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2. The ‘Greenhouse Effect’ and the ‘Enhanced Greenhouse Effect’. SUN. Relative Distance from Sun. Mercury. Venus. Earth. Mars. 1. 0.39. 0.72. 1.5. Scales with 1 distance 2. What controls climate?. Energy from the Sun – Radiation Consider the 4 inner planets of the solar system:.

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what controls climate

SUN

Relative

Distance from Sun

Mercury

Venus

Earth

Mars

1

0.39

0.72

1.5

Scales with 1

distance2

What controls climate?
  • Energy from the Sun – Radiation
  • Consider the 4 inner planets of the solar system:

Receives

342 W m-2 solar radiation

2250 W m-2

660 W m-2

150 W m-2

planetary albedo
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.
slide4

Land has higher albedo than ocean

Clouds have high albedo

Ice and snow have high albedo

slide5

Christmas fires in Sydney 2001/2002

Smoke aerosolmore reflective than ocean

radiative equilibrium
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

¼

{

}

Rearranging: T = S(1- α)

σ

T(°C) = T(K) - 273

Temperature of the inner planets

S(1-α) = σ T4

(σ = 5.67 x 10-8 W m-2 K-4)

slide8

Temperature of the inner planets

¼

{

}

Rearranging: T = S(1- α)

σ

S(1-α) = σ T4

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
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

Solar

radiation

Terrestrial

radiation

69% absorbed at surface

Earth’s ClimateSystem

Sun

Atmosphere

Land

Ocean

Ice

Sub-surface Earth

enhanced greenhouse effect

To get same amount of net radiation, need higher surface temperatures

Terrestrial

radiation

Extract and burn fossil fuels

add CO2 to atmosphere

Enhanced greenhouse effect

More greenhouse gases, more radiation absorbed

composition of the atmosphere
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 O310-500 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

slide14

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

Aerosols

also fromhumanactivity

aerosols
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
slide16

Warming from increasesin greenhouse gases

General coolingfrom increasesin aerosols –but high uncertainty

IPCC(2007)

the enhanced greenhouse effect
The Enhanced Greenhouse Effect

S L

236 236

S L

236 232

S L

236 236

S L

236 236

Solar (S) and longwave (L) radiation in Wm-2 at the top of the atmosphere

T = -18°C

CO2 x 2

+ Feedbacks

H2O (+60%) Ice/Albedo (+20%)

Cloud?

Ocean?

CO2 x 2

CO2 x 2

TS = 15°C

TS = 15°C

DTS ~ 1.2K

DTS ~ 2.5K

summary 2 greenhouse effect
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