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. 1
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. 2
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. 3
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. 4
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 ---------
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. 6
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 7
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). 8
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. 9
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. 10
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
