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
Overview
• 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
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 Spectrum
Electromagnetic Radiation
• Energy constantly emitted from every surface
• Can be in many different forms, e.g. light or
heat
What happens when light gets
absorbed?
What causes the
atmosphere to be
opaque?
Solar 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↑
Scattering
• 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 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
• 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 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
Poleward 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
Path of the Sun in the Sky (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
50°
12 hrs
350 W/m2
Equinoxes
Path of the Sun in the Sky (Equator)
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
90°
12 hrs
440 W/m2
Equinoxes
Path of the Sun in the Sky (North Pole)
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
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
• Sensible Heat – can be felt & measured
Conduction
– Transferred by conduction (touching surface)
– Transferred by convection (carried by rising air)
– Example: Moving air masses
Convection
• 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
Latent heat absorbed
in evaporation
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
• 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 0.290.34
So what happens to all the energy
absorbed by these various processes?
• 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”
• Longwave radiation absorbed & re-radiated
to surface by atmosphere
• Lower atmosphere acts like blanket
Global Energy Budget
Energy balanced for each level: surface, atmosphere, & space
Climate & 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)