Chapter 2
The Earth’s Global Energy Balance
This chapter focuses on solar radiation which flows through the atmosphere to the Earth’s
surface. This energy is responsible for driving the Earth’s physical and biological
systems.

The Earth’s energy balance is accounted for by the difference in the flow of energy
reaching the Earth’s surface and the flow of energy emitted by the Earth’s surface.

Solar energy is the driving force for most natural phenomena at the Earth’s surface.

Electromagnetic radiation is a spectrum of wavelengths emitted by all objects
maintaining temperatures greater than absolute zero (0°Kelvin).

Two principles that govern the emission of electromagnetic radiation are:

Hot objects radiate more total energy than cooler objects.

Hot objects radiate energy at lower wavelengths.
The sun is a star of average size with a nuclear fusion (four hydrogen atoms fused
into helium) generated surface temperature of approximately 6000° C.

The solar constant is the amount of energy received per square meter of Earth’s
upper (1370 watts per square meter; 1370 W/m2).

The sun emits a large amount of energy concentrated in the ultraviolet, visible, and
shortwave infrared wavelengths. This spectrum collection is called short wave
radiation.

The Earth is much cooler than the sun. It therefore emits less energy as longwave
radiation.

Insolation, or incoming solar radiation, varies with the angle of the sun above the
horizon (solar angle) and the length of daylight.

Locations between 23½° north and 23½° south of the equator experience two
insolation maxima per year, while locations poleward of these latitudes experience
only one insolation maximum per year.

Locations poleward of the arctic and Antarctic circles experience daily insolation
values of zero for part of the year.

Daily insolation values are greatest at both poles during the summer solstice.

The seasonal pattern of daily insolation is the basis for dividing the Earth into world
latitude zones which include equatorial, tropical, subtropical, midlatitude, subarctic
(subantarctic), arctic (antarctic), and north (south) polar zones.

Although the Earth’s atmosphere extends to approximately 10,000 kilometers above
the Earth, ninety-seven percent of the atmosphere lies within 30 kilometers of the
Earth’s surface.

Pure dry air consists of seventy-eight percent nitrogen and twenty-one percent
oxygen by volume. Argon, CO2, and other trace gases account for the remaining one
percent.

CO2 is a very important gas due to its ability to absorb radiant heat and its role in
photosynthesis.

The ozone layer is found in the stratosphere, where it attenuates ultraviolet
radiation.

Human activity has increased the amount of gases such as chloroflourocarbons,
nitrous oxides, bromine oxides, and hydrogen oxides which act to deplete the ozone
layer.

On average, for every one percent decrease in global ozone there is a two percent
increase in ultraviolet radiation.

Sensible heat is the quantity of heat held by an object that can be sensed by touch,
measured by a thermometer, and transferred by conduction from warmer to cooler
objects.

Latent heat is energy that is absorbed and stored when a substance changes state
from a liquid to a gas or a solid to a liquid. Latent heat is transferred when water
evaporates from a land or water surface and is important in moving large amounts of
energy from one region to another.

As solar radiation flows through the atmosphere, energy is scattered and absorbed
by gas molecules and dust particles in the air.

Clouds which absorb five to twenty percent and reflecting thirty to sixty percent of
insolation are a major factor in determining how much energy reaches the Earth’s
surface.

Albedo refers to the percentage of shortwave energy reflected by a surface. The
average albedo of the Earth is twenty-nine to thirty-four percent.

CO2 and water vapor absorb incoming shortwave radiation and outgoing longwave
radiation from the Earth. They re-emit this radiation in all directions with part of it
returning to the Earth’s surface as counterradiation, resulting in a warmer than
normal surface of the Earth.

Energy entering the Earth’s atmosphere is reflected by molecules, dust, clouds, and
the ground surface and absorbed by molecules, dust, and clouds resulting in
approximately forty-nine percent of the incoming energy that is absorbed by the
Earth’s land and water surfaces.

The energy entering the Earth’s system must be balanced by energy leaving the
Earth’s system. Energy leaves the Earth’s surface as longwave radiation as well as
through transfers of sensible heat and latent heat. Human changes to the Earth that
affect albedo, cloud cover, or other aspects of the energy transfers may have an
impact on this balance.

Net radiation is the difference between all incoming and all outgoing radiation.
Although net radiation is zero for the Earth as a whole, it is positive between 40°
north and south latitude and negative poleward of these latitudes. As a result, global
and atmospheric circulation systems transport energy from lower to higher latitudes.