Earth’s Global Energy Balance Overview

Earth’s Global Energy Balance Overview

Earth’s Global Energy Balance Overview

• Electromagnetic Radiation– Radiation and temperature– Solar Radiation– Longwave radiation from the Earth– Global radiation balance

• Geographic Variations in Energy Flow– Insolation over the globe– Net radiation, latitude and energy balance– Sensible and latent heat transfer

• Electromagnetic Radiation– Radiation and temperature– Solar Radiation– Longwave radiation from the Earth– Global radiation balance

• Geographic Variations in Energy Flow– Insolation over the globe– Net radiation, latitude and energy balance– Sensible and latent heat transfer

• The global energy system– Solar energy losses in the atmosphere– Albedo– Counterradiation and the greenhouse effect– Global energy budgets of the atmosphere &

surface– Climate & global change

• The global energy system– Solar energy losses in the atmosphere– Albedo– Counterradiation and the greenhouse effect– Global energy budgets of the atmosphere &

surface– Climate & global change

OverviewOverview

What is light?

Light is an Electromagnetic Wave

&a Particle

Photons: “pieces” of light, each with precise wavelength, frequency, and energy.

Our eyes recognize frequency (or wavelength) as color!

Photons

• Photons – are little packets of energy.

• The energy carried by each photon depends on its frequency (color)

• Blue light carries more energy per photon than red light.

Electromagnetic SpectrumElectromagnetic Spectrum

Electromagnetic RadiationElectromagnetic Radiation

• Energy constantly emitted from every surface

• Can be in many different forms, e.g. light or heat

• Energy constantly emitted from every surface

• Can be in many different forms, e.g. light or heat

What happens when light gets absorbed?

What happens when light gets absorbed?

What causes the atmosphere to be

opaque?

Solar RadiationSolar Radiation

•Shortwave Radiation

from Sun (dark purple)

•Absorption of UV by O3

•Absorption by CO2 and

water vapor (H2O↑)

shown as valleys

•Longwave Radiation

from

Earth (dark red)

•Much absorbed by CO2

& H2O↑

ScatteringScattering• Solar radiation can be scattered by atmosphere

– Deflected off a molecule, cloud droplet, or particle– May go up toward space, or down toward Earth– Scattering most prevalent in blue wavelengths– Thus, clear, blue skies

• Some solar radiation goes directly to surface– Called transmission– Solar radiation arrives as 0.3μm to 3μm wavelengths– This is shortwave radiation

• Solar radiation can be scattered by atmosphere– Deflected off a molecule, cloud droplet, or particle– May go up toward space, or down toward Earth– Scattering most prevalent in blue wavelengths– Thus, clear, blue skies

• Some solar radiation goes directly to surface– Called transmission– Solar radiation arrives as 0.3μm to 3μm wavelengths– This is shortwave radiation

Remember you live on a rotating sphere

Geographic Variation in Solar EnergyGeographic Variation in Solar Energy• Insolation – Incoming

solar radiation– More intense where sun

angle is highest– Less intense with lower

sun angle• Same energy spread over a

larger area

• Insolation – Incoming solar radiation– More intense where sun

angle is highest– Less intense with lower

sun angle• Same energy spread over a

larger area

InsolationInsolation• Daily insolation – avg radiation total in 24 hours

– Depends on :• Sun angle – higher sun angle → greater insolation

• Length of day – higher latitudes get long summer days

• Annual insolation – avg radiation total for year– Also depends on sun angle and length of day– Both of these determined by latitude– So, latitude determines annual insolation

• Daily insolation – avg radiation total in 24 hours– Depends on :

• Sun angle – higher sun angle → greater insolation

• Length of day – higher latitudes get long summer days

• Annual insolation – avg radiation total for year– Also depends on sun angle and length of day– Both of these determined by latitude– So, latitude determines annual insolation

Net RadiationNet Radiation• Energy not usually balanced

at any location

• Net Radiation - Difference between incoming and outgoing radiation

• Between 40°N and 40°S, incoming > outgoing– Creates energy surplus

• Poleward of 40°N & S, outgoing > incoming– Creates energy deficit

• Deficit = Surplus, so net radiation for Earth = 0

• Energy not usually balanced at any location

• Net Radiation - Difference between incoming and outgoing radiation

• Between 40°N and 40°S, incoming > outgoing– Creates energy surplus

• Poleward of 40°N & S, outgoing > incoming– Creates energy deficit

• Deficit = Surplus, so net radiation for Earth = 0

Poleward Heat TransportPoleward Heat Transport• Surplus energy moves

toward poles (deficit regions)

• Carried by:• Warm, moist air• Warm sea water• Tropical cyclones

• Poleward heat transport is driving force behind:

•Global atmospheric circulation• Weather systems• Ocean currents

Why are there seasons?

• The Earth is tilted 23.5° from it orbital plane

• Combine tilt with orbit– Northern hemisphere gets

more direct Sun part of year (northern summer)

– Southern hemisphere gets more direct Sun part of year (northern winter)

• Tilt & orbit create seasons, not distance to Sun

Northern Summer

Northern Winter

Solstices & Equinoxes

Path of the Sun in the Sky

• June solstice:– Sun rises

north of east & sets north of west

– Peaks at 73.5° above horizon at noon

– 15 hours of daylight

– Highest daily insolation of year

40° North

Date Noon Sun Angle

Daylight Daily

Insolation

June Solstice 73.5° 15 hrs 460 W/m2

Dec. Solstice 26.5° 9 hrs 160 W/m2

Equinoxes 50° 12 hrs 350 W/m2

Path of the Sun in the Sky (40° North)

Date Noon Sun Angle

Daylight Daily

Insolation

June Solstice 66.5° 12 hrs ~400 W/m2

Dec. Solstice 66.5° 12 hrs ~400 W/m2

Equinoxes 90° 12 hrs 440 W/m2

Path of the Sun in the Sky (Equator)

Date Noon Sun Angle

Daylight Daily

Insolation

June Solstice 23.5° 24 hrs 500 W/m2

Dec. Solstice No Sun 0 hrs 0 W/m2

Equinoxes Horizon 12 hrs ~0 W/m2

Path of the Sun in the Sky (North Pole)

Daily Insolation through the Year• Yearly change in insolation greatest toward poles• In Arctic & Antarctic Circles, Sun is below horizon part of year• At Equator, 2 maxs & 2 mins for daily insolation

– At equinoxes & solstices

• Between tropics, also 2 maxs & 2 mins per year• Yearly insolation change important to climate

Insolation at equinox

Annual Insolation by Latitude• Tilted Earth shown as red

line– Equator greatest annual

insolation– Considerable insolation at

highest latitudes

• Untilted Earth (blue line)– Equator greatest annual

insolation– Highest latitudes little

insolation– Big changes in climate– Very cold pole– Massive poleward heat

transport

Heat Transfer: Surplus energy is transported in two forms

Heat Transfer: Surplus energy is transported in two forms

• Sensible Heat – can be felt & measured– Transferred by conduction (touching surface) – Transferred by convection (carried by rising air)– Example: Moving air masses

• Latent Heat – cannot be felt or measured– Stored as molecular motion when water changes

phase– Absorbed in evaporation, melting, and sublimation– Released in condensation, freezing, and deposition– Very important form of heat transfer over long

distances

– Example: Storm systems, hurricanes

• Sensible Heat – can be felt & measured– Transferred by conduction (touching surface) – Transferred by convection (carried by rising air)– Example: Moving air masses

• Latent Heat – cannot be felt or measured– Stored as molecular motion when water changes

phase– Absorbed in evaporation, melting, and sublimation– Released in condensation, freezing, and deposition– Very important form of heat transfer over long

distances

– Example: Storm systems, hurricanes

Conduction

Convection

Latent heat absorbedin evaporation

Solar energy losses in the atmosphere

Solar energy losses in the atmosphere

•Scattering due to:• Gas molecules• Dust or other particles

•O2, O3, & H2O↑ most important absorbers of insolation

•Global avg – 49% of insolation makes it to surface

Once at the surface what happens?

Albedo

Once at the surface what happens?

Albedo• Proportion of shortwave radiation

reflected

• Shown as a proportion (0-1)

• Examples:– Snowfield 0.45-0.85

– Black pavement 0.03

– Clouds 0.30-0.60

– Water (calm, high angle 0.02), (low angle 0.80)

• Avg for Earth and atmosphere 0.29-0.34

• Proportion of shortwave radiation reflected

• Shown as a proportion (0-1)

• Examples:– Snowfield 0.45-0.85

– Black pavement 0.03

– Clouds 0.30-0.60

– Water (calm, high angle 0.02), (low angle 0.80)

• Avg for Earth and atmosphere 0.29-0.34

So what happens to all the energy absorbed by these various processes?

So what happens to all the energy absorbed by these various processes?

• Counterradiation – heat absorbed by atmosphere reflected down to surface

• Counterradiation – heat absorbed by atmosphere reflected down to surface

A – energy radiated to

space from surface

B – energy from

surface absorbed by

atmosphere

C – energy radiated to

space from atmosphere

D – Counterradiation

Part of Counterradiation is the “Greenhouse Effect”

Part of Counterradiation is the “Greenhouse Effect”

• Longwave radiation absorbed & re-radiated to surface by atmosphere

• Lower atmosphere acts like blanket

• Longwave radiation absorbed & re-radiated to surface by atmosphere

• Lower atmosphere acts like blanket

Global Energy BudgetGlobal Energy BudgetEnergy balanced for each level: surface, atmosphere, & space

Climate & Global ChangeClimate & Global Change

• Quantifying human impacts on climate difficult• Climate and society have complex relationship

• e.g., Industrial processes• add CO2 to atmosphere (warming)

• add aerosols to atmosphere (cooling)