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What controls climate?
Energy from the Sun – Radiation Consider the 4 inner planets of the solar system:SUN
Receives342 W m-2
solar radiation
1
RelativeDistance from Sun
0.39 0.72 1.5
2250 W m-2 660 W m-2 150 W m-2Scales with
1distance2
Mercury Venus MarsEarth
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.
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
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
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
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 ClimateSystem
Sun
IceOceanLand
Sub-surface Earth
Atmosphere
Terrestrial radiation
About 31%reflected into space
69% absorbed at surface
Solarradiation
Enhanced greenhouse effect
Terrestrial radiation
Extract and burn fossil fuelsadd CO2 to atmosphere
More greenhouse gases, more
radiation absorbed
To get same amount of net radiation, need higher surface temperatures
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%
GreenhouseGases
A greenhouse gas is one that absorbs terrestrial (LW)radiation, i.e. emitted from the Earth’s surface/atmosphere
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
IPCC(2007)
Warming from increasesin greenhouse gases
General coolingfrom increasesin aerosols –but high uncertainty
The Enhanced Greenhouse EffectSolar (S) and longwave (L) radiation in Wm2 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 TS ~ 1.2K TS ~ 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