Appendix I - Glossary
Editor: A.P.M. Baede
A indicates that the following
term is also contained in this Glossary.
See: Lifetime; see also: Response
A collection of airborne solid or liquid particles, with a typical size between
0.01 and 10 µm and residing in the atmosphere for at least several hours.
Aerosols may be of either natural or anthropogenic origin. Aerosols may influence
climate in two ways: directly through scattering and absorbing radiation, and
indirectly through acting as condensation nuclei for cloud formation or modifying
the optical properties and lifetime of clouds. See: Indirect
The term has also come to be associated, erroneously, with the propellant used
in “aerosol sprays”.
Planting of new forests on lands that historically have not contained forests.
For a discussion of the term forest
and related terms such as afforestation, reforestation,
and deforestation: see the
IPCC Report on Land Use, Land-Use Change and Forestry (IPCC, 2000).
The fraction of solar radiation reflected by a surface or object, often expressed
as a percentage. Snow covered surfaces have a high albedo; the albedo of soils
ranges from high to low; vegetation covered surfaces and oceans have a low albedo.
The Earth’s albedo varies mainly through varying cloudiness, snow, ice, leaf
area and land cover changes.
A technique for the measurement of the elevation of the sea, land or ice surface.
For example, the height of the sea surface (with respect to the centre of the
Earth or, more conventionally, with respect to a standard “ellipsoid of revolution”)
can be measured from space by current state-of-the-art radar altimetry with
centrimetric precision. Altimetry has the advantage of being a measurement relative
to a geocentric reference frame, rather than relative to land level as for a
tide gauge, and of affording
Resulting from or produced by human beings.
The gaseous envelope surrounding the Earth. The dry atmosphere consists almost
entirely of nitrogen (78.1% volume mixing ratio) and oxygen (20.9% volume mixing
ratio), together with a number of trace gases, such as argon (0.93% volume mixing
ratio), helium, and radiatively active greenhouse
gases such as carbon dioxide
(0.035% volume mixing ratio), and ozone. In addition the atmosphere contains
water vapour, whose amount is highly variable but typically 1% volume mixing
ratio. The atmosphere also contains clouds and aerosols.
See: Detection and attribution.
Respiration by photosynthetic
The total mass of living organisms in a given area or volume; recently dead
plant material is often included as dead biomass.
Biosphere (terrestrial and marine)
The part of the Earth system comprising all ecosystems
and living organisms, in the atmosphere, on land (terrestrial biosphere) or
in the oceans (marine biosphere), including derived dead organic matter, such
as litter, soil organic matter and oceanic detritus.
Operationally defined species based on measurement of light absorption and chemical
reactivity and/or thermal stability; consists of soot, charcoal, and/or possible
light-absorbing refractory organic matter. (Source: Charlson and Heintzenberg,
1995, p. 401.)
The total mass of a gaseous substance of concern in the atmosphere.
Aerosol consisting predominantly of organic substances and various forms of
black carbon. (Source: Charlson
and Heintzenberg, 1995, p. 401.)
The term used to describe the flow of carbon (in various forms, e.g. as carbon
dioxide) through the atmosphere, ocean, terrestrial biosphere
Carbon dioxide (CO2)
A naturally occurring gas, also a by-product of burning fossil fuels and biomass,
as well as land-use changes
and other industrial processes. It is the principal anthropogenic greenhouse
gas that affects the earth’s radiative balance. It is the reference gas against
which other greenhouse gases are measured and therefore has a Global
Warming Potential of 1.
Carbon dioxide (CO2) fertilisation
The enhancement of the growth of plants as a result of increased atmospheric
CO2 concentration. Depending on their mechanism of photosynthesis,
certain types of plants are more sensitive to changes in atmospheric CO2
concentratioin. In particular, C3
plants generally show a larger response to CO2 than C4
Material resulting from charring of biomass, usually retaining some of the microscopic
texture typical of plant tissues; chemically it consists mainly of carbon with
a disturbed graphitic structure, with lesser amounts of oxygen and hydrogen.
See: Black carbon; Soot particles.
(Source: Charlson and Heintzenberg, 1995, p. 402.)
Climate in a narrow sense is usually defined as the “average weather”, or more
rigorously, as the statistical description in terms of the mean and variability
of relevant quantities over a period of time ranging from months to thousands
or millions of years. The classical period is 30 years, as defined by the World
Meteorological Organization (WMO). These quantities are most often surface variables
such as temperature, precipitation, and wind. Climate in a wider sense is the
state, including a statistical description, of the climate
Climate change refers to a statistically significant variation in either the
mean state of the climate or in its variability, persisting for an extended
period (typically decades or longer). Climate change may be due to natural internal
processes or external forcings, or to persistent anthropogenic changes in the
composition of the atmosphere or in land use.
Note that the Framework Convention
on Climate Change (UNFCCC), in its Article 1, defines “climate change” as: “a
change of climate which is attributed directly or indirectly to human activity
that alters the composition of the global atmosphere and which is in addition
to natural climate variability observed over comparable time periods”. The UNFCCC
thus makes a distinction between “climate change” attributable to human activities
altering the atmospheric composition, and “climate variability” attributable
to natural causes.
See also: Climate variability.
An interaction mechanism between processes in the climate
system is called a climate feedback, when the result of an initial process triggers
changes in a second process that in turn influences the initial one. A positive
feedback intensifies the original process, and a negative feedback reduces it.
Climate model (hierarchy)
A numerical representation of the climate
system based on the physical, chemical and biological properties of its components,
their interactions and feedback processes, and accounting for all or some of
its known properties. The climate system can be represented by models of varying
complexity, i.e. for any one component or combination of components a hierarchy
of models can be identified, differing in such aspects as the number of spatial
dimensions, the extent to which physical, chemical or biological processes are
explicitly represented, or the level at which empirical parametrizations
are involved. Coupled atmosphere/ocean/sea-ice General Circulation Models (AOGCMs)
provide a comprehensive representation of the climate system. There is an evolution
towards more complex models with active chemistry and biology.
Climate models are applied, as a research tool, to study and simulate the climate,
but also for operational purposes, including monthly, seasonal and interannual
A climate prediction or climate forecast is the result of an attempt to produce
a most likely description or estimate of the actual evolution of the climate
in the future, e.g. at seasonal, interannual or long-term time scales. See also:
Climate projection and Climate
A projection of the response
of the climate system to emission
or concentration scenarios of greenhouse gases and aerosols, or radiative
forcing scenarios, often based upon simulations by climate
models. Climate projections are distinguished from climate
predictions in order to emphasise that climate projections depend upon the emission/concentration/
radiative forcing scenario used, which are based on assumptions, concerning,
e.g., future socio-economic and technological developments, that may or may
not be realised, and are therefore subject to substantial uncertainty.
A plausible and often simplified representation of the future climate, based
on an internally consistent set of climatological relationships, that has been
constructed for explicit use in investigating the potential consequences of
anthropogenic climate change,
often serving as input to impact models. Climate
projections often serve as the raw material for constructing climate scenarios,
but climate scenarios usually require additional information such as about the
observed current climate. A climate change scenario is the difference between
a climate scenario and the current climate.
In IPCC Reports, equilibrium climate sensitivity refers to the equilibrium change
in global mean surface temperature following a doubling of the atmospheric (equivalent)
CO2 concentration. More generally, equilibrium climate sensitivity
refers to the equilibrium change in surface air temperature following a unit
change in radiative forcing
(°C/Wm-2). In practice, the evaluation of the equilibrium climate
sensitivity requires very long simulations with Coupled General Circulation
Models (Climate model).
The effective climate sensitivity is a related measure that circumvents this
requirement. It is evaluated from model output for evolving non-equilibrium
conditions. It is a measure of the strengths of the feedbacks
at a particular time and may vary with forcing history and climate state. Details
are discussed in Section 9.2.1 of Chapter
9 in this Report.
The climate system is the highly complex system consisting of five major components:
the atmosphere, the hydrosphere,
the cryosphere, the land surface
and the biosphere, and the
interactions between them. The climate system evolves in time under the influence
of its own internal dynamics and because of external forcings such as volcanic
eruptions, solar variations and human-induced forcings such as the changing
composition of the atmosphere and land-use
Climate variability refers to variations in the mean state and other statistics
(such as standard deviations, the occurrence of extremes, etc.) of the climate
on all temporal and spatial scales beyond that of individual weather events.
Variability may be due to natural internal processes within the climate system
(internal variability), or to variations in natural or anthropogenic external
forcing (external variability). See also: Climate
Cloud condensation nuclei
Airborne particles that serve as an initial site for the condensation of liquid
water and which can lead to the formation of cloud droplets. See also: Aerosols.
See Carbon dioxide (CO2)
Cooling degree days
The integral over a day of the temperature above 18°C (e.g. a day with an
average temperature of 20°C counts as 2 cooling degree days). See also:
Heating degree days.
The component of the climate
system consisting of all snow, ice and permafrost on and beneath the surface
of the earth and ocean. See: Glacier;
Plants that produce a three-carbon compound during photo-synthesis; including
most trees and agricultural crops such as rice, wheat, soyabeans, potatoes and
Plants that produce a four-carbon compound during photo-synthesis; mainly of
tropical origin, including grasses and the agriculturally important crops maize,
sugar cane, millet and sorghum.
Conversion of forest to non-forest. For a discussion of the term forest
and related terms such as afforestation,
reforestation, and deforestation:
see the IPCC Report on Land Use, Land-Use Change and Forestry (IPCC, 2000).
Land degradation in arid, semi-arid, and dry sub-humid areas resulting from
various factors, including climatic variations and human activities. Further,
the UNCCD (The United Nations Convention to Combat Desertification) defines
land degradation as a reduction or loss, in arid, semi-arid, and dry sub-humid
areas, of the biological or economic productivity and complexity of rain-fed
cropland, irrigated cropland, or range, pasture, forest, and woodlands resulting
from land uses or from a process or combination of processes, including processes
arising from human activities and habitation patterns, such as: (i) soil erosion
caused by wind and/or water; (ii) deterioration of the physical, chemical and
biological or economic properties of soil; and (iii) long-term loss of natural
Detection and attribution
Climate varies continually on all time scales. Detection of climate
change is the process of demonstrating that climate has changed in some defined
statistical sense, without providing a reason for that change. Attribution of
causes of climate change is the process of establishing the most likely causes
for the detected change with some defined level of confidence.
Diurnal temperature range
The difference between the maximum and minimum temperature during a day.
Dobson Unit (DU)
A unit to measure the total amount of ozone in a vertical column above the Earth’s
surface. The number of Dobson Units is the thickness in units of 10-5
m, that the ozone column would occupy if compressed into a layer of uniform
density at a pressure of 1013 hPa, and a temperature of 0°C.
One DU corresponds to a column of ozone containing 2.69 x1020 molecules
per square meter. A typical value for the amount of ozone in a column of the
Earth’s atmosphere, although very variable, is 300 DU.
A system of interacting living organisms together with their physical environment.
The boundaries of what could be called an ecosystem are somewhat arbitrary,
depending on the focus of interest or study. Thus the extent of an ecosystem
may range from very small spatial scales to, ultimately, the entire Earth.
El Niño-Southern Oscillation (ENSO)
El Niño, in its original sense, is a warm water current which periodically
flows along the coast of Ecuador and Peru, disrupting the local fishery. This
oceanic event is associated with a fluctuation of the intertropical surface
pressure pattern and circulation in the Indian and Pacific oceans, called the
Southern Oscillation. This coupled atmosphere-ocean phenomenon is collectively
known as El Niño-Southern Oscillation, or ENSO. During an El Niño
event, the prevailing trade winds weaken and the equatorial countercurrent strengthens,
causing warm surface waters in the Indonesian area to flow eastward to overlie
the cold waters of the Peru current. This event has great impact on the wind,
sea surface temperature and precipitation patterns in the tropical Pacific.
It has climatic effects throughout the Pacific region and in many other parts
of the world. The opposite of an El Niño event is called La Niña.
A plausible representation of the future development of emissions of substances
that are potentially radiatively active (e.g. greenhouse
gases, aerosols), based on
a coherent and internally consistent set of assumptions about driving forces
(such as demographic and socio-economic development, technological change) and
their key relationships.
Concentration scenarios, derived from emission scenarios, are used as input
into a climate model to compute climate
In IPCC (1992) a set of emission scenarios was presented which were used as
a basis for the climate projections
in IPCC (1996). These emission scenarios are referred to as the IS92 scenarios.
In the IPCC Special Report on Emission Scenarios (Nakic´enovic´
et al., 2000) new emission scenarios, the so called SRES
scenarios, were published some of which were used, among others, as a basis
for the climate projections presented in Chapter 9 of
this Report. For the meaning of some terms related to these scenarios, see SRES
Averaged over the globe and over longer time periods, the energy budget of the
climate system must be in
balance. Because the climate system derives all its energy from the Sun, this
balance implies that, globally, the amount of incoming solar
radiation must on average be equal to the sum of the outgoing reflected solar
radiation and the outgoing infrared
radiation emitted by the climate system. A perturbation of this global radiation
balance, be it human induced or natural, is called radiative
Equilibrium and transient climate experiment
An equilibrium climate experiment is an experiment in which a climate
model is allowed to fully adjust to a change in radiative
forcing. Such experiments provide information on the difference between the
initial and final states of the model, but not on the time-dependent response.
If the forcing is allowed to evolve gradually according to a prescribed emission
scenario, the time dependent response of a climate model may be analysed. Such
experiment is called a transient climate experiment. See: Climate
Equivalent CO2 (carbon dioxide)
The concentration of CO2
that would cause the same amount of radiative
forcing as a given mixture of CO2 and other greenhouse
Eustatic sea-level change
A change in global average sea level brought about by an alteration to the volume
of the world ocean. This may be caused by changes in water density or in the
total mass of water. In discussions of changes on geological time-scales, this
term sometimes also includes changes in global average sea level caused by an
alteration to the shape of the ocean basins. In this Report the term is not
used with that sense.
The combined process of evaporation from the Earth’s surface and transpiration
See: Climate system.
Extreme weather event
An extreme weather event is an event that is rare within its statistical reference
distribution at a particular place. Definitions of “rare” vary, but an extreme
weather event would normally be as rare as or rarer than the 10th or 90th percentile.
By definition, the characteristics of what is called extreme weather may vary
from place to place.
An extreme climate event is an average of a number of weather events over a
certain period of time, an average which is itself extreme (e.g. rainfall over
Bright patches on the Sun. The area covered by faculae is greater during periods
of high solar activity.
See: Climate feedback.
To avoid the problem of coupled atmosphere-ocean general circulation models
drifting into some unrealistic climate state, adjustment terms can be applied
to the atmosphere-ocean fluxes of heat and moisture (and sometimes the surface
stresses resulting from the effect of the wind on the ocean surface) before
these fluxes are imposed on the model ocean and atmosphere. Because these adjustments
are precomputed and therefore independent of the coupled model integration,
they are uncorrelated to the anomalies which develop during the integration.
In Chapter 8 of this Report it is concluded that present
models have a reduced need for flux adjustment.
A vegetation type dominated by trees. Many definitions of the term forest are
in use throughout the world, reflecting wide differences in bio-geophysical
conditions, social structure, and economics. For a discussion of the term forest
and related terms such as afforestation,
reforestation, and deforestation:
see the IPCC Report on Land Use, Land-Use Change and Forestry (IPCC, 2000).
Fossil CO2 (carbon dioxide) emissions
Emissions of CO2 resulting from the combustion of fuels from fossil
carbon deposits such as oil, gas and coal.
Framework Convention on Climate Change
See: United Nations Framework
Convention on Climate Change (UNFCCC).
The large scale motions of the atmosphere and the ocean as a consequence of
differential heating on a rotating Earth, aiming to restore the energy
balance of the system through transport of heat and momentum.
General Circulation Model (GCM)
See: Climate model.
The surface which an ocean of uniform density would assume if it were in steady
state and at rest (i.e. no ocean circulation and no applied forces other than
the gravity of the Earth). This implies that the geoid will be a surface of
constant gravitational potential, which can serve as a reference surface to
which all surfaces (e.g., the Mean Sea Surface) can be referred. The geoid (and
surfaces parallel to the geoid) are what we refer to in common experience as
A mass of land ice flowing downhill (by internal deformation and sliding at
the base) and constrained by the surrounding topography e.g. the sides of a
valley or surrounding peaks; the bedrock topography is the major influence on
the dynamics and surface slope of a glacier. A glacier is maintained by accumulation
of snow at high altitudes, balanced by melting at low altitudes or discharge
into the sea.
Global surface temperature
The global surface temperature is the area-weighted global average of (i) the
sea-surface temperature over the oceans (i.e. the subsurface bulk temperature
in the first few meters of the ocean), and (ii) the surface-air temperature
over land at 1.5 m above the ground.
Global Warming Potential (GWP)
An index, describing the radiative characteristics of well mixed greenhouse
gases, that represents the combined effect of the differing times these gases
remain in the atmosphere and their relative effectiveness in absorbing outgoing
infrared radiation. This index
approximates the time-integrated warming effect of a unit mass of a given greenhouse
gas in today’s atmosphere, relative to that of carbon
Greenhouse gases effectively
absorb infrared radiation,
emitted by the Earth’s surface, by the atmosphere itself due to the same gases,
and by clouds. Atmospheric radiation is emitted to all sides, including downward
to the Earth’s surface. Thus greenhouse gases trap heat within the surface-troposphere
system. This is called the natural greenhouse effect.
Atmospheric radiation is strongly coupled to the temperature of the level at
which it is emitted. In the troposphere
the temperature generally decreases with height. Effectively, infrared radiation
emitted to space originates from an altitude with a temperature of, on average,
-19°C, in balance with the net incoming solar radiation, whereas the Earth’s
surface is kept at a much higher temperature of, on average, +14°C.
An increase in the concentration of greenhouse gases leads to an increased infrared
opacity of the atmosphere, and therefore to an effective radiation into space
from a higher altitude at a lower temperature. This causes a radiative
forcing, an imbalance that can only be compensated for by an increase of the
temperature of the surface-troposphere system. This is the enhanced greenhouse
Greenhouse gases are those gaseous constituents of the atmosphere, both natural
and anthropogenic, that absorb and emit radiation at specific wavelengths within
the spectrum of infrared radiation emitted by the Earth’s surface, the atmosphere
and clouds. This property causes the greenhouse
effect. Water vapour (H2O), carbon dioxide (CO2), nitrous
oxide (N2O), methane (CH4) and ozone (O3) are
the primary greenhouse gases in the Earth’s atmosphere. Moreover there are a
number of entirely human-made greenhouse gases in the atmosphere, such as the
halocarbons and other chlorine
and bromine containing substances, dealt with under the Montreal
Protocol. Beside CO2, N2O and CH4, the Kyoto
Protocol deals with the greenhouse gases sulphur hexafluoride (SF6),
hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs).
Gross Primary Production (GPP)
The amount of carbon fixed from the atmosphere through photosynthesis.
The junction between ice sheet
and ice shelf or the place
where the ice starts to float.
Compounds containing either chlorine, bromine or fluorine and carbon. Such compounds
can act as powerful greenhouse
gases in the atmosphere. The chlorine and bromine containing halocarbons are
also involved in the depletion of the ozone
Heating degree days
The integral over a day of the temperature below 18°C (e.g. a day with an
average temperature of 16°C counts as 2 heating degree days). See also:
Cooling degree days.
The conversion of organic matter to CO2 by organisms other than plants.
The component of the climate system comprising liquid surface and subterranean
water, such as: oceans, seas, rivers, fresh water lakes, underground water etc.
A dome shaped ice mass covering a highland area that is considerably smaller
in extent than anice sheet.
A mass of land ice which is sufficiently deep to cover most of the underlying
bedrock topography, so that its shape is mainly determined by its internal dynamics
(the flow of the ice as it deforms internally and slides at its base). An ice
sheet flows outwards from a high central plateau with a small average surface
slope. The margins slope steeply, and the ice is discharged through fast-flowing
ice streams or outlet glaciers, in some cases into the sea or into ice-shelves
floating on the sea. There are only two large ice sheets in the modern world,
on Greenland and Antarctica, the Antarctic ice sheet being divided into East
and West by the Transantarctic Mountains; during glacial periods there were
A floating ice sheet of considerable
thickness attached to a coast (usually of great horizontal extent with a level
or gently undulating surface); often a seaward extension of ice sheets.
Indirect aerosol effect
Aerosols may lead to an indirect
radiative forcing of the climate
system through acting as condensation nuclei or modifying the optical properties
and lifetime of clouds. Two indirect effects are distinguished:
First indirect effect
A radiative forcing induced by an increase in anthropogenic aerosols which cause
an initial increase in droplet concentration and a decrease in droplet size
for fixed liquid water content, leading to an increase of cloud albedo.
This effect is also known as the Twomey effect. This is sometimes referred to
as the cloud albedo effect. However this is highly misleading since the second
indirect effect also alters cloud albedo.
Second indirect effect
A radiative forcing induced by an increase in anthropogenic aerosols which cause
a decrease in droplet size, reducing the precipitation efficiency, thereby modifying
the liquid water content, cloud thickness, and cloud life time. This effect
is also known as the cloud life time effect or Albrecht effect.
A period of rapid industrial growth with far-reaching social and economic consequences,
beginning in England during the second half of the eighteenth century and spreading
to Europe and later to other countries including the United States. The invention
of the steam engine was an important trigger of this development. The industrial
revolution marks the beginning of a strong increase in the use of fossil fuels
and emission of, in particular, fossil carbon dioxide. In this Report the terms
pre-industrial and industrial refer, somewhat arbitrarily, to the periods before
and after 1750, respectively.
Radiation emitted by the earth’s surface, the atmosphere and the clouds. It
is also known as terrestrial or long-wave radiation. Infrared radiation has
a distinctive range of wavelengths (“spectrum”) longer than the wavelength of
the red colour in the visible part of the spectrum. The spectrum of infrared
radiation is practically distinct from that of solar
or short-wave radiation because of the difference in temperature between the
Sun and the Earth-atmosphere system.
A method of analysis that combines results and models from the physical, biological,
economic and social sciences, and the interactions between these components,
in a consistent framework, to evaluate the status and the consequences of environmental
change and the policy responses to it.
See: Climate variability.
A mathematical procedure by which the input to a model is estimated from the
observed outcome, rather than vice versa. It is, for instance, used to estimate
the location and strength of sources and sinks of CO2 from measurements
of the distribution of the CO2 concentration in the atmosphere, given
models of the global carbon
cycle and for computing atmospheric transport.
Isostatic land movements
Isostasy refers to the way in which the lithosphere
and mantle respond to changes in surface loads. When the loading of the lithosphere
is changed by alterations in land ice mass, ocean mass, sedimentation, erosion
or mountain building, vertical isostatic adjustment results, in order to balance
the new load.
The Kyoto Protocol to the United Nations Framework
Convention on Climate Change (UNFCCC) was adopted at the Third Session of the
Conference of the Parties (COP) to the United Nations Framework
Convention on Climate Change, in 1997 in Kyoto, Japan. It contains legally binding
commitments, in addition to those included in the UNFCCC. Countries included
in Annex B of the Protocol (most OECD countries and countries with economies
in transition) agreed to reduce their anthropogenic greenhouse
gas emissions (CO2, CH4, N2O, HFCs, PFCs, and
SF6) by at least 5% below 1990 levels in the commitment period 2008
to 2012. The Kyoto Protocol has not yet entered into force (April 2001).
The total of arrangements, activities and inputs undertaken in a certain land
cover type (a set of human actions). The social and economic purposes for which
land is managed (e.g., grazing, timber extraction, and conservation).
A change in the use or management of land by humans, which may lead to a change
in land cover. Land cover and land-use change may have an impact on the albedo,
and sinks of greenhouse
gases, or other properties of the climate
system and may thus have an impact on climate, locally or globally. See also:
the IPCC Report on Land Use, Land-Use Change, and Forestry (IPCC, 2000).
See: El Niño-Southern
Lifetime is a general term used for various time-scales characterising the rate
of processes affecting the concentration of trace gases. The following lifetimes
may be distinguished:
Turnover time (T) is the ratio of the mass M of a reservoir (e.g., a gaseous
compound in the atmosphere) and the total rate of removal S from the reservoir:
T = M/S. For each removal process separate turnover times can be defined. In
soil carbon biology this is referred to as Mean Residence Time (MRT).
Adjustment time or response time (Ta) is the time-scale characterising the decay
of an instantaneous pulse input into the reservoir. The term adjustment time
is also used to characterise the adjustment of the mass of a reservoir following
a step change in the source strength. Half-life or decay constant is used to
quantify a first-order exponential decay process. See: Response
time, for a different definition pertinent to climate variations. The term lifetime
is sometimes used, for simplicity, as a surrogate for adjustment time.
In simple cases, where the global removal of the compound is directly proportional
to the total mass of the reservoir, the adjustment time equals the turnover
time: T = Ta. An example is CFC-11 which is removed from the atmosphere only
by photochemical processes in the stratosphere. In more complicated cases, where
several reservoirs are involved or where the removal is not proportional to
the total mass, the equality T = Ta no longer holds. Carbon
dioxide (CO2) is an extreme example. Its turnover time is only about
4 years because of the rapid exchange between atmosphere and the ocean and terrestrial
biota. However, a large part of that CO2 is returned to the atmosphere
within a few years. Thus, the adjustment time of CO2 in the atmosphere
is actually determined by the rate of removal of carbon from the surface layer
of the oceans into its deeper layers. Although an approximate value of 100 years
may be given for the adjustment time of CO2 in the atmosphere, the
actual adjustment is faster initially and slower later on. In the case of methane
(CH4) the adjustment time is different from the turnover time, because
the removal is mainly through a chemical reaction with the hydroxyl radical
OH, the concentration of which itself depends on the CH4 concentration.
Therefore the CH4 removal S is not proportional to its total mass
The upper layer of the solid Earth, both continental and oceanic, which comprises
all crustal rocks and the cold, mainly elastic, part of the uppermost mantle.
Volcanic activity, although part of the lithosphere, is not considered as part
of the climate system, but
acts as an external forcing factor. See: Isostatic
LOSU (Level of Scientific Understanding)
This is an index on a 4-step scale (High, Medium, Low and Very Low) designed
to characterise the degree of scientific understanding of the radiative forcing
agents that affect climate change. For each agent, the index represents a subjective
judgement about the reliability of the estimate of its forcing, involving such
factors as the assumptions necessary to evaluate
the forcing, the degree of knowledge of the physical/ chemical mechanisms determining
the forcing and the uncertainties surrounding the quantitative estimate.
Mean Sea Level
See: Relative Sea Level.
A human intervention to reduce the sources
or enhance the sinks of greenhouse
See: Mole fraction.
See: Climate model.
Mole fraction, or mixing ratio, is the ratio of the number of moles of a constituent
in a given volume to the total number of moles of all constituents in that volume.
It is usually reported for dry air. Typical values for long-lived greenhouse
gases are in the order of mmol/mol (parts per million: ppm), nmol/mol (parts
per billion: ppb), and fmol/mol (parts per trillion: ppt). Mole fraction differs
from volume mixing ratio, often expressed in ppmv etc., by the corrections for
non-ideality of gases. This correction is significant relative to measurement
precision for many greenhouse gases. (Source: Schwartz and Warneck, 1995).
The Montreal Protocol on Substances that Deplete the Ozone Layer was adopted
in Montreal in 1987, and subsequently adjusted and amended in London (1990),
Copenhagen (1992), Vienna (1995), Montreal (1997) and Beijing (1999). It controls
the consumption and production of chlorine- and bromine-containing chemicals
that destroy stratospheric ozone, such as CFCs, methyl chloroform, carbon tetrachloride,
and many others.
Net Biome Production (NBP)
Net gain or loss of carbon from a region. NBP is equal to the Net
Ecosystem Production minus the carbon lost due to a disturbance, e.g. a forest
fire or a forest harvest.
Net Ecosystem Production (NEP)
Net gain or loss of carbon from an ecosystem.
NEP is equal to the Net Primary
Production minus the carbon lost through heterotrophic
Net Primary Production (NPP)
The increase in plant biomass
or carbon of a unit of a landscape. NPP is equal to the Gross
Primary Production minus carbon lost through autotrophic
Enhancement of plant growth through the addition of nitrogen compounds. In IPCC
Reports, this typically refers to fertilisation from anthropogenic sources of
nitrogen such as human-made fertilisers and nitrogen oxides released from burning
A process is called “non-linear” when there is no simple proportional relation
between cause and effect. The climate
system contains many such non-linear processes, resulting in a system with a
potentially very complex behaviour. Such complexity may lead to rapid
North Atlantic Oscillation (NAO)
The North Atlantic Oscillation consists of opposing variations of barometric
pressure near Iceland and near the Azores. On average, a westerly current, between
the Icelandic low pressure area and the Azores high pressure area, carries cyclones
with their associated frontal systems towards Europe. However, the pressure
difference between Iceland and the Azores fluctuates on time-scales of days
to decades, and can be reversed at times.
Aerosol particles consisting
predominantly of organic compounds, mainly C, H, O, and lesser amounts of other
elements. (Source: Charlson and Heintzenberg, 1995, p. 405.) See: Carbonaceous
Ozone, the triatomic form of oxygen (O3), is a gaseous atmospheric
constituent. In the troposphere
it is created both naturally and by photochemical reactions involving gases
resulting from human activities (“smog”). Tropospheric ozone acts as a greenhouse
gas. In the stratosphere it
is created by the interaction between solar ultraviolet radiation and molecular
oxygen (O2). Stratospheric ozone plays a decisive role in the stratospheric
radiative balance. Its concentration is highest in the ozone
See: Ozone layer.
The stratosphere contains
a layer in which the concentration of ozone is greatest, the so called ozone
layer. The layer extends from about 12 to 40 km. The ozone concentration reaches
a maximum between about 20 and 25 km. This layer is being depleted by human
emissions of chlorine and bromine compounds. Every year, during the Southern
Hemisphere spring, a very strong depletion of the ozone layer takes place over
the Antarctic region, also caused by human-made chlorine and bromine compounds
in combination with the specific meteorological conditions of that region. This
phenomenon is called the ozone hole.
In climate models, this term
refers to the technique of representing processes, that cannot be explicitly
resolved at the spatial or temporal resolution of the model (sub-grid scale
processes), by relationships between the area or time averaged effect of such
sub-grid scale processes and the larger scale flow.
Patterns of climate variability
Natural variability of the climate
system, in particular on seasonal and longer time-scales, predominantly occurs
in preferred spatial patterns, through the dynamical non-linear characteristics
of the atmospheric circulation and through interactions with the land and ocean
surfaces. Such spatial patterns are also called “regimes” or “modes”. Examples
are the North Atlantic Oscillation
(NAO), the Pacific-North American pattern (PNA), the El
Niño-Southern Oscillation (ENSO), and the Antarctic Oscillation (AO).
The process by which plants take CO2 from the air (or bicarbonate
in water) to build carbohydrates, releasing O2 in the process. There
are several pathways of photosynthesis with different responses to atmospheric
CO2 concentrations. See: Carbon
The vertical movement of the continents and sea floor following the disappearance
and shrinking of ice sheets,
e.g. since the Last Glacial Maximum (21 ky BP). The rebound is an isostatic
Ppm, ppb, ppt
See: Mole fraction.
Atmospheric compounds which themselves are not greenhouse
gases or aerosols, but which
have an effect on greenhouse gas or aerosol concentrations by taking part in
physical or chemical processes regulating their production or destruction rates.
See: Industrial revolution.
A projection is a potential future evolution of a quantity or set of quantities,
often computed with the aid of a model. Projections are distinguished from predictions
in order to emphasise that projections involve assumptions concerning, e.g.,
future socio-economic and technological developments that may or may not be
realised, and are therefore subject to substantial uncertainty. See also Climate
projection; Climate prediction.
A proxy climate indicator is a local record that is interpreted, using physical
and biophysical principles, to represent some combination of climate-related
variations back in time. Climate related data derived in this way are referred
to as proxy data. Examples of proxies are: tree ring records, characteristics
of corals, and various data derived from ice cores.
Radiative forcing is the change in the net vertical irradiance (expressed in
Watts per square metre: Wm-2) at the tropopause
due to an internal change or a change in the external forcing of the climate
system, such as, for example, a change in the concentration of carbon
dioxide or the output of the Sun. Usually radiative forcing is computed after
allowing for stratospheric temperatures to readjust to radiative equilibrium,
but with all tropospheric properties held fixed at their unperturbed values.
Radiative forcing is called instantaneous if no change in stratospheric temperature
is accounted for. Practical problems with this definition, in particular with
respect to radiative forcing associated with changes, by aerosols, of the precipitation
formation by clouds, are discussed in Chapter 6 of this
Radiative forcing scenario
A plausible representation of the future development of radiative
forcing associated, for example, with changes in atmospheric composition or
land-use change, or with external factors such as variations in solar
activity. Radiative forcing scenarios can be used as input into simplified climate
models to compute climate
The surface and bedrock, and hence the thickness, of a glacier can be mapped
by radar; signals penetrating the ice are reflected at the lower boundary with
rock (or water, for a floating glacier tongue).
Rapid climate change
The non-linearity of the climate
system may lead to rapid climate change, sometimes called abrupt events or even
surprises. Some such abrupt events may be imaginable, such as a dramatic reorganisation
of the thermohaline circulation,
rapid deglaciation, or massive melting of permafrost leading to fast changes
in the carbon cycle. Others
may be truly unexpected, as a consequence of a strong, rapidly changing, forcing
of a non-linear system.
Planting of forests on lands that have previously contained forests but that
have been converted to some other use. For a discussion of the term forest
and related terms such as afforestation,
reforestation, and deforestation:
see the IPCC Report on Land Use, Land-Use Change and Forestry (IPCC, 2000).
Preferred patterns of climate
Relative Sea Level
Sea level measured by a tide
gauge with respect to the land upon which it is situated. Mean Sea Level (MSL)
is normally defined as the average Relative Sea Level over a period, such as
a month or a year, long enough to average out transients such as waves.
(Relative) Sea Level Secular Change
Long term changes in relative sea level caused by either eustatic
changes, e.g. brought about by thermal
expansion, or changes in vertical land movements.
A component of the climate
system, other than the atmosphere, which has the capacity to store, accumulate
or release a substance of concern, e.g. carbon, a greenhouse
gas or a precursor. Oceans,
soils, and forests are examples
of reservoirs of carbon. Pool is an equivalent term (note that the definition
of pool often includes the atmosphere). The absolute quantity of substance of
concerns, held within a reservoir at a specified time, is called the stock.
The process whereby living organisms convert organic matter to CO2,
releasing energy and consuming O2.
The response time or adjustment time is the time needed for the climate
system or its components to re-equilibrate to a new state, following a forcing
resultinsg from external and internal processes or feedbacks.
It is very different for various components of the climate system. The response
time of the troposphere is
relatively short, from days to weeks, whereas the stratosphere
comes into equilibrium on a time-scale of typically a few months. Due to their
large heat capacity, the oceans have a much longer response time, typically
decades, but up to centuries or millennia. The response time of the strongly
coupled surface-troposphere system is, therefore, slow compared to that of the
stratosphere, and mainly determined by the oceans. The biosphere
may respond fast, e.g. to droughts, but also very slowly to imposed changes.
See: Lifetime, for a different
definition of response time pertinent to the rate of processes affecting the
concentration of trace gases.
A plausible and often simplified description of how the future may develop,
based on a coherent and internally consistent set of assumptions about driving
forces and key relationships. Scenarios may be derived from projections,
but are often based on additional information from other sources, sometimes
combined with a “narrative storyline”. See also: SRES
scenarios; Climate scenario;
Sea level rise
See: Relative Sea Level Secular
Change; Thermal expansion.
Significant wave height
The average height of the highest one-third of all sea waves occurring in a
particular time period. This serves as an indicator of the characteristic size
of the highest waves.
Any process, activity or mechanism which removes a greenhouse
gas, an aerosol or a precursor
of a greenhouse gas or aerosol from the atmosphere.
Water stored in or at the land surface and available for evaporation.
The Sun exhibits periods of high activity observed in numbers of sunspots,
as well as radiative output, magnetic activity, and emission of high energy
particles. These variations take place on a range of time-scales from millions
of years to minutes. See: Solar
Solar (“11 year”) cycle
A quasi-regular modulation of solar
activity with varying amplitude and a period of between 9 and 13 years.
Radiation emitted by the Sun. It is also referred to as short-wave radiation.
Solar radiation has a distinctive range of wavelengths (spectrum) determined
by the temperature of the Sun. See also: Infrared
Particles formed during the quenching of gases at the outer edge of flames of
organic vapours, consisting predominantly of carbon, with lesser amounts of
oxygen and hydrogen present as carboxyl and phenolic groups and exhibiting an
imperfect graphitic structure. See: Black
carbon; Charcoal. (Source: Charlson and Heintzenberg, 1995, p. 406.)
Any process, activity or mechanism which releases a greenhouse gas, an aerosol
or a precursor of a greenhouse gas or aerosol into the atmosphere.
Spatial and temporal scales
Climate may vary on a large range of spatial and temporal scales. Spatial scales
may range from local (less than 100,000 km2), through regional (100,000
to 10 million km2) to continental (10 to 100 million km2).
Temporal scales may range from seasonal to geological (up to hundreds of millions
SRES scenarios are emission
scenarios developed by Nakic´enovic´ et al. (2000) and used, among
others, as a basis for the climate projections in Chapter
9 of this Report. The following terms are relevant for a better understanding
of the structure and use of the set of SRES scenarios:
Scenarios that have a similar demographic, societal, economic and technical-change
storyline. Four scenario families comprise the SRES scenario set: A1, A2, B1
Scenarios within a family that reflect a consistent variation of the storyline.
The A1 scenario family includes four groups designated as A1T, A1C, A1G and
A1B that explore alternative structures of future energy systems. In the Summary
for Policymakers of Nakic´enovic´ et al. (2000), the A1C and A1G
groups have been combined into one Fossil Intensive’ A1FI scenario group.
The other three scenario families consist of one group each. The SRES scenario
set reflected in the Summary for Policymakers of Nakic´enovic´ et
al. (2000) thus consist of six distinct scenario groups, all of which are equally
sound and together capture the range of uncertainties associated with driving
forces and emissions.
A scenario that is illustrative for each of the six scenario groups reflected
in the Summary for Policymakers of Nakic´enovic´ et al. (2000).
They include four revised scenario markers’ for the scenario groups A1B,
A2, B1, B2, and two additional scenarios for the A1FI and A1T groups. All scenario
groups are equally sound.
A scenario that was originally posted in draft form on the SRES website to represent
a given scenario family. The choice of markers was based on which of the initial
quantifications best reflected the storyline, and the features of specific models.
Markers are no more likely than other scenarios, but are considered by the SRES
writing team as illustrative of a particular storyline. They are included in
revised form in Nakic´enovic´ et al. (2000). These scenarios have
received the closest scrutiny of the entire writing team and via the SRES open
process. Scenarios have also been selected to illustrate the other two scenario
groups (see also Scenario Group’ and Illustrative Scenario’).
A narrative description of a scenario (or family of scenarios) highlighting
the main scenario characteristics, relationships between key driving forces
and the dynamics of their evolution.
The temporary increase, at a particular locality, in the height of the sea due
to extreme meteorological conditions (low atmospheric pressure and/or strong
winds). The storm surge is defined as being the excess above the level expected
from the tidal variation alone at that time and place.
The highly stratified region of the atmosphere above the troposphere
extending from about 10 km (ranging from 9 km in high latitudes to 16 km in
the tropics on average) to about 50 km.
Small dark areas on the Sun. The number of sunspots is higher during periods
of high solar activity, and
varies in particular with the solar
In connection with sea level, this refers to the increase in volume (and decrease
in density) that results from warming water. A warming of the ocean leads to
an expansion of the ocean volume and hence an increase in sea level.
Large-scale density-driven circulation in the ocean, caused by differences in
temperature and salinity. In the North Atlantic the thermohaline circulation
consists of warm surface water flowing northward and cold deep water flowing
southward, resulting in a net poleward transport of heat. The surface water
sinks in highly restricted sinking regions located in high latitudes.
A device at a coastal location (and some deep sea locations) which continuously
measures the level of the sea with respect to the adjacent land. Time-averaging
of the sea level so recorded gives the observed Relative
Sea Level Secular Changes.
Transient climate response
The globally averaged surface air temperature increase, averaged over a 20 year
period, centred at the time of CO2 doubling, i.e., at year 70 in
a 1% per year compound CO2 increase experiment with a global coupled
The boundary between the troposphere
and the stratosphere.
The lowest part of the atmosphere from the surface to about 10 km in altitude
in mid-latitudes (ranging from 9 km in high latitudes to 16 km in the tropics
on average) where clouds and “weather” phenomena occur. In the troposphere temperatures
generally decrease with height.
An expression of the degree to which a value (e.g. the future state of the climate
system) is unknown. Uncertainty can result from lack of information or from
disagreement about what is known or even knowable. It may have many types of
sources, from quantifiable errors in the data to ambiguously defined concepts
or terminology, or uncertain projections of human behaviour. Uncertainty can
therefore be represented by quantitative measures (e.g. a range of values calculated
by various models) or by qualitative statements (e.g., reflecting the judgement
of a team of experts). See Moss and Schneider (2000).
United Nations Framework Convention on Climate Change (UNFCC)
The Convention was adopted on 9 May 1992 in New York and signed at the 1992
Earth Summit in Rio de Janeiro by more than 150 countries and the European Community.
Its ultimate objective is the “stabilisation of greenhouse gas concentrations
in the atmosphere at a level that would prevent dangerous anthropogenic interference
with the climate system”. It contains commitments for all Parties. Under the
Convention, Parties included in Annex I aim to return greenhouse gas emissions
not controlled by the Montreal Protocol to 1990 levels by the year 2000. The
convention entered into force in March 1994. See: Kyoto
The addition of a substance of concern to a reservoir.
The uptake of carbon containing substances, in particular carbon dioxide, is
often called (carbon) sequestration.
Volume mixing ratio
See: Mole fraction.
Charlson, R. J., and J. Heintzenberg (Eds.): Aerosol Forcing of Climate,
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