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Lecture 16: HEAT BUDGET OF THE EARTH

Basic budget
Latitude variations
Surface temperatures
Sea ice

Powerpoint Lecture Slides


"HEAT BUDGET"?? -- fate of solar radiation
reflection, absorbtion, re-radiation
Importance
surface temperatures
drives ocean and atm. circulation

GLOBAL HEAT BUDGET

Let global average solar radiation = 100%
Reflection = 35% [31% by atm.; 4% by land & sea]
Absorbtion = 65% [17.5% in atm.; 47.5% by land & sea]

Fate of energy absorbed by land & sea
Re-radiated (infrared) directly to space = 5.5%
Transfered to atm. = 42%
Evap. and condensation of water vapor = 29.5%
heat required for evaporation -- cools surface
heat released in condensation -- warms atm.
Conduction and radiation = 12.5%
Absorbtion of infrared by "greenhouse gases" warms atm. further (in absence of greenhouse effect, Earth would be -10°C!)

Fate of energy absorbed by atm. = 59.5% re-radiated
Direct absorbtion = 17.5%
Transfered from surface = 42%

Latitude variations in heat budget
Intensity of solar rad. decreases with latitude
max. at Equator
min. at poles
Global heat budget must be balanced:
solar rad. = reflection + re-radiation
But at any given latitude, input output
low latitudes: input > output
high latitudes: input < output
A runaway thermal catastrophe!?.................

......NO!
Heat is transfered by circulation of oceans and atmosphere
Warm water and air currents: Equator --> poles
Cool water and air currents: poles --> Equator

The latitude imbalance in solar-energy gains and losses is the driving mechanism for the circulation of the oceans and atmosphere.

SURFACE TEMPERATURES
Latitude "belts" of ~ constant T --
Offset in oceans by surface currents
Seasonal temp. variations:
Continents -- large T range
Oceans -- small T range
Reason for this difference?
Contrast in heat capacities of land and water. Oceans "stabilize" surface T of Earth.

SEA ICE
Sea ice versus icebergs
Extent of sea-ice coverage
Thickness of sea-ice
Energy must be extracted for freezing
But ice is a thermal insulator
Max. thickness (in a single season) -- ~ 2 meters
Effect of sea-ice formation on salinity and density
Exclusion of salt ions.
Unfrozen saline (and dense) sea water sinks.
Important in forming the densest and deepest water masses in the oceans.


Detailed Notes start here

Introduction

You may be asking yourself, "What is the 'heat budget?'" It is simply a way of describing the fate of solar radiation that reaches the Earth's surface -- How solar radiation is "processed" (reflected, absorbed) before it is re-radiation back to space. The global heat budget is important in the operation of Earth's physical systems:
* The heat budget controls surface temperatures (both air and sea).
* Imbalances in the heat budget is the driving mechanism for atmospheric and oceanic circulation.

Global Heat Budget

Because of Earth's curvature, equatorial regions receive much more solar radiation on an annual basis than polar latitudes. But to work out the heat budget for the entire Earth, let's deal with the global average solar radiation and call that value 100 %.

* Of that total, 35% is reflected back into space (31% from the atmosphere, 4% from the surface of land and sea). Reflected radiation has little influence on temperature and circulation of the oceans and atmosphere.
* 65% of the incoming radiation is absorbed (17.5% in the atmosphere, 47.5% by land and sea surfaces).

What happens to the energy absorbed by land and sea?
* A small portion (5.5%) is re-radiated directly back to space. (as infared radiation)
* The rest (42%) is transferred to atmosphere by.....

Evaporation (29.5%).
* Recall that heat energy is required for evaporation; this cools the surface of land and sea.
* The heat energy stored in atmospheric water vapor (often called "latent heat") is released when vapor condenses to rain and snow.

Conduction and Radiation (12.5%).
* The (warm) land-sea surface transfers some heat by conduction to the (cool) atmosphere.
* In addition, remember that the surface emits infrared radiation, which is partly absorbed in the atmosphere by "greenhouse" gases, such as CO2 and water vapor.
- This process is important in maintaining a "nice" average temperature of 18 deg. C at Earth's surface.
- In the absence of greenhouse-gas absorbtion, the surface would be a chilly -10 deg. C.

What happens to the energy absorbed by the atmosphere?
* Of the global total of 100 %, direct absorbtion of sunlight accounts for 17.5 % and transfer from the Earth's surface is 42 %. This net gain (59.5 %) is re-radiated back into space.
* Our global heat budget is complete and balanced!

Variations in solar radiation with latitude, and its influence on the Earth's heat budget.

Because of Earth's curvature, the angle of incidence of the Sun's rays and thus the intensity of solar radiation per unit area is a strong function of latitude -- maximum intensity at the Equator, minimum intensity at the poles. There is a further decrease in intensity at high latitudes due to the greater path length of sunlight through the atmosphere -- more is absorbed before reaching the surface.

The global average heat budget must be balanced. In other words, solar energy input = reflection and re-radiation from Earth. [What would be the consequence if this were not true?] But at most locations on Earth, input and output are not equal on an annual basis.

* At low latitudes, there is a net surplus radiation (input > output).
* At high latitudes, there is a net deficit in radiation (input < output).
* This situation could lead to a runaway thermal catastrophe, with low latitudes becoming increasingly hot and high latitudes becoming increasingly cold!

But the Earth System has a nifty way of accomodating this radiation imbalance. Heat is transfered across latitudes through the circulation of the oceans and the atmosphere.
Warm air and water moves from the Equator to poles, and cool air and water in the opposite direction. (About 1/3 of the "excess heat" is transfered by the atmosphere and 2/3 by the oceans.)
Stated differently, the latitude imbalance in solar-energy gains and losses is the driving mechanism for the circulation of the oceans and atmosphere.

Surface temperatures

Latitudinal variations in solar radiation received and in the net heat budget result in average annual temperatures that decrease from the Equator to the poles. In the oceans, these latitude "belts" of ~ constant temperature are offset slightly by prevailing surface currents.
Currents originating in low latitudes transport warm water poleward, while currents originating in high latitudes transport cool water toward the Equator.

Seasonal variations in temperatures differ from continents to oceans.

* Temperature changes from summer to winter on land are relatively high, particularly for continental interiors at mid- and high latitudes.
* In contrast, seasonal variations in surface ocean temperature are relatively low because of the high heat capacity of water.
* This illustrates an important point - oceans can store and release great quantities of heat with great changes in temperature. The existence of a global ocean acts to stabilize the surface temperature of Earth.

Sea Ice

Sea ice is - simply put - frozen sea water. Don't confuse "sea ice" with "icebergs" - icebergs are broken off pieces of glaciers that flow to the sea.

Sea ice is a year-round feature of the central Arctic Ocean and around Antarctica. During winter cooling, sea ice covers the entire Artic Ocean and extends far out into the Southern Ocean.

Sea ice is important in the heat and "salt" budgets of high-latitude oceans.

* Recall that heat energy must be extracted for water (and sea water) to freeze.
* But ice is a thermal insulator -- it does not conduct heat very well. Therefore, when a layer of sea ice forms, it slows the removal of heat (by conduction) from sea water in contact with the base of the layer, where freezing occurs.
* As a consequence, the thickness of ice that forms in a winter season is typically limited to about 2 meters.
* Recall also that dissolved salts are not incorporated into frozen sea water (or at least not very much - the salinity of typical sea ice is 5-10 g/kg).
* Therefore, sea-ice formation promotes vertical circulation beneath the ice: the residual, saline water sinks (because it is dense) and less saline, warmer water rises. Seasonal sea-ice formation is an important process in the formation of the deepest and densest water masses in the oceans.


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