What controls climate? Energy from the Sun – Radiation Consider the 4 inner planets of the solar system: SUN 1 Relative Distance from Sun 0.39 0.72 1.5 Mercury Venus Earth Mars Receives 342 W m-2 solar radiation Scales with 1 distance2 2250 W m-2 660 W m-2 150 W m-2

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 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)

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 69% absorbed at surface Solar radiation Sun Terrestrial radiation Atmosphere Land Ocean Ice Sub-surface Earth

Enhanced greenhouse effect Terrestrial radiation To get same amount of net radiation, need higher surface temperatures More greenhouse gases, more radiation absorbed Extract and burn fossil fuels add CO2 to atmosphere

The Enhanced Greenhouse Effect Solar (S) and longwave (L) radiation in Wm-2 at the top of the atmosphere S L 236 236 S L 236 232 S L 236 236 S L 236 236 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…) 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