Climate

Earth’s Radiation Balance

Solar Radiation Budget

• Life on earth is supported by energy from the sun

• Energy from the sun is not simply absorbed by earth, but also retransmitted

• Without retransmission, all water would boil away & all life would fry

• The balance between absorption and retransmission of the sun’s energy is called the radiation budget

Basic Processes Governing the Radiation Balance

• Insolation measures the amount of energy coming from the sun via short wave radiation.

• Measured in watts/sec• Top graphic is winter• Bottom graphic is

summer

Insolation

Annual Insolation

Albedo

• Degree of reflectivity of the Earth-Atmosphere system (ranges from 0-1)

• Ice and sand are highly reflective (high albedo)

• Oceans and forests don’t reflect much (low albedo)

• top-winter• bottom-summer

Annual Albedo

Short Wave Reflected Radiation

• Measures the amount of the sun’s energy that is reflected due to the albedo

• Measured in watts/m2

• Does not affect processes on earth since it is never absorbed

• Top-winter• Bottom-summer

Annual Short Wave Reflection

Short Wave Absorption

• The energy absorbed by the earth-atmosphere system

• Primary cause of earth’s surface temperature

Annual Short Wave Absorption

Surface Temperature

• Provides a measure of the temperature all over the Earth

• Temperatures measured in Kelvin

• Heat at the surface is translated into long wave radiation

Annual Surface Temperature

Radiation Spectra for Sun & Earth

• The wavelength energy emitted by a body is inversely related to the temperature of the body.

(nm) = 2.9 x 106/T

Outgoing Long Wave Radiation

• Measures the amount of energy leaving the Earth

• Results from the surface temperature

• Also called terrestrial radiation

• Measured in watts/m2

Annual Long Wave Terrestrial Emission

Net Radiation

• Difference between incoming and outgoing radiation

• Over time and space this is zero

Annual Net Radiation

Greenhouse Amount

• Amount of energy retained by Earth’s atmosphere

• This storage of energy is called the Greenhouse Effect

• Necessary for maintaining life on Earth

Greenhouse Percent

• Amount of energy stored in atmosphere

• Shows Greenhouse Effect as a fraction of the total possible energy that could be stored in the atmosphere

• Useful for detecting changes over time

• Annual graphic

Partitioning of Incoming SW Radiation• ~19 units absorbed in

atmosphere to become heat & LW radiation – 2 units by ozone

– 17 units by clouds & gases

• ~23 units scattered & then absorbed as diffused insolation

• ~28 units directly absorbed at surface

• Total absorbed = 70 units

Processes of Retransmission

• Three modes– latent heat (drives

hydrological cycle)

– sensible heat (atmosphere heated by updrafts)

– radiation of IR into space

• Total of 70 units emitted

Comparison of Radiation Budgets of the Terrestrial Planets• Compare Earth, Venus & Mars

• Earth & Venus have similar mass & dimensions; Mars 10% Earth’s mass & half as large

• Temperatures expected to be in the order:Venus > Earth > Mars

• Observed:Earth (255 K) > Venus (234 K) > Mars (217 K)

• The surface temperatures bear little resemblance to their effective temperaturesVenus (427oC) > Earth (15oC) > Mars (-53oC)

• Due to greenhouse effect• Venus

– extensive atmosphere (100x more mass than Earth’s)

– composed mainly of CO2, H2O, SO2

• Mars– atmosphere contains more CO2 Earth’s

– has 100x less water than Earth’s atmosphere