Earth’s Energy Budget
The Earth receives energy from the Sun, and it also reflects and radiates energy back into
space. This balance of incoming and outgoing energy creates our climate that supports life
as we know it on Earth. The Law of Conservation of Energy states that energy can neither
be created nor destroyed, but it can be transformed from one state to another. Energy
from the Sun is delivered as light energy, and some of that energy is used to warm the
Earth, and the differences in densities of air and water between warm and cold regions of
the atmosphere and oceans induce currents. Heat also drives the evaporation of water from
the oceans and drives the water cycle. Some light energy is converted into chemical energy
through photosynthesis, and stored as biomass. The petroleum we use today is the result
of photosynthesis long ago. A small amount of the Sun's energy that reaches the Earth's
surface is converted to electrical energy through photovoltaic cells and used to power lights
and machines.
All of the energy that warms the atmosphere, oceans and land must be radiated back into
space in order to maintain our current climate. If the amount of energy radiating back into
space is decreased by even a very small amount, it can lead to warming. It is believed that
increasing levels of carbon dioxide in the atmosphere has a 'greenhouse effect' of reducing
the amount of energy radiated into space.
Earth's Energy Budget
Part 1
Principle
Absorption and re-emission of radiation at the earth's surface is
only one part of an intricate web of heat transfer in the earth's
planetary domain. Equally important are selective absorption and
emission of radiation from molecules in the atmosphere. If the
earth did not have an atmosphere, surface temperatures would be
too cold to sustain life. If too many gases which absorb and emit
infrared radiation were present in the atmosphere, surface
temperatures would be too hot to sustain life.
Figure 1 - Radiation "Budget" for Incoming Solar Radiation
Incoming Solar Radiation

The sun radiates mostly in the visible band, but also in the
ultraviolet (shorter wavelength).

When averaged globally and annually, only 51% of the solar
radiation striking the earth and its atmosphere is absorbed
at the surface.
The atmosphere absorbs 19% of incoming solar radiation and the
remaining 30% is reflected back into space.

The shortest wavelengths of solar radiation (those most
dangerous to life) are absorbed by molecules in the upper
and middle atmosphere.
In particular, ozone selectively absorbs ultraviolet radiation while
allowing visible radiation to pass through relatively unhindered.

Reflection significantly affects the solar radiation that
reaches the ground, as the sun's rays could be reflected off
of air molecules (termed scattering), clouds or the ground
itself.
Light-colored or shiny objects reflect more radiation than dark
objects. Energy that is reflected cannot be absorbed or transmitted
through an object.

Different surfaces have different albedos (see Table 1).
Meteorologists refer to the percent of radiation returning from a
surface compared to the incident radiation as the "albedo". For
example, the earth reflects an average of 30% of the incoming
radiation, so the average albedo is 30%, or 0.3. Most of the earth's
average reflection results from clouds.

Visible satellite imagery can be used only during the day,
when visible solar radiation is striking the earth's
atmosphere and surface.
Table 1- Average
Reflectivity of Surfaces to
Solar Radiation
Surface Type
% Reflected
Albedo
fresh snow
~90%
~0.9
thick clouds
~80%
~0.8
thin clouds
~40%
~0.4
ice
~35%
~0.35
soil
~20%
~0.2
forest
~5%
~0.05
ocean (high sun angle) ~5%
~0.05
ocean (low sun angle)
~0.95
~95%
The Sun's Energy
The Sun releases an estimated 384.6 yotta watts (3.846×1026 watts) of energy in the form of light
and other forms of radiation. We are able to survive on Earth because the energy is spread over
the area of a sphere with a radius of approximately 93,000,000 miles.
At the upper reaches of our atmosphere, the energy density of solar radiation is
approximately 1,368 W/m2 (watts per square meter). At the Earth's surface, the energy density is
reduced to approximately 1,000 W/m2 for a surface perpendicular to the Sun's rays at sea level on
a clear day[1].
Solar energy is harvested by capturing light for direct photovoltaic conversion into electricity, or as
thermal energy (heat) water heating, space heating and other uses.
If all the sunlight energy striking the Earth's surface in Texas alone could be converted to
electricity, it would be up to 300 times the total power output of all the power plants in the
world!
Total world electricity production was 20,261 Terawatt hours (TWh) in 2008[2].
Power plant output in watts is: 20,261 TWh ÷ 365 days/year ÷ 24 hours/day = 2.31 TW
Texas is 696,241 km2, so, 1,000 w/m2 x 1,000,000 m2/km2 x 696,241 km2 = 696,241,000,000,000
Watts = 696.241 TW
So, the sunlight falling on Texas at noon is equivalent to 696 TW solar energy ÷ 2.31 TW power
plant output = 301 times the output of power plants.
Earth's Energy Budget
Part 2
Principle
Figure 2 - Globally Averaged Energy Budget
Absorption and re-emission of radiation at the earth's
surface is only one part of an intricate web of heat
transfer in the earth's planetary domain. Equally
important are selective absorption and emission of
radiation from molecules in the atmosphere. If the
earth did not have an atmosphere, surface
temperatures would be too cold to sustain life. If too
many gases which absorb and emit infrared radiation
were present in the atmosphere, surface
temperatures would be too hot to sustain life.
Outgoing Terrestrial Radiation

The earth's surface, atmosphere, and clouds
emit radiation in the infrared band and nearinfrared band.

Outgoing infrared (IR) radiation from the
earth's surface (also called terrestrial
radiation) is selectively absorbed by certain
molecules, particularly water vapor and
carbon dioxide.
Gases which absorb IR radiation are termed
collectively as "greenhouse gases".

Water vapor and carbon dioxide emit infrared
radiation.
Infrared radiation from greenhouse gases in the
atmosphere is emitted in all directions, including back
to the earth's surface. It is this re-emission to the
earth's surface that maintains a higher temperature
on our planet than what would be possible without
the atmosphere.
Global Energy Balance

Condensed water is also an efficient absorber
and emitter of IR radiation. Thus, clouds act in
a manner similar to greenhouse gases.

Satellite infrared imagery detects infrared
emission from clouds and the earth's surface
and can be used during both day and night.

When averaged over a year, the incoming
energy in both the earth and its atmosphere
equals the outgoing energy.
If we consider the entire Earth-atmosphere system,
then the amount of radiation entering the system
must equal to the amount leaving, or the system
would continually heat or cool. Not all of this energy is
radiative energy; some is sensible and latent heat.

If we consider the atmosphere alone, we find
that the atmosphere
experiences radiative cooling.
The atmosphere is kept from a net cooling by the
addition of energy by latent and sensible heating.

The atmosphere has a warming effect on
Earth's surface -- the "atmospheric
greenhouse effect".
If Earth had no atmosphere, the globally averaged
surface temperature would be -18 degrees Celsius.
Because Earth does have an atmosphere, the average
surface temperature actually is 15 degrees Celsius.
The atmosphere acts as a greenhouse because of
gases that selectively allow solar radiation to pass
through but absorb and then re-emit terrestrial
radiation. These gases are collectively called
"greenhouse gases" and include water vapor, carbon
dioxide, ozone, molecular oxygen, methane and
nitrous oxide. These gases are selective as to which
wavelengths they will absorb. For example, ozone
absorbs shortwave ultraviolet radiation whereas
water vapor absorbs infrared radiation more readily.

Most of the sun's radiation that passes
through the atmosphere to hit the earth is in
the visible part of the spectrum.

Most of the earth's radiation that escapes the
atmosphere is in the infrared band between 8
microns and 11 microns.
This region of the spectrum is called the "atmospheric
window".
What is Solar Energy?
While some solar energy is reflected back into space, but about 70 percent is absorbed by the
atmosphere, clouds, oceans, land, plants and other surfaces. Some is converted by photosynthesis into
stored energy in plants as biomass. Some of the energy, especially ultraviolet light, damages and
degrades materials, as well as causing sunburn and other adverse health effects. Much of the energy is
absorbed by surfaces and heats them, and this heat can be transferred to other materials for our use,
such as space heating and water heating. Converting light directly into electricity using photovoltaic
materials, first observed in 1839(3), allows us to harvest solar energy for direct-use or to sell electricity
onto the grid.
After learning about how we receive and harvest solar energy, move on to the next section to conduct
an energy audit to determine how much energy you use.
Sunlight
The solar energy reaching Earth's surface includes visible light, ultraviolet and infrared
radiation. Ultraviolet light is high energy and causes damage including sunburn, fading of pigments and
degradation of compounds such as fabrics, plastic and rubber. Ultraviolet radiation can also be used to
kill harmful bacteria and parasites in water, rendering it safe for drinking.
Infrared radiation is felt as heat, and is given off by all objects. If your body is warmer than surrounding
objects, your body gives up more radiant heat than it receives from the surroundings, and you will feel
cool or cold. If nearby objects are warmer, such as a heat lamp or space heater, you receive more
radiant heat on that side of your body than your body gives off, and you will feel warm on that side. The
other side may feel quite cold, however.
Much of the energy from the Sun arrives on Earth in the form of infrared radiation. Sunlight in space at
the top of Earth's atmosphere at a power of 1366 watts/m2 is composed (by total energy) of about 50%
infrared light, 40% visible light, and 10% ultraviolet light[4]. At ground level, this decreases to about
1120-1000 watts/m2, and consists of 44% visible light, 3% ultraviolet (with the Sun at the zenith (directly
overhead), but less at other angles), and the remainder infrared. Thus, sunlight's composition at ground
level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of
visible light, and 32 watts of ultraviolet radiation. The balance between absorbed and emitted infrared
radiation has a critical effect on the Earth's climate.
Photosynthesis
Photosynthesis is a very important method of capturing and storing solar energy. Sunlight provides the
energy plants use to convert carbon dioxide, water and nutrients into chemical energy that is stored as
sugars, starches and cellulose. Plants use this chemical energy as food. We use the resulting biomass
for food, clothing, building materials, and as feedstock for many chemical processes. All of the
petroleum we use today from natural gas and crude oil is the result of photosynthesis.
Today, we produce ethanol and biodiesel from plant materials such as corn, soybeans, sugar cane and a
few other crops. Researchers are continuing research to find efficient and economically viable methods
to produce ethanol from cellulosic materials such as switch grass, wood debris, etc.
Resources
http://okfirst.mesonet.org/train/meteorology/EnergyBudget.html
https://ag.tennessee.edu/solar/Pages/What%20Is%20Solar%20Energy/Earth-Energy-Budget.aspx
[1] Sunlight: Composition and Power - Wikipedia
[2] Electricity Generation: Production by country - Wikipedia
(3) http://en.wikipedia.org/wiki/Alexandre-Edmond_Becquerel
[4] Sunlight: Composition and Power - Wikipedia