2. The ‘Greenhouse Effect’
and the
‘Enhanced Greenhouse
Effect’
What controls climate?

SUN

Energy from the Sun – Radiation
Consider the 4 inner planets of the solar system:
Relative
Distance
from Sun
Scales with
1
distance2
Mercury
0.39
2250 W m-2
Venus
0.72
660 W m-2
Earth
1
Mars
1.5
Receives
150 W m-2
342 W m-2
solar radiation
Planetary Albedo

A fraction of the incoming solar radiation (S) is
reflected back into space, the rest is absorbed
by the planet. Each planet has a different
reflectivity, or albedo (α):






Earth
α = 0.31 (31% reflected, 69% absorbed)
Mars
α = 0.15
Venus α = 0.59
Mercury α = 0.1
Net incoming solar radiation = S(1 - α)
One possible way of changing Earth’s climate
is by changing its albedo.
Land has
higher
albedo than
ocean
Clouds have
high albedo
Ice and snow
have high
albedo
Christmas fires in Sydney 2001/2002
Smoke aerosol
more reflective
than ocean
Radiative Equilibrium


Each planet must balance net incoming solar
radiation with outgoing radiation, determined by its
temperature.
Stefan-Boltzmann Law:


“A body at temperature T radiates energy at a rate
proportional to T4 ” (T in Kelvin)
Balance incoming and outgoing radiation:
Net incoming radiation=Outgoing radiation
S(1-α) = σ T4
(σ is the Stefan-Boltzmann constant = 5.67 x 10-8 W m-2 K-4)
Temperature of the inner planets
S(1-α) = σ
Rearranging: T =
T4
(σ = 5.67 x 10-8 W m-2 K-4)
{
S(1- α)
σ
¼
}
Net solar
radiation
S(1- α)
T(°C) = T(K) - 273
Relative
distance
Solar
radiation
(S) W m-2
Albedo
(α)
Mercury
0.39
2250
0.1
180
Venus
0.72
660
0.59
453
Earth
1
342
0.31
Mars
1.5
150
0.15
236
Equilib
-rium
T (°C)
-19
Actual
surface
T (°C)
15
-43
Temperature of the inner planets
S(1-α) = σ
Rearranging: T =
T4
(σ = 5.67 x 10-8 W m-2 K-4)
Mercury
{
S(1- α)
σ
¼
}
T(°C) = T(K) - 273
Relative
distance
Solar
radiation
(S) W m-2
Albedo
(α)
Net solar
radiation
S(1- α)
Equilib
-rium
T (°C)
Actual
surface
T (°C)
0.39
2250
0.1
2025
162
180
Just about agrees
Venus
0.72
660
0.59
271
-10
453
Disagrees badly
Earth
1
342
0.31
236
-19
15
Disagrees
Mars
1.5
150
0.15
128
-55
-43
Nearly agrees
The ‘Greenhouse Effect’
Radiative equilibrium works for Mercury (no atmosphere) and just
about for Mars (thin atmosphere)

The disagreement for Venus and the Earth is because these two
planets have atmospheres containing certain gases which modify
their surface temperatures.

This is the ‘Greenhouse Effect’ in action:
Earth’s surface is 34°C warmer than if there were no atmosphere
Venus has a ‘runaway’ Greenhouse effect, and is over 400°C
warmer
Mars atmosphere slightly warms its surface, by about 10°C
• The existence of the Greenhouse Effect is universally accepted (it
is not controversial), and it links the composition of a planet’s
atmosphere to its surface temperature.

Earth’s Climate
System
About 31%
reflected
into space
Terrestrial
radiation
Sun
Solar
radiation
Atmosphere
69% absorbed at surface
Land
Ocean
Sub-surface Earth
Ice
Earth’s Energy Balance
Enhanced greenhouse effect
Terrestrial
radiation
More greenhouse
gases, more
radiation absorbed
Extract and burn fossil fuels
add CO2 to atmosphere
To get same
amount of net
radiation, need
higher surface
temperatures
Composition of the Atmosphere
N2
O2
Ar
CO2
CH4
H2
N2O
O3
78.084%
20.948%
0.934%
0.036% (360 ppmv)
1.7 ppmv
0.55 ppmv
0.31 ppmv

Nitrogen
Oxygen
Argon
Carbon Dioxide
Methane
Hydrogen
Nitrous Oxide
Ozone

Water
H2O
100 pptv – 4%



Greenhouse
Gases
10-500 ppbv (troposphere)
0.5-10 ppmv (stratosphere)
A greenhouse gas is one that absorbs terrestrial (LW)
radiation, i.e. emitted from the Earth’s surface/atmosphere
Rising levels of CO2, N2O, and CH4 as a result of human activity
Aerosols
also from
human
activity
14
Aerosols







Clumps of molecules – typically of order 1 micron (1 μm = 10-6 m) in
diameter, e.g., ‘sulphate aerosol’, formed when SO2 is oxidised.
Main effect is to reflect incoming solar radiation – effectively increasing
albedo (e.g. Sydney fires image earlier)
Haze in the atmosphere is due to aerosols – most aerosols are directly
linked to air pollution (but also natural sources, e.g. volcanoes)
Generally have a cooling influence on climate – they act to offset the
warming from greenhouse gases
Aerosols have short residence times in the atmosphere (days). This
means they are not well-mixed through the atmosphere (unlike, e.g.,
CO2). So aerosols are mainly found close to their sources (e.g., over
industrialised countries).
Aerosol impact on climate is much more uncertain than the effect of
greenhouse gases
Measures to reduce air pollution (e.g., SO2), are removing the cooling
influence of aerosols, i.e. adding to the warming from GHGs
Warming from
increases
in greenhouse
gases
General cooling
from increases
in aerosols –
but high uncertainty
IPCC(2007)
The Enhanced Greenhouse Effect
Solar (S) and longwave (L) radiation in Wm-2 at the top of the atmosphere
S
236
L
236
S
236
L
232
S
236
L
236
T = -18°C
CO2 x 2
CO2 x 2
S
236
L
236
CO2 x 2
+ Feedbacks
H2O (+60%)
Ice/Albedo (+20%)
Cloud?
Ocean?
TS = 15°C
TS = 15°C
DTS ~ 1.2K
DTS ~ 2.5K
Summary 2 (Greenhouse Effect…)







Radiation from the Sun drives our climate
Our distance from the Sun, and the reflectivity of the Earth
determines how much radiation is absorbed
Earth’s atmosphere traps outgoing radiation (the Greenhouse
Effect), warming the surface by about 34°C
On Venus, a runaway Greenhouse Effect warms its surface by
over 400°C; Mars thin atmosphere warms its surface by about
10°C
So there is good evidence from the other planets that the
atmospheric composition is important in determining the surface
temperature
Global Warming is often called ‘The Greenhouse Effect’ – really it
is the Enhanced Greenhouse Effect – the addition of more
Greenhouse Gases (mainly from burning fossil fuels) to the
atmosphere enhances the existing effect.
Humans have also changed the Earth’s albedo – mainly by adding
aerosols to the atmosphere – these tend to cool climate, offsetting
the GHG warming