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The enhanced greenhouse effect
Greenhouse gases are a natural part of the atmo-
sphere. Without these gases the global average
temperature would be around -20ºC. The problem
we now face is that human actions – particularly
burning fossil fuels (coal, oil and natural gas) and
land clearing – are increasing their concentrations.
The more of these gases there are, the more heat
is trapped. This is known as the enhanced green-
house effect. Naturally occurring greenhouse gas-
es include water vapour, carbon dioxide, methane,
nitrous oxide, and ozone. Greenhouse gases that
are not naturally occurring include hydro-fluorocar-
bons (HFCs), perfluorocarbons (PFCs), and sulphur
hexafluoride (SF6), which are generated in a variety
of industrial processes.
On average, about one-third of the solar radiation
that hits the Earth is reflected back into space. The
land and the oceans mostly absorb the rest, with
the remainder trapped in the atmosphere. The solar
radiation that strikes the Earth’s surface heats it up,
and as a result infrared radiation is emitted.
Cooling or heating, a balancing act
THE GREENHOUSE EFFECT
10
Solar radiationpasses through
the clear atmosphere
Incoming solar radiation:
343 Watt per m
2
Solar energy is absorbed by the
Earths surface and warms it...
... and is converted into heat causing
the emission of longwave (infrared)
radiation back to the atmosphere
Surface gains more heat and
infrared radiation is emitted again
Some of the infrared radiation is
absorbed and re-emitted by the
greenhouse gas molecules. The
direct effect is the warming of the
Earthssurfaceand the troposphere.
Some solar radiation is
reflected by the atmosphere
and Earths surface
Outgoing solar radiation:
103 Watt per m
2
Some of the infrared
radiationpasses through
the atmosphere and is
lost in space
Net outgoing infrared radiation:
240 Watt per m
2
Net incoming solar radiation:
240 Watt per m
2
A T M O S P H E R E
E A R T H
G
R
E
E
N
H
O
U
S
E
G
A
S
E
S
The Greenhouse effect
168 Watt per m
2
Sources:OkanaganUniversityCollege,University ofOxford,EPA, IPCC.
UnitedNationsEnvironmentProgramme /GRID-Arendal
-2
-1
0
1
2
3
Global mean radiative forcing (Wm
-2
)
Anthropogenic and natural forcing of the climate for the year 2000, relative to 1750
N
2
O
CO
2
Greenhouse gases
Stratospheric
ozone
The height of a bar indicates a best estimate of the forcing, and the
accompanying vertical line a likely range of values. Where no bar is present
the vertical line only indicates the range in best estimates with no likelihood.
CH
4
UnitedNationsEnvironmentProgramme /GRID-Arendal
Cooling
Halocarbons
Tropospheric
ozone
Sulphate
Organic
carbon
from
fossil
fuel
burning
Black
carbon
from
fossil
fuel
burning
Biomass
burning
Mineral
Dust
Aerosol
indirect
effect
Contrails Cirrus
Aviation
Aerosols + clouds
LEVEL OF SCIENTIFIC
UNDERSTANDING
Warming
Land use
(albedo only)
Solar
Every year the sun delivers an average of 340 watts of energy to every square metre of the Earth. To
produce this amount of energy we would need 440 million large electric power plants, each gener-
ating 100 million watts of power (NASA).
It would get uncomfortably hot on Earth with all this energy, but fortunately for us, the amount of heat we receive from
the sun is balanced by heat radiated back into space by the atmosphere. Radiative forcing is the change in the balance
between radiation coming into the atmosphere and radiation going out. A positive radiative forcing tends on average to
warm the surface of the Earth, and negative forcing tends on average to cool the surface. Greenhouse gases, for example,
produce positive radiative forcing – they trap outgoing terrestrial (infrared) radiation, which causes a temperature rise at
the Earth’s surface – the “greenhouse effect”. In contrast, negative radiative forcing from clouds and aerosols, which can
reflect back into space, acts as a cooling mechanism.
Radiative forcing
The radiative forcing from the increase in
anthropogenic greenhouse gases since the
pre-industrial era is positive (warming) with a
small uncertainty range; that from the direct
effects of aerosols is negative (cooling) and
smaller; whereas the negative forcing from
the indirect effects of aerosols (on clouds and
the hydrologic cycle) might be large but is not
well quantified. Key anthropogenic and natu-
ral factors causing a change in radiative forc-
ing from year 1750 to year 2000 are shown
in this figure, where wide, colored bars mark
the factors whose radiative forcing can be
quantified. Only some of the aerosol effects
are estimated here and denoted as ranges.
Other factors besides atmospheric constitu-
ents -- solar irradiance and land-use change
-- are also shown. Stratospheric aerosols
from large volcanic eruptions have led to
important, but short-lived, negative forcings
(particularly during the periods 1880-1920
and 1960-1994), which are not important
over the time scale since the pre-industrial
era and not shown. The sum of quantified
factors in the figure is positive, but this does
not include the potentially large, negative
forcing from aerosol indirect effects.