JEG100H1 Lecture 2: JEG100 - lec2 - Earth's Surface Energy Balance
21 Jan 2019
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March equinox: March 20-21st, subsolar point will be at the equator, circle of illumination
passing through both poles (same as September equinox)
• Daylength everywhere is 12 hours
If Earth has no tilt relative to the plane of orbit:
• Earth would have no seasons (equinox all the time), always facing same direction
towards Sun
• At a given point, similar temperature year round
• Very different biodiversity
• More ice in northern regions because no seasonal melt, northern hemisphere never
pointing towards solar radiation
• Less forest, more dessert around equator (sun directly overhead equator year long)
Earth’s Surface Energy Balance:
• Energy available to power Earth’s systems, effects of anthropogenic surface/atmosphere
changes on radiation budgets and climate
• Sun: emits shortwave, absorbed by Earth atm./surface or reflected into space
• Earth: emits longwave radiation, absorbed/reradiated/lost into space
• Incoming SW highest at northern edge of tropics/dessert (not directly at equator), no
clouds/moisture in the air over desserts
• Lower SW values at high and low latitudes (forested areas produce more
evapotranspiration, therefore, more clouds)
Fate of incoming SW radiation
• Rayleigh scattering: not absorbed, gives optical properties of atmosphere ie. smog -
aerosols in atm scattering light, depends on wavelengths (moon has no atmosphere
therefore sky is black)
• Earthshine/moonshine: Albedo (moon’s albedo: 6-8%), Earth reflects 31% albedo
therefore much brighter
• Remaining 69% interacting with surface/being absorbed (assimilation of radiation by
molecules of matter, converted into energy - LW radiation, chemical reactions ie,
photosynthesis
Clouds:
• White therefore reflective, cooling effect because less radiation reaching surface, (cloud-
albedo forcing)
• Cloud-greenhouse forcing, reradiates some LW radiation coming off of Earth’s surface
Earth-Atmosphere Energy Balance:
• 31% reflected,scattered (albedo)
• 69% absorbed (reradiated back to space - what comes in must equal what goes out)
Overall Earth Energy Imbalance:
• Long run: incoming radiation = outgoing radiation (not in reality)
• Currently imbalanced, not all incoming solar radiation being emitted
• NET R
Albedo Calculation:
• Albedo (a) = outgoing solar radiation(Wm^2)/incoming solar radiation(Wm^2)
Document Summary
March equinox: march 20-21st, subsolar point will be at the equator, circle of illumination passing through both poles (same as september equinox: daylength everywhere is 12 hours. Earth"s surface energy balance: energy available to power earth"s systems, effects of anthropogenic surface/atmosphere changes on radiation budgets and climate, sun: emits shortwave, absorbed by earth atm. /surface or reflected into space, earth: emits longwave radiation, absorbed/reradiated/lost into space. Incoming sw highest at northern edge of tropics/dessert (not directly at equator), no clouds/moisture in the air over desserts: lower sw values at high and low latitudes (forested areas produce more evapotranspiration, therefore, more clouds) Clouds: white therefore reflective, cooling effect because less radiation reaching surface, (cloud- albedo forcing, cloud-greenhouse forcing, reradiates some lw radiation coming off of earth"s surface. Earth-atmosphere energy balance: 31% reflected,scattered (albedo, 69% absorbed (reradiated back to space - what comes in must equal what goes out)
