Download 1 The primary energy source driving Earth`s climate is sunlight

WHAT ENERGIES DRIVE OUR CLIMATE?
T
he primary energy source driving Earth’s climate is sunlight. Some of the sunlight
that reaches Earth is reflected back to space, mostly by clouds, but much of it is
absorbed by the land and ocean in the tropics. Heat is lost by radiation. Overall, the
tropics absorb more heat than they lose, and the poles lose more heat than they absorb. The
imbalance between poles and equator drives the planetary heat engine. That energy is
redistributed within the climate system through various processes. One of the most important is convection, which drives the circulation of the atmosphere on both local and global
scales. Solar energy is also distributed through ocean circulation and winds. The atmospheric and oceanic systems are intimately connected. The atmosphere drives ocean currents, and the heat supplied from the ocean is vital to the release of energy into the atmosphere. Atmospheric wind patterns govern oceanic flows, which in turn influence where
and how much heat is released into the atmosphere. Furthermore, atmospheric cloud cover
determines where and how much the ocean will be heated.
This theme (Climate - Energy) discusses the energy that drives climate and some of the ways
that energy is redistributed throughout the atmospheric and oceanic systems.
Related Themes:
• Earth’s hydrologic cycle and the properties of water are addressed in Climate - Scale
and Structure.
• Climate changes are examined in Climate - Process and Change.
• How ocean circulation, upwelling, and downwelling affect climate is explained in
Climate - Systems and Interactions.
• How satellites are used to study climate is presented in Climate - Measurements.
• Seasonal changes in sea level are included in Oceans - Process and Change.
• The Coriolis Effect is thoroughly covered in Oceans - Energy.
• Geostrophic ocean circulation is discussed in Oceans - Systems and Interactions.
Related Activities:
• Absorbing Light: Dark Versus Bright
• Solar Energy and Distance
• Convection
INTRODUCTION
When we casually think of the weather, we are usually interested in local predictions. Will it
be rainy or sunny tomorrow? Climate is long-term weather averaged over seasons, years, and
even longer time periods. Climate is determined by a large system that includes the atmosphere,
the hydrosphere (liquid and frozen water), the lithosphere (solid land and the ocean floors), the
biosphere (living things), the orientation of the earth in space, and external factors like the Sun.
Climate changes take place over the course of years, thousands or even millions of years. Climate changes are related to changes in atmospheric and oceanic temperature and motion.
1
SOLAR ENERGY AND RADIATION BALANCE
The solar energy incident on Earth is about 1,370 watts per square meter, on an area oriented
perpendicular to the radiation. This value is determined by the Sun’s luminosity and Earth’s
distance from the Sun. About 40% of this energy is absorbed by the land and ocean. Some of that
is reradiated from the surface to the atmosphere, thereby heating the atmosphere. Approximately 40% of incoming solar radiation is reflected by clouds directly back into space. Clouds
reflect more solar energy back into space than do clear skies. Finally, only 20% of the incoming
solar energy is absorbed directly by the atmosphere through various absorbing components such
as ozone, carbon dioxide, and water vapor molecules [Fig. 1]. Water vapor, a very strong absorber of infrared radiation, is created at low levels in the atmosphere mostly by the evaporation
of surface water from the oceans. Seawater also absorbs a great deal of solar radiation and
circulates heat around Earth.
Figure 1. Radiative transfer. This schematic shows the amount of sunlight incident on Earth being
reflected and absorbed.
2
The average temperature of Earth depends on many factors, including Earth’s distance from
the Sun. Because Earth has a curved surface, solar radiation input varies with latitude. A ray of
sunlight is spread over a larger area at high latitudes, where the Sun is closer to the horizon, than
it is in areas less than 23.5º latitude where the Sun is almost directly overhead. Therefore, there is
always a significant temperature difference between the poles and the equator [ Fig. 2].
Seasonal effects also play a role in the degree of solar heating. Because Earth’s rotational axis
is tilted, day is longer than night in the summer hemisphere, and night is longer than day in the
winter hemisphere [Movie 1]. If, like Venus, Earth had almost no axial tilt, day and night length
would be equal at all latitudes and the Sun would only shine directly overhead at the equator.
Thus, on Earth, uneven solar heating with latitude is affected by: (1) variations in the angle at
which the sunlight hits the ground; and (2) inequalities in the length of day.
Figure 2. Incident sunlight in the tropics. This figure shows the incident solar radiation at northern
summer solstice. A ray of sunlight is spread over a larger area of ground at high latitudes, where the Sun
is close to the horizon, than at lower latitudes (i.e., less than 23.5º) where the Sun is almost overhead. The
day is longer than the night in the summer hemisphere and the night is longer than the day in the winter
hemisphere. Both factors influence the degree of incident solar radiation.
3
Movie 1. The influence of Earth’s
tilt on seasons. Day and night lengths
vary throughout the year because of
the orientation of Earth’s tilted spin
axis in relationship to the Sun. Associated changes in solar energy input
to each hemisphere cause seasons. In
the northern hemisphere, summer begins at the summer solstice when the
north pole is titled toward the Sun.
Winter in the northern hemisphere
begins when the north pole is oriented
away from the Sun, around December 21. In both hemispheres, day and
night lengths are equal on two days
each year, the autumnal and vernal
equinoxes.
HEAT RETENTION: OCEAN VERSUS ATMOSPHERE
The ocean is slower to change temperature than the atmosphere and thus helps to moderate
climate. Heat is gained and lost through the atmosphere over the span of hours. On the other
hand, oceanic heat transport over large areas occurs over weeks, months, and years. This is because the ocean has a tremendous capacity to store heat, and acts as the climate’s thermal memory.
The amount of heat that the atmosphere stores is equivalent to the heat stored by only the top 3
meters of the ocean.
More than 98% of the ocean is so deep (the ocean’s average depth is 4000 meters) that surface
heating from sunlight does not penetrate it and, overall, its temperature is relatively independent of season. However, the oceans’ upper 100 meters or so are significantly heated in the summer and cooled in the winter. These changes in heating and cooling of the upper ocean cause sea
level to vary by a few centimeters from season-to-season [Fig. 3]. Whereas the highest and lowest
atmospheric temperatures occur in the summer and winter seasons, the highest and lowest sea
levels occur in the fall and spring. The lag between atmospheric and ocean seasons results from
the differences in heat storage capability of air (low) and water (high).
ENERGY REDISTRIBUTION
Because Earth is round, most of the sunlight that reaches Earth is deposited in the tropical regions
of the planet. This makes tropical regions warmer, but the difference is much smaller than it
would be if significant amounts of heat were not redistributed towards the poles. Heat is
redistributed around Earth and to various parts of the climate system through various processes,
including ocean processes, atmospheric processes, and interactions between the two.
4
Figure 3. Ocean seasons. These four maps display how sea level changes (which are related
to ocean heat storage) vary from season to season as monitored by the Earth-orbiting TOPEX/
Poseidon satellite. The observations cover four seasons from September 1992 to August 1993.
Each map represents a seasonal variation of sea level from its annual average. The highest sea
surface elevation occurs in the fall, after a full summer's heating. The surface then cools off in
the winter, and reaches its lowest point in the spring before starting to warm up again in the
summer.
Heat is transferred from the oceans to the atmosphere through evaporation and condensation.
Changes in water phase play an important role in atmospheric circulation and heat transfer.
Energy is taken up when water goes from its liquid to vapor phase. This energy is usually
supplied by the reservoir, such as the oceans, from which the evaporation is taking place. When
water vapor condenses into water droplets (which may fall to the ground as rain), heat is released
to the atmosphere, causing the air to warm and driving atmospheric circulation, including convection. Because the energy available to evaporation is variable, atmospheric water vapor content varies around globe [Fig. 4].
5
EARTH’S ATMOSPHERE
IN MOTION
Atmospheric motion
(wind) is caused by differential heating of air
(including the poles versus the equator), especially by heat released
when water condenses as
rain. To understand
what causes wind, consider one example of sunlight striking the planet’s
surface. A dark patch of
ground will get hotter
than a lighter patch because it absorbs more of
the incident sunlight.
The air above the dark
patch will be heated
through the process of
conduction. As the air
warms, it expands, making it less dense, or
Figure 4. Derived atmospheric water vapor. These data from the TOPEX/
Poseidon satellite show that the highest values of atmospheric water vapor
occur above the tropical oceans where the waters and atmosphere are the
warmest, enabling the most evaporation.
lighter than the air above it.
This causes the heated air to
start to move upwards. The
same principal is what makes
hot air balloons float up. Convection is the process by which
heated gas or liquid moves upward due to density changes.
When the warmed air begins to
convect, then surrounding air
flows in horizontally to fill the
void.
Figure 5. Onshore wind formation. During the afternoon, heat rises
over the land to draw in cool sea breezes.
6
For example, the process of
convection is important for producing local winds [Fig. 5]. Onshore winds often result when
land is warmer than the ocean.
During the day, land heats up
faster, which causes the air
above it to convect. Then cooler,
more dense air over the ocean begins
to blow towards the land to fill in the
space being vacated by the rising air.
GLOBAL ATMOSPHERIC CIRCULATION
PATTERNS
About half the heat transported
to the poles is carried by the ocean
currents. The other half is transported by winds. The global wind
and current patterns that transport
heat are complicated by the rotation
of Earth. Thus it is illustrative to begin the concept of global heat transport patterns with a simple model: a
non-rotating, water-covered Earth.
If Earth were not rotating and
completely covered with water, wind
patterns would be relatively simple
[Fig. 6]. Heated air near the equator
would rise and condense water vapor, in the form of rain. The water vapor-depleted air would flow toward
the poles and cool. Near the poles, the
Figure 6. Simple model of atmospheric winds. On a nonrotating, water-covered Earth, unequal heating between the equator and poles would cause simple two-celled circulation. In the
northern hemisphere, surface winds would blow from north to
south and winds at high altitude would blow in the opposite
direction. The scenario would be reversed in the southern hemisphere.
cold, dry air would sink and flow along
the surface toward the equator, where
the process would begin again.
Figure 7. The Coriolis effect. As Earth turns toward the
east, currents in the northern hemisphere are deflected to the
right; currents in the southern hemisphere are deflected to
the left.
7
Earth’s rotation and gravity affect
atmospheric and oceanic circulation
patterns. The rotation of Earth leads to
the Coriolis effect; this causes air and
ocean currents to be deflected to the
right in the northern hemisphere and
to the left in the southern hemisphere
[Fig. 7]. Pressure gradient forces drive
wind and ocean currents across lines
of equal pressure. However, when air
or ocean currents begin to move, the
Coriolis effect deflects it. Soon winds
and currents are deflected 90° (perpendicular) and travel along the lines of
equal pressure, or isobars [Movie 2].
In the atmosphere and oceans,
the Coriolis effect causes geostrophic (Earth turning) winds
and currents to form. In nature,
the overall effect is that particles
in the atmosphere and oceans
follow approximately circular
paths of constant pressure. On
Earth, the atmosphere can be
modeled as a six-celled pattern
with two tropical cells, two midlatitude cells, and two polar cells
[Fig. 8].
Unequal heating of Earth’s
surface between the equator and
poles sets the atmosphere in motion. Warm air rises in the equatorial regions and begins to move Movie 2. Effect of Coriolis force on wind and ocean currents.
toward the poles. This forms The Coriolis force deflects winds and currents until they are paralband of clouds near the equator lel to isobars, then the pressure gradient balances Coriolis force.
called the Intertropical Convergence Zone [Fig. 8].
Near 30º north
and south latitude,
cooling of the atmosphere at high
altitudes makes
the air sink and
form a belt of high
pressure. Vertical
atmosphere circulation cells in the
tropics (i.e., between the equator
and 30º north and
south) are known
as Hadley cells [Fig.
9].
At 30º latitude
in both hemiFigure 8. Six-celled model of atmospheric convection. The Coriolis effect influ- spheres, air sinks
ences global wind patterns to form three major convection cells in each hemisphere. and flows along
8
Earth’s surface toward the equator.
These are the trade
winds. The Coriolis
effect deflects the
trade winds to the
right in the northern
hemisphere and to
the left in the southern
hemisphere.
Thus, north the of
equator, tropical
trade winds flow
from the northeast.
Similarly, tropical
trade winds flow
from the southeast in
the southern hemisphere. Along the
equator, trade winds
blow generally from
east to west.
Figure 9. Equatorial Cloud Formation and Tropical Trade Winds. Hadley
Cells are characterized by rising air and cloud formation at the equator. At Earth’s
Two mid-latitude
surface, Hadley cells form tropical trade winds whose directions are deflected cells, generally found
by the Coriolis effect (as shown).
between 30º and 60º latitudes in
both hemispheres, drive westerly
winds (i.e., winds that blow from
the west) [Movie 3]. Two polar
cells, generally located between
60º north and south and the poles,
include easterly (i.e., winds that
blow from the east) winds [Fig.
8]. At about 50º north latitude,
westerly winds meet cold dense
air blowing from the polar regions in a zone called the polar
front. The southern hemisphere
has a similar feature called the
Antarctic front. These boundaries
between cold and warm air
masses are susceptible to highly Movie 3. Wind patterns in the Pacific. These data from the NASA
variable weather.
Scatterometer (NSCAT) on September 20, 1996 show ocean surface wind speeds and directions over the Pacific.
9
CONCLUSION
The primary driver of Earth’s weather is energy from the Sun. The average weather over time
is the climate. Earth’s distance from the Sun allows water to exist in solid, liquid, and vapor
phase. The transition between these phases is key for transferring energy around the globe.
Liquid water in the oceans stores and redistributes heat, thus moderating climate on Earth. Heat
is also redistributed around Earth by the evaporation and condensation of water and by atmospheric circulation. The Earth’s rotation influences the patterns of atmospheric circulation and
ocean currents and thus affects regional climate, as well.
VOCABULARY:
biosphere
conduction
equinox
gradient
infrared radiation
isobar
solstice
climate
condense (condensation)
convection
Coriolis effect
evaporation
geostrophic
Hadley cells
hydrosphere
Intertropical Convergence Zone (ITCZ)
lithosphere
ozone
trade winds
vapor phase
10