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CLASS 16
HEAT BUDGET OF THE EARTH
Basic budget
Latitude variations
Surface temperatures
SEA ICE
Why is the Earth warm?
Why is Mars Colder? (avg. = - 60˚C)
"HEAT BUDGET"
Heat comes in.....
Heat goes out....
"HEAT BUDGET"
Where does incoming solar radiation go?
Incoming Solar Energy can be:
•Reflected
•Absorbed (changed into heat)
• Re-radiated out to space
•The balance or imbalance of these:
Controls earth surface temperatures
Drives ocean and atmosphere circulation
Reflected light
THE GLOBAL HEAT BUDGET
•Of incoming solar radiation how much is....
•Reflected back out to space?
~ 30% (24% by atmos., 6% by land and sea)
•Absorbed?
~ 70% (23% in atm.; 47% by land & sea)
•ABOUT HALF OF THE INCOMING ENERGY IS
ABSORBED BY THE LAND AND SEA
1/3 reflected;
½ absorbed by land and sea
Of the energy absorbed by land & sea (47%),
how much is.....
1) Re-radiated (infrared) directly to space?.... 10%
2) Transferred to the atmosphere? ...... 37%
MOST OF THE SOLAR RADIATION ABSORBED
BY LAND AND SEA IS TRANSFERRED BACK
TO THE ATMOSPHERE
Transfer of heat from land and sea to the
atmosphere happens in two ways:
1) Evaporation and condensation of water....
24% (i.e., 24 out of the original 100 units)
• Heat uptake by evaporation -- cools surface
• Later…heat released by condensation -- warms atm.
• WATER VAPOR VERY IMPORTANT!!!!
2) Conduction and radiation to atmos. = 13%
• Much infrared energy radiated by earth’s surface is
absorbed in the atmosphere by "greenhouse gases"
• In absence of this greenhouse effect, the earth
would be about -10˚C!
Greenhouse
effect
This is a Global Average,
but....
Solar energy coming in is
max. at Equator
min. at poles
So… at any given latitude, heat input does not
match output
low latitudes: solar input > IR output
high latitudes: solar input < IR output
•If heat input > output, then temperature
should increase constantly
•This does not happen….
•What maintains the balance?
•Heat transferred from lower latitudes to
higher latitudes by oceans and atm. currents
–Warm water/air currents: Equator --> poles
–Cool water/air currents: poles --> Equator
•Convection!!!
Heat is transferred from lower latitudes to higher
latitudes by currents in oceans and atmosphere
–Warm water/air currents: Equator --> poles
–Cool water/air currents: poles --> Equator
•These are convection currents
•They are driven by temperature differences
Warm
Cold
Water Vapor Transport => Heat Transport
Temperature Belts
Note effects of ocean currents
Temperature variation- Winter vs. Summer
•Continents -- large T range
•Oceans -- small T range
– Reason : Heat capacities, land vs. water
Oceans
stabilize
surface T of
Earth
SEA ICE (NOT Icebergs)
•Layer of ice that forms annually at high lat.
–Near Antarctica; In Arctic Ocean
•SEA ICE: Sheets of ice at high latitude
•Grow in winter, shrink in summer
•Thickness is limited...
– < 2 meters (single season)
–Ice is a thermal insulator
•Sea-ice formation creates high-S water:
–Reminder: Ice formation --> high-S water.
–This forms the densest and deepest water
masses in the oceans.
These are icebergs, not sea ice