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Chapter 6: Earth Systems and Resources
According to the College Board, about 10 to 15% of the test is based directly on the content covered in this chapter. If you are unfamiliar with a topic presented here, consult your textbook for more in-depth information.
Let’s take a step back and review the fundamental themes of this chapter. First of all, a resource is strictly defined as any substance, capability (such as work performed by humans or animals), or other asset that is available in a supply that can be accessed and drawn on as needed. Resources are utilized for the effective functioning of an organism or community, often for economic gain. Within the scope of Environmental Science, we typically understand resources to mean natural resources—resources that occur in nature.
We also discuss the importance of each sphere to humans. The four spheres are:
The Solid Earth—the Earth’s solid, rocky outer shell
Topics: the movement of tectonic plates, volcanoes and -earthquakes, and the different types of rock
The upper shell of the Solid Earth is called the Lithosphere and is the part that interacts most with the other spheres
The Pedosphere—more commonly known as soil
Topics: soil’s characteristics, formation, layers, and development
The Atmosphere—the envelope of gases that surrounds the Earth
Topics: the greenhouse effect, climate, and weather events
The Hydrosphere—the Earth’s oceans and freshwater bodies
Topics: fresh- and saltwater bodies and ocean currents
The four physical spheres provide resources that support the Biosphere, the fifth of the Earth’s spheres, which comprises all the living organisms that inhabit the planet and draw on the physical resources of the other four spheres.
WELCOME TO PLANET EARTH
The first thing you should know about the Earth is its history. The Earth is thought to be between 4.5 and 4.8 billion years old. That amount of time is pretty inconceivable to humans, but the following geologic time scale will help you get a sense of the vast amount of time that has gone by since the Earth was formed. You will not be responsible for memorizing all of the eons, eras, periods, and epochs for this exam, but you should be familiar with the major ones—they will come in handy.
We are currently in the Holocene Epoch.
The Quaternary and Tertiary are the two most recent geologic periods.
Non-avian dinosaurs lived during the Mesozoic Era. (Note that evolutionary biologists consider birds to be avian, or flying, dinosaurs. Pterosaurs such as pterodactyls are a group of reptiles less closely related to dinosaurs and birds.)
The Precambrian eons represent the vast majority of the geologic time scale.
The next epoch will be called the Anthropocene, to recognize humankind’s accelerating effects on the planet’s physical resources, climate, and life forms. Many geologists propose that we have already entered this new, post-Holocene epoch.
Where Is the Earth in the Solar System?
The Earth is the third planet from the sun in our solar system, which contains a total of eight currently known and recognized planets. From the sun outward, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Each planet has its own orbit around the sun in the shape of an ellipse (a “stretched” circle). And you probably already know that it takes the Earth about 365.25 days, or 1 year, to complete its orbit of the sun.
The Solid Earth
Planet Earth is made up of three concentric zones of rocks that are either solid or liquid (molten). The innermost zone is the core. The core has two parts: a solid inner core and a molten outer core. The inner core is composed mostly of nickel and iron and is solid due to the tremendous pressure from overlying matter. The outer core is composed mostly of iron, also mixed with nickel as well as some lighter elements, and is semi-solid due to lower pressure. Surrounding the outer core is the mantle, which is made mostly of solid rock. Near the top of the mantle lies a layer of slowly flowing rock called the asthenosphere. The lithosphere, a thin, rigid layer of rock, is the Earth’s outer shell. The lithosphere includes the rigid upper mantle above the asthenosphere and the crust, the solid surface of the Earth. Think of it as floating atop the asthenosphere like a cracker atop a thick layer of hot pudding.
Tectonic Plates
Scientists theorize that during the Paleozoic and Mesozoic Eras, the continents were joined together, forming a supercontinent known as Pangaea. Roughly 200 million years ago, Pangaea began to break apart.
Today, it is believed that the Earth’s crust is composed of several large pieces of lithosphere—called tectonic plates—that move slowly over the mantle of the Earth.
There are a total of a dozen or so tectonic plates that move independently of one another. The majority of the land on Earth sits above six giant plates; the remainder of the plates lie under the ocean as well as the continents. The edges of the plates are called plate boundaries, and the places where two plates about each other are where events like sea floor spreading and most volcanoes and earthquakes occur.
There are three types of plate boundary interactions:
Convergent boundary: Two plates are pushed toward and into each other. One of the plates slides beneath the other, pushed deep into the mantle.
Divergent boundary: Two plates move away from each other. This creates a gap between plates that may be filled with rising magma (molten rock). When this magma cools, it forms new crust.
Transform fault boundary: Two plates slide against each other in opposite directions—as when you rub your hands back and forth to warm them up. These are also called simply transform boundaries.
Volcanoes and Earthquakes
Volcanoes are mountains formed by pressure from magma rising from the Earth’s interior. Active volcanoes are those that are currently erupting or have erupted within recorded history (that is, within the last 10,000 years), while dormant volcanoes have not been known to erupt during this period. It’s thought that extinct volcanoes will never erupt again. Active volcanoes are categorized by the kind of tectonic events that produce them. They are associated with the following:
Subduction zones occur at convergent boundaries between oceanic and continental plates, or sometimes between two oceanic plates. The subducting plate is recycled into new magma, which rises through the overlying plate to create volcanoes inland.
Rift valleys occur at divergent boundaries, usually between two oceanic plates. New ocean floor is formed as magma fills in the gap between separating plates. Thick magma rising from rift valleys is made of basaltic minerals and forms pillow lava upon contact with the cold ocean water. Rift valleys may also occur between continental plates; a prominent example is the Great Rift Valley of eastern Africa, which gave rise to Mount Kilimanjaro and other volcanoes.
Hot spots do not form at plate boundaries. Instead, they are found in the middle of tectonic plates, in locations where columns of unusually hot magma melt through the mantle and weaken the Earth’s crust. The Hawaiian islands continue to form over a hot spot beneath the Pacific plate. Volcanoes over oceanic hot spots are basaltic, resulting in milder eruptions; while volcanoes over continental hot spots are characterized by rhyolitic rocks, which produce more violent eruptions.
There are four types of volcanoes:
Shield volcanoes have broad bases and are tall with gentle slopes. They generally form over oceanic hot spots and usually have mild eruptions with slow lava flow. Sometimes, however, when water enters the vent, they can be very explosive, forming pyroclastic flows, a fluidized mixture of hot ash and rock.
Composite volcanoes have broad bases and are also tall but with steeper slopes. They are formed at subduction zones and are associated with violent eruptions that eject lava, water, and gases as superheated ash and stones.
Cinder volcanoes are small, short, and steeply sloped cones. They form when molten lava erupts and cools quickly in the air, hardening into porous rocks (called cinders or scoria) that fracture as they hit the Earth’s surface. Cinder volcanoes generally form near other types of volcanoes.
Lava domes are small and short with steep slope and rounded tops. They are formed from lava that is too viscous to travel far but instead hardens into a dome shape. This type of volcano occurs near or even inside other types of volcanoes.
The Rock Cycle
Rocks are all around us, in the soil, our buildings, and the ore used in industry. So, where do all those rocks come from? The answer is: other rocks. The oldest rocks on Earth are 3.8 billion years old, while others are only a few million years old. This means that rocks are recycled. These transformations are described by the rock cycle. In the rock cycle, time, pressure, and the Earth’s heat interact to create three basic types of rocks.
Igneous rock results when rock is melted (by heat and pressure below the crust) into a liquid and then resolidifies when cooled. The molten rock (magma) comes to the surface of the Earth, and when it emerges it is called lava; cooled lava becomes solid igneous rock. An example of an igneous rock is basalt.
Sedimentary rock is formed as sediment (eroded rocks and the remains of plants and animals) builds up and is compressed. Sedimentary rock forms under water as sediments or dissolved minerals deposit on a stream bed or ocean floor. They are compressed as more material is deposited and then cemented together. An example of a sedimentary rock is limestone.
Metamorphic rock is formed as a great deal of pressure and heat produces physical and/or chemical changes in existing rock. This can happen as sedimentary rocks sink deeper into the Earth and are heated by the high temperatures found in the Earth’s mantle. An example of a metamorphic rock is slate, which results from the metamorphosis of shale.
Where Does Soil Come From?
Basically, soil is a combination of organic material and rock that has been broken down by chemical and biological weathering—which is, unsurprisingly, the process by which rock and other material decomposes. Therefore, it should also not be surprising to learn that the types of minerals found in soil in a particular region will depend on the identity of the base rock of that region. Note that erosion is a distinct process, by which broken-down material is removed from one place and transported to another, across the Earth’s surface, usually by wind or water. Sometimes the same process can be responsible for both erosion and weathering, as when a glacier grinds material from surface rock and drags it along with its own movement, depositing it far away.
Water, wind, temperature, and living organisms are all prominent agents of weathering, and all weathering processes are placed into the following three rather broad categories.
Physical weathering (also known as mechanical weathering): Any process that breaks rock down into smaller pieces without changing the chemistry of the rock. The forces responsible for physical weathering are typically wind and water.
Chemical weathering: Occurs as a result of chemical reactions of rock with water, air, or dissolved minerals. Chemical processes result in minerals that are broken down or restructured into different minerals. This type of weathering tends to dominate in warm or moist environments. One example of chemical weathering is rust, which forms when iron and other metallic elements come in contact with water.
Biological weathering: Weathering that takes place as the result of the activities of living organisms, which may act through physical or chemical means. When tree roots enlarge the cracks in rocks as they grow, that is a physical process. When plant roots or lichens growing on rocks release organic acids that dissolve minerals, that is a chemical process.
Soil Layers
Soil comprises distinct layers known as horizons, each of which has distinct physical, chemical, and biological properties. Study the following diagram which illustrates and describes the different horizons. Not every soil contains all of these horizons.
O horizon: This layer is made up of organic matter at various stages of decomposition. It includes animal waste, leaves and other plant tissues (such as dead roots), and the decomposing bodies of organisms. The stable residue left after most organic matter has decomposed is a dark, crumbly material called humus.
A horizon: This is the topsoil—the topmost mineral horizon and the most intensively weathered soil layer. Its dark color is due to accumulation of organic matter from the O horizon. In soils lacking an E horizon, this may also be called the zone of leaching.
E horizon: This is the eluviated horizon. It is light in color and coarse in texture; no organic matter has traveled down from the A horizon, while clays and minerals like iron and aluminum oxides have been washed out by leaching and eluviation. The E horizon isn’t found in all soils: it’s mostly found in soils developed under forest.
B horizon: Sometimes called the subsoil, this is where organic matter, clay, and minerals washed out of the upper horizons accumulate. Thus, it is called the zone of accumulation or the zone of illuviation.
C horizon: This layer is the parent material—unconsolidated material, loose enough to be dug up with a shovel. Weathering at this depth is minimal so the soil retains identifiable features of the parent material (rock from which the A and B horizons formed). This horizon has much less biological activity than the horizons above it.
R horizon: Beneath the soil lies a layer of consolidated (cemented), Un weathered rock. Because it hasn’t been weathered, this isn’t part of the soil, strictly speaking. If the material from which horizons A through C formed was not transported from elsewhere, the R horizon has the same source minerals as the parent material above. The R stands for regolith, a word meaning “blanket or surface rock” in ancient Greek.
Soil Development
Four Processes of Soil Development. Soil development is an intricate business! The development of distinct soil horizons is accomplished by four basic processes that work on soil minerals and particles, organic matter, and soil chemistry. These four processes include additions of materials from off-site; losses through erosion or leaching or biological activity (plant uptake); vertical translocations or movement from one soil horizon to another; and transformations of materials in place by weathering or chemical activity.
Six Soil-Forming Factors. The processes of soil development are influenced by six soil-forming factors. It takes hundreds to thousands to millions of years for a soil to develop its characteristic layers or profile. To wrap it all up in one big sentence: any soil you see is a dynamic formation produced by the effects of climate and biological activity (organisms), as modified by topography (relief) and human influences, acting on parent materials over time. The six soil-forming factors can be remembered by the awkward acronym Cl–O–R–P–T–H. Here is a summary of how each factor works.
Climate: Climate involves differences in temperature and precipitation across the globe, and both heat and water facilitate chemical and biochemical reactions. Seasonal fluctuation of heat and moisture affects processes such as freeze-thaw cycles that weather rock. Climate also helps to determine what organisms grow in a particular location.
Organisms (Biological Activity): Different local conditions support different organisms, which influence the soil in a multitude of ways. Microorganisms perform biochemical functions such as decomposition of organic matter and transformation of minerals into different forms. Animals move soils, consume vegetation, and add nutrients through waste and decomposing bodies. Plants perform physical weathering through root growth, take up soil nutrients and water, alter soil chemistry in various ways, and add nutrients when they die and decompose.
Relief (Topography): Topographical relief affects where water moves on the landscape and also the depth of the water table in a given location. Relief similarly affects erosion—which locations are likely to lose surface material through the action of wind and rain, and which locations are likely to accumulate eroded material. Topographical relief also leads to differences in how much sun different locations receive. Through these characteristics, relief influences which organisms grow in a particular location.
Parent Material: This is the starting point for soil development. Its mineral properties, hardness, and topographical form affect how it is weathered into soil. As parent material varies from location to location, so will the soil that develops at each location. As an example, parent material rich in quartz, such as granite and sandstone, weathers into sandy soil. Shale weathers into soil richer in silt and clay.
Time: More time equals more change! Hard parent material weathers more slowly and softer material more quickly. A flat, stable topographic position develops horizons more quickly than do slopes and depressions where material is lost and gained.
Human Influence: The effects of human activity must increasingly be acknowledged as a factor in soil development. Use of fertilizer, pollution, and acid rain alter soil chemistry on a broad scale. Construction activities such as digging, and plowing tend to mix soils and blur the distinctions between horizons. Human activities also lead to compaction (through the traffic of vehicles and machinery), erosion (through removal of stabilizing vegetation), and salinization (increase in salt content, through irrigation and depletion of groundwater).
THE ATMOSPHERE
In the broadest definition, the atmosphere is a layer of gases that’s held close to the Earth by the force of gravity. The inner four layers of the atmosphere reach an altitude that’s just about 12.5% of the Earth’s radius. The layer of gases that lies closest to the Earth is the troposphere; it extends from the Earth’s surface to about, on average, 12 km (7.5 miles) at the poles and 20 km (12.4 miles) at the equator.
The troposphere is where all the weather that we experience takes place. The layer also contains 99% of the atmosphere’s water vapor and clouds. Generally, the troposphere is well-mixed from bottom to top—with the exception of periodic temperature inversions. The troposphere gets colder with altitude, decreasing 6.5°C for every kilometer of altitude (or 3.5°F for every thousand feet).
Because of its density, the troposphere contains about 75–80% of the Earth’s atmosphere by mass. You’ve probably heard about the troposphere before in the news because of the greenhouse effect.
The troposphere contains the air we breathe, which is made up of 78% nitrogen and 21% oxygen. The remaining 1% includes the so-called “greenhouse” gases (GHGs). The proportion of these gases in the troposphere is minuscule, but their effects on conditions on Earth are disproportionately significant.
The most important of them are water vapor (H2O), carbon dioxide (CO2), and methane (CH4). As the sun’s rays strike the Earth, some of the solar radiation is reflected back into space; however, greenhouse gases in the troposphere intercept and absorb a lot of this radiation. This warming effect of greenhouse gases was a good thing, until their concentration in the atmosphere shot up after the Industrial Revolution.
Crowning the troposphere is the tropopause, which is a layer that acts as a buffer between the troposphere and the next layer up, the stratosphere. This buffer zone is where the jet streams, air currents that are important drivers of weather patterns and important factors in planning airline routes, travel.
The stratosphere sits on top of the tropopause and extends about 20–50 km (7.5–31 miles) above the Earth’s surface. As opposed to those in the troposphere, gases in the stratosphere are not well mixed and temperatures increase with distance from the Earth. This warming effect is due to the ozone layer, a thin band of ozone (O3) that exists in the lower half of this layer.
The ozone traps the high-energy radiation of the sun, holding some of the heat and protecting the troposphere and the Earth’s surface from this radiation. The stratosphere is similar to the troposphere in gas composition, only less dense and drier, with a thousand times less water vapor. Commercial jets may also fly in the lower part of this layer.
Above the stratosphere are two layers called the mesosphere and the thermosphere. The mesosphere extends to about 80 km (50 miles) above the Earth’s surface and is the area where meteors usually burn up. Temperatures again decrease here, to the coldest point in the atmosphere at the top of this layer, around –90°C (–130°F).
The thermosphere extends from 80 to around 500 km above the Earth (50–435 miles). Gases are very thin (rare) and it’s in this layer that the spectacular and colorful auroras (northern lights and southern lights) take place. The furthest layer is the exosphere, extending to 10,000 km (6,200 miles) or more above the Earth, although the upper limit of this layer is not definitively settled.
The concentration of gases is thinnest here. Human-made satellites orbit in the exosphere and in the upper thermosphere.
The ionosphere is not a distinct layer but dispersed throughout the upper mesosphere, the thermosphere, and the lower exosphere. The ionosphere comprises regions of ionized gases that absorb most of the energetic charged particles from the sun—the protons and electrons of the solar wind. Interestingly, the ionosphere also reflects radio waves, making long-distance radio communication possible. You’ll need to know how the climates that we experience on Earth are created by the atmosphere, so let’s go into this next.
Climate
The Earth’s atmosphere has physical features that change from day to day as well as patterns that are consistent over a space of many years. The day-to-day properties such as wind speed and direction, temperature, amount of sunlight, pressure, and humidity are referred to as weather. The patterns that are constant over many years (30 years or more) are referred to as climate. The two most important factors in describing climate are average temperature and average precipitation amounts. Meteorologists are scientists who study weather and climate.
Air Circulation in the Atmosphere
The motion of air around the globe is the result of solar heating, the rotation of the Earth, and the physical properties of air, water, and land. There are three major reasons why the Earth is unevenly heated.
More of the sun’s rays strike the Earth at the equator in each unit of surface area than strike the poles in the same unit area. This is because the angle of the suns rays strike the Earth more directly at the equator.
The tilt of the Earth’s axis points regions toward or away from the sun. When pointed toward the sun, those areas receive more direct or intense light than when pointed away. This causes the seasons.
The Earth’s surface at the equator is moving faster than at the poles, because the circumference is larger but the rotation time is the same. Because an object or air mass nearer to the equator is moving more rapidly (from east to west), it will maintain this eastward momentum as it moves away from the equator to where the surface is moving more slowly, winding up further east. Therefore, winds moving north from the equator near the surface are deflected to the right (east), and winds moving south from the equator are deflected to the left (east).
Conversely, winds blowing toward the equator will be deflected to the west because they are moving eastward more slowly than is the surface in the lower latitudes where they are moving to. So, in the Northern Hemisphere, this westward deflection will be to the right, and in the Southern Hemisphere, it will be to the left. This deflection pattern is known as the Coriolis effect. The resulting wind patterns are known as the prevailing winds: belts of air that distribute heat and moisture unevenly around the globe.
Solar energy warms the Earth’s surface. The heat is transferred to the atmosphere by radiation heating. The warmed gases expand, become less dense, and rise, creating vertical air flow called convection currents. The warm currents can also hold a lot of moisture compared to the surrounding air. As these large masses of warm, moist air rise, cool air flows along the Earth’s surface to occupy the area vacated by the warm air. This flowing air or horizontal airflow is one way that surface winds are created. As warm, moist air rises into the cooler atmosphere, it cools to the dew point, the temperature at which water vapor condenses into liquid water. This condensation creates clouds. If condensation continues and the water drops get bigger, they can no longer be held up by the convection in the Earth’s atmosphere and they fall as precipitation (which can be frozen or liquid). This cold, dry air is now denser than the surrounding air. This air mass then sinks to the Earth’s surface, where it is warmed and can gather more moisture, thus starting the convection cell rotation again.
On a local level, this phenomenon accounts for land and sea breezes. On a global scale, these cells are called Hadley cells. A large Hadley cell starts its cycle over the equator, where the warm, moist air evaporates and rises into the atmosphere. The precipitation in that region is one cause of the abundant equatorial rainforests. The cool, dry air then descends about 30 degrees north and south of the equator, forming the belts of deserts occurring around the Earth at those latitudes.
Albedo (Reflectance)
Another important property that affects the climates of different regions on Earth is albedo, the percentage of insolation (incoming solar radiation) reflected by a surface. The lower the surface albedo, the more solar radiation is absorbed.
An albedo value of 0 corresponds to zero reflectance and absorption of all radiation, whereas an albedo value of 1 corresponds to reflection of all incoming radiation. Snow and ice have high albedo values, while land and trees have lower albedo values.
Changes in albedo can lead to alterations in temperature. For example, snowfall may raise the albedo of an area, leading to an increase in the reflection of solar radiation and a decrease in temperature.
Types of Winds
So, what is “wind”? Why does everyone refer to wind when they’re discussing weather? Well, the term “wind” is widely used to refer to air currents, and we already know that air currents tend to flow from regions of high pressure to regions of low pressure. But let’s review some important details you’ll need to know about wind before we move on to our review of the hydrosphere. Formally speaking, wind is air that’s moving as a result of the unequal heating of the Earth’s atmosphere. It is part of the Earth’s circulatory system and moves heat, moisture, soil, and even pollution around the planet.
One crucial wind-related phenomenon that you’ll need to know about for the AP Environmental Science Exam is trade winds.
Trade winds were named for their ability to quickly propel trading ships across the ocean. The trade winds that blow between about 30 degrees latitude and the equator are steady and strong, and travel at a speed of about 11 to 13 mph. They are caused by the surface currents of the Hadley cells, described above, along with the Earth’s direction of rotation (counterclockwise if viewed toward the North Pole). In the Northern Hemisphere, the trade winds blow from the northeast and are known as the Northeast Trade Winds; in the Southern Hemisphere, the winds blow from the southeast and are called the Southeast Trade Winds.
Another important type of moving air mass, called a westerly (named for the direction from which it originates), travels north and east in the Northern Hemisphere and south and east in the Southern Hemisphere in the latitudes between 30 degrees and 60 degrees north and south of the equator. The movement of air that accounts for the westerlies, called the Ferrel cell, is the reverse of the Hadley cell but operates on the same thermodynamic principles.
The eastward movement of westerlies are a result of the Coriolis effect. Polar easterlies are formed by similar forces: in polar easterlies, winds between latitudes of 60 degrees and the North Pole blow from the north and east, and winds between 60 degrees and the South Pole blow from the south and east.
Between the wind belts mentioned above, air movement is less predictable, and often no wind blows at all for days. For example, between about 30 degrees to 35 degrees north and 30 degrees to 35 degrees south of the equator lie the regions known as the horse latitudes (or the subtropical high). Subsiding dry air and high pressure result in very weak winds in these regions.
Some people say that sailors gave the regions of the subtropical highs the name “horse latitudes” because ships relying on wind were unable to sail in these areas—so, afraid of running out of food and water, sailors would throw their horses (and other live cargo) overboard to save on food and water and to make the ship lighter and easier to move. Similarly, the air near the equator is relatively still because air there is constantly rising rather than blowing. For this reason, early sailors called this region the doldrums.
The region of the doldrums, occurring between 5 degrees north and 5 degrees south of the equator, is also known as the Intertropical Convergence Zone, or ITCZ for short. The trade winds converge there, producing convectional storms that give the ITCZ some of the world’s heaviest precipitation.
The last type of moving air system that you’ll need to be familiar with for the exam is the jet stream. Jet streams are high-speed currents of wind that occur in the tropopause; these fast-moving air currents have a large influence on local weather patterns.
Weather Events
Weather and climate are affected not only by the insolation in a given area but also by geologic and geographic factors, such as terrain (mountains, plains, distance from the ocean) and ocean temperatures.
Monsoons, or seasonal winds that are usually accompanied by very heavy rainfall, occur when land heats up and cools down more quickly than water does. In a monsoon, hot air rises from the heated land and a low-pressure system is created.
The rising air is quickly replaced by cooler moist air that blows in from over the ocean’s surface. As this air rises over land, it cools, and the moisture it carries is released in a steady seasonal rainfall. This process happens in reverse during the dry season, when masses of air that have cooled over the land blow out over the ocean .
On a smaller—local or regional—scale, this effect can be seen on the shores of large lakes or bays. In these areas, again the land warms faster than does the water during the day, so the air mass over the land rises. Air from over the lake moves in to replace it, and this creates a breeze.
At night, the reverse happens: the land cools more quickly than the water, and the air over the lake rises. The air mass from the land moves out over the lake to replace the rising air, and this creates a breeze as well. This small-scale monsoon effect is called the lake effect (something of a misnomer since the phenomenon isn’t limited to inland lakes). If you live near one of the Great Lakes or in the Bay Area of San Francisco, you may have experienced the lake effect yourself!
As we mentioned above, the air that moves in from over the ocean or a large body of water contains large amounts of water. If an air mass is forced to climb in altitude—if, for instance, it encounters an obstruction such as a mountain—the air will be forced to rise. When the air mass rises, it will cool, and water will precipitate out on the ocean side of the mountain. By the time the air mass reaches the opposite side of the mountain, it will be virtually devoid of moisture. This phenomenon is known as the rain shadow effect and is responsible for the impressive growth of the Olympic rainforest (within Olympic National Park) on the coast of Washington State.
Interestingly, Olympic National Park has a temperate rainforest on its west side where annual precipitation is about 150 inches (making it perhaps the wettest area in the continental United States) and forests on its much drier east side, where the annual precipitation is around 15 inches.
How the Rain Shadow Effect Works
Remember trade winds from the last section? Well, they occur in steady and somewhat predictable wind patterns, but they may cause local disturbances when they blow over very warm ocean water.
When this occurs, the air warms and forms an intense, isolated, low-pressure system while also picking up more water vapor from the ocean surface.
The wind will circle around this isolated low-pressure air area (counterclockwise in the Northern Hemisphere and the opposite in the Southern Hemisphere—once again due to the Coriolis effect!). The low-pressure system will continue to move over warm water, increasing in strength and wind speed; this will eventually result in a tropical storm.
Certain tropical storms are of sufficient intensity to be classified as hurricanes. Hurricanes must have winds with speeds greater than 120 km/hr. The rotating winds of a hurricane remove water vapor from the ocean’s surface, and heat is released as the water vapor condenses.
This addition of heat energy continues to contribute to the increase in wind speed, and some hurricanes have winds traveling at speeds of nearly 400 km per hour! A major hurricane contains more energy than that released during a nuclear explosion, but since the force is released more slowly, the damage is generally less concentrated.
Another important note about this type of storm is that they are referred to as hurricanes in the Atlantic Ocean, but they are called typhoons or cyclones when they occur in the Pacific Ocean. Go figure!
El Niño is a climate variation that takes place in the tropical Pacific about once every three to seven years, and it lasts for about one year. Under normal weather conditions, trade winds move the warm surface waters of the Pacific away from the west coast of Central and South America.
As a result, the cold ocean water that lies under the displaced water moves to the surface (causing the thermocline, or line of demarcation between two layers of water with different temperatures, to rise), bringing nutrients with it and keeping the temperature of the coastal water relatively cool. This phenomenon is called upwelling.
During El Niño, the normal trade winds are weakened or reversed because of a reversal of the high- and low-pressure regions on either side of the tropical Pacific. This reversal of pressure systems is known as the Southern Oscillation.
Without these regular trade winds off the Central and South American coast, the process of upwelling slows or stops, and the water off the coast becomes warmer and contains fewer nutrients.
This means that during El Niño, the northern United States and Canada experience warmer winters and a less intense hurricane season; the eastern United States and regions of Peru and Ecuador that are typically dry have higher-than-average rainfall; and the Philippines, Indonesia, and Australia are drier than normal.
One environmentally important effect that El Niño has on humans is that, because of the suppression of upwelling, the offshore fish populations of certain coastal areas decline. In countries like Peru, which relies heavily on fishing, El Niño has devastating economic effects.
The reverse of El Niño is known as La Niña. The Coriolis effect contributes to La Niña conditions. As air moves toward the equator to replace rising hot air, the moving air deflects to the west and helps move the surface water, allowing the upwelling.
During La Niña, the surface waters of the ocean surrounding Central and South America are colder than normal.
The term El Niño comes from the fact that traditionally these conditions were observed to begin around Christmas time (“El Niño” means “infant boy” in Spanish).
The alternations of atmospheric conditions that lead to El Niño’s and La Niña’s are referred to as ENSO (El Niño/Southern Oscillation) events.
Keep in mind that El Niño and La Niña are large-scale climate patterns, but they are influenced by geological and geographic factors and affect different locations in different ways.
THE HYDROSPHERE
Water covers about 71 percent of planet Earth. Most of the water on the Earth’s surface is salt water. On average, the salt water in the world’s oceans has a salinity of about 3.5 percent.
This means that for every 1 liter (1,000 ml) of sea water, there are 35 grams of salts (mostly, but not entirely, sodium chloride) dissolved in it. (1 ml of water weighs approximately 1 gram.) In fact, 1 cubic foot of seawater would evaporate to leave about 2 pounds of sea salt! However, sea water is not uniformly saline throughout the world. The planet’s freshest sea water is in the Gulf of Finland, part of the Baltic Sea.
The most saline open sea is the Red Sea, where high temperatures and confined circulation result in high rates of surface evaporation.
Freshwater is water that contains only minimal quantities of dissolved salts, especially sodium chloride.
All freshwater ultimately comes from precipitation of atmospheric water vapor, which reaches inland lakes, rivers, and groundwater bodies directly, or after melting of snow or ice. Let’s start with a discussion of freshwater before discussing the world’s oceans.
Freshwater is deposited on the surface of the Earth through precipitation. Water that falls on the Earth and doesn’t move through the soil to become groundwater moves along the Earth’s surface via gravity, forms small streams, and then eventually forms larger ones.
The size of the stream will continue to increase as water is added to it, until the stream becomes a river, and the river will flow until it reaches the ocean.
The land area that drains into a particular stream is known as a watershed, or drainage basin. A particular watershed has some characteristics that define it: its area, length, slope, soil and vegetation types, and how it’s separated from adjoining watersheds.
As water moves into streams, it carries with it sediment and other dissolved substances, including small amounts of oxygen.
Turbulent waters are especially laden with dissolved oxygen and carbon dioxide, such as those found at the source, or headwaters, of a stream. As a general rule, the more turbulent the water, the more dissolved gases it will contain.
Freshwater Bodies
Freshwater bodies include streams, rivers, ponds, and lakes. As you probably know, freshwater that travels on land is largely responsible for shaping the Earth’s surface.
Erosion occurs when the movement of water etches channels into rocks and soil. The moving water then carries eroded material farther downstream. Because of obstructions on land, moving water does not move in a straight line. Instead, it follows the lowest topographical path, and as it flows continuously along the same path it cuts farther into its banks to eventually form a curving channel.
As the water travels around these bends, its velocity decreases, and the stream drops some of its sedimentary load. Water always follows the path of least resistance as it travels from the highlands to the sea.
Rivers drop most of their sedimentary load as they meet the ocean because their velocity decreases significantly at this juncture. At these locations, landforms called deltas—which are made of deposited sediments—are created.
Ocean Currents
Ocean currents play a major role in modifying conditions around the Earth that can affect where certain climates are located.
As the sun warms water in the equatorial regions of the globe, prevailing winds, differences in salinity (saltiness), and the Earth’s rotation set masses of ocean water in motion. For example, in the Northern Hemisphere, the Gulf Stream carries sun-warmed water northward along the east coast of the Unites States and across the Atlantic Ocean as far as Great Britain.
This warm water displaces the colder, denser water in the polar regions, which can move south to be re-warmed by the equatorial sun. Northern Europe is kept 5° to 10°C warmer than it would be were the current not present.
Oceanographers also study a major current, the “ocean conveyor belt,” that moves cold water in the depths of the Pacific Ocean while creating major upwellings in other areas of the Pacific.
Chapter 6: Earth Systems and Resources
According to the College Board, about 10 to 15% of the test is based directly on the content covered in this chapter. If you are unfamiliar with a topic presented here, consult your textbook for more in-depth information.
Let’s take a step back and review the fundamental themes of this chapter. First of all, a resource is strictly defined as any substance, capability (such as work performed by humans or animals), or other asset that is available in a supply that can be accessed and drawn on as needed. Resources are utilized for the effective functioning of an organism or community, often for economic gain. Within the scope of Environmental Science, we typically understand resources to mean natural resources—resources that occur in nature.
We also discuss the importance of each sphere to humans. The four spheres are:
The Solid Earth—the Earth’s solid, rocky outer shell
Topics: the movement of tectonic plates, volcanoes and -earthquakes, and the different types of rock
The upper shell of the Solid Earth is called the Lithosphere and is the part that interacts most with the other spheres
The Pedosphere—more commonly known as soil
Topics: soil’s characteristics, formation, layers, and development
The Atmosphere—the envelope of gases that surrounds the Earth
Topics: the greenhouse effect, climate, and weather events
The Hydrosphere—the Earth’s oceans and freshwater bodies
Topics: fresh- and saltwater bodies and ocean currents
The four physical spheres provide resources that support the Biosphere, the fifth of the Earth’s spheres, which comprises all the living organisms that inhabit the planet and draw on the physical resources of the other four spheres.
WELCOME TO PLANET EARTH
The first thing you should know about the Earth is its history. The Earth is thought to be between 4.5 and 4.8 billion years old. That amount of time is pretty inconceivable to humans, but the following geologic time scale will help you get a sense of the vast amount of time that has gone by since the Earth was formed. You will not be responsible for memorizing all of the eons, eras, periods, and epochs for this exam, but you should be familiar with the major ones—they will come in handy.
We are currently in the Holocene Epoch.
The Quaternary and Tertiary are the two most recent geologic periods.
Non-avian dinosaurs lived during the Mesozoic Era. (Note that evolutionary biologists consider birds to be avian, or flying, dinosaurs. Pterosaurs such as pterodactyls are a group of reptiles less closely related to dinosaurs and birds.)
The Precambrian eons represent the vast majority of the geologic time scale.
The next epoch will be called the Anthropocene, to recognize humankind’s accelerating effects on the planet’s physical resources, climate, and life forms. Many geologists propose that we have already entered this new, post-Holocene epoch.
Where Is the Earth in the Solar System?
The Earth is the third planet from the sun in our solar system, which contains a total of eight currently known and recognized planets. From the sun outward, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Each planet has its own orbit around the sun in the shape of an ellipse (a “stretched” circle). And you probably already know that it takes the Earth about 365.25 days, or 1 year, to complete its orbit of the sun.
The Solid Earth
Planet Earth is made up of three concentric zones of rocks that are either solid or liquid (molten). The innermost zone is the core. The core has two parts: a solid inner core and a molten outer core. The inner core is composed mostly of nickel and iron and is solid due to the tremendous pressure from overlying matter. The outer core is composed mostly of iron, also mixed with nickel as well as some lighter elements, and is semi-solid due to lower pressure. Surrounding the outer core is the mantle, which is made mostly of solid rock. Near the top of the mantle lies a layer of slowly flowing rock called the asthenosphere. The lithosphere, a thin, rigid layer of rock, is the Earth’s outer shell. The lithosphere includes the rigid upper mantle above the asthenosphere and the crust, the solid surface of the Earth. Think of it as floating atop the asthenosphere like a cracker atop a thick layer of hot pudding.
Tectonic Plates
Scientists theorize that during the Paleozoic and Mesozoic Eras, the continents were joined together, forming a supercontinent known as Pangaea. Roughly 200 million years ago, Pangaea began to break apart.
Today, it is believed that the Earth’s crust is composed of several large pieces of lithosphere—called tectonic plates—that move slowly over the mantle of the Earth.
There are a total of a dozen or so tectonic plates that move independently of one another. The majority of the land on Earth sits above six giant plates; the remainder of the plates lie under the ocean as well as the continents. The edges of the plates are called plate boundaries, and the places where two plates about each other are where events like sea floor spreading and most volcanoes and earthquakes occur.
There are three types of plate boundary interactions:
Convergent boundary: Two plates are pushed toward and into each other. One of the plates slides beneath the other, pushed deep into the mantle.
Divergent boundary: Two plates move away from each other. This creates a gap between plates that may be filled with rising magma (molten rock). When this magma cools, it forms new crust.
Transform fault boundary: Two plates slide against each other in opposite directions—as when you rub your hands back and forth to warm them up. These are also called simply transform boundaries.
Volcanoes and Earthquakes
Volcanoes are mountains formed by pressure from magma rising from the Earth’s interior. Active volcanoes are those that are currently erupting or have erupted within recorded history (that is, within the last 10,000 years), while dormant volcanoes have not been known to erupt during this period. It’s thought that extinct volcanoes will never erupt again. Active volcanoes are categorized by the kind of tectonic events that produce them. They are associated with the following:
Subduction zones occur at convergent boundaries between oceanic and continental plates, or sometimes between two oceanic plates. The subducting plate is recycled into new magma, which rises through the overlying plate to create volcanoes inland.
Rift valleys occur at divergent boundaries, usually between two oceanic plates. New ocean floor is formed as magma fills in the gap between separating plates. Thick magma rising from rift valleys is made of basaltic minerals and forms pillow lava upon contact with the cold ocean water. Rift valleys may also occur between continental plates; a prominent example is the Great Rift Valley of eastern Africa, which gave rise to Mount Kilimanjaro and other volcanoes.
Hot spots do not form at plate boundaries. Instead, they are found in the middle of tectonic plates, in locations where columns of unusually hot magma melt through the mantle and weaken the Earth’s crust. The Hawaiian islands continue to form over a hot spot beneath the Pacific plate. Volcanoes over oceanic hot spots are basaltic, resulting in milder eruptions; while volcanoes over continental hot spots are characterized by rhyolitic rocks, which produce more violent eruptions.
There are four types of volcanoes:
Shield volcanoes have broad bases and are tall with gentle slopes. They generally form over oceanic hot spots and usually have mild eruptions with slow lava flow. Sometimes, however, when water enters the vent, they can be very explosive, forming pyroclastic flows, a fluidized mixture of hot ash and rock.
Composite volcanoes have broad bases and are also tall but with steeper slopes. They are formed at subduction zones and are associated with violent eruptions that eject lava, water, and gases as superheated ash and stones.
Cinder volcanoes are small, short, and steeply sloped cones. They form when molten lava erupts and cools quickly in the air, hardening into porous rocks (called cinders or scoria) that fracture as they hit the Earth’s surface. Cinder volcanoes generally form near other types of volcanoes.
Lava domes are small and short with steep slope and rounded tops. They are formed from lava that is too viscous to travel far but instead hardens into a dome shape. This type of volcano occurs near or even inside other types of volcanoes.
The Rock Cycle
Rocks are all around us, in the soil, our buildings, and the ore used in industry. So, where do all those rocks come from? The answer is: other rocks. The oldest rocks on Earth are 3.8 billion years old, while others are only a few million years old. This means that rocks are recycled. These transformations are described by the rock cycle. In the rock cycle, time, pressure, and the Earth’s heat interact to create three basic types of rocks.
Igneous rock results when rock is melted (by heat and pressure below the crust) into a liquid and then resolidifies when cooled. The molten rock (magma) comes to the surface of the Earth, and when it emerges it is called lava; cooled lava becomes solid igneous rock. An example of an igneous rock is basalt.
Sedimentary rock is formed as sediment (eroded rocks and the remains of plants and animals) builds up and is compressed. Sedimentary rock forms under water as sediments or dissolved minerals deposit on a stream bed or ocean floor. They are compressed as more material is deposited and then cemented together. An example of a sedimentary rock is limestone.
Metamorphic rock is formed as a great deal of pressure and heat produces physical and/or chemical changes in existing rock. This can happen as sedimentary rocks sink deeper into the Earth and are heated by the high temperatures found in the Earth’s mantle. An example of a metamorphic rock is slate, which results from the metamorphosis of shale.
Where Does Soil Come From?
Basically, soil is a combination of organic material and rock that has been broken down by chemical and biological weathering—which is, unsurprisingly, the process by which rock and other material decomposes. Therefore, it should also not be surprising to learn that the types of minerals found in soil in a particular region will depend on the identity of the base rock of that region. Note that erosion is a distinct process, by which broken-down material is removed from one place and transported to another, across the Earth’s surface, usually by wind or water. Sometimes the same process can be responsible for both erosion and weathering, as when a glacier grinds material from surface rock and drags it along with its own movement, depositing it far away.
Water, wind, temperature, and living organisms are all prominent agents of weathering, and all weathering processes are placed into the following three rather broad categories.
Physical weathering (also known as mechanical weathering): Any process that breaks rock down into smaller pieces without changing the chemistry of the rock. The forces responsible for physical weathering are typically wind and water.
Chemical weathering: Occurs as a result of chemical reactions of rock with water, air, or dissolved minerals. Chemical processes result in minerals that are broken down or restructured into different minerals. This type of weathering tends to dominate in warm or moist environments. One example of chemical weathering is rust, which forms when iron and other metallic elements come in contact with water.
Biological weathering: Weathering that takes place as the result of the activities of living organisms, which may act through physical or chemical means. When tree roots enlarge the cracks in rocks as they grow, that is a physical process. When plant roots or lichens growing on rocks release organic acids that dissolve minerals, that is a chemical process.
Soil Layers
Soil comprises distinct layers known as horizons, each of which has distinct physical, chemical, and biological properties. Study the following diagram which illustrates and describes the different horizons. Not every soil contains all of these horizons.
O horizon: This layer is made up of organic matter at various stages of decomposition. It includes animal waste, leaves and other plant tissues (such as dead roots), and the decomposing bodies of organisms. The stable residue left after most organic matter has decomposed is a dark, crumbly material called humus.
A horizon: This is the topsoil—the topmost mineral horizon and the most intensively weathered soil layer. Its dark color is due to accumulation of organic matter from the O horizon. In soils lacking an E horizon, this may also be called the zone of leaching.
E horizon: This is the eluviated horizon. It is light in color and coarse in texture; no organic matter has traveled down from the A horizon, while clays and minerals like iron and aluminum oxides have been washed out by leaching and eluviation. The E horizon isn’t found in all soils: it’s mostly found in soils developed under forest.
B horizon: Sometimes called the subsoil, this is where organic matter, clay, and minerals washed out of the upper horizons accumulate. Thus, it is called the zone of accumulation or the zone of illuviation.
C horizon: This layer is the parent material—unconsolidated material, loose enough to be dug up with a shovel. Weathering at this depth is minimal so the soil retains identifiable features of the parent material (rock from which the A and B horizons formed). This horizon has much less biological activity than the horizons above it.
R horizon: Beneath the soil lies a layer of consolidated (cemented), Un weathered rock. Because it hasn’t been weathered, this isn’t part of the soil, strictly speaking. If the material from which horizons A through C formed was not transported from elsewhere, the R horizon has the same source minerals as the parent material above. The R stands for regolith, a word meaning “blanket or surface rock” in ancient Greek.
Soil Development
Four Processes of Soil Development. Soil development is an intricate business! The development of distinct soil horizons is accomplished by four basic processes that work on soil minerals and particles, organic matter, and soil chemistry. These four processes include additions of materials from off-site; losses through erosion or leaching or biological activity (plant uptake); vertical translocations or movement from one soil horizon to another; and transformations of materials in place by weathering or chemical activity.
Six Soil-Forming Factors. The processes of soil development are influenced by six soil-forming factors. It takes hundreds to thousands to millions of years for a soil to develop its characteristic layers or profile. To wrap it all up in one big sentence: any soil you see is a dynamic formation produced by the effects of climate and biological activity (organisms), as modified by topography (relief) and human influences, acting on parent materials over time. The six soil-forming factors can be remembered by the awkward acronym Cl–O–R–P–T–H. Here is a summary of how each factor works.
Climate: Climate involves differences in temperature and precipitation across the globe, and both heat and water facilitate chemical and biochemical reactions. Seasonal fluctuation of heat and moisture affects processes such as freeze-thaw cycles that weather rock. Climate also helps to determine what organisms grow in a particular location.
Organisms (Biological Activity): Different local conditions support different organisms, which influence the soil in a multitude of ways. Microorganisms perform biochemical functions such as decomposition of organic matter and transformation of minerals into different forms. Animals move soils, consume vegetation, and add nutrients through waste and decomposing bodies. Plants perform physical weathering through root growth, take up soil nutrients and water, alter soil chemistry in various ways, and add nutrients when they die and decompose.
Relief (Topography): Topographical relief affects where water moves on the landscape and also the depth of the water table in a given location. Relief similarly affects erosion—which locations are likely to lose surface material through the action of wind and rain, and which locations are likely to accumulate eroded material. Topographical relief also leads to differences in how much sun different locations receive. Through these characteristics, relief influences which organisms grow in a particular location.
Parent Material: This is the starting point for soil development. Its mineral properties, hardness, and topographical form affect how it is weathered into soil. As parent material varies from location to location, so will the soil that develops at each location. As an example, parent material rich in quartz, such as granite and sandstone, weathers into sandy soil. Shale weathers into soil richer in silt and clay.
Time: More time equals more change! Hard parent material weathers more slowly and softer material more quickly. A flat, stable topographic position develops horizons more quickly than do slopes and depressions where material is lost and gained.
Human Influence: The effects of human activity must increasingly be acknowledged as a factor in soil development. Use of fertilizer, pollution, and acid rain alter soil chemistry on a broad scale. Construction activities such as digging, and plowing tend to mix soils and blur the distinctions between horizons. Human activities also lead to compaction (through the traffic of vehicles and machinery), erosion (through removal of stabilizing vegetation), and salinization (increase in salt content, through irrigation and depletion of groundwater).
THE ATMOSPHERE
In the broadest definition, the atmosphere is a layer of gases that’s held close to the Earth by the force of gravity. The inner four layers of the atmosphere reach an altitude that’s just about 12.5% of the Earth’s radius. The layer of gases that lies closest to the Earth is the troposphere; it extends from the Earth’s surface to about, on average, 12 km (7.5 miles) at the poles and 20 km (12.4 miles) at the equator.
The troposphere is where all the weather that we experience takes place. The layer also contains 99% of the atmosphere’s water vapor and clouds. Generally, the troposphere is well-mixed from bottom to top—with the exception of periodic temperature inversions. The troposphere gets colder with altitude, decreasing 6.5°C for every kilometer of altitude (or 3.5°F for every thousand feet).
Because of its density, the troposphere contains about 75–80% of the Earth’s atmosphere by mass. You’ve probably heard about the troposphere before in the news because of the greenhouse effect.
The troposphere contains the air we breathe, which is made up of 78% nitrogen and 21% oxygen. The remaining 1% includes the so-called “greenhouse” gases (GHGs). The proportion of these gases in the troposphere is minuscule, but their effects on conditions on Earth are disproportionately significant.
The most important of them are water vapor (H2O), carbon dioxide (CO2), and methane (CH4). As the sun’s rays strike the Earth, some of the solar radiation is reflected back into space; however, greenhouse gases in the troposphere intercept and absorb a lot of this radiation. This warming effect of greenhouse gases was a good thing, until their concentration in the atmosphere shot up after the Industrial Revolution.
Crowning the troposphere is the tropopause, which is a layer that acts as a buffer between the troposphere and the next layer up, the stratosphere. This buffer zone is where the jet streams, air currents that are important drivers of weather patterns and important factors in planning airline routes, travel.
The stratosphere sits on top of the tropopause and extends about 20–50 km (7.5–31 miles) above the Earth’s surface. As opposed to those in the troposphere, gases in the stratosphere are not well mixed and temperatures increase with distance from the Earth. This warming effect is due to the ozone layer, a thin band of ozone (O3) that exists in the lower half of this layer.
The ozone traps the high-energy radiation of the sun, holding some of the heat and protecting the troposphere and the Earth’s surface from this radiation. The stratosphere is similar to the troposphere in gas composition, only less dense and drier, with a thousand times less water vapor. Commercial jets may also fly in the lower part of this layer.
Above the stratosphere are two layers called the mesosphere and the thermosphere. The mesosphere extends to about 80 km (50 miles) above the Earth’s surface and is the area where meteors usually burn up. Temperatures again decrease here, to the coldest point in the atmosphere at the top of this layer, around –90°C (–130°F).
The thermosphere extends from 80 to around 500 km above the Earth (50–435 miles). Gases are very thin (rare) and it’s in this layer that the spectacular and colorful auroras (northern lights and southern lights) take place. The furthest layer is the exosphere, extending to 10,000 km (6,200 miles) or more above the Earth, although the upper limit of this layer is not definitively settled.
The concentration of gases is thinnest here. Human-made satellites orbit in the exosphere and in the upper thermosphere.
The ionosphere is not a distinct layer but dispersed throughout the upper mesosphere, the thermosphere, and the lower exosphere. The ionosphere comprises regions of ionized gases that absorb most of the energetic charged particles from the sun—the protons and electrons of the solar wind. Interestingly, the ionosphere also reflects radio waves, making long-distance radio communication possible. You’ll need to know how the climates that we experience on Earth are created by the atmosphere, so let’s go into this next.
Climate
The Earth’s atmosphere has physical features that change from day to day as well as patterns that are consistent over a space of many years. The day-to-day properties such as wind speed and direction, temperature, amount of sunlight, pressure, and humidity are referred to as weather. The patterns that are constant over many years (30 years or more) are referred to as climate. The two most important factors in describing climate are average temperature and average precipitation amounts. Meteorologists are scientists who study weather and climate.
Air Circulation in the Atmosphere
The motion of air around the globe is the result of solar heating, the rotation of the Earth, and the physical properties of air, water, and land. There are three major reasons why the Earth is unevenly heated.
More of the sun’s rays strike the Earth at the equator in each unit of surface area than strike the poles in the same unit area. This is because the angle of the suns rays strike the Earth more directly at the equator.
The tilt of the Earth’s axis points regions toward or away from the sun. When pointed toward the sun, those areas receive more direct or intense light than when pointed away. This causes the seasons.
The Earth’s surface at the equator is moving faster than at the poles, because the circumference is larger but the rotation time is the same. Because an object or air mass nearer to the equator is moving more rapidly (from east to west), it will maintain this eastward momentum as it moves away from the equator to where the surface is moving more slowly, winding up further east. Therefore, winds moving north from the equator near the surface are deflected to the right (east), and winds moving south from the equator are deflected to the left (east).
Conversely, winds blowing toward the equator will be deflected to the west because they are moving eastward more slowly than is the surface in the lower latitudes where they are moving to. So, in the Northern Hemisphere, this westward deflection will be to the right, and in the Southern Hemisphere, it will be to the left. This deflection pattern is known as the Coriolis effect. The resulting wind patterns are known as the prevailing winds: belts of air that distribute heat and moisture unevenly around the globe.
Solar energy warms the Earth’s surface. The heat is transferred to the atmosphere by radiation heating. The warmed gases expand, become less dense, and rise, creating vertical air flow called convection currents. The warm currents can also hold a lot of moisture compared to the surrounding air. As these large masses of warm, moist air rise, cool air flows along the Earth’s surface to occupy the area vacated by the warm air. This flowing air or horizontal airflow is one way that surface winds are created. As warm, moist air rises into the cooler atmosphere, it cools to the dew point, the temperature at which water vapor condenses into liquid water. This condensation creates clouds. If condensation continues and the water drops get bigger, they can no longer be held up by the convection in the Earth’s atmosphere and they fall as precipitation (which can be frozen or liquid). This cold, dry air is now denser than the surrounding air. This air mass then sinks to the Earth’s surface, where it is warmed and can gather more moisture, thus starting the convection cell rotation again.
On a local level, this phenomenon accounts for land and sea breezes. On a global scale, these cells are called Hadley cells. A large Hadley cell starts its cycle over the equator, where the warm, moist air evaporates and rises into the atmosphere. The precipitation in that region is one cause of the abundant equatorial rainforests. The cool, dry air then descends about 30 degrees north and south of the equator, forming the belts of deserts occurring around the Earth at those latitudes.
Albedo (Reflectance)
Another important property that affects the climates of different regions on Earth is albedo, the percentage of insolation (incoming solar radiation) reflected by a surface. The lower the surface albedo, the more solar radiation is absorbed.
An albedo value of 0 corresponds to zero reflectance and absorption of all radiation, whereas an albedo value of 1 corresponds to reflection of all incoming radiation. Snow and ice have high albedo values, while land and trees have lower albedo values.
Changes in albedo can lead to alterations in temperature. For example, snowfall may raise the albedo of an area, leading to an increase in the reflection of solar radiation and a decrease in temperature.
Types of Winds
So, what is “wind”? Why does everyone refer to wind when they’re discussing weather? Well, the term “wind” is widely used to refer to air currents, and we already know that air currents tend to flow from regions of high pressure to regions of low pressure. But let’s review some important details you’ll need to know about wind before we move on to our review of the hydrosphere. Formally speaking, wind is air that’s moving as a result of the unequal heating of the Earth’s atmosphere. It is part of the Earth’s circulatory system and moves heat, moisture, soil, and even pollution around the planet.
One crucial wind-related phenomenon that you’ll need to know about for the AP Environmental Science Exam is trade winds.
Trade winds were named for their ability to quickly propel trading ships across the ocean. The trade winds that blow between about 30 degrees latitude and the equator are steady and strong, and travel at a speed of about 11 to 13 mph. They are caused by the surface currents of the Hadley cells, described above, along with the Earth’s direction of rotation (counterclockwise if viewed toward the North Pole). In the Northern Hemisphere, the trade winds blow from the northeast and are known as the Northeast Trade Winds; in the Southern Hemisphere, the winds blow from the southeast and are called the Southeast Trade Winds.
Another important type of moving air mass, called a westerly (named for the direction from which it originates), travels north and east in the Northern Hemisphere and south and east in the Southern Hemisphere in the latitudes between 30 degrees and 60 degrees north and south of the equator. The movement of air that accounts for the westerlies, called the Ferrel cell, is the reverse of the Hadley cell but operates on the same thermodynamic principles.
The eastward movement of westerlies are a result of the Coriolis effect. Polar easterlies are formed by similar forces: in polar easterlies, winds between latitudes of 60 degrees and the North Pole blow from the north and east, and winds between 60 degrees and the South Pole blow from the south and east.
Between the wind belts mentioned above, air movement is less predictable, and often no wind blows at all for days. For example, between about 30 degrees to 35 degrees north and 30 degrees to 35 degrees south of the equator lie the regions known as the horse latitudes (or the subtropical high). Subsiding dry air and high pressure result in very weak winds in these regions.
Some people say that sailors gave the regions of the subtropical highs the name “horse latitudes” because ships relying on wind were unable to sail in these areas—so, afraid of running out of food and water, sailors would throw their horses (and other live cargo) overboard to save on food and water and to make the ship lighter and easier to move. Similarly, the air near the equator is relatively still because air there is constantly rising rather than blowing. For this reason, early sailors called this region the doldrums.
The region of the doldrums, occurring between 5 degrees north and 5 degrees south of the equator, is also known as the Intertropical Convergence Zone, or ITCZ for short. The trade winds converge there, producing convectional storms that give the ITCZ some of the world’s heaviest precipitation.
The last type of moving air system that you’ll need to be familiar with for the exam is the jet stream. Jet streams are high-speed currents of wind that occur in the tropopause; these fast-moving air currents have a large influence on local weather patterns.
Weather Events
Weather and climate are affected not only by the insolation in a given area but also by geologic and geographic factors, such as terrain (mountains, plains, distance from the ocean) and ocean temperatures.
Monsoons, or seasonal winds that are usually accompanied by very heavy rainfall, occur when land heats up and cools down more quickly than water does. In a monsoon, hot air rises from the heated land and a low-pressure system is created.
The rising air is quickly replaced by cooler moist air that blows in from over the ocean’s surface. As this air rises over land, it cools, and the moisture it carries is released in a steady seasonal rainfall. This process happens in reverse during the dry season, when masses of air that have cooled over the land blow out over the ocean .
On a smaller—local or regional—scale, this effect can be seen on the shores of large lakes or bays. In these areas, again the land warms faster than does the water during the day, so the air mass over the land rises. Air from over the lake moves in to replace it, and this creates a breeze.
At night, the reverse happens: the land cools more quickly than the water, and the air over the lake rises. The air mass from the land moves out over the lake to replace the rising air, and this creates a breeze as well. This small-scale monsoon effect is called the lake effect (something of a misnomer since the phenomenon isn’t limited to inland lakes). If you live near one of the Great Lakes or in the Bay Area of San Francisco, you may have experienced the lake effect yourself!
As we mentioned above, the air that moves in from over the ocean or a large body of water contains large amounts of water. If an air mass is forced to climb in altitude—if, for instance, it encounters an obstruction such as a mountain—the air will be forced to rise. When the air mass rises, it will cool, and water will precipitate out on the ocean side of the mountain. By the time the air mass reaches the opposite side of the mountain, it will be virtually devoid of moisture. This phenomenon is known as the rain shadow effect and is responsible for the impressive growth of the Olympic rainforest (within Olympic National Park) on the coast of Washington State.
Interestingly, Olympic National Park has a temperate rainforest on its west side where annual precipitation is about 150 inches (making it perhaps the wettest area in the continental United States) and forests on its much drier east side, where the annual precipitation is around 15 inches.
How the Rain Shadow Effect Works
Remember trade winds from the last section? Well, they occur in steady and somewhat predictable wind patterns, but they may cause local disturbances when they blow over very warm ocean water.
When this occurs, the air warms and forms an intense, isolated, low-pressure system while also picking up more water vapor from the ocean surface.
The wind will circle around this isolated low-pressure air area (counterclockwise in the Northern Hemisphere and the opposite in the Southern Hemisphere—once again due to the Coriolis effect!). The low-pressure system will continue to move over warm water, increasing in strength and wind speed; this will eventually result in a tropical storm.
Certain tropical storms are of sufficient intensity to be classified as hurricanes. Hurricanes must have winds with speeds greater than 120 km/hr. The rotating winds of a hurricane remove water vapor from the ocean’s surface, and heat is released as the water vapor condenses.
This addition of heat energy continues to contribute to the increase in wind speed, and some hurricanes have winds traveling at speeds of nearly 400 km per hour! A major hurricane contains more energy than that released during a nuclear explosion, but since the force is released more slowly, the damage is generally less concentrated.
Another important note about this type of storm is that they are referred to as hurricanes in the Atlantic Ocean, but they are called typhoons or cyclones when they occur in the Pacific Ocean. Go figure!
El Niño is a climate variation that takes place in the tropical Pacific about once every three to seven years, and it lasts for about one year. Under normal weather conditions, trade winds move the warm surface waters of the Pacific away from the west coast of Central and South America.
As a result, the cold ocean water that lies under the displaced water moves to the surface (causing the thermocline, or line of demarcation between two layers of water with different temperatures, to rise), bringing nutrients with it and keeping the temperature of the coastal water relatively cool. This phenomenon is called upwelling.
During El Niño, the normal trade winds are weakened or reversed because of a reversal of the high- and low-pressure regions on either side of the tropical Pacific. This reversal of pressure systems is known as the Southern Oscillation.
Without these regular trade winds off the Central and South American coast, the process of upwelling slows or stops, and the water off the coast becomes warmer and contains fewer nutrients.
This means that during El Niño, the northern United States and Canada experience warmer winters and a less intense hurricane season; the eastern United States and regions of Peru and Ecuador that are typically dry have higher-than-average rainfall; and the Philippines, Indonesia, and Australia are drier than normal.
One environmentally important effect that El Niño has on humans is that, because of the suppression of upwelling, the offshore fish populations of certain coastal areas decline. In countries like Peru, which relies heavily on fishing, El Niño has devastating economic effects.
The reverse of El Niño is known as La Niña. The Coriolis effect contributes to La Niña conditions. As air moves toward the equator to replace rising hot air, the moving air deflects to the west and helps move the surface water, allowing the upwelling.
During La Niña, the surface waters of the ocean surrounding Central and South America are colder than normal.
The term El Niño comes from the fact that traditionally these conditions were observed to begin around Christmas time (“El Niño” means “infant boy” in Spanish).
The alternations of atmospheric conditions that lead to El Niño’s and La Niña’s are referred to as ENSO (El Niño/Southern Oscillation) events.
Keep in mind that El Niño and La Niña are large-scale climate patterns, but they are influenced by geological and geographic factors and affect different locations in different ways.
THE HYDROSPHERE
Water covers about 71 percent of planet Earth. Most of the water on the Earth’s surface is salt water. On average, the salt water in the world’s oceans has a salinity of about 3.5 percent.
This means that for every 1 liter (1,000 ml) of sea water, there are 35 grams of salts (mostly, but not entirely, sodium chloride) dissolved in it. (1 ml of water weighs approximately 1 gram.) In fact, 1 cubic foot of seawater would evaporate to leave about 2 pounds of sea salt! However, sea water is not uniformly saline throughout the world. The planet’s freshest sea water is in the Gulf of Finland, part of the Baltic Sea.
The most saline open sea is the Red Sea, where high temperatures and confined circulation result in high rates of surface evaporation.
Freshwater is water that contains only minimal quantities of dissolved salts, especially sodium chloride.
All freshwater ultimately comes from precipitation of atmospheric water vapor, which reaches inland lakes, rivers, and groundwater bodies directly, or after melting of snow or ice. Let’s start with a discussion of freshwater before discussing the world’s oceans.
Freshwater is deposited on the surface of the Earth through precipitation. Water that falls on the Earth and doesn’t move through the soil to become groundwater moves along the Earth’s surface via gravity, forms small streams, and then eventually forms larger ones.
The size of the stream will continue to increase as water is added to it, until the stream becomes a river, and the river will flow until it reaches the ocean.
The land area that drains into a particular stream is known as a watershed, or drainage basin. A particular watershed has some characteristics that define it: its area, length, slope, soil and vegetation types, and how it’s separated from adjoining watersheds.
As water moves into streams, it carries with it sediment and other dissolved substances, including small amounts of oxygen.
Turbulent waters are especially laden with dissolved oxygen and carbon dioxide, such as those found at the source, or headwaters, of a stream. As a general rule, the more turbulent the water, the more dissolved gases it will contain.
Freshwater Bodies
Freshwater bodies include streams, rivers, ponds, and lakes. As you probably know, freshwater that travels on land is largely responsible for shaping the Earth’s surface.
Erosion occurs when the movement of water etches channels into rocks and soil. The moving water then carries eroded material farther downstream. Because of obstructions on land, moving water does not move in a straight line. Instead, it follows the lowest topographical path, and as it flows continuously along the same path it cuts farther into its banks to eventually form a curving channel.
As the water travels around these bends, its velocity decreases, and the stream drops some of its sedimentary load. Water always follows the path of least resistance as it travels from the highlands to the sea.
Rivers drop most of their sedimentary load as they meet the ocean because their velocity decreases significantly at this juncture. At these locations, landforms called deltas—which are made of deposited sediments—are created.
Ocean Currents
Ocean currents play a major role in modifying conditions around the Earth that can affect where certain climates are located.
As the sun warms water in the equatorial regions of the globe, prevailing winds, differences in salinity (saltiness), and the Earth’s rotation set masses of ocean water in motion. For example, in the Northern Hemisphere, the Gulf Stream carries sun-warmed water northward along the east coast of the Unites States and across the Atlantic Ocean as far as Great Britain.
This warm water displaces the colder, denser water in the polar regions, which can move south to be re-warmed by the equatorial sun. Northern Europe is kept 5° to 10°C warmer than it would be were the current not present.
Oceanographers also study a major current, the “ocean conveyor belt,” that moves cold water in the depths of the Pacific Ocean while creating major upwellings in other areas of the Pacific.
