5 Climates and Soils
5Climates and Soils A beautiful garden. This garden is located at the Sequim Gardens, in Washington State. Where Does Your Garden Grow? If you have ever planted a garden, you know that you have a lot of decisions to make. You can choose from a vast selection of fruits and vegetables, not to mention a dizzying array of flowers, shrubs, and trees. Although you have a large number of choices, not all plants grow well in all locations. To help gardeners determine what plants can survive and flourish in their location, the U.S. Department of Agriculture has developed a map of plant hardiness zones. Plant hardiness zones take into account the coldest temperatures that occur during the winter. The hardiest plants tolerate very cold temperatures, whereas others are too sensitive to survive harsh winters. The plant hardiness zones follow the minimum temperature typically reached in locations throughout North America. Zone 10, for
example, is found in southern Florida, where the average minimum temperature in winter is between –1 °C and 4 °C. In contrast, zone 1 is found in Alaska, with an average minimum winter temperature below – 45 °C. In looking at the map of plant hardiness zones, several patterns emerge. First, there appears to be an ordering of zones that is related to latitude, particularly through the middle of the continent. Higher latitudes receive less sunlight in the winter and have lower winter temperatures. However, a second pattern of plant hardiness zones curves up along the coastlines. For instance, in the interior of the continent, zone 8 spans states such as Louisiana, Alabama, and Georgia. Along the East Coast, however, zone 8 extends all the way up to Virginia. Along the West Coast, zone 8 extends north, all the way through the state of Washington. These patterns occur because the two coasts are adjacent to oceans, which contain warm tropical waters that circulate up from the equator. These warm waters heat the air during winter and make the land along the coasts warmer than the interior of the continent at the same latitude. A third pattern can be seen in elevation; mountain tops have colder temperatures than lower elevations. “The map of plant hardiness zones illustrates how climates around the world are the result of a complex combination of sunlight, latitude, elevation, air currents, and ocean currents.” The hardiness zone map also shows that the West Coast has warmer winter temperatures than the East Coast. This allows farmers in California to grow fruits and vegetables during the winter. The difference in temperatures between the East and West Coasts is caused by prevailing winds that blow from west to east. In the winter along the West Coast, the winds carry the warmer ocean air toward the coast, making it warm. Along the East Coast, however, winds
carry cold air from the middle of the continent toward the coast and push the warm ocean air away from the coast. As a result, the East Coast remains colder than the West Coast during the winter. The map of plant hardiness zones illustrates how climates around the world are the result of a complex combination of sunlight, latitude, elevation, air currents, and ocean currents. In this chapter, we will explore how global processes affect the distribution of climates on Earth and how climates affect the types of soils that form. Plant hardiness zones for North America. The warmest zones occur in the southern United States and along the coasts.
Learning Objectives
After reading this chapter, you should be able to:
5.1 Describe how Earth is warmed by the greenhouse effect.
5.2 Explain the unequal heating of Earth by the Sun.
5.3 Illustrate how the unequal heating of Earth drives air currents in the atmosphere.
5.4 Demonstrate how ocean currents also affect the distribution of climates.
5.5 Describe the role of smaller-scale geographic features in affecting regional and local climates.
5.6 Explain how climate and the underlying bedrock interact to create a diversity of soils. As we saw in Chapter 3, the climate of a region on Earth refers to the average atmospheric conditions measured over many years. Climates can range widely, from very cold areas near the North and South Poles, to hot and dry deserts at approximately 30° N and 30° S latitudes, to hot and wet areas near the equator. In this chapter, we will examine factors that determine the location of climates around the world. With an understanding of climates, we will move on to look at how soils are formed. As we will see in subsequent chapters, these differences in climates and soils help determine the distribution of organisms around the globe. A number of factors contribute to the different climate patterns. Some of the most important are the greenhouse effect, the unequal heating of Earth by solar energy, atmospheric convection currents, the rotation of Earth, ocean currents, and a variety of smaller-scale topographic features.
5.1 Earth is warmed by the greenhouse effect Solar radiation provides the vast majority of the energy that warms Earth and that organisms use. However, solar radiation alone is not sufficient to warm the planet; gases in the atmosphere also play a critical role.
The Greenhouse Effect
Solar radiation warms Earth through a series of steps, illustrated in Figure 5.1. About one-third of the solar radiation emitted toward Earth is reflected by the atmosphere—the 600-km thick layer of air that surrounds the planet—and heads back into space. The remaining solar radiation penetrates the atmosphere. Much of the high-energy radiation—including ultraviolet radiation—is absorbed in the atmosphere. The rest passes with most of the visible light through the atmosphere. When this radiation subsequently strikes clouds and the surface of Earth, a portion is reflected back into space and the rest is absorbed. As clouds and Earth’s surface absorb this radiation, they begin to warm and emit lower-energy infrared radiation. The heat you feel radiating into the air if you stand on a hot asphalt road is an example of this infrared radiation.
Figure 5.1 The greenhouse effect. Of the solar radiation that strikes Earth, some is reflected back into space and the rest penetrates the atmosphere, where much of it warms clouds and the planet’s surface. These warmed objects emit infrared radiation back toward the atmosphere, where it is absorbed by greenhouse gases. The warmed greenhouse gases reemit infrared radiation back toward Earth, which causes the surface to warm further. Atmosphere The 600-km thick layer of air that surrounds the planet. Infrared radiation is readily absorbed by gases in the atmosphere. The gases are warmed by the infrared radiation and then re-emit infrared radiation in all directions. Some of this energy goes out into space and some goes back toward the planet’s surface. This process of solar radiation striking Earth, being converted to infrared radiation, and then being absorbed and reradiated by atmospheric gases back to Earth is known as the greenhouse effect. The name comes from the fact that the effect resembles a gardener’s greenhouse with windows that hold in the heat from solar radiation. Greenhouse effect The process of solar radiation striking Earth, being converted to infrared radiation, and being absorbed and re-emitted by atmospheric gases.
Greenhouse Gases
There are many different gases in the atmosphere, but only those that absorb and re-emit infrared radiation and contribute to the greenhouse effect are known as greenhouse gases. In fact, if we exclude water vapor, 99 percent of the gases in the atmosphere are oxygen (O2) and nitrogen (N2), and neither gas functions as a greenhouse gas. This means that greenhouse gases, which play such a major role in keeping our planet warm, compose only a small fraction of the atmosphere. This also means that even small changes in the concentrations of greenhouse gases can have large impacts on Earth’s temperature. The two most prevalent greenhouse gases are water vapor (H2O) and carbon dioxide (CO2). Other naturally occurring greenhouse gases include methane (CH4), nitrous oxide (N2O), and ozone (O3). These gases have a variety of natural sources: H2O comes from large bodies of water and the
transpiration of plants; CO2 comes from decomposition, respiration of organisms, and volcanic eruptions; CH4 comes from anaerobic decomposition; N2O comes from wet soils and low-oxygen regions of water bodies; and O3 comes from ultraviolet radiation breaking apart O2 molecules in the atmosphere and causing each molecule to combine with another O2 molecule. The naturally occurring greenhouse effect is quite beneficial to organisms on Earth. Without this phenomenon, the average temperature on Earth, which is currently 14 °C, would be much colder, about −18 °C. The concentrations of greenhouse gases in the atmosphere are increasing. As we noted in Chapter 4, the concentration of CO2 in the atmosphere has substantially increased over the past two centuries due to increased combustion of fossil fuels by automobiles, electric generating plants, and other industrial processes. At the same time, there have also been increases in methane and nitrous oxide from a variety of anthropogenic sources that include agriculture, landfills, and the combustion of fossil fuels. Finally, there are greenhouse gases that are not naturally produced, such as chlorofluorocarbons that have been manufactured to serve as propellants in aerosol cans and as refrigerants in freezers, refrigerators, and air conditioners. Although these human-created compounds exist at much lower concentrations than water vapor or CO2, each molecule can absorb much more infrared radiation than water vapor and CO2. In addition, each of these human-created molecules persists in the atmosphere for hundreds of years. A steady increase of these manufactured gases has raised concerns among scientists, environmentalists, and policy makers. Because greenhouse gases absorb and re-emit infrared radiation to Earth and its atmosphere, it makes logical sense that an increase in the concentration of greenhouse gases in the atmosphere would cause an increase in the average temperature of Earth. This expectation has been borne out. Based on thousands of measurements made throughout the world, the average air temperature of the planet’s surface increased by approximately 1 °C from 1880 to 2016. While some regions, such as Antarctica, have become 1 °C to 2 °C cooler, other regions, such as Northern Canada, have become up to 4 °C warmer. In fact, over the 136-year period of monitoring temperatures around the world, 16 of the 17 warmest
years occurred from 2001 to 2016. As we will see at the end of this chapter, these temperature changes are altering global climates.
Concept Check
1. What are the steps involved in the greenhouse effect? 2. How does the human production of greenhouse gases lead to global warming? 3. If 99% of all gases in the atmosphere (excluding water vapor) are nitrogen and oxygen, why aren’t climate researchers focused on changes in the concentrations of these gases?
#### 5.2 There Is an Unequal Heating of Earth by the Sun
5.2 There is an unequal heating of Earth by the Sun The differences in temperature around the globe are the result of how much solar radiation strikes the surface of Earth at a given location. Differences in solar radiation are determined by the angle of the Sun striking different regions of the globe, the depth of the atmosphere that the energy passes through, and seasonal changes in the position of Earth relative to the Sun.
The Path and Angle of the Sun
Consider the position of the Sun during the March and September equinoxes when the Sun is positioned directly over the equator. At these times of the year, the equator receives the greatest amount of solar radiation and the poles receive the least. Three factors dictate this pattern: the distance that sunlight must pass through Earth’s atmosphere, the angle at which the Sun’s rays hit Earth, and the reflectivity of Earth’s surface. As illustrated in Figure 5.2, before the rays of the Sun reach Earth, they must travel through Earth’s atmosphere. When they pass through the atmosphere, gases absorb some of the solar energy. Following the path of the rays, you can see that the distance traveled through the atmosphere is shorter at the equator than at the poles. This means that less solar energy is removed by the atmosphere before it strikes Earth at the equator.
Figure 5.2 Unequal heating of Earth. When the Sun is directly over the equator, its rays travel through less atmosphere and are spread over a smaller area. Near the poles, however, the rays of the Sun must travel through more atmosphere and are spread over a larger area.
The intensity of solar radiation that strikes an area also depends on the angle of the Sun’s rays. Looking again at Figure 5.2, you can see that when the Sun is positioned directly above the equator, the rays of the Sun strike Earth at a right angle. This causes a large quantity of solar energy to strike a small area. In contrast, near the poles the rays of the Sun strike Earth at an oblique angle, which causes the solar energy to spread over a larger area. As a result, Earth’s surface receives more solar energy per square meter near the equator than near the poles. These differences in solar intensity translate into differences in temperatures with latitude, as we saw in our opening story on plant hardiness zones. Finally, some surfaces of the globe reflect solar energy more than others. Light-colored objects reflect a higher percentage of solar energy than darkcolored objects, which absorb most incoming solar energy. For example, asphalt absorbs 90 to 95 percent of the total solar energy that strikes its surface, which explains why asphalt pavement becomes so hot on a sunny summer afternoon. On the other hand, cropland reflects 10 to 25 percent of the total solar energy that strikes its surface and fresh snow reflects 80 to 95 percent. The fraction of solar energy reflected by an object is its albedo. As you can see in Figure 5.3, the more solar energy reflected, the higher the albedo.
Figure 5.3 Albedo effect. Light-colored objects such as fresh snow reflect a high percentage of incoming solar energy, and dark-colored objects reflect very little. The average albedo of Earth is 30 percent. Albedo The fraction of solar energy reflected by an object. The unequal heating of Earth explains the general pattern of declining temperatures as we move from the equator to the poles. At the equator, the Sun’s rays lose less energy to the atmosphere, solar energy is spread over a smaller area, and the low albedo of dark-colored forests causes much of this energy to be absorbed. Near the poles, however, the Sun’s rays lose much more of their energy to the atmosphere, solar energy is spread over a larger area, and the high albedo of the snow-covered land causes much of this solar energy to be reflected. This helps explain why the plant hardiness zone numbers we discussed at the beginning of this chapter generally decrease as you move to higher latitudes.
Seasonal Heating of Earth
The relationship between the Sun and Earth also causes seasonal differences in temperatures on Earth. The axis of Earth is tilted 23.5° with respect to the
path Earth follows in its orbit around the Sun. Figure 5.4 illustrates how this tilt affects the seasonal heating of Earth. During the March equinox, the Sun is directly over the equator. As we approach the June solstice, the orbit and tilt of Earth cause the Sun to be directly over 23.5° N latitude, which is also known as the Tropic of Cancer. In September, the Sun is back to being directly over the equator, and in December, the Sun is directly over 23.5° S latitude, which is also known as the Tropic of Capricorn.
Figure 5.4 Seasonal heating of Earth. The central axis of Earth is tilted 23.5°. Because of the tilt, the Northern Hemisphere receives the most direct sunlight during the June solstice and the Southern Hemisphere receives the most direct sunlight during the December solstice. Locations near the equator receive the most direct sunlight during the March and September equinoxes. The tilt of Earth as it orbits around the Sun causes the Northern Hemisphere to receive more solar energy between March and September than the Southern Hemisphere. During this time the daylight period in the Northern Hemisphere is greater than the nighttime period, and the Sun’s angle is 90° somewhere over the Northern Hemisphere. This means that more intense solar radiation is produced per unit area and for a longer period of time. Between the fall equinox in September and the spring equinox in March, the situation reverses and the Southern Hemisphere has longer days
and receives more direct solar energy than the Northern Hemisphere. The latitude that receives the most direct rays of the Sun, known as the solar equator, shifts throughout the year—from 23.5° N latitude in June to 23.5° S latitude in December. These are the warmest latitudes on Earth and are known as the tropical latitudes. Solar equator The latitude receiving the most direct rays of the Sun. Seasonal changes in temperature vary as Earth traces its annual path around the Sun. While the average temperatures of the warmest and coldest months in the tropics differ by as little as 2 °C to 3 °C, at higher latitudes in the Northern Hemisphere, average monthly temperatures vary by an average of 30 °C over the year and extreme temperatures vary by more than 50 °C annually.
1. Why is solar energy per unit more intense near the equator than near the poles? 2. What is the albedo effect? 3. What is the solar equator?
Analyzing Ecology
Regressions As we have discussed, latitudes closer to the equator receive more solar radiation than latitudes closer to the poles. Given this observation, lower latitudes should also have warmer temperatures than higher latitudes. In fact, understanding the nature of this relationship would help us determine exactly how much temperature changes with latitude. When we want to know how one variable changes in relation to another, we use a statistical
tool called regression. In Chapter 4, we saw that a correlation determines if there is a relationship between two variables. A regression determines whether there is a relationship and also describes the nature of that relationship. Regression A statistical tool that determines whether there is a relationship between two variables and that also describes the nature of that relationship. To help illustrate this idea, we can use data on the average January temperature from 56 cities around the United States, spanning the latitudes of the contiguous 48 states. If we plot the relationship between city latitude and average city temperature in January, we obtain the following graph: In this case, the relationship between the two variables follows a straight line, and we have drawn a line of best fit through the distribution of the data points. This is a regression line because it represents the relationship between the two variables. It informs us about the nature of the relationship through the slope and intercept of the line. For these data, the regression can be described using the equation of a straight line, where Y is the dependent variable, X is the independent variable, m is the
slope of the line, and b is the Y-intercept of the line at the point where X = 0. In this example, the slope is −1.2 and the intercept is 43: Y=mX+bTemperature = -1.2×latitude + 43 This regression equation tells us that for every 1 degree increase in latitude, the average temperature in January decreases by 1.2 °C. Note that while the simplest form of a regression is a straight line, regression lines can also be curvilinear. YOUR TURN Based on the relationship between latitude and temperature, use the regression equation to estimate the average January temperature at 10, 20, and 30 degrees of latitude.
#### 5.3 the Unequal Heating of Earth Drives Air Currents in the Atmosphere
5.3 The unequal heating of Earth drives air currents in the atmosphere Earth’s uneven heating helps determine atmospheric convection currents, which are the circulations of air between the surface of Earth and the atmosphere. The patterns of air circulation play a major role in the location of tropical rainforests, deserts, and grasslands throughout the world. In this section, we will explore how the interaction of Earth’s unequal heating and the properties of air creates atmospheric convection currents. Atmospheric convection currents The circulations of air between the surface of Earth and the atmosphere.
Properties of Air
Four properties of air influence atmospheric convection currents: density, water vapor saturation point, latent heat release, and adiabatic heating or cooling. Air density In regard to density, when air warms, it expands and becomes less dense. As a result, when air becomes warmer next to the surface of Earth, it becomes less dense than the air above it. This causes the warm air to rise. This is the initial step in creating convection currents. Water vapor saturation point As air temperature increases, its capacity to contain water vapor—the gaseous form of water—increases. The graph in Figure 5.5 shows the relationship between air temperature and the maximum amount of water vapor that the air can contain. Although the capacity to contain water increases at higher temperatures, there is always a limit, known as the saturation point. When the water vapor content of air exceeds the saturation point at any given temperature, the excess water vapor condenses and changes into either liquid water or ice. When the water vapor content is below the saturation point, liquid water or ice can be converted to water vapor. For example, at 30 °C, air can contain up to 30 g of water vapor per
m3. Air that contains the maximum amount of water vapor has reached its saturation point. If the air at 30 °C cools to 10 °C, the saturation point of the air will decrease to 10 g of water vapor per m3. As a result, the excess vapor changes phases to liquid water and produces clouds and precipitation. The relationship between temperature and water vapor saturation affects patterns of evaporation and precipitation around the world. This, in combination with air currents, determines the distribution of wet and dry environments around the world.
Figure 5.5 Saturation point of water vapor in air. As the temperature of the air increases, the air is able to contain greater amounts of water vapor. Saturation point The limit of the amount of water vapor the air can contain. Latent heat release
Another property of air to consider when contemplating atmospheric convection currents is the release of heat. As you might recall from our discussion of the thermal properties of water in Chapter 2, converting liquid water to water vapor requires a great deal of energy. In the reverse process, known as latent heat release, water vapor converted back to liquid water releases energy in the form of heat. Latent heat release is significant because whenever water vapor exceeds its saturation point, condensation will cause a release of heat that warms the surrounding air. Latent heat release When water vapor is converted back to liquid, water releases energy in the form of heat. Adiabatic heating and cooling The final factor to consider in regard to convection currents is the movement of air in response to changes in pressure. Near the surface of Earth, the gravitational pull on all molecules in the atmosphere brings many of the molecules close to Earth’s surface. An increase in the number of molecules causes an increase in air pressure near Earth’s surface. As one moves higher into the atmosphere, the air contains fewer total molecules, which lowers the air pressure. As a result, when air moves between the surface of Earth and the atmosphere, it experiences changes in pressure. Air pressure is related to the frequency of collisions among air molecules, which also influences temperature. Lower rates of collision cause lower temperatures. As a result, when air moves higher up into the atmosphere and experiences lower pressure, the air expands and the temperature decreases—a process known as adiabatic cooling. Conversely, when air moves down to Earth’s surface and experiences higher pressure, the air compresses and the temperature increases in a process known as adiabatic heating. Adiabatic cooling The cooling effect of reduced pressure on air as it rises higher in the atmosphere and expands. Adiabatic heating The heating effect of increased pressure on air as it sinks toward the surface of Earth and decreases in volume.
Formation of Atmospheric Convection Currents
Understanding the above four properties of air helps us understand how atmospheric convection currents are formed. Let’s begin by looking at the equator during the March or September equinoxes when the Sun is directly over the equator. As you can see in Figure 5.6, solar energy warms the air at the surface of Earth. This warming causes the air to expand and rise. As air rises into regions of decreased atmospheric pressure, it expands. As the air expands, the temperature of the air cools through the mechanism of adiabatic cooling. This cooled air has a reduced capacity to contain water vapor, so the excess water vapor condenses and falls back to Earth as rain. This process, in which the surface air heats, rises, and releases excess water vapor in the form of rain, is the primary reason that latitudes near the equator experience high amounts of rainfall.
Figure 5.6 The circulation of air in Hadley cells. In this example, the Sun is directly over the equator, as happens during the March and September equinoxes. At the latitude receiving the most direct sun, a column of warm air rises and the intertropical convergence zone (ITCZ) drops its precipitation. After rising more than 10 km into the atmosphere, the now cool, dry air circulates back to Earth at approximately 30° N and 30° S latitudes. Returning to Figure 5.6, we can see that when the water vapor condenses, it causes latent heat release, which further warms the air. This enhances the upward motion of the air, condensation, and rainfall. As the air pressure continues to fall with rising altitude, the air temperature continues to drop. At
high altitudes, the cool, dry air is pushed from below by more rising air and begins to move horizontally toward the poles. The upward movement of air is the driving force behind atmospheric convection currents, but it is just the first of a series of steps in the process. Once the cool, dry air is displaced horizontally toward the poles, it begins to sink back toward Earth at approximately 30° N and 30° S latitudes. As Figure 5.6 illustrates, this dry air sinks toward Earth where increased pressure causes it to compress. As the air compresses, it experiences adiabatic heating. By the time the air falls back to the surface of Earth, it is hot and dry. This explains why many of the major deserts of the world—which are characterized by hot, dry air—are located at approximately 30° N and 30° S latitudes. Once this hot, dry air reaches the ground, it flows back toward the equator, completing the air circulation cycle. The two circulation cells of air between the equator and 30° N and 30° S latitudes are known as Hadley cells. The area where the two Hadley cells converge and cause large amounts of precipitation is known as the intertropical convergence zone (ITCZ). Hadley cells The two circulation cells of air between the equator and 30° N and 30° S latitudes. Intertropical convergence zone (ITCZ) The area where the two Hadley cells converge and cause large amounts of precipitation. The intense sunlight at the solar equator drives the Hadley cells and the ITCZ, causing the warmed air to rise and precipitation to be released in the form of rain. As we saw in our earlier discussion about unequal seasonal heating of Earth, we know that the solar equator shifts throughout the year, from 23.5° N latitude in June to 23.5° S latitude in December. Because the latitude of the solar equator moves throughout the year and the latitude of the solar equator determines the latitude of the ITCZ, the latitude of the ITCZ also moves throughout the year. This also means that the seasonal movement of the solar equator influences seasonal patterns of rainfall. You can see the effect of the ITCZ movement in Figure 5.7 by examining the patterns of rainfall across three locations in the Western Hemisphere. The city of Mérida, Mexico, lies about 20° N of the equator. The intertropical convergence reaches Mérida only during June, which is why June is the rainy season for Mérida. In comparison, Rio de Janeiro, Brazil, lies about 20° S
latitude. The intertropical convergence reaches Rio de Janeiro in December, which is the middle of the rainy season for that city. Close to the equator, in Bogotá, Colombia, the intertropical convergence zone passes overhead twice each year, during the March and September equinoxes. As a result, Bogotá experiences two rainy seasons.
Figure 5.7 Rainy seasons and the ITCZ. As the solar equator moves throughout the year, so does the ITCZ. As a result, latitudes north and south of the equator have a single distinct rainy season, whereas latitudes close to the equator have two rainy seasons. The pattern of air circulation near the equator also exists near the two poles. At approximately 60° N and 60° S latitudes, air rises up into the atmosphere and drops moisture. The cold, dry air then moves toward the poles and sinks back to Earth at approximately 90° N and 90° S latitudes. This air then moves along the ground back to 60° N and 60° S latitudes where it rises again. The atmospheric convection currents that move air between 60° and 90° latitudes are called polar cells. Polar cells The atmospheric convection currents that move air between 60° and 90° latitudes in the Northern and Southern Hemispheres. Between the Hadley cells and polar cells, from latitudes of approximately 30° to 60°, is an area of air circulation that lacks distinct convection currents. In this range of latitudes in the Northern Hemisphere—which includes much
of the United States, Canada, Europe, and Central Asia—some of the warm air from the Hadley cells that descends at 30° latitude moves toward the North Pole, while some of the cold air from the polar cells that is traveling toward 60° moves toward the equator. The movement of air in this region also helps redistribute the warm air of the tropics and the cold air of the polar regions toward the middle latitudes. The region between Hadley cells and polar cells can have dramatic changes in wind direction and therefore can experience large fluctuations in temperature and precipitation. However, winds generally move from west to east. This wind direction contributes to warmer conditions on the west coast of North America than on the east coast, as you can see from the plant hardiness zones. EARTH’S ROTATION AND THE CORIOLIS EFFECT Hadley cells and polar cells are important drivers of wind direction on Earth. However, wind direction is also affected by the speed of Earth’s rotation, which changes with latitude. Earth completes a single rotation in 24 hours. Because the circumference of the planet at the equator is much larger than its circumference near the poles, the speed of rotation is faster at the equator. As
Figure 5.8 shows, an object at the equator rotates at 1,670 km/hr, an object at 30° N rotates at 1,445 km/hr, and an object at 80° N rotates at 291 km/hr.
Figure 5.8 Earth’s speed of rotation. An object sitting at the equator travels at a much higher speed to complete a rotation in 24 hours than do objects at higher latitudes. The different rotation speeds deflect the direction of surface air circulation in the Hadley and polar cells. Imagine standing at the North Pole and throwing a baseball straight south to the equator, as shown in Figure 5.9. While the ball is flying through the air, the planet continues to rotate. As a result, the ball does not land straight south at the equator. Instead, it travels along a path that appears to deflect to the west. In fact, the ball is traveling straight, but because the planet rotates while the ball is in motion, the ball lands west of its intended target. With respect to the planet, the ball’s path appears to be deflected. The deflection of an object’s path due to the rotation of Earth is known as the Coriolis effect.
Figure 5.9 The Coriolis effect. (a) Because Earth rotates, the path of any object that travels north or south is deflected. (b) This deflection causes the predominant air circulation currents along Earth’s surface to be deflected. Coriolis effect The deflection of an object’s path due to the rotation of Earth. The deflected path of the ball in our example mimics the Coriolis effect on air moving to the north or south. For example, Hadley cells north of the equator move air along the surface from north to south. As we can see in
Figure 5.10, the Coriolis effect causes this path to deflect so that it moves from the northeast to the southwest. These winds are known as the northeast trade winds. Below the equator, the Hadley cells are moving air along the ground
from the south to the equator. The Coriolis effect causes this path to deflect so that it moves from the southeast to the northwest. These winds are known as the southeast trade winds. A similar phenomenon occurs in the polar cells. In the latitudes between Hadley cells and polar cells, wind direction can be quite variable. However, these winds often move away from the equator and toward the poles, only to be deflected by the Coriolis effect. This causes winds known as westerlies. Thus, weather in the middle latitudes tends to move from west to east. When considering the Coriolis effect, the general rule is that surface winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Figure 5.10 Air circulation patterns on Earth. Through a combination of atmospheric convection currents and the Coriolis effect, different latitudes have predominant winds that travel in different directions.
Concept Check
1. What are the four properties of air that are important in driving
atmospheric convection currents? 2. Given that the position of the solar equator moves throughout the year, what does its changing position suggest about the location of the intertropical convergence zone throughout the year? 3. What is the Coriolis effect and how does it affect atmospheric convection currents?
#### 5.4 Ocean Currents Also Affect the Distribution of Climates
5.4 Ocean currents also affect the distribution of climates Like air currents, ocean currents distribute unequal heating of Earth’s water and therefore influence the location of different climates. Figure 5.11 shows some general patterns of circulation: Warm tropical water circulates up through the western reaches of ocean basins toward the poles and the cold polar water circulates down the eastern edges of ocean basins toward the tropics. Many factors create these currents, including unequal heating, Coriolis effects, predominant wind directions, the topography of the ocean basins, and differences in salinity. In this section, we will examine the drivers of major ocean currents, including gyres and upwelling. We will then investigate how natural changes in ocean currents can have large effects on global climates through a process known as the El Niño–Southern Oscillation. Finally, we will explore the thermohaline ocean circulation, a deep ocean current that can take hundreds of years to complete a single path around the globe.
Figure 5.11 Ocean currents. Ocean currents circulate as a result of unequal heating, Coriolis effects, predominant wind directions, and the topography of the ocean basins. Each of the five major ocean basins contains a gyre. These gyres are driven by the trade winds in the tropics and the westerlies at mid-latitudes. This produces a clockwise circulation pattern in the Northern Hemisphere and a counterclockwise circulation pattern in the Southern Hemisphere. Along the west coasts of many continents, currents diverge and cause the upwelling of deeper and more fertile water.
We have noted how tropical regions of Earth receive more direct sunlight than regions at higher latitudes. This causes ocean waters near the equator generally to be warmer than ocean waters at higher latitudes. Because of the unequal heating, the tropical waters expand as they warm. This expansion causes the water near the equator to be approximately 8 cm higher in elevation than the water in mid-latitudes. Although the difference may seem small, it is enough for the force of gravity to cause movement of water away from the equator. Around the globe, ocean circulation patterns are also affected by the
dominant wind directions and Coriolis effects. North of the equator, for example, the northeast trade winds push surface water from the northeast to the southwest. At the same time, the Coriolis forces deflect ocean currents to the right. The combination of the two forces causes tropical water above the equator to move from east to west. The topography of ocean basins, particularly the locations of continents, forces these currents to change their direction. At mid-latitudes, the westerlies push surface waters to the northeast. As this occurs, Coriolis forces deflect the ocean currents to the right, which causes the ocean currents to move from west to east at midlatitudes in the Northern Hemisphere. These large-scale water circulation patterns between continents are called gyres. The direction of the deflections caused by Coriolis forces depends on latitude; as you can see in Figure 5.11, gyres move in a clockwise direction in the Northern Hemisphere and in a counterclockwise direction in the Southern Hemisphere. Gyre A large-scale water circulation pattern between continents. Gyres redistribute energy by transporting both warm and cold ocean water around the globe. The proximity of these ocean waters to continents can make the continents considerably warmer or colder and therefore influence terrestrial climates. You can see this impact on the coastal patterns of the plant hardiness zones we discussed in the beginning of the chapter. For example, England and Newfoundland, Canada, are at similar latitudes. However, England is adjacent to a warm-water current, the Gulf Stream, that comes out of the Gulf of Mexico, whereas Newfoundland is adjacent to a cold-water current that comes down from the west side of Greenland and pushes the warmer Gulf Stream water offshore. As a result, England experiences winter temperatures that are, on average, 20 °C warmer than Newfoundland.
Upwelling
Any upward movement of ocean water is referred to as upwelling. Illustrated as dark blue areas in Figure 5.11, upwelling occurs in locations along continents where surface currents move away from the coastline. As surface water moves away from land, cold water from beneath is drawn upward.
Strong upwelling zones occur on the western coasts of continents where gyres move surface currents toward the equator and then veer from the continents. As surface water moves away from the continents, it is replaced by water rising from greater depths. Because deep water tends to be rich in nutrients, upwelling zones are often regions of high biological productivity. Major commercial fisheries are often located in these zones. Upwelling An upward movement of ocean water. THE EL NIÑO–SOUTHERN OSCILLATION Sometimes ocean currents are substantially altered and this can affect climatic conditions. One of the best-known examples is the El Niño– Southern Oscillation (ENSO), in which periodic changes in winds and ocean currents in the South Pacific cause weather changes throughout much of the world. This is illustrated in Figure 5.12. During most years in the South Pacific Ocean, southeast trade winds and Coriolis forces push the surface waters of the Peru Current, which causes them to flow northwest along the west coast of South America, with upwelling of cold water along the coast. Equatorial winds—powered by high air pressures in the eastern Pacific and low air pressures in the western Pacific—then push these surface waters offshore at Ecuador and west across the Pacific Ocean. As this water moves west, it warms. This warm water drives thunderstorm activity in the western Pacific, which results in high amounts of precipitation.
Figure 5.12 The El-Niño–Southern Oscillation (ENSO). Changes in the strength of trade winds near the equator can have major impacts on the climates of the world. (a) In most years, strong trade winds push warm surface waters away from the west coast of South America. This causes cold, deep waters to upwell along the coast. (b) During an ENSO year, the trade winds weaken or reverse and the warm surface water moves from west to east. As a result, warm water builds up along the west coast of South America and prevents the upwelling of the cold, deep water. This change in ocean circulation alters climates around the world. El Niño–Southern Oscillation (ENSO) The periodic changes in winds and ocean currents in the South Pacific, causing weather changes throughout much of the world. Every 3 to 7 years, however, this series of events changes. In the atmosphere, the normal difference in air pressures reverses and the equatorial winds weaken. In some years, these winds can even reverse direction. This change in air pressures in the Southern Hemisphere is the Southern Oscillation element of the ENSO. With either weakened or reversed equatorial winds, the warm surface waters of the western Pacific move east toward South America. As a result, the upwelling of nutrients shuts down and
the normally productive fisheries in the area become much less productive. The accumulating warm water also serves as a source of increased precipitation for this region. The unusually warm water is the El Niño (“the baby boy”) element of the ENSO, so named because it typically occurs around Christmas time. Because air and water currents are responsible for distributing energy throughout the world, the effects of an ENSO event extend over much of the world. In North America, ENSO events bring cooler, wetter, and often stormy weather to the southern United States and northern Mexico, and warm, dry conditions to the northern United States and southern Canada. Some ENSO events have been particularly strong. For example, a strong ENSO event in 1982‒83 disrupted fisheries and destroyed kelp beds in California, caused reproductive failure of seabirds in the central Pacific Ocean, and killed off large areas of coral in Panama. Precipitation was also dramatically affected in many terrestrial ecosystems. Another ENSO event from 1991‒92 was accompanied by the worst drought of the twentieth century in Africa, which caused poor crop production and widespread starvation. The event also brought extreme dryness to many areas of tropical South America, Australia, and the islands of the South Pacific. Heat and drought in Australia reduced populations of red kangaroos to less than half their pre-ENSO levels. The El Niño event of 1997‒98 was blamed for 23,000 human deaths—mostly from famine—and $33 billion in damages to crops and property worldwide. The most recent ENSO event happened in 2015‒16 and was one of the strongest ENSO events in recent decades. It caused 1 to 5 °C warmer winter temperatures in Canada and increased precipitation in Northern California. The impacts around the Pacific were even larger, including record-setting heat and drought in Thailand, Malaysia, and India. The El Niño portion of an ENSO event is typically followed by a La Niña portion in which conditions in the southern Pacific Ocean oscillate strongly in the opposite direction. During La Niña, equatorial winds blow much stronger to the west and all the effects of El Niño event are reversed. Regions that become hotter and drier during the El Niño become cooler and wetter during La Niña. After the cycle of El Niño and La Niña, we typically experience several years of more normal weather conditions.
Thermohaline Circulation
Ocean currents are also driven by the thermohaline circulation, a global pattern of surface- and deep-water currents that flow as a result of variations in temperature and salinity that change the density of water. The thermohaline circulation, shown in Figure 5.13, is responsible for the global movement of great masses of water between the major ocean basins. As wind-driven surface currents—for example, the Gulf Stream—move toward higher latitudes, the water cools and becomes denser. In the far north, toward Iceland and Greenland, the surface of the ocean freezes in winter. Because ice does not contain salts, the salt concentration of the underlying water rises, which causes the cold water to become even denser. This high-density water begins to sink and acts as the driving force behind a deep-water current in the Atlantic Ocean known as the North Atlantic Deep Water. Similar sinking currents are formed around the edges of Antarctica in the Southern Ocean. These cold, dense waters then flow through the deep ocean basins and back into equatorial regions, where they eventually surface as upwelling currents. These upwelling currents become warm and begin to make their way back to the North Atlantic. Like a giant conveyor belt, the thermohaline circulation slowly redistributes energy and nutrients among the oceans of the world in a trip that can take hundreds of years.
Figure 5.13 Thermohaline circulation. This slow circulation of deep water and surface waters is driven by the sinking of cold, high-salinity water near Greenland and Iceland.
Thermohaline circulation A global pattern of surface- and deep-water currents that flow as a result of variations in temperature and salinity that change the density of water.
Concept Check
1. Where are the five major gyres in the world? 2. Why are areas of ocean upwelling important to commercial fishing? 3. For El Niño–Southern Oscillation events, what causes the oscillation?
#### 5.5 Smaller-Scale Geographic Features Can Affect Regional and Local Climates
5.5 Smaller-scale geographic features can affect regional and local climates As we have seen, the primary global patterns in Earth’s climate are the result of the unequal solar heating of Earth’s surface. However, a number of other factors have secondary effects on local temperature and precipitation. These include continental land area, proximity to coasts, and rain shadows.
Continental Land Area
The positions of continents exert important secondary effects on temperature and precipitation. For example, oceans and lakes, the sources of most atmospheric water vapor, cover 81 percent of the Southern Hemisphere but only 61 percent of the Northern Hemisphere. Because of this difference, more rain falls at any given latitude in the Southern Hemisphere than in the Northern Hemisphere. The presence of water has a moderating influence on land temperatures, so temperatures in the Northern Hemisphere vary more than they do in the Southern Hemisphere.
Proximity to Coasts
The interior of a continent usually experiences less precipitation than its coasts, simply because the interior lies farther from the oceans, which are the major sources of atmospheric water. Furthermore, as we saw in our discussion of plant hardiness zones, coastal climates vary less than interior climates because the heat storage capacity of ocean water reduces temperature fluctuations along the coasts. The ocean warms the air near the coasts during the winter and cools the air near the coasts during the summer. For example, the hottest and coldest average monthly temperatures near the Pacific coast of North America at Portland, Oregon, differ by only 16 °C. As we move farther inland, we observed that this range increases to 23 °C at Spokane, Washington; 26 °C at Helena, Montana; and 33 °C at Bismarck, North Dakota.
Rain Shadows
Mountains also play a secondary role in determining climates, as we can see in Figure 5.14. When winds blowing inland from the ocean encounter coastal
mountains, the mountains force the air upward, which causes adiabatic cooling, condensation, and precipitation. The air, which is now dry and warmed by latent heat release, descends the other side of the mountain, warms adiabatically, and travels across the lowlands beyond, where it creates relatively warm, arid environments called rain shadows. The Great Basin Desert of the western United States, for example, lies in the rain shadow of the Sierra Nevadas and the Cascade Mountains, and covers a large area that includes nearly all of Nevada and much of western Utah. The processes involved in creating rain shadows have much in common with the processes we saw occurring in Hadley cells, including adiabatic cooling, adiabatic heating, and release of latent heat.
Figure 5.14 Rain shadows. When winds carry warm moist air up over a mountain, the air cools and releases much of its moisture as precipitation. After crossing the mountain, the now dry air descends down the mountain, which causes the environment on this side of the mountain to be very dry. Rain shadow A region with dry conditions found on the leeward side of a mountain range as a result of humid winds from the ocean, causing precipitation on the windward side. We can now use what we have learned to draw a complete picture of worldwide climate distribution, which can be categorized by temperature and precipitation. Looking at Figure 5.15, we see repeated patterns that show where different climates exist. Tropical climates, characterized by warm temperatures and high precipitation, occur in regions near the equator. At
approximately 30° N and 30° S latitudes, we commonly find the dry climates that experience a wide range of temperatures. Dry climates are not only affected by latitude, however. Many dry climates are caused by rain shadows, for example, the extensive regions that lie just east of the Andes Mountains in western South America. Moist subtropical mid-latitude climates are characterized by warm, dry summers and cold, wet winters. Moist continental mid-latitude climates exist at the interior of continents and typically have warm summers, cold winters, and moderate amounts of precipitation. Finally, closest to the poles, we find polar climates that experience very cold temperatures and relatively little precipitation.
Figure 5.15 Broad climate patterns around the world. Near the tropics, the climate is warm with high amounts of precipitation. The world’s great deserts are near 30° N and 30° S latitudes. Cold and snowy polar regions are located at even higher latitudes. In addition, we can see that regions of high precipitation sometimes occur on the western side of mountains, as in western Canada, and that deserts occur in the rain shadows of mountains, such as on the eastern sides of the Cascade Mountains and Sierra Nevadas in North America and the Andes Mountains in South America. Tropical climate A climate characterized by warm temperatures and high precipitation, occurring in regions near the equator. Dry climate A climate characterized by low precipitation and a wide range of temperatures, commonly found at approximately 30° N and 30° S latitudes. Moist subtropical mid-latitude climate A climate characterized by warm, dry summers and cold, wet winters. Moist continental mid-latitude climate A climate that exists at the interior of continents and is typically characterized by warm
summers, cold winters, and moderate amounts of precipitation. Polar climate A climate that experiences very cold temperatures and relatively little precipitation. We have seen the processes that account for different climates around the world. Before we look more closely at the individual climate types and the plants that they support in the next chapter, we need to know something about the formation of soil, which supports all life.
1. What is the impact of continental land masses on the amount of precipitation falling in the Northern versus Southern Hemisphere? 2. Why do coastal continental temperatures typically fluctuate less than inland temperatures? 3. How do rain shadows cause desert formation?
#### 5.6 Climate and the Underlying Bedrock Interact to Create a Diversity of Soils
5.6 Climate and the underlying bedrock interact to create a diversity of soils Climate indirectly affects the distributions of plants and animals through its influence on the development of soil, which provides the substratum for plant roots to grow and in which many animals burrow. Soil defies a simple definition, but we can describe it as the layer of chemically and biologically altered material that overlies bedrock or other unaltered material at Earth’s surface. Because the layer of bedrock that underlies soils plays a major role in determining the type of soil that will form above it, soil scientists call bedrock the parent material. Soil The layer of chemically and biologically altered material that overlies bedrock or other unaltered material at Earth’s surface. Parent material The layer of bedrock that underlies soil and plays a major role in determining the type of soil that will form above it.
Soil Formation
Soil includes minerals derived from the parent material; modified minerals formed within the soil; organic material contributed by plants, air, and water within the pores of the soil; living roots of plants; microorganisms; and the larger worms and arthropods that make the soil their home. For example, if you have ever seen a recently excavated road cutting through a hillside, you may have observed that soils have distinct layers, called horizons, as shown in Figure 5.16. Soil horizons are categorized by the components and processes that occur at each level.
Figure 5.16 Soil horizons. Soils develop distinct horizons that differ in thickness, depending on climates and parent material. Horizon A distinct layer of soil. Soils exist in a dynamic state and their characteristics are determined by climate, parent material, vegetation, local topography, and, to some extent, age. Groundwater removes some substances by dissolving them and moving them down through the soil to lower layers, a process known as leaching. Other materials enter the soil from vegetation, in precipitation, as dust from above, and from the parent rock below. Where little rain falls, the parent material breaks down slowly, and sparse plant production means that little organic material is added to the soil. Thus, dry climates typically have
shallow soils, with bedrock lying close to the surface. In places where decomposed bedrock and organic material erode as rapidly as they form, soils may not form at all. Soil development also stops short on alluvial deposits, where fresh layers of silt deposited each year by floodwaters bury older material. At the other extreme, soil formation proceeds rapidly in tropical climates, where chemical alteration of parent material may extend to depths of 100 m. Most soils of mid-latitude climates are intermediate in depth, extending to an average of about 1 m. Leaching A process in which groundwater removes some substances by dissolving them and moving them down through the soil to lower layers.
Weathering
Weathering is the physical and chemical alteration of rock material near Earth’s surface. It occurs whenever surface water penetrates the parent material. In cold climates, for example, the repeated freezing and thawing of water in crevices cause the rock to break into smaller pieces and expose a greater surface area of the rock to chemical reactions. The initial chemical alteration of the rock occurs when water dissolves some of the more soluble minerals, such as sodium chloride (NaCl) and calcium sulfate (CaSO4). Further chemical reactions continue the soil building process. Weathering The physical and chemical alteration of rock material near Earth’s surface. The weathering of granite illustrates some basic processes of soil formation. The minerals responsible for the grainy texture of granite— feldspar, mica, and quartz—consist of various combinations of oxides of aluminum, iron, silicon, magnesium, calcium, and potassium. The key aspect of the process of weathering is the displacement of many of these elements by hydrogen ions, followed by the reorganization of the remaining minerals into new types of minerals. The hydrogen ions involved in weathering are derived from two sources. One source is the carbonic acid that forms when carbon dioxide dissolves in rainwater, as discussed in Chapter 2. The other source of hydrogen ions is the decomposition of organic material in the soil itself. The metabolism of carbohydrates, for example, produces carbon
dioxide. This carbon dioxide is converted into carbonic acid in water, which generates additional hydrogen ions. As granite weathers, many of the positively charged elements—such as iron (Fe3+) and calcium (Ca2+)—are replaced by hydrogen ions to form new, insoluble materials, such as the clay particles we discussed in Chapter 3. Clay particles are important to the water-holding capacity of soils. They accumulate negative charges on their surfaces that attract positively charged ions called cations. Cations—including calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+)—are important nutrients for plants. The ability of a soil to retain these cations, called its cation exchange capacity, provides an index to the fertility of that soil. Young soils have relatively few clay particles and little added organic material; this low cation exchange capacity leads to relatively low fertility. Older soils generally have a higher cation exchange capacity and therefore relatively high fertility. Soil fertility improves with time, up to a point. Eventually, weathering breaks down clay particles, cation exchange capacity decreases, and soil fertility drops. Cation exchange capacity The ability of a soil to retain cations. Podsolization Under mild temperatures and moderate precipitation, sand grains and clay particles resist weathering and become stable components of the soil. This allows soils to retain relatively high fertility. However, in acidic soils typical of cool, moist regions, clay particles break down in the E horizon, and their soluble ions are transported down to the lower B horizon. This process, known as podsolization, reduces the fertility of the soil’s upper layers. Podsolization A process occurring in acidic soils typical of cool, moist regions, where clay particles break down in the E horizon, and their soluble ions are transported down to the lower B horizon. Acidic soils occur primarily in cool regions where needle-leaved trees such as spruces and firs dominate the forests. The slow decomposition of the spruce and fir needles produces organic acids that promote high concentrations of hydrogen ions. In the moist regions where podsolization
occurs, rainfall usually exceeds evaporation. Because water continuously moves downward through the soil profile, little clay-forming material is transported upward from the weathered bedrock. In North America, podsolization is most advanced under spruce and fir forests of New England, in the Great Lakes region, and across a wide belt of southern and western Canada. A typical profile of a highly podsolized soil, shown in Figure 5.17, reveals striking bands corresponding to the regions of leaching and redeposition. The A horizon is dark and rich in organic matter. It is underlain by a light-colored E horizon that has been leached of most of its clay, leaving behind sandy material that holds neither water nor nutrients well. One usually finds a dark band immediately below the E horizon. This is the uppermost layer of the B horizon, where iron and aluminum oxides are redeposited. Other, more mobile minerals may accumulate to some extent in lower parts of the B horizon, which then grades almost imperceptibly into a C horizon and the parent material.
Figure 5.17 Podsolization. In cool, moist conditions with highly acidic soils, clay particles normally found in the E horizon are weathered and leached down, leaving a very sandy layer with little capacity to retain nutrients for plants. Laterization In the warm, humid climates of many tropical and subtropical regions, soils weather to great depths. One of the most conspicuous features of weathering
under these conditions is the breakdown of clay particles, which causes silicon to leach from the soil and leaves oxides of iron and aluminum to dominate throughout the soil profile—a process that is called laterization. The iron and aluminum oxides give such soils a characteristic reddish coloration, as illustrated in Figure 5.18. Even though the rapid decomposition of organic material in tropical soils contributes an abundance of hydrogen ions, bases formed by the breakdown of clay particles neutralize them. Consequently, lateritic soils are not usually acidic, even though they may be deeply weathered. Regardless of the parent material, weathering reaches deepest, and laterization proceeds farthest, on low-lying soils, such as those of the Amazon basin, where highly weathered surface layers are not eroded away and the soil profiles are very old.
Figure 5.18 Laterization. Under conditions of warm temperatures and high precipitation, clay particles are broken down and leave behind a soil that has a low cation exchange capacity and low fertility. Laterization The breakdown of clay particles, which results in the leaching of silicon from the soil, leaving oxides of iron and aluminum to predominate throughout the soil profile.
Laterization causes many tropical soils to have a low cation exchange capacity. In the absence of clay and organic matter, mineral nutrients are readily leached from the soil. Where soils are deeply weathered, new minerals formed by the decomposition of the parent material are simply too far from the surface to contribute to soil fertility. In addition, heavy rainfall in the tropics keeps water moving down through the soil profile, preventing the upward movement of nutrients. In general, the deeper the ultimate sources of nutrients in the unaltered bedrock, the lower the fertility of the surface layers. The high productivity of tropical rainforests depends more on rapid cycling of nutrients close to the surface of the soil than on the nutrient content of the soil itself. Rich soils do, however, develop in many tropical regions, particularly in mountainous areas where erosion continually removes nutrient-depleted surface layers of soil, and in volcanic areas where the parent material of ash and lava is often rich in nutrients such as potassium. From our discussion of soils, you can see that the composition of the soils present in various parts of the world depends on differences in climate, underlying parent material, and vegetation. In the next chapter, we will discuss how these regional differences in climate and the associated effects on soils affect the types of plants and animals that can live in each region.
Concept Check
1. What are the different soil horizons and of what are they composed? 2. Why is cation exchange capacity important for determining soil fertility? 3. Why are tropical soils weathered to greater depths than soils in the northern United States?
Concepts
Global Climate Change A polar bear hunting for seals on the Arctic sea ice of Norway. Warming trends over the last few decades have caused the Arctic ice to melt earlier in the year; this means that polar bears have less time to hunt for seals, which make up a large part of their diet. As we have seen in this chapter, a substantial number of interacting factors determine the different climates of the world. For example, the differential heating of Earth drives the movements of air and water, which are further modified by the Coriolis effect and the position of continents. Because these interactions are complex, any changes in these factors can have farreaching effects on the entire system. Such is the case for global warming and global climate change. Global warming is the increase in the average temperature of the planet due to an increased concentration of greenhouse gases in the atmosphere. Global climate change is a much broader phenomenon that refers to changes in Earth’s climates and includes global warming, changes in the global distribution of precipitation and temperature, changes in the intensity of storms, and altered ocean circulation. Throughout the history of Earth, long periods of gradual global warming and cooling have been associated with substantial global climate change. During the past two
centuries, however, human activities have caused a rapid change in conditions that have led to global warming and global climate change. Global climate change A phenomenon that refers to changes in Earth’s climates, including global warming, changes in the global distribution of precipitation and temperature, changes in the intensity of storms, and altered ocean circulation. Global warming is a major driver of current changes in global climates. One direct impact is the increase in temperatures in many parts of the world, particularly at high latitudes in the Northern Hemisphere. The rise in temperature in these regions has had a wide range of effects. For example, in high latitudes and at high altitudes, the lower layers of soil may be permanently frozen, a phenomenon that is known as permafrost. Warmer temperatures cause these highly organic soils to thaw and begin decomposing. Because these soils are waterlogged and anaerobic, the decomposition produces methane, a greenhouse gas that can further contribute to global warming. Permafrost A phenomenon whereby layers of soil are permanently frozen Increased global temperatures, which affect high latitudes more than lower latitudes, also affect the amount of ice melting around the world. From 1979 to 2016, the polar ice cap that exists between the United States, Canada, Europe, and Russia has declined at a rate of 13 percent per year. The remaining ice has also become
thinner. In 2016, the warmest year around the world on record since 1880, the Arctic ice cap melted to the secondsmallest area since records began in 1979. The ice of Greenland and Antarctica is also melting. NASA scientists found, from 2003 to 2016 that the two regions lost an average of 412 gigatonnes (Gt) of ice per year and that the annual rate of ice loss is accelerating. By 2016, Greenland was losing 350 Gt of ice per year. Similarly, glaciers are melting in many regions of the world. In Montana’s Glacier National Park, for example, there were 150 glaciers in 1850. Today, only 25 glaciers remain. All this ice melting, combined with warmer oceans expanding in volume, has caused sea levels to rise 200 mm since 1870, and scientists predict that continued melting could raise sea levels an additional 280 to 430 mm by the year 2100. In 2016, researchers reported that based on the close correlation between increased CO2 emissions and ice loss in the polar ice cap, a complete loss of ice could occur in summer by approximately 2050. The melting polar ice cap. The area covered by the Arctic ice cap has declined at a rate of 13 percent annually.
Because the complex nature of the global climate system can make it difficult to predict how climate will change in the coming decades, scientists have developed computer models that incorporate our best understanding of the processes that govern climate along with the changes being caused by increased atmospheric greenhouse gas concentrations. Although different models make somewhat different predictions, there is general agreement regarding several aspects of predicted change. For example, the increase in temperatures is expected to cause longer periods of hot weather and fewer days of extremely cold weather. Because heat is the driver of evaporation and air circulation that determine precipitation, precipitation patterns are also predicted to change across the globe, with some regions of the world receiving increased amounts of rain and snow while other regions receive less. The intensity of storms, such as hurricanes, is also predicted to increase due to an increased warming of the world’s oceans. Ocean currents may also be affected by global warming. As we discussed earlier in this chapter, ocean currents are driven by the differential heating of Earth and, in turn, play a major role in determining the temperature of nearby continents. Of particular concern is the potential impact on thermohaline circulation. As you may recall, thermohaline circulation is the slow, deep circulation of ocean water around the globe that is driven by the dense, salty water that sinks near Greenland. With the increased melting of the polar ice cap and the ice sheets of Greenland, climate scientists are concerned that the water in the North Atlantic may not be dense enough to sink and therefore may cause the thermohaline circulation to shut down. While
researchers do not have a prediction for when the thermohaline circulation might stop, the disappearance of this deep-water current would effectively stop the circulation of warm water from the Gulf of Mexico to Europe and cause a substantial cooling of Europe—with potentially devastating consequences for the people and environment of that region. SOURCES: Notz, D., and J. Stroeve. 2016, November 11. Observed Arctic sea- ice loss directly follows anthropogenic CO2 emission. Science 354, no. 6313:747-750. doi:10.1126/science.aag2345. Climate Change 2007: Synthesis Report. Fourth Assessment Report of the Intergovernmental Panel on Climate Change. http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf. Global Climate Change Impacts in the United States. 2009. U.S. Global Change Research Program. http://downloads.globalchange.gov/usimpacts/pdfs/climate- impacts-report.pdf.
Summary of Learning Objectives
5.1 Earth is warmed by the greenhouse effect. Much of the ultraviolet and visible light emitted by the Sun passes through the atmosphere and strikes clouds and the surface of Earth. Clouds and the planet begin to warm and emit infrared radiation back toward the atmosphere. The gases in the atmosphere absorb the infrared radiation, become warmer, and re-emit infrared radiation back toward Earth. These greenhouse gases allow the planet to become warmer than would otherwise be possible. The increase in production of greenhouse gases due to human activities increases the greenhouse effect and leads to global warming. Key Terms: Atmosphere, Greenhouse effect
5.2 There is an unequal heating of Earth by the Sun. Each year, high-latitude regions of the world receive solar radiation that is weaker in intensity due to a longer path through the atmosphere with a less direct angle, which causes the energy of the Sun to be spread over a larger area. In addition, the axis of Earth is tilted 23.5° and this causes seasonal changes in temperature. Key Terms: Albedo, Solar equator, Regression
5.3 The unequal heating of Earth drives air currents in the atmosphere. Due to the properties of air, the warmer temperatures near the equator drive atmospheric convection currents known as Hadley cells between approximately 0° to 30° latitude in the Northern and Southern Hemispheres. Polar cells are at higher latitudes, between approximately 60° to 90°. These air convection currents cause the distribution of heat and precipitation around the globe. Their path is also influenced by Coriolis forces that are created by the rotation of Earth. Key Terms: Atmospheric convection currents, Saturation point,
Latent heat release, Adiabatic cooling, Adiabatic heating, Hadley cells, Intertropical convergence zone (ITCZ), Polar cells, Coriolis effect
5.4 Ocean currents also affect the distribution of climates. Ocean currents are driven by the unequal warming of Earth combined with Coriolis effects, atmospheric convection currents, and differences in salinity. Gyres exist on both sides of the equator and help distribute heat and nutrients to higher latitudes. El Niño–Southern Oscillation (ENSO) events represent a disruption in normal ocean currents in the South Pacific, and the impacts on climates can be felt around the world. Thermohaline circulation is a deep and slow circulation of the world’s oceans driven by changes in salt concentration in the waters of the North Atlantic. Key Terms: Gyre, Upwelling, El Niño–Southern Oscillation (ENSO), Thermohaline circulation
5.5 Smaller-scale geographic features can affect regional and local climates. Increased land area of continents reduces the amount of evaporation possible, which causes the Northern Hemisphere to experience less precipitation than the Southern Hemisphere. Proximity to the coast can also affect climates; regions that are more distant from coastlines typically have lower precipitation and higher variation in temperature. Mountain ranges force air to rise over them, causing higher precipitation on one side of the mountain range and rain shadows on the opposite side. Key Terms: Rain shadow, Tropical climate, Dry climate, Moist subtropical mid-latitude climate, Moist continental mid-latitude climate, Polar climate
5.6 Climate and the underlying bedrock interact to create a diversity of soils. Soils are made up of horizons that contain different amounts of organic matter, nutrients, and minerals. Soils can be weathered by processes including freezing, thawing, and leaching. In
acidic soils of cool, moist regions, soils can experience podsolization, a process that breaks down clay particles and reduces fertility. In warm, humid climates, soils can experience laterization, a process that breaks down clay particles and leaches nutrients from the soil. Key Terms: Soil, Parent material, Horizon, Leaching, Weathering, Cation exchange capacity, Podsolization, Laterization, Global climate change, Permafrost
Critical Thinking Questions
1. Of the many different types of greenhouse gases, which ones will likely decline faster if we reduce their production? 2. Some sources of air pollution produce tiny black particles that can be transported around the world and settle on regions covered in snow and ice. Based on the albedo effect, how might this air pollution contribute to warming global temperatures, melting polar ice caps, and rising sea levels? 3. If Earth was not tilted on its axis, how would it affect the seasonality of rainfall at the equator? 4. What parallels can you draw between the processes that drive Hadley cells versus rain shadows? 5. If there was no Coriolis effect, how would it affect atmospheric convection currents and ocean currents? 6. How do ocean gyres affect the pattern of plant hardiness zones in North America? 7. Why do El Niño and La Niña events cause opposite weather climates around the world? 8. Based on your knowledge of the thermohaline circulations, how might melting of the ice in the Arctic Ocean affect the climate of Europe? 9. What are the processes that are responsible for the locations of the world’s major deserts? 10. Compare and contrast podsolization and laterization.
#### Graphing the Data: Precipitation in Mexico City, Quito, and La Paz
GRAPHING THE DATA Precipitation in Mexico City, Quito, and La Paz As we have seen in this chapter, cities around the world often differ in their pattern of monthly precipitation. Using the data provided in the table, create a bar graph for each of the three cities. (a) Based on these graphs, how many peaks in precipitation does each city receive? (b) Based on their geographic locations, why does the number of peaks in precipitation in these cities differ? Average Monthly Precipitation (mm) in Three Cities Month Mexico City, Mexico Quito, Ecuador La Paz, Bolivia January 10.2 114.3 129.5 February 10.2 129.5 104.1 March 12.7 152.4 71.1 April 27.9 175.3 35.6 May 58.4 124.5 12.7 June 157.5 48.3 5.1 July 182.9 20.3 7.6 August 172.7 25.4 15.2 September 144.8 78.7 30.5 October 61.0 127.0 40.6 November 5.1 109.2 50.8 December 0.8 104.1 94.0