Download 1 The ocean is an important factor in determining Earth`s climate and

HOW AND WHY DOES THE OCEAN CONTROL EARTH’S CLIMATE?
T
he ocean is an important factor in determining Earth’s climate and its variations. The
ocean ties up gases that influence climate, particularly carbon dioxide. It stores heat
much more effectively than the land or atmosphere, helping to make Earth’s climate
moderate and habitable. The oceans and atmosphere distribute heat around Earth, strongly
affecting regional and global climates.
This theme (Climate - Scale and Structure) compares and contrasts Earth’s climate with that of
Mars and Venus. It also addresses how the oceans lock up carbon dioxide, the properties of
water that affect climate, the hydrological cycle, and the effect of the oceans on local and regional
climates.
Related Themes:
• Greenhouse gases and global warming are described in Climate - Human Interactions.
• The greenhouse effect is addressed in Climate - Human Interactions and Climate Process and Change.
• The role of water phase changes in transporting energy is featured in Climate - Energy.
• How convection is tied to climate is discussed in Climate - Energy.
• A more complete discussion of the global conveyer belt model is available in Climate
- Systems and Interactions.
• The difference in heat capacity of ocean versus land is covered in Climate - Systems
and Interactions.
• How phytoplankton communities convert carbon dioxide into oxygen is addressed
in the Life - Scale and Structure and Life - Systems and Interactions.
• More information about ocean gyres can be obtained in the Oceans - Systems and
Interactions.
Related Activities:
• Properties of Fresh Water and Sea Water
• Earth’s Hydrologic Cycle
• Coastal versus Inland Temperatures
• Ocean Currents and Coastal Temperatures
INTRODUCTION
Earth has the only climate in the Solar System that is hospitable to life as we know it. Our
nearest planetary neighbors, Mars and Venus, are both very unfriendly to life, with Mars being
extremely cold with a very thin atmosphere and Venus being extremely hot with a very thick
atmosphere. Why are there differences? Certainly, distance from the Sun is important. The
presence of the oceans is significant, as well, because the oceans play a key role in controlling
climate. The oceans moderate global climate — keeping it from getting too hot or too cold. And
life is important. Animals in the ocean convert carbon dioxide into shells. The thick layers of
limestone are the result of marine animals removing carbon dioxide from the atmosphere-ocean
system, keeping Earth from overheating.
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EARTH VERSUS MARS AND VENUS
To better understand the importance of the ocean to Earth’s climate, let us first look at our
nearest planetary neighbors, Mars and Venus, neither of which have liquid water on their surfaces [Fig. 1].
Mars has a very thin atmosphere with a surface pressure less than 1% that of Earth’s, and
very cold temperatures. Water is not stable as a liquid on Mars because of this combination of
temperature and pressure. It exists only as ice and vapor, much as carbon dioxide does on Earth
(dry ice and carbon dioxide gas).
Daily temperature swings on Mars’ surface tend to be extremely large, except in polar regions where ice caps act to stabilize the temperature. In the non-polar regions, daily temperature
Figure 1. Global images of Earth, Mars, and Venus. Sometimes called the triad planets, because of
characteristics that they share, these three planets have very different appearances, and very different
climates. Notice the dominance of the oceans and clouds in the image of Earth. Both result from liquid
water.
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swings of 80ºC (144ºF) are common. In part, this is because of the very thin atmosphere, which
does not trap heat very well. What atmosphere there is consists mostly of carbon dioxide gas, a
greenhouse gas. However, there is so little of it that much of the heat escapes anyway. In addition,
the land itself is a very poor heat sink. It heats up very quickly, but it also cools very quickly at
night.
Geologic evidence, such as large outflow channels likely caused by liquid water, indicates
that Mars may have had liquid water and a thicker atmosphere early in its history, both of which
would have acted to moderate the temperature swings on the planet [Fig. 2].
Venus, in contrast, has an atmosphere that is much thicker than Earth’s. The surface pressure
on Venus is almost 100 times greater than that on Earth, equivalent to the pressure 1 kilometers
(0.6 miles) beneath the surface of our oceans. However, Venus has no oceans, and liquid water
has no chance of existing anywhere on its surface. Venus has a very effective greenhouse effect.
The carbon dioxide gas that makes up most of its atmosphere allows visible light from the Sun to
enter the atmosphere, but traps much of the infrared radiation emitted by the surface and in the
lower atmosphere. This effectively traps heat and acts like a blanket.
The surface temperature on Venus is approximately 450ºC (900ºF), hundreds of degrees hotter than needed to boil water at those pressures, so water is only stable as a vapor. Water, possibly as oceans, may have existed in early Venus history, before there was as much carbon dioxide
in the atmosphere. Water would have helped trap carbon dioxide from the atmosphere in other
forms, minimizing the greenhouse effect. However, as Venus warmed, and sources of water
from within Venus and other sources (e.g., from cometary impacts) decreased, the water evapo-
Figure 2. Image of Mars’ surface from Mars Pathfinder. Pathfinder landed in ancient outflow channel
of Ares Vallis July, 1997.
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rated away. Once all the water was transformed into a gas, the heating process accelerated.
Carbon dioxide continued to vent from Venusian volcanoes. Because there was no longer water
to help trap it in other forms, carbon dioxide began to dominate the atmosphere. A runaway
greenhouse condition had begun. As more carbon dioxide entered the atmosphere, it trapped
more and more heat, eventually causing extremely high surface temperatures. Although Earth
is farther from the Sun than Venus, it too would be much warmer if it were not for the oceans,
which help trap carbon dioxide.
LOCKING UP CARBON DIOXIDE
Plants and animals living in
Earth’s oceans have helped to tie
up carbon dioxide in solid forms.
In fact, 99% of all the carbon dioxide that has existed in atmosphere is tied up in ocean sediments. This has occurred through
both non-biologic and biologic
processes; processes made possible because of liquid water.
Coral reefs and the shells of many
sea creatures are created from carbon and oxygen [Fig. 3]. The
great limestone rock formations
on Earth are composed of the
shells of marine animals. The
oceans also contain a much
smaller, but significant fraction of Figure 3. Coral reef.
carbon dioxide dissolved in the
water itself. The locking up of carbon dioxide is crucial for moderating Earth’s climate. However, there are now concerns that increased carbon dioxide emissions from human activity may
be contributing to global warming.
PROPERTIES OF WATER AFFECTING CLIMATE
In addition to locking up carbon dioxide, several properties of water are key to regulating
Earth’s climate. Table 1 lists properties of water; notice how many are unique among similar
materials. At Earth’s atmospheric pressures and temperatures, water is able to exist in three
phases: solid, liquid, and gas. This is not the case on Mars or Venus. Conversions between these
phases assist in transport of energy around Earth.
Water has a very high heat capacity. This means that it takes more energy to heat or cool water
than a similar mass of rock or air. Thus, the oceans heat and cool slower than land, and can store
much more heat than the atmosphere. Without the oceans, Earth’s climate would experience
much larger temperature swings.
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Table 1: Properties of water.
Ta ble 2 : Re s e rv oirs of Av a ila ble Wa t e r on E a rt h
Reservoir
Oceans
Glaciers (liquid equivalent)
Aquifers
Lakes and Rivers
Soil moisture
Atmosphere
Living Biosphere (liquid equivalent)
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Volume
(106 km3)
1350
29
8
0.1
0.1
0.013
0.001
Percentage of
Total
97.3
2.1
0.6
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Water also has a high latent heat of vaporization, the amount of energy required to convert it
from a liquid to a gas. It is also the amount of energy released when water condenses from a gas
to a liquid. Water’s high latent heat of vaporization makes it effective at transferring heat into
the atmosphere through evaporation and for warming the atmosphere when water rains out. So,
rain heats the atmosphere and drives great storms such as hurricanes, typhoons, and thunderstorms. Moreover, the heat released by rain drives much of our atmospheric circulation.
THE HYDROLOGIC CYCLE
In the hydrologic cycle, water goes through various phases as it moves around the globe. This
process takes heat and energy from some places and distributes it elsewhere. The oceans are the
most significant part of the hydrologic cycle as they contain over 97% of the available water on
Earth (including over 99% of all liquid water [Table 2]). In general, the oceans serve to heat and
cool the atmosphere in various locations, strongly affecting patterns of wind and rain.
Figure 4. Water cycle. This figure illustrates the role of water in atmospheric circulation and heat transfer.
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Heat is transferred from the oceans to the atmosphere through the physical processes of evaporation and condensation [Fig. 4]. At any temperature, equilibrium exists when water evaporation
equals condensation. As temperature rises, the more energetic water molecules escape into the
air, or evaporate. This continues until the amount of water vapor in the air is so high that any
additional evaporation is balanced by condensation.
Energy is expended when water goes from its liquid to vapor phase. This energy is usually
supplied by the reservoir from which the evaporation is taking place (i.e., oceans, lakes, rivers,
etc.), thereby cooling the body. This is why we feel cooler when sweat evaporates from our skin.
When water vapor condenses into water droplets, heat is released to the atmosphere, fueling
circulation processes such as convection.
GLOBAL HEAT TRANSPORT BY CURRENTS
In addition to providing water
and heat to the atmosphere as part
of the hydrologic cycle, the oceans
also redistribute heat around Earth
through their own movement. The
oceans transport heat from the
equatorial regions to the polar regions in a global circuit.
In general, ocean currents moderate global climate by transporting
shallow, warm tropical waters to the
cold polar seas. As heat is lost to
the atmosphere in the north, the
colder water sinks below the
warmer surface layer and migrates
throughout the depths of the global
oceans [Fig. 5]. The entire circuit Figure 5. The Global Conveyor Belt. Schematic showing the
takes as long as 1,000 years to com- flow of water as part of the simplified model of a global conveyor belt, which redistributes energy from the equatorial regions
plete.
to the poles.
COASTAL VERSUS INLAND CLIMATES
Ocean currents affect not only global climate, but also regional climate. For example, coastal
climates tend to be much more moderate than inland climates. Consider two cities at approximately the same latitude, one located on an ocean coast, the other located well inland. San
Diego, California, and Phoenix, Arizona, for example, both lie at about thirty-two degrees north
latitude. San Diego is located on the Pacific Coast, whereas Phoenix, situated in North America’s
Sonora Desert, is several hundred kilometers from the nearest ocean shore.
Notice in Figure 6 that the average high temperature for San Diego changes very little from
month to month throughout the year, but the average high temperature in Phoenix changes
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Figure 6. Annual weather charts for San Diego, California (triangles) and Phoenix, Arizona (circles).
Note that, although both cities are at about the same latitude, their monthly high temperatures and rainfall
amounts vary dramatically. This illustrates the difference between coastal and inland climates.
considerably over the course of a year. Because they are at nearly the same latitude, both cities
receive about the same insolation. That is, both receive about the same amount of incident sunlight over the course of a year (disregarding differences in average cloud cover). And yet Phoenix is much hotter in the summer and colder in the winter than San Diego.
The reason for this is the high heat capacity of the Pacific Ocean that moderates San Diego’s
climate. Thus air temperatures over coastal cities are less extreme than those farther inland.
Landmasses have a much lower heat capacity than water. Desert areas are particularly notorious for their inability to retain heat. In the Sonora Desert, for example, it is possible for afternoon
high temperatures to approach 38ºC (100ºF), and then drop to below 10ºC (50ºF) at night. The
difference between seasonal temperature extremes for inland climates is also considerable. Summer high temperatures in Phoenix and the surrounding desert can exceed 46ºC (115ºF); winter
low temperatures can drop below -7ºC (20ºF).
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San Diego and Phoenix are several hundred kilometers apart, but significant differences in
climate can also occur within only a few kilometers of the coast. Even within the city of San
Diego, temperatures a few kilometers inland will be several degrees warmer than the coast on a
summer day.
ONSHORE AND OFFSHORE WINDS
Winds that develop
because of temperature
differences over land and
water also help to moderate near-coastal climates. The climate in areas many kilometers inland from a coast can also
be affected by ocean temperatures because of onshore and offshore winds.
As discussed above,
ocean water temperatures do not change as
dramatically over the Figure 7. Onshore and offshore wind formation. Day-night differences
course of a day as land in the relative temperature of land and ocean sets up onshore and offshore
temperatures. The air winds. During the afternoon, heat rises over the land to draw in cool sea
over the ocean can there- breezes. At night, relatively warm ocean temperatures cause offshore land
fore be substantially breezes to blow.
warmer or cooler than the
air over inland regions. Warm air is less dense than cool air, and therefore tends to rise. As a
column of warm air rises, the air pressure at the base of the column decreases, and cooler air is
pulled in [Fig. 7].
Late in the afternoon, the land is much hotter than the nearby ocean. The hot land heats the
air causing it to rise. Cool air is drawn onshore to replace the rising hot air. This produces an
onshore sea breeze. At night, the process is reversed. The land cools more quickly than the sea
and wind blows from the land toward the ocean. This is the land breeze which is strongest in the
late night and early morning hours. Generally speaking, coastal areas tend to be breezy due to
the difference in air temperatures over land and water.
EFFECT OF OCEAN CURRENT TEMPERATURES
One might think that coastal cities in different parts of the world at the same latitude would
have similar climates. While this may seem reasonable, it is not true because nearby ocean currents may have very different temperatures. Thus, warm or cold water offshore currents can
change the local climate. For example, the climate in San Francisco is much cooler than the
climate in Norfolk, Virginia, which is at the same latitude.
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On the west coast of the United States, cool water from northern latitudes flows south toward warmer latitudes (the California Current). Hence, the waters on the west coast create
cooler coastal temperatures. Another key climate factor along California’s west coast is the seasonal upwelling of cold water from depth. When upwelling is strong, weather is much cooler and
cloudier than when it stops. On the east coast of the United States, warm water flows north
toward cooler latitudes (the Gulf Stream) and creates warmer coastal temperatures. Thus, San
Francisco, California, on the west coast and Norfolk, Virginia, on the east coast, are at approximately the same latitude, but average temperatures in Norfolk tend to be warmer [Fig. 8].
In general, ocean currents (and therefore the coastal climate) along the east coasts of continents are warmer than those along the west coasts. This is because ocean currents move in large
circular patterns known as gyres. Gyres move water pole-ward along the western boundary of
oceans and equator-ward along the eastern boundary. The influence of the North Atlantic’s western boundary current extends well beyond the east coast of the United States. This current is
called the Gulf Stream between Cape Hatteras, North Carolina and the Grand Banks of New-
Figure 8. Plot of average temperatures of San Francisco and Norfolk. Note that, although both cities
are at about the same latitude, the range of temperatures for each city is quite different over a year.
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foundland, Canada. It carries heat across the Atlantic, and warms the average climate of northern Europe by several degrees. This makes San Francisco’s climate hardly warmer than that of
Dublin (Ireland), despite the fact that San Francisco is over 1,600 kilometers (960 miles) closer to
the equator than Dublin.
CONCLUSION
The influence of oceans on our planet is profound, providing a relatively moderate climate
for millions of living species. The properties of water itself allow it to efficiently transport heat
around the globe via the hydrologic cycle, as well as atmospheric and ocean circulation systems.
Ocean currents influence global, regional, and local climates in a variety of ways. Much of Earth’s
human population lives near the seashore, partly because of the moderating influence the oceans
have on coastal climates.
VOCABULARY
climate
evaporation
greenhouse gas
heat sink
latent heat of vaporization
runaway greenhouse
condense (condensation)
global conveyor belt
gyre
hydrologic cycle
offshore winds
upwelling
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convection
greenhouse effect
heat capacity
insolation
onshore winds