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

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

2. 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

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 aerosolmore 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. 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)

8. 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

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

11. Earth’s Energy Balance

12. 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

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 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

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

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. Warming from increasesin greenhouse gases General coolingfrom increasesin aerosols –but high uncertainty IPCC(2007)

17. 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

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