Download Energy Balance - GEO

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 the balance between the flow of energy reaching the
Earth and the flow of energy leaving the Earth.

Solar energy is the driving force for most natural phenomena at the Earth’s surface.

Electromagnetic radiation is emitted from all objects as a collection of wavelengths
traveling away from the surface of an object.

Two principles that govern the emission of electromagnetic radiation are:

An inverse relationship exists between the temperature of an object and the range
of wavelengths that object emits as electromagnetic radiation.

Hot objects radiate more energy than cooler objects.

The sun is a star of average size with a surface temperature of 6000° C generated by
nuclear fusion.

The solar constant is the amount of energy received per square meter just outside the
Earth’s atmosphere. The value is 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 is called short wave radiation.

The Earth is much cooler than the sun. It therefore emits less energy and emits that
energy as longwave radiation.

Insolation, or incoming solar radiation, varies with the angle of the sun above the
horizon and daylength.

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.

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 the pole 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 make up 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 absorbs ultraviolet radiation
and shields the Earth from the stratosphere’s harmful effects.

Human activity has increased the amount of gases such as chloroflourocarbons,
nitrous oxides, bromine oxides, and hydrogen oxides which are depleting the ozone
layer.

For every one percent decrease in global ozone, ultraviolet radiation may increase by
two percent.

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 are a major factor in determining how much energy reaches the Earth’s
surface absorbing five to twenty percent and reflecting thirty to sixty percent of
insolation.

Albedo refers to the percentage of shortwave (SW) energy reflected by a surface.
The albedo of the Earth is twenty-nine to thirty-four percent.

CO2 and water vapor absorb incoming SW radiation and outgoing LW radiation from
the Earth. They re-emit this radiation in all directions with part of it returning to the
Earth’s surface in counterradiation, making the surface of the Earth warmer than it
would otherwise be.

Energy entering the Earth’s atmosphere is reflected by molecules, dust, clouds, and
the surface and absorbed by molecules, dust, and clouds leaving only forty-nine
percent of the incoming energy to be 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 latitude
40° north and 40° south and negative poleward of these latitudes. As a result, global
and atmospheric circulation systems transport energy from lower to higher latitudes.