Earths’ Energy Budget: Radiation From the Sun and Earth
(Reading #3 for my course on Climate Change, Alan Holyoak, PhD)
Daily Objectives
- Be
able to explain what radiation is, including the difference between direct
solar radiation and infrared radiation.
- Be
able to identify the wavelengths of radiation that are important to global
climate.
- Be
able to explain why there is a latitudinal difference in the amount of
energy received each year.
- Be
able to explain what the Earth’s energy budget is, and the factors that
affect the flow of energy into and out of the atmosphere.
- Be able to list important greenhouse gases and explain the term `global warming potential’.
Introduction
The climate of the Earth is
affected by the rate of gain and loss of energy. This dynamic exchange of energy entering and
leaving Earth is used to describe its energy budget. Solar radiation is the main form of energy
that is important to climate. While the
sun is the major source of radiant energy for the Earth, the Earth, the
atmosphere, and everything warmer than absolute zero also emit radiation at all times and in all directions. It is therefore important to understand what
radiation is, as well as the processes involved in energy gains and losses at
Earth’s surface in order better understand climate change.
About Radiation
Radiation is energy, and is therefore
subject to the Laws of Thermodynamics.
Since these laws apply, it is important to remember that energy cannot
be created or destroyed, but it can change forms, and every time it changes
form entropy increases and is released as waste heat. Energy is released by everything with a
temperature above absolute zero (Fig.
1). The temperature of an object
determines the wavelength of energy it releases. Absolute zero is the lowest temperature that
can theoretically exist, and is indicated by 0o Kelvin, -273oC,
or -460oF.
Figure 1. Thermal
image approximating what we might look like to each other if we could see
infrared radiation. (Image courtesy of Wikimedia Commons.)
Think of
radiation as a three-dimensional wave of energy. These waves travel at the speed of light,
which is 3x108 meters/second or 186,000 miles/second. Note that any wave has a particular wavelength or distance from crest to
crest (Fig. 2). The units for a wavelength
of radiation are usually micrometers
(a millionth of a meter and designated by “μm”) or nanometers
(a billionth of a meter and designated by “nm”).
Figure 2. Each spectrum of radiation has its own unique
wavelength. Spectra with longer
wavelengths (upper image) carry less energy than those with shorter wavelengths
(lower image). (Image courtesy of Dr. Hipps, Utah State University.)
Wavelengths of Radiation
The range of
wavelengths of electromagnetic radiation in the universe is enormous. Figure 3 shows this range and indicates
wavelengths of radiation important to Earth’s climate. Many people are surprised when they learn
that visible radiation or “light” comprises only a tiny segment of the total
range of the electromagnetic spectrum of radiation. Your eyes sense only this narrow range of
wavelengths. Even though there is an
enormous range of electromagnetic radiation, only small section of the range is
important to Earth’s energy budget.
Figure 3. The electromagnetic spectrum of radiation. Wavelengths of the spectrum of particular
interest to global climate include ultraviolet (UV) radiation of ~10-380nm,
visible light from ~380-740nm, and infrared (IR) radiation of ~740nm-300μm. (Image
courtesy of Wikimedia Commons.)
The amount of
radiation emitted and the wavelengths of that emission are determined by the
temperature of an object. Table 1 shows
the key principles of radiation emission, wavelength distribution, and
temperature of emissions from the sun and Earth.
Table 1. Characteristics of radiation released by the
sun and Earth.
Emitting body
|
Temperature
|
Radiation
characteristics
|
Sun
|
6000 K (~5700oC)
|
Larger emission at shorter wavelengths
|
Earth
|
290 K (~17oC)
|
Smaller emission at longer wavelengths
|
Radiation Emission of
Sun and Earth
Since the sun and
Earth have significantly different temperatures we expect their radiation emissions
to be quite different. The sun is about
6000oK (5730oC or 10,340oF). The average temperature on Earth is about 290oK
(17oC or 62oF).
There are therefore large differences in both the intensity and
wavelengths emitted by these two objects. Visible light comprises about half
the total emission from the sun (Fig. 4), while the vast majority of radiation
released by the much cooler Earth is in the range of infrared radiation.
Figure 4. The sun emits mainly
high-energy, short-wavelength radiation, including the visible spectrum of
light, and the Earth emits mainly low-energy, long-wavelength radiation, mostly
in the form of infrared radiation or heat. (Image courtesy of Dr. Hipps, Utah
State University.)
Some important points to know
1.
The size of emission and distribution of wavelengths are related
to temperature. The sun emits more energy
and in shorter wavelengths than the Earth.
The Earth, which is cooler than the sun, emits less energy and at longer
wavelengths.
2.
About half of solar emission lies in the wavelengths of visible light (380-740nm), and it peaks
near 500nm.
3.
Earth emissions cover a large range of wavelengths, and peak at
about 10 μm. And because infrared
radiation falls outside of the range of visible light, we cannot see it.
Total Radiation
Emission For All Wavelengths
The total
sum of the emission of radiation over all wavelengths can be calculated from a
law of physics in which the energy emitted is calculated by multiplying a
constant “k” that includes the ability of an object to absorb energy that is a
value between 0 and 1, the area of the object, and Stephan’s constant (don’t
get hung up on this) by the temperature “T” of the object in degrees Kelvin
taken to the 4th power as shown in the this equation:
Emission = k * T4
This means that when we know the
temperature of an object we can readily calculate the total radiation (energy)
it emits. Because emission changes with
temperature to the fourth power, small changes in temperature translate into
large changes in emission! The ability
to make these kinds of measurements and carry out these calculations makes it
possible for climatologists to track the flow of energy throughout Earth’s
climate system.
Units of Energy
Since
radiation is energy, we should use the appropriate units to refer to it: Joules.
A Joule is a unit of energy or work.
Maybe this will help you understand how much energy is in one Joule. Imagine that there is an average sized apple
sitting on the ground. You expend about one
Joule of energy when you lift that apple to a height of one meter at a constant
speed. Most of the time, though, we
refer to energy in terms of Watts. Does
this sound more familiar? Going back to
our example of lifting an apple, you expend about one Watt of energy when you
lift the apple to the height of one meter in one second. So if you lifted 100 apples at the rate of
one meter/second you’d expend 100 Watts of energy. Though Joules are the best unit for referring
to energy, Watts are used when we want to measure the rate at which energy is
moved or used. The units most conveniently
used to refer to radiant energy are therefore Watts per meter squared. This lets us know how much energy we are
talking about in terms of an area of interest.
So don’t be surprised to see various values in Watts and Watts m-2
throughout the rest of the semester. You
therefore need to become familiar and comfortable with this unit. By the way, if you have ever ridden a bicycle
that has a friction generator used to power a light, you can really feel it
when you ride! Most of these generators
produce only a few Watts of power (Fig. 5).
Figure 5. A
friction generator used to provide power for bicycle head and tail lights (3
Watts).
(Image courtesy of Amazon.com)
Seasonal Changes in
Solar Energy Received on Earth
You already
know that the tilt of the Earth produces our seasons, but here are some
additional things that were not covered in that reading that you need to be
aware of.
As a result
of Earth’s axial tilt the sun’s rays can be directly overhead at noon only between
latitudes of 23.5oN (the Tropic
of Cancer) and 23.5oS (the Tropic
of Capricorn). The latitude where
the sun is directly overhead at noon moves north and south with the seasons. At all other places on the planet the
elevation of the sun from the horizon can never reach vertical or 90o
overhead.
On exactly
two days a year the sun is vertical at noon directly at the equator. These events are called equinoxes. The autumnal equinox is about 22 September,
and the vernal equinox is about 22
March. Let’s follow the movement of the
sun’s location over a year.
After the autumnal equinox the
location of the sun’s vertical rays moves gradually south until about 22
December when it is vertical at the Tropic of Capricorn. This day is the winter solstice. After this
date the latitude where the sun is vertical moves north until it reaches the
equator at about 22 March. This is the
vernal equinox. The location where the
sun’s rays are vertical at noon continues to move north until it reaches the
Tropic of Cancer on about 22 June. This
is the summer solstice. The location where the sun is vertical at
noon then moves south until it reaches the equator near 22 September on the
autumnal equinox (Table 2).
Table 2. Summary
of dates important to seasonal change on Earth.
Event
|
Date
|
Latitude sun is vertical at noon
|
Autumnal equinox
|
22 September
|
Equator
|
Winter solstice
|
22 December
|
Tropic of Capricorn
|
Vernal equinox
|
22 March
|
Equator
|
Summer solstice
|
22 June
|
Tropic of Cancer
|
Remember
that 50% of the Earth’s surface is always
being illuminated. Even so, because the
sun is vertical above the equator at noon only on the equinoxes, on only those
dates the day length is 12 hours everywhere on Earth.
Because of
the tilt of Earth’s axis, polar regions of the Earth have periods of complete
darkness and others that have constant light at various times of the year. This results in two more latitudes that are
special when we think about seasons and climate. On the summer solstice no solar radiation
reaches farther south than 90 minus 23.5o, or 66.5o S. This latitude is the Antarctic Circle. So at any
latitude greater than 66.5o S there is a period of time when it
receives no solar radiation at all.
Conversely, on the winter solstice the region of complete light reaches only as
far north as 66.5o S. Similarly on the summer solstice, the region
of 24-hour daylight reaches as far south as 66.5o N. This latitude is called the Arctic Circle. Within the Antarctic Circle there is one day
per year of no light, 22 June, and one day per year of 24-hour light, 22
December. As you move toward the South
Pole from 66.5o S or the North Pole from 66.5o N, the number
of days of darkness or light per day grows until at the North and South Poles 6
months pass when the sun never rises above the horizon, and 6 months of when
the sun doesn’t drop below the horizon.
The rate of
change of the sun angle overhead varies with the seasons. The position of the sun in the sky changes
most slowly at the solstices and fastest at the equinoxes. You may have noticed that our day length
changes slowly during the middle of the summer and the middle of the winter,
and that the duration of day length changes much faster during the fall and
spring. Near the equator there is little
seasonal change in sun angle or day length.
The day length in the tropics is therefore always about 12 hours.
There are also
seasonal changes in the path the sun takes across the sky, especially the farther
you go from the equator. The sun will
rise from exactly the east and set exactly in the west only on equinoxes. For example, let’s consider the track of the
sun across the northern hemisphere between 22 September and 22 March. On each day the sun will rise south of true
east and set south of true west. The
maximum elevation reached by the sun at local noon will reduce every day until
it reaches its lowest height on 22 December – the shortest day of the year in
the northern hemisphere. So during the
winter not only is the height of the sun in the sky lower, but the length of
the arc it makes across the sky is also shorter. Both of these factors result in less total solar
radiation being received by the surface during the fall and winter months in the
northern hemisphere than during the Spring Summer. During the northern hemisphere summer the sun
rises north of true east and sets north of true west. The arc of the sun’s path across the sky is
now much longer than in the Fall/Winter.
In addition, the maximum elevation of the sun reached at noon also
increases. So total solar radiation is greater
in the summer than in the winter.
What is the
largest elevation angle the sun can reach at noon at any given latitude? You can calculate this by subtracting 23.5o
from the latitude of interest, and then subtracting this difference from 90o. For example, the latitude of Rexburg is 43.8oN. The maximum solar elevation at this latitude is:
90 – (43.8 – 23.5) = about 69.7o.
So, on 22 June the sun will reach its maximum elevation at noon, and
will be at an angle of 69.7o overhead.
Why bother
going into all of this? Latitudinal variations
in solar radiation have a massive effect on climate!
Earth’s Energy Budget
Earth’s energy budget is the dynamic balance
between total energy received and total energy emitted. The atmosphere affects the rate of movement
of radiation as it passes through it so calculating Earth’s energy budget can
be challenging. One very important thing
to understand is that all energy received is eventually emitted back into
space.
We will start by looking at the
rate at which solar radiation enters and infrared radiation leaves. Figure 6 shows that 100% of the energy
entering Earth’s system eventually leaves and goes back into space. The relative rates at which energy enters,
the length of time it stays in the atmosphere, and the length of time it takes
energy to leave determine whether Earth is warming or cooling (Table 3).
Table 3. Effects
of the rates of radiation entering and leaving Earth’s atmosphere.
Differential rate of energy flow
|
Average global temperature
|
Energy
entering > Energy leaving
|
Warming
|
Energy entering =
Energy leaving
|
Unchanging
|
Energy entering
< Energy leaving
|
Cooling
|
Radiation can have many fates as
it passes through the atmosphere. Some
of it can pass all the way through the atmosphere and strike the Earth’s
surface. However, radiation can be also absorbed, reflected, or scattered by
the atmosphere. It is also absorbed or reflected
by the surface.
Fates of Solar
Radiation
Solar radiation entering Earth’s
atmosphere has a variety of fates. Solar
radiation can be absorbed, reflected, or scattered by the atmosphere before it
reaches the surface (Figs. 6 & 7).
The remaining solar radiation that passes all the way through the
atmosphere and strikes the Earth’s surface and called direct solar radiation; this is the light you see when you glance
up at the sun. Direct solar radiation
can be absorbed or reflected by Earth’s surface.
Figure 6. Simplified view of Earth’s energy budget.
Yellow arrows represent solar radiation.
Orange arrows represent solar energy that has been absorbed. Red arrows represent the release of absorbed
energy as infrared radiation. (Image courtesy of NASA.)
Figure 7. Fates and pathways of incident solar radiation, including absorption, reflection, and scattering. (Image courtesy of Dr. Hipps, Utah State University.)
Absorption
The main
factor that determines the rate at which energy moves through the atmosphere is
the composition of the atmosphere itself.
Some gases can absorb certain wavelengths of solar radiation. Most of this absorption occurs in the ultraviolet
(UV) region. Most UV radiation is
absorbed high in the stratosphere. Major
players involved in this absorption are ozone, oxygen, and N2O
(nitrous oxide). Some UV radiation reaches
the Earth’s surface as part of direct solar radiation, and this is what causes sunburns
and skin cancers.
Small
particles called aerosols and clouds
can absorb and reflect radiation. Clouds
are highly reflective to solar radiation.
Interestingly, the atmosphere does not absorb much energy in the visible
wavelengths of light (380-740nm). Solar
radiation absorbed in the atmosphere results in an increase in the temperature
of the absorbing gases.
Scattering
When solar radiation
approaches some small particles or air molecules, a complex interaction takes
place that causes the incident beam (the ray of solar energy) to be redirected in
random directions. This is scattering. Scattered light can be scattered again and
again, depending on its path and what it encounters. Ultimately, this redirected light finds its
way back into space or to the Earth’s surface.
Scattered light that reaches the surface in this way is called diffuse radiation, and thus differs
from direct radiation that reaches the surface without interference. On a cloudy day, and at times just before sunrise
and just after sunset, all radiation is diffuse. Whenever direct sunlight is present, a
combination of direct and diffuse radiation reaches the surface.
Particles
and air molecules smaller than the wavelengths of visible light cause something
called Rayleigh scattering. Causes shorter wavelengths of visible light
(greens, blues, and violets) to be scattered more than longer wavelengths of
visible light (reds, oranges, and yellows).
As a result when the sun is more directly overhead the sky looks
blue. When the light has to pass through
more of the atmosphere before it reaches us, as at dawn and dusk, shorter
wavelengths are all scattered, leaving only the reds and oranges, and this can
give us colorful sunrises and sunsets.
Since
scattering redirects radiation into all directions, some of radiation is always
reflected into space. So, scattering
always reduces the total solar radiation reaching the surface.
Surface Reflection
The reflectivity
of a surface refers to the proportion of solar radiation reflected by the
surface in relation to the total amount of radiation reaching the surface. This
factor is called the albedo. Every object will have an albedo between 0.0
and 1.0. An albedo of 0.0 means that a
surface is perfectly black and absorbs all wavelengths of radiation; none is
reflected. An albedo of 1.0, on the
other hand, means that a surface is perfectly white and all wavelengths of
radiation are reflected. The albedo of
objects in the atmosphere and on Earths’ surface is critical in the climate
system.
When the
entire Earth-atmosphere system is considered, Earth’s average albedo is about
0.3. So about 30% of the incident solar radiation is reflected
back into space (see Fig. 6). This 30%
includes reflection off of Earth’s atmosphere, clouds, and surface.
Infrared Radiation and the Atmosphere
The Earth
and molecules in the atmosphere absorb high-energy solar radiation, and then
emit longer wavelength, low-energy infrared radiation (IR). This IR eventually makes its way back out
into space. The rate at which IR makes
its what through the atmosphere, however, is determined mainly by the following
factors:
- IR is
absorbed by: liquid water in clouds, water vapor, carbon dioxide (CO2),
methane (CH4), nitrous oxide (N2O), ozone (O3),
chlorofluorocarbons (CFCs), and other greenhouse gases.
- IR absorbed
by atmospheric components increases atmosphere retention and then emission
of IR in all directions
- This
non-directional emission of IR slows the rate of IR loss to space
The Greenhouse Effect
The absorption of some of the IR
from the Earth by the atmosphere has a major effect on climate. The IR energy absorbed by the atmosphere increases
the overall energy content of the atmosphere.
This causes an increase in the IR emission by the absorbing gas or
cloud. However, the IR travels in all
directions when it is emitted. So, some
of the IR energy ends up moving out into space, but some finds its way back toward
the surface. These IR absorbing
molecules act to reduce the net rate of loss of energy from the Earth to space. This process is the Greenhouse Effect. The gases
that perform this absorption in the atmosphere are called Greenhouse Gases.
Greenhouse gases keep the Earth
warmer that it would otherwise be. When the
amount of greenhouse gases in the atmosphere increases, the rate of loss of IR into
space slows down and atmospheric average temperatures rise. The presence of greenhouse gases and clouds
maintain the Earth at a much warmer temperature that it would otherwise
have. Earth’s average temperature is
about 17oC or 63oF, but calculations have shown that
without greenhouse gases the Earth would have an average temperature of about
-18oC or 0oF!
At this point it is important to
note that not all greenhouse gases behave the same way in the atmosphere. Water vapor, for example, is a strong
greenhouse gas, but it does not accumulate indefinitely in the atmosphere. Once the atmosphere reaches its full capacity
of water vapor and the air cools even slightly, water vapor will condense into
water droplets and fall as precipitation.
Other kinds of greenhouse gases such as carbon dioxide, however, can
accumulate more or less indefinitely in the atmosphere, allowing their effects
to increase over time.
Fates of Infrared Radiation (IR)
Figure 8 shows the pathways of IR
in the Earth-atmosphere system. The
major processes that affect IR radiation balance are also displayed. Recall that the surface and atmosphere both
emit energy. The source of IR is mainly
from the Earth surface. The surface is heated
by solar radiation, and later releases that energy when it emits IR into the
atmosphere. The amount of IR emitted by
the surface is proportional to surface temperature.
There is significant absorption of
IR by water in clouds, water vapor, and other greenhouse gases once the Earth
releases the IR. Because the air
releases IR in all directions, a significant proportion of IR ends up returning
back to the surface.
Clouds play a major role in the
climate system because they contain droplets of liquid water. Clouds reflect incident solar radiation and
reduce the amount of direct solar radiation that reaches the surface. However, clouds are also strong absorbers of
IR, which slows the loss of energy from the surface. The combined effects of clouds on the total
budget of solar radiation and IR are therefore complex. A big issue at present is whether changes in
clouds due to increased atmospheric temperature will act to accelerate of
diminish further warming since they simultaneously absorb IR energy and reflect
incident solar energy back into space.
There is a strong relationship
between IR balance and temperature. For
example, an increase in the concentration of greenhouse gases results in
imbalance between energy received and energy emitted into space. The average temperature would consequently
increase because there would be a smaller IR loss. But the higher temperature would also produce
increases in IR surface emission and increase IR loss into space. Eventually IR loss would reach a level where
total losses would again balance solar input.
At that point a new equilibrium is reached, but at a higher average
global temperature.
Figure 8. Fates of infrared radiation. (Image courtesy of Dr. Hipps, Utah State
University.)
More about Greenhouse
Gases
Direct
measurements of the atmosphere have revealed that human activities are
increasing the concentrations of some greenhouse gases in the atmosphere, and
there is reason for concern about their effects on the global climate (Fig. 9).
For example, atmospheric scientists examined all known natural and anthropogenic (human-produced) sources
of CO2 and concluded that virtually all recent increases in CO2
is attributable to two main sources: 1) burning fossil fuels for energy,
transportation, and industry, and 2) cement production. This trend is exacerbated by the destruction
or degradation of key terrestrial ecosystems, such as forests, that have the
potential to capture and store excess carbon emissions. Methane has also been steadily rising during
the same time period as CO2.
The cause of methane rise is less well documented than for CO2,
but it also appears to be linked to human activities such as draining wetlands
and swamps, increasing cattle production, and other activities. Increases in N2O are related to
fertilizer use, combustion, and industry.
Figure 9. Trends
of greenhouse gas concentrations in the atmosphere, 1978-2011. (Image courtesy
of NOAA.)
Chlorofluorocarbons
(CFCs) are experiencing an interesting trend of concentrations in the
atmosphere (Fig. 9). There was a rapid
increase in CFC concentrations through the mid-1990s. Since that time, however, CFC concentrations
appear to have leveled off. This is not
surprising, since the Montreal Protocol,
an international agreement reached in 1987 banning the production and
distribution of CFCs went into effect in 1989.
This agreement was formalized after governments recognized overwhelming
scientific evidence showing that CFCs are the primary agent destroying the stratospheric
ozone layer.
CFCs were used initially as a
replacement for ammonium as a refrigerant.
CFCs have several advantages over ammonia. For example, a refrigeration system
containing ammonia could leak, and the ammonia readily evaporates. The problem with this is that ammonia is a
poisonous gas. How would it be to be
killed by your refrigerator!? CFCs, on
the other hand, are excellent refrigerants and they are not toxic. They were also believed to be chemically
stable, and they are under most conditions, but when CFCs make their way into
the stratosphere they are bombarded by UV radiation. This causes CFCs break down and release
individual atoms of chlorine. These chlorine
atoms react with ozone, splitting one atom of oxygen away and forming chlorine
monoxide (ClO). The ClO then also
breaks apart in the presence of UV light, and the chlorine ion then breaks down
another atom of ozone, and so on and so son.
It is estimated that a single chlorine atom in the stratosphere will
break down as many as 100,000 molecules of ozone before being bound up in a
longer-lived molecule. This is why ozone
depletion occurs in the presence of CFCs.
This CFC-driven reduction of the stratospheric ozone layer allowed
increased amounts of UV radiation to reach the surface. UV radiation can cause biological problems ranging
from reduced photosynthetic ability in plants to skin cancers and eye cataracts
in animals. And, CFCs are powerful
greenhouse gases.
You may be wondering why there are
still CFCs in the atmosphere if they were banned as of 1989. CFCs are long-lived molecules (e.g., CFC-11 =
45 yrs, CFC-12 = 100 yrs), and there are still a lot of CFCs still in people’s
refrigerators, air conditioners, etc., which continue to leak and be released
into the atmosphere.
Anyway, the range of wavelengths of
energy and amount of energy individual molecules can absorb varies from one
kind of greenhouse gas to another. Since
this is the case, we need to find a way we can compare the effects of different
greenhouse gases to each other in an apples-to-apples type of comparison. This is where the global warming potential (GWP) of greenhouse gases helps out. Table 4 shows the GWP of key greenhouse gases
in relation to CO2, which has been assigned a baseline GWP of
1.0.
Note the
large differences in the effect of different greenhouse gases on temperature. Compared to CO2, a single molecule
of methane has a 21x greater GWP. But,
also notice the concentrations! The
value for CO2 is almost 200x more abundant in the atmosphere than
methane, and about 1 million times more abundant than CFC-12. This makes CO2 is the most
important of the greenhouse gases when you consider its GWP and its overall
concentration in the atmosphere, and this is the greenhouse gas humans are
currently emitting into the atmosphere at the greatest rate.
Table 4. Global
warming potential (GWP) of selected greenhouse gases.
Greenhouse gas
|
Atmospheric concentration
|
GWP
|
CO2
|
~390 pp million
|
1
|
CH4
|
~ 2 pp million
|
21
|
N2O
|
~ 320 pp billion
|
310
|
CFC-12
|
~540 pp trillion
|
8500
|
HCFC-22
|
~100 pp trillion
|
1350
|
Net Radiation
The value
of radiation that concerns us when we consider Earth’s energy budget is the
actual amount of radiant energy available at the surface, called net radiation (Rnet). This is calculated by accounting for all
solar and infrared radiation as follows:
Rnet =
(Incident Solar – Reflected Solar) + (Incident IR – Surface IR)
The sum of
these radiation streams represents the amount of energy available to drive Earth’s
climate. OK, what is this energy used
for?
About 77%
of Rnet is used to evaporate water, and the remaining 23% is used to
heat the surface. In other words, the
ocean absorbs the vast majority of solar energy. These results are not surprising when we
consider that about 71% of Earth’s surface is covered by water. The large amount of energy associated with
the evaporation process means that this factor has large effects on global and
regional climate systems.
If the
temperature of Earth were constant, then the Rnet, when averaged
over the entire planet for an entire year, would equal zero. Sadly, such is not the case. Earth currently has a positive Rnet.
Patterns of Radiation
Over the Earth
Because of the tilt of the Earth
and its orbit around the sun, there is a large seasonal change in radiation
over the surface. There is also a large
variation in received and emitted radiation with latitude. Figure 10 shows the annual distribution of
solar energy received over an entire year by latitude. It is not a surprise that the tropics receive
the greatest amount of solar radiation and has the highest Rnet over
a year. These values decrease with
distance from the equator until a minimum is reached at the poles.
Figure 10. Total
amount of solar radiation received by latitude. (Image courtesy of Dr. Hipps,
Utah State University.)
It is useful to look at the net
value of absorbed minus emitted radiation for each latitude band, over a
year. This requires plotting both the
solar radiation received and IR lost by the surface. Figure 11 shows this difference by
latitude. In the tropics there is more
energy gained at the surface than is lost every year. At higher latitudes the opposite is observed
where more IR is lost than the amount of solar energy gained. So there is a continuous deficit of energy at
higher latitudes. The regions of surplus
change to deficits at about 37o north and south.
Figure 11. Annual
net radiation by latitude. (Image
courtesy of the Univ. of Wisconsin.)
If this unequal distribution of
radiation by latitude were the only process that affects climate, what would
Earth’s temperature profile look like?
For one thing, the tropics would not just be hot, they would be unbearably
hot, and the rest of the planet would be either cold or unbearably cold. This is not observed because heat is
constantly transported away from the tropics to higher latitudes via air and
ocean currents.
The unequal
distribution of radiation over the Earth is the most important process of the
global climate system. The physical
processes of the atmosphere and ocean are continually working to transfer
energy from the tropics towards the poles.
These massive transports of heat have a massive impact on global
climate.
It is useful to look at how the
distribution of this energy varies by season.
Figure 12 is an image showing net radiation over the Earth in January
and in June. Red and orange colors
represent positive net radiation values, and blue shows negative values. Remember that the sun is near vertical at
23.5oS during the northern hemisphere winter.
Figure 12.
Intensity of net radiation during January (left) and June (right). (Images courtesy of the Univ. of Wisconsin.)
In the next few readings you will learn
about processes that transport energy (heat) around the
planet.
Source material
Hipps, LE. 2010. Personal
communication and readings produced by Dr. Hipps. Professor of Atmospheric
Science, Department of Plants, Soils, and Climate. Utah State University.
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