2. The ‘Greenhouse Effect’

and the ‘Enhanced Greenhouse

Effect’

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.

Land has higher albedo than ocean

Clouds have high albedo

Ice and snow have high albedo

Christmas fires in Sydney 2001/2002

Smoke aerosolmore 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

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

Earth’s Energy Balance

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

14

Aerosolsalso fromhumanactivity

Rising levels of CO2, N2O, and CH4 as a result of human activity

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