Greenhouse Effect .

1. The ‘Greenhouse Effect’
and the
‘Enhanced Greenhouse
Effect’

2. What controls climate?
 Energy from the Sun – Radiation
 Consider the 4 inner planets of the solar system:
SUN
Receives
342 W m-2
solar radiation
1
Relative
Distance
from Sun
0.39 0.72 1.5
2250 W m-2
660 W m-2
150 W m-2
Scales with
1
distance2
Mercury Venus Mars
Earth

3. 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.

4. Land has
higher
albedo than
ocean
Clouds have
high albedo
Ice and snow
have high
albedo

5. Christmas fires in Sydney 2001/2002
Smoke aerosol
more reflective
than ocean

6. 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)

7. Temperature of the inner planets
Relative
distance
Solar
radiation
(S) W m-2
Albedo
(α)
Net solar
radiation
S(1- α)
Equilib
-rium
T (°C)
Actual
surface
T (°C)
Mercury 0.39 2250 0.1 180
Venus 0.72 660 0.59 453
Earth 1 342 0.31 236 -19 15
Mars 1.5 150 0.15 -43
S(1-α) = σ T4
(σ = 5.67 x 10-8 W m-2 K-4)
Rearranging: T = S(1- α)
σ
{ }
¼
T(°C) = T(K) - 273

8. Temperature of the inner planets
Relative
distance
Solar
radiation
(S) W m-2
Albedo
(α)
Net solar
radiation
S(1- α)
Equilib
-rium
T (°C)
Actual
surface
T (°C)
Mercury 0.39 2250 0.1 2025 162 180
Venus 0.72 660 0.59 271 -10 453
Earth 1 342 0.31 236 -19 15
Mars 1.5 150 0.15 128 -55 -43
S(1-α) = σ T4 Rearranging: T = S(1- α)
σ
{ }
¼
T(°C) = T(K) - 273
(σ = 5.67 x 10-8 W m-2 K-4)
Just about agrees
Disagrees badly
Disagrees
Nearly agrees

9. 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.

10. Earth’s Climate
System
Sun
Ice
Ocean
Land
Sub-surface Earth
Atmosphere
Terrestrial
radiation
About 31%
reflected
into space
69% absorbed at surface
Solar
radiation

11. Earth’s Energy Balance

12. Enhanced greenhouse effect
Terrestrial
radiation
Extract and burn fossil fuels
add CO2 to atmosphere
More greenhouse
gases, more
radiation absorbed
To get same
amount of net
radiation, need
higher surface
temperatures

13. Composition of the Atmosphere
Nitrogen N2 78.084%
Oxygen O2 20.948%
Argon Ar 0.934%
 Carbon Dioxide CO2 0.036% (360 ppmv)
 Methane CH4 1.7 ppmv
Hydrogen H2 0.55 ppmv
 Nitrous Oxide N2O 0.31 ppmv
 Ozone O3 10-500 ppbv (troposphere)
0.5-10 ppmv (stratosphere)
 Water H2O 100 pptv – 4%
Greenhouse
Gases
A greenhouse gas is one that absorbs terrestrial (LW)
radiation, i.e. emitted from the Earth’s surface/atmosphere

14. 14
Aerosols
also from
human
activity
Rising levels of CO2, N2O, and CH4 as a result of human activity

15. 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

16. IPCC(2007)
Warming from
increases
in greenhouse
gases
General cooling
from increases
in aerosols –
but high uncertainty

17. The Enhanced Greenhouse Effect
Solar (S) and longwave (L) radiation in Wm-2 at the top of the atmosphere
S L
236 236
T = -18°C
S L
236 232
CO2 x 2
S L
236 236
CO2 x 2
S L
236 236
CO2 x 2
+ Feedbacks
H2O (+60%)
Ice/Albedo (+20%)
Cloud?
Ocean?
TS = 15°C TS = 15°C DTS ~ 1.2K DTS ~ 2.5K

18. 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