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 Receives 342 W m -2 solar radiation 1 Relative Distance from Sun W m W m W m -2 Scales with 1 distance 2 MercuryVenusMarsEarth

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 aerosol more 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 T 4 ” (T in Kelvin) Balance incoming and outgoing radiation: Net incoming radiation=Outgoing radiation S(1-α)= σ T 4 (σ is the Stefan-Boltzmann constant = 5.67 x 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 Venus Earth Mars S(1-α) = σ T 4 (σ = 5.67 x 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 Venus Earth Mars S(1-α) = σ T 4 Rearranging: T = S(1- α) σ {} ¼ T(°C) = T(K) (σ = 5.67 x 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 Climate System Sun IceOceanLand Sub-surface Earth Atmosphere Terrestrial radiation About 31% reflected into space 69% absorbed at surface Solar radiation

Earth’s Energy Balance

Enhanced greenhouse effect Terrestrial radiation Extract and burn fossil fuels add CO 2 to atmosphere More greenhouse gases, more radiation absorbed To get same amount of net radiation, need higher surface temperatures

Composition of the Atmosphere NitrogenN % Oxygen O % ArgonAr0.934% Carbon DioxideCO % (360 ppmv) MethaneCH ppmv HydrogenH ppmv Nitrous OxideN 2 O0.31 ppmv OzoneO ppbv (troposphere) ppmv (stratosphere) WaterH 2 O100 pptv – 4% Greenhouse Gases A greenhouse gas is one that absorbs terrestrial (LW) radiation, i.e. emitted from the Earth’s surface/atmosphere

14 Aerosols also from human activity Rising levels of CO 2, N 2 O, and CH 4 as a result of human activity

Aerosols Clumps of molecules – typically of order 1 micron (1 μm = m) in diameter, e.g., ‘sulphate aerosol’, formed when SO 2 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., CO 2 ). 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., SO 2 ), are removing the cooling influence of aerosols, i.e. adding to the warming from GHGs

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

The Enhanced Greenhouse Effect Solar (S) and longwave (L) radiation in Wm  2 at the top of the atmosphere SL236 T =  18°C SL CO 2 x 2 SL236 CO 2 x 2 SL236 CO 2 x 2 + Feedbacks H 2 O (+60%) Ice/Albedo (+20%) Cloud? Ocean? T S = 15°C  T S ~ 1.2K  T S ~ 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