Glossary
Aerosols

Airborne particles. The term has also come to be associated, erroneously, with
the propellant used in "aerosol sprays".
Climate change (FCCC usage)
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.
Climate change (IPCC usage)
Climate change as referred to in the observational record of climate occurs
because of internal changes within the climate system or in the interaction
between its components, or because of changes in external forcing either for
natural reasons or because of human activities. It is generally not possible clearly
to make attribution between these causes. Projections of future climate change
reported by I P C C generally consider only the influence on climate of
anthropogenic increases in greenhouse gases and other human-related factors.
Climate sensitivity
In IPCC reports, climate sensitivity usually refers to the long-term (equilibrium)
change in global mean surface temperature following a doubling of atmospheric
C O j (or equivalent C O 2 ) concentration. More generally, it refers to the
equilibrium change in surface air temperature following a unit change in
radiative forcing (°CAVm~2).
Diurnal temperature range
The difference between maximum and minimum temperature over a period of 24
hours
Equilibrium climate experiment
An experiment where a step change is applied to the forcing of a climate model
and the model is then allowed to reach a new equilibrium. Such experiments
provide information on the difference between the initial and final states of the
model, but not on the time-dependent response.
Equivalent C O j
The concentration of CO^ that would cause the same amount of radiative forcing
as the given mixture of C O j and other greenhouse gases.
Evapotranspiration
The combined process of evaporation from the Earth's surface and transpiration
from vegetation.
Greenhouse gas
A gas that absorbs radiation at specific wavelengths within the spectrum of
radiation (infrared radiation) emitted by the Earth's surface and by clouds. The
gas in turn emits infrared radiation from a level where the temperature is colder
than the surface. The net effect is a local trapping of part of the absorbed energy
and a tendency to warm the planetary surface. Water vapour (HjO), carbon
dioxide (COj), nitrous oxide ( N 2 O ) , methane (CH^) and ozone ( O 3 ) are the
primary greenhouse gases in the Earth's atmosphere.
Ice-cap A dome-shaped glacier usually covering a highland near a water divide.
Ice sheet A glacier more than 50,000 km^ in area forming a continuous cover over a land
surface or resting on a continental shelf.
Radiative forcing A simple measure of the importance of a potential climate change mechanism.
Radiative forcing is the perturbation to the energy balance of the Earth-
atmosphere system (in Wm^^) following, for example, a change in the
concentration of carbon dioxide or a change in the output of the Sun; the climate
system responds to the radiative forcing so as to re-establish the energy balance.
A positive radiative forcing tends to warm the surface and a negative radiative
forcing tends to cool the surface. The radiative forcing is normally quoted as a
global and annual mean value. A more precise definition of radiative forcing, as
used in IPCC reports, is the perturbation of the energy balance of the surface-
troposphere system, after allowing for the sti'atosphere to re-adjust to a state of
global mean radiative equilibrium (see Chapter 4 of IPCC (1994)). Sometimes
called "climate forcing".
Spatial scales continental 10 - 100 million square kilometres (km^)
regional 100 thousand - 10 million km^
local less than 100 thousand km^
Soil moisture Water stored in or at the continental surface and available for evaporation. In
IPCC (1990) a single store (or "bucket") was commonly used in climate models.
Today's models which incorporate canopy and soil processes view soil moisture
as the amount held in excess of plant "wilting point".
Stratosphere The highly stratified and stable region of the atmosphere above the troposphere
{qv.) extending from about 10 km to about 50 km.
Thermohaline circulation Large scale density-driven circulation in the oceans, driven by differences in
temperature and salinity.
Transient climate experiment A n analysis of the time-dependent response of a climate model to a time-varying
change of forcing.
Troposphere 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. The troposphere is
defined as the region where temperatures generally decrease with height.