activities such as fossil fuel burning, forest clearing, and other land use changes.

Carbon sinks:
reservoirs that take in and store more carbon than they release, thereby partially offsetting greenhouse gas emissions. Examples; Forests and oceans

Carbon cycle:
the flow of carbon (in its various forms, i.e. carbon dioxide) through the atmosphere, ocean, terrestrial biosphere, and lithosphere.

Chloroflorocarbons (CFCs):
compounds containing carbon, chlorine, flourine, which are very stable in the troposphere, but in the stratosphere are broken down by strong ultraviolet light, releasing chlorine atoms that then deplete stratospheric ozone. They act also as greenhouse gases, absorbing outgoing infrared radiation in the atmosphere. They are commonly used as refrigerants propellants, blowing agents (for producing foam), and solvents.

average weather pattern for a particular region and time period that varies from place to place. It depends on factors such as latitude, distance from the sea. And it varies over time: by season, year, decades or even longer periods of time.

Climate change:
as defined by the IPCC, is any change in climate over time, whether resulting from natural causes or from human activity. These changes typically persist for decades or longer, and may affect either the mean state of the climate or its variability. The UN Framework Convention on Climate Change (UNFCCC) draws a distinction between climate change attributable to human activities, and climate variability attributable to natural causes.

Climate models:
mathematical models of the earth’s past, present and future climate systems. They are used as a research tool to study and simulate natural climate variability, and project the climate response to human activities (i.e. human induced forcing). They are also used operationally for monthly, seasonal and multi-year climate predictions Climate models of varying complexity depict climate system components singly and in combination (coupled models).

Climate system:
is defined as an interactive system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the land surface, and the biosphere. The climate system continues to evolve over time, influenced by: its own internal dynamics, external forcings such as volcanic eruptions, solar variations, and human-induced forcings such as fossil fuel burning and land use change.

Climate variability:
refers to variations in the mean state and other statistics that occur on time and spatial scales beyond those of individual weather events. Some of this variability is “forced” from outside the climate system itself—by things like greenhouse gases or solar variability. Other variability, such as oscillations in atmospheric-oceanic circulation, is internal to the climate system.

the Earth’s surface where water is in solid form, including sea ice, lake ice, river ice, snow cover, glaciers, ice caps and ice sheets, and frozen ground. It has high reflectivity for solar radiation, low thermal conductivity and large thermal inertia which are important factors for the climate system.

Diurnal temperature range:
difference between the maximum and minimum temperatures during a day.

Dobson Unit (DU):
also known as total column ozone, DU is a measure of the total amount of ozone in a column of the atmosphere from ground level to the top of the atmosphere, based on analysis of absorbed ultraviolet light. The number of Dobson units corresponds directly with the “thickness” of the ozone layer. Measurements vary widely according to time and place, but a typical reading for a healthy polar ozone layer might be in the 300-450 Dobson unit range.

El Niño-Southern Oscillation (ENSO):
pattern of climate/weather variation, that results from coupled atmosphere-ocean interactions, and recurs at two- to seven-year intervals. The ENSO pattern is driven partly by alternating warmer and cooler temperatures of the sea surface in the eastern and central tropical Pacific Ocean, which in turn are caused by changes in upwelling currents. It affects precipitation and temperatures over a large portion of the globe, with drastic consequences to human activities like farming and fishing. The opposite of an El Niño event is called La Niña.

Eustatic sea level change:
average sea level change caused by changes in water density or in the total mass of water.

Extreme weather events:
Weather phenomena that occur infrequently, such as heat waves, heavy rainfall droughts, floods, tornadoes and hurricanes.

General Circulation Model (GCM):
a global, three dimensional computer climate system model, which can be used to simulate human-induced change. GCMs are highly complex, depicting the interactions of such factors as greenhouse gas concentrations, clouds, annual and daily solar heating, mean ocean temperatures and ice boundaries, and the reflective and absorptive properties of atmospheric water vapor.

Glacial cycles:
is a climate cycle, made up of alternating ice ages and thaws—called glacial and interglacial periods. The last glacial cycle in human experience occurred about 20,000 years ago. Glacial cycles have come at fairly regular intervals of about 100,000 years.

Global surface temperature:
average of near surface air temperature over land, and sea surface temperature. It is found by calculating sea-surface measurements, and land surface readings taken 1.5 meters above the ground.

Global Warming Potential (GWP):
the amount of global warming caused by substance, expressed as the ratio of the warming caused by one substance relative to that caused by a similar mass of carbon dioxide over a given period of time.

Greenhouse effect:
The phenomenon whereby the earth's atmosphere traps solar radiation, caused by the presence in the atmosphere of gases such as carbon dioxide, water vapor, and methane that allow incoming sunlight to pass through but prevent the outgoing radiation, (infrared) which is re-radiated from Earth, from escaping into outer space.

Greenhouse gases:
greenhouse gases in the Earth’s atmosphere include carbon dioxide, nitrous oxide, methane, ozone, and water vapor. These gases in the atmosphere absorb and then emit infrared radiation in all directions including downward to the Earth’s surface thus warming the lower atmosphere.

climate system comprising of all liquid surface and subterranean water—includes both fresh water and salt water.

these are compounds that combine carbon with either fluorine, chlorine, or bromine. These compounds can act as powerful greenhouse gases. Halocarbons containing chlorine and bromine also cause ozone depletion in the stratosphere.

Intergovernmental Panel on Climate Change (IPCC):
organization established by the United Nations Environmental Programme and the World Meteorological Organization in 1988 to assess scientific and technical information related to all significant components of the issue of climate change.

Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC):
international agreement, adopted in 1997 in Kyoto Japan, wherein signatories agreed to reduce their anthropogenic greenhouse gas emissions (CO2, CH4, N2O, HFCs, PFCs, and SF6) to at least 5 percent below 1990 levels in the commitment period 2008 to 2012. Taking effect without ratification or approval by the U.S.

Montreal Protocol on Substances that Deplete the Ozone Layer:
international agreement, adopted in Montreal in 1987. It was modified five times since then, that called for a freeze on production and use of halocarbons at 1986 levels by mid-1989, and over the next 10 years a reduction in CFC production by half. The U.S. and more than 180 other nations have ratified the agreement.

An atmospheric area about 80 kilometers between the stratosphere and the thermosphere.

North Atlantic Oscillation (NAO):
is an irregular climate fluctuation, the phases of which may span months to decades. It is the Periodic shifts in relative pressure between the southern high and the northern low.

Ozone (O3):
bluish gas that is harmful to breathe, consisting of three bound atoms of oxygen. Nearly 90 percent of Earth’s ozone is in the stratosphere, where it provides important benefits in absorbing harmful UV-B radiation, preventing most of it from reaching Earth’s surface.

Ozone hole:
observed depletion of the ozone layer over the Antarctic region that occurs yearly during the Southern Hemisphere spring. It is thought to be caused by the joint effects of human-made chlorine and bromine compounds and meteorological conditions that are specific to the region.

Ozone layer:
A region of the upper atmosphere, between about 15 and 30 kilometers (10 and 20 miles) in altitude, containing a relatively high concentration of ozone that absorbs solar ultraviolet radiation in a wavelength range.

Pacific Decadal Oscillation:
cyclic variations in sea-surface temperature in the Northern Pacific—an example of natural climate variability. These occur on decadal timescales and affect the weather in places like the U.S.Pacific Northwest region.

Proxy record:
historical record of climate-related variations obtained by examining tree rings, corals, ice cores, etc.

Radiative forcing:
change in the balance between incoming solar radiation and outgoing infrared radiation. It is caused by internal changes and external forcing, such as changes in solar output or carbon dioxide concentrations. A positive forcing warms the Earth, a negative forcing cools it.

Solar radiation:
this is energy from the sun—includes infrared radiation, ultraviolet radiation and visible radiation.

Solar “11-year” Cycle:
recurring pattern of solar output modulation, on scales of nine to 13 years.

zone of the atmosphere between the troposphere and the mesosphere, extending from about 10 km to about 50 km above Earth’s surface. Commercial airplanes routinely fly in the lower stratosphere.

Thermohaline circulation (THC):
global-scale overturning of the ocean, driven by density differences arising from temperature and salinity effects. One of the best known examples of thermohaline circulation is the Gulf Stream, a river of warmer, fresher surface water that flows to the North Atlantic, where it gives up its heat and sinks, making much of Western Europe considerably warmer than it would otherwise be.

Trace gas:
any one of the less common gases, together making up less than 1 percent of the Earth’s atmosphere. Among these are carbon dioxide, water vapor, methane, nitrous oxide, ozone, and ammonia. Though small in absolute volume, they have significant effects on the Earth’s weather and climate.

lowest part of the atmosphere, extending from the Earth’s surface to about 10 km in altitude in mid-latitude. In the troposphere, temperatures generally decrease with altitude.

Ultraviolet (UV) radiation:
Invisible rays that are part of the energy that comes from the sun. UV is commonly split into three bands: UV-A, UVB, and UV-C. UV-A is not absorbed by ozone, UV-B is mostly absorbed by ozone, and UV-C is completely absorbed by ozone and normal oxygen.

United Nations Environment Programme (UNEP):
the UN agency with a mission “to provide leadership and encourage partnership in caring for the environment by inspiring, informing, and enabling nations and peoples to improve their quality of life without compromising that of future generations.”

United Nations Framework Convention on Climate Change (UNFCC):
agreement signed at the 1992 Earth Summit in Rio de Janeiro by more than 150 countries and the European Community, with ultimate objective That the “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”

the fluctuating state of the atmosphere at a given time and place with respect to variables such as moisture, temperature, wind, precipitation, clouds, and other weather elements.

World Meteorological Organization (WMO):
Geneva based 185 member United Nations organization that provides “authoritative scientific voice on the state and behavior of the Earth’s atmosphere and climate.” Its stated purpose is to facilitate international cooperation in the establishment of networks of stations for making meteorological, hydrological, and other observations, and to promote the rapid exchange of meteorological information, the standardization of meteorological observations, and the uniform publication of observations and statistics.