What is the Greenhouse Effect?

Review of last lectureReview of last lecture

– The two basic motions of the EarthThe two basic motions of the Earth– What causes the four seasons: the What causes the four seasons: the

Earth’s tilt and the 3 ways it affects the Earth’s tilt and the 3 ways it affects the solar insolation (change of length of the solar insolation (change of length of the day, beam spreading, beam depletion)day, beam spreading, beam depletion)

– Change of the Earth’s orbit at longer Change of the Earth’s orbit at longer time scales (Milankovitch cycles): time scales (Milankovitch cycles): eccentricity, axial tilt, and precessioneccentricity, axial tilt, and precession

Satellite Measurements of the Satellite Measurements of the Earth’s Radiation BudgetEarth’s Radiation Budget

NASA’s Earth Radiation Budget Satellite (ERBS) 1985-1989

Earth’s energy budget (averaged over Earth’s energy budget (averaged over the whole globe and over a long time)the whole globe and over a long time)

• At the top of the atmosphere: At the top of the atmosphere: Incoming shortwave = Reflected Shortwave + Emitted longwaveIncoming shortwave = Reflected Shortwave + Emitted longwave• At the surface:At the surface: Incoming shortwave = Reflected shortwave + Net emitted longwave (emitted - incoming) Incoming shortwave = Reflected shortwave + Net emitted longwave (emitted - incoming) + Latent heat flux + sensible heat flux+ Latent heat flux + sensible heat flux

Sensible heat 7%

Latent heat 23%

Net Longwave 21%

Yellow: shortwave

Red: longwave

Atmospheric influences on Atmospheric influences on radiationradiation

ReflectionReflection ScatteringScattering Absorption Absorption (absorber (absorber warms)warms)

ReflectionReflection

• reflectionreflection – redirection of energy – redirection of energy w/o absorption w/o absorption

• all objects reflect visible light all objects reflect visible light effectiveness varieseffectiveness varies

• albedo – albedo – % of visible light reflected % of visible light reflected

• There are two types of reflection There are two types of reflection (solid surface):(solid surface):– SpecularSpecular: light is reflected with : light is reflected with

equal intensity (e.g. mirror)equal intensity (e.g. mirror)– Diffuse reflection Diffuse reflection OR OR

scatteringscattering: light is reflected in : light is reflected in multiple directions, weakly (e.g. multiple directions, weakly (e.g. snow)snow)

3 Types of Scattering: 3 Types of Scattering: 1.1. RaleighRaleigh

2.2. MieMie

3.3. Non-SelectiveNon-Selective

Atmospheric Atmospheric ScatteringScattering

A discussion of each type follows…A discussion of each type follows…

• involves gases smaller than insolation wavelength• scatters light in all directions • most effective at short wavelengths (violet, blue)

… hence, blue sky

Rayleigh Scattering Rayleigh Scattering

The Earth has an atmosphere. So it has Rayleigh scattering and its sky appears blue

The Moon has no atmosphere. So it has no Rayleigh scattering and its sky appears dark

• Rayleigh scattering also explains reddish-orange sunsets when light travels through thick slice of atmosphere

Monet: Impressions, SunriseMonet: Impressions, Sunrise

Grainstacks at Grainstacks at different time of the different time of the

dayday

2) Mie scattering2) Mie scattering– involves aerosols (e.g. dust, smoke) involves aerosols (e.g. dust, smoke)

larger than gas moleculeslarger than gas molecules– forward scatterforward scatter– equally effective across visible spectrumequally effective across visible spectrum– explains hazy, gray daysexplains hazy, gray days

3) Non-selective scattering3) Non-selective scattering– Happens when atmospheric particles are much Happens when atmospheric particles are much

larger than the wavelength of incoming larger than the wavelength of incoming radiation (e.g. water droplets in clouds)radiation (e.g. water droplets in clouds)

– Act like lenses; scatter all wavelengths equally Act like lenses; scatter all wavelengths equally to create a white appearanceto create a white appearance

– That’s why clouds appear whiteThat’s why clouds appear white

Video: The greenhouse effectVideo: The greenhouse effect• http://www.youtube.com/watch?v=ZzCA60WnoMkhttp://www.youtube.com/watch?v=ZzCA60WnoMk

Atmospheric Absorption - The Greenhouse EffectAtmospheric Absorption - The Greenhouse Effect

Transparent Transparent to solar to solar (shortwave) (shortwave) radiationradiation

Opaque to Opaque to earth’s earth’s (longwave) (longwave) radiationradiation

Major GH gases: CO2, H20(v), CH4

The importance of methane The importance of methane (CH(CH44))

• 23 times more powerful as a greenhouse 23 times more powerful as a greenhouse gas than COgas than CO22

• The livestock sector is a major player, which The livestock sector is a major player, which accounts for 35-40% global anthropogenic accounts for 35-40% global anthropogenic emissions of methane (their burps!)emissions of methane (their burps!)

• The livestock sector is responsible for 18% The livestock sector is responsible for 18% of total greenhouse gas emissionsof total greenhouse gas emissions

• Therefore, consuming less meat can help Therefore, consuming less meat can help reducing global warming than not driving reducing global warming than not driving cars.cars.

Seasonal variation of surface Seasonal variation of surface radiationradiation

Surface “Sensible” and Surface “Sensible” and “Latent” heat transfers“Latent” heat transfers

1.1. ConductionConduction– This is how excess heat in ground This is how excess heat in ground

is transferred to the atmosphere is transferred to the atmosphere via an extremely thin layer of air via an extremely thin layer of air in contact with the surfacein contact with the surface

2.2. ConvectionConvection– Once the heat is transferred from Once the heat is transferred from

the surface to the air via the surface to the air via conduction, convection takes over conduction, convection takes over from here via “sensible” and from here via “sensible” and “latent” heat transfers“latent” heat transfers

First, recall 2 other methods of First, recall 2 other methods of energy transfer in addition to energy transfer in addition to radiation:radiation:

Sensible HeatSensible Heat • Heat energy which is readily detectedHeat energy which is readily detected

• Magnitude is related to an object’s specific Magnitude is related to an object’s specific heatheat– The amount of energy needed to change the The amount of energy needed to change the

temperature of an object a particular amount in temperature of an object a particular amount in J/kg/KJ/kg/K

• Related to massRelated to mass– Higher mass requires more energy for heatingHigher mass requires more energy for heating

• Sensible heat transfer occurs from warmer to Sensible heat transfer occurs from warmer to cooler areas (i.e., from ground upward)cooler areas (i.e., from ground upward)

Latent HeatLatent Heat• Energy required to induce changes of Energy required to induce changes of

state in a substancestate in a substance

• In atmospheric processes, invariably In atmospheric processes, invariably involves waterinvolves water

• When water is present, latent heat of When water is present, latent heat of evaporation redirects some energy evaporation redirects some energy which would be used for sensible heatwhich would be used for sensible heat– Wet environments are cooler Wet environments are cooler

relative to their insolation amountsrelative to their insolation amounts

• Latent heat of evaporation is stored in Latent heat of evaporation is stored in water vaporwater vapor– Released as latent heat of Released as latent heat of

condensation when that change of condensation when that change of state is inducedstate is induced

• Latent heat transfer occurs from Latent heat transfer occurs from regions of wetter-to-drierregions of wetter-to-drier

Seasonal variation of surface energy Seasonal variation of surface energy budgetbudget

Storage change = net radiation - latent heat flux - sensible heat flux

SummarySummary

• Earth’s energy balance at the top of the atmosphere Earth’s energy balance at the top of the atmosphere and at the surface. What percentage of solar energy is and at the surface. What percentage of solar energy is absorbed by the surface?absorbed by the surface?

• Atmospheric influences on radiation (3 ways)Atmospheric influences on radiation (3 ways)

• The three types of atmospheric scattering. What The three types of atmospheric scattering. What causes the blue sky? Why causes the reddish-orange causes the blue sky? Why causes the reddish-orange sunsets? sunsets?

• What cause the greenhouse effect? What are the major What cause the greenhouse effect? What are the major greenhouse gases? Why is methane important? greenhouse gases? Why is methane important?

• Sensible heat flux (dry flux from warm to cold regions) Sensible heat flux (dry flux from warm to cold regions) and latent heat flux (wet flux from wet to dry regions)and latent heat flux (wet flux from wet to dry regions)