List and describe Earth’s four major spheres. Define system, and explain why Earth is considered a system.

Anyone who studies Earth soon learns that our planet is a dynamic body with many separate but highly interactive parts, or spheres. The atmosphere, hydrosphere, biosphere, and lithosphere, along with all of their components, can be studied separately. However, the parts are not isolated. Each is related in many ways to the others, producing a complex and continuously interacting whole that we call the Earth system.

Earth’s Spheres

The images in Figure 1.8 are classics because, for the first time, they let humanity see Earth differently from ever before. These photos profoundly altered our conceptualizations of Earth and remain powerful images decades after they were first viewed. Seen from space, Earth is breathtaking in its beauty and startling in its solitude. The images remind us that our home is, after all, a planet—small, self-contained, and in some ways even fragile. Bill Anders, the Apollo 8 astronaut who took the “Earthrise” photo, expressed it this way: “We came all this way to explore the Moon, and the most important thing is that we discovered the Earth.”

The closer view of Earth from space shown in Figure 1.8 helps us appreciate why the physical environment is traditionally divided into three major parts: Earth’s gaseous envelope, the atmosphere; the water portion of our planet, the hydrosphere; and Earth’s solid outer layer, the lithosphere. It should be emphasized that our environment is highly integrated and is not dominated by air, water, or rock alone. Instead, the biosphere, the life-forms on our planet, extends into each of the three physical realms and is an equally integral part of the planet.

Moreover, the interactions among Earth’s four spheres are incalculable. Figure 1.9 provides one easy-to-visualize example. The shoreline is an obvious meeting place for rock, water, and air, and these spheres in turn support life-forms in and near the water. In this scene, ocean waves created by the drag of air moving across the water are breaking against the rocky shore. The force of water, in turn, erodes the shoreline.

The Atmosphere

Earth is surrounded by a life-giving gaseous envelope called the atmosphere (Figure 1.10). When we watch a high-flying jet plane cross the sky, it seems that the atmosphere extends upward for a great distance. However, when compared to the thickness (radius) of the solid Earth, the atmosphere is a very shallow layer. This thin blanket of air is nevertheless an integral part of the planet. It not only provides the air we breathe, but also acts to protect us from the dangerous radiation emitted by the Sun.

Furthermore, the energy exchanges that continually occur between the atmosphere and Earth’s surface and between the atmosphere and space produce the effects we call weather. If, like the Moon, Earth had no atmosphere, our planet would not only be lifeless, but many of the processes and interactions that make the surface such a dynamic place could not operate.

The Hydrosphere

More than anything else, water makes Earth unique. The hydrosphere is a dynamic mass that is continually on the move, evaporating from the oceans to the atmosphere, precipitating to the land, and running back to the ocean again. The global ocean is certainly the most prominent feature of the hydrosphere, blanketing nearly 71 percent of Earth’s surface (Figure 1.11). The hydrosphere also includes the freshwater found in clouds, streams, lakes, and glaciers, as well as that found underground. Although these latter sources constitute only a tiny fraction of the total, they are much more important than their meager percentage indicates. Clouds, of course, play a vital role in many weather and climate processes. In addition, clouds provide the rainfall so essential to life on land.

The Lithosphere

Beneath the atmosphere and the ocean is Earth’s rocky outer layer, called the lithosphere. The surface of the lithosphere is very uneven and contains high mountainous topography, as well as low areas such as Death Valley—portions of which lie below sea level. Sometimes the lithosphere is referred to as the geosphere, in which case scientists include Earth’s mantle and core in its description.

Soil, the thin veneer of material at Earth’s surface that supports the growth of plants, may be thought of as part of all four spheres. The solid portion is a mixture of weathered rock debris (lithosphere) and organic matter from decayed plant and animal life (biosphere). The decomposed and disintegrated rock debris is the product of weathering processes that require air (atmosphere) and water (hydrosphere). Air and water also occupy the open spaces between the solid particles.

The Biosphere

The biosphere includes all life on Earth, including the vast oceans (Figure 1.12). Plants and animals depend on the physical environment for the basics of life. However, organisms do more than just respond to their physical environment. Through countless interactions, life-forms help maintain and alter their physical environment. Without life, the makeup and nature of the atmosphere, hydrosphere, and lithosphere would be very different.

The Earth System

Scientists have recognized that to more fully understand our planet, they must learn how its individual components (air, water, land, and life-forms) are interconnected. This endeavor aims to study Earth as a system. A system is a collection of numerous interacting parts, or subsystems, that form a complex whole. Using an interdisciplinary approach, scientists attempt to understand and address many of our global environmental problems.

Most of us hear and use the term system frequently. We may service our car’s cooling system, make use of the city’s transportation system, and participate in the political system. A news report might inform us of an approaching weather system. Further, we know that Earth is just a small part of a larger system known as the solar system.

Earth as a System

The Earth system has a nearly endless array of subsystems in which matter is recycled over and over again. One familiar loop, or subsystem, is the hydrologic cycle. It represents the unending circulation of Earth’s water among the hydrosphere, atmosphere, biosphere, and lithosphere (Figure 1.13). Water enters the atmosphere through evaporation from Earth’s surface and transpiration from plants. Water vapor (water in the gaseous state) condenses in the atmosphere to form clouds, which in turn produce precipitation that falls back to Earth’s surface. Some of the rain that falls onto the land infiltrates (soaks into the ground) and is later taken up by plants or is stored as groundwater, while some flows across the surface toward the ocean.

The parts of the Earth system are linked so that a change in one part can produce changes in any or all of the other parts. For example, during most winter seasons, moisture evaporates from the Pacific Ocean and subsequently falls as rain in the hills of southern California. Sometimes the rainfall is heavy enough to trigger destructive debris flows (Figure 1.14). The processes that move water from the hydrosphere to the atmosphere and then to the lithosphere have a profound impact on the physical environment and on the plants and animals (including humans) that inhabit the affected regions.

Humans are part of the Earth system, a system in which the living and nonliving components are profoundly interconnected. Therefore, our actions in one sphere can produce changes in all the other spheres. When we burn gasoline and coal, dispose of wastes, and clear the land, we cause other parts of the system to respond, often in unforeseen ways. Throughout this book, you will learn about some of Earth’s subsystems, including the hydrologic system and the climate system. Remember that these components and we humans are all part of the complex interacting whole we call the Earth system.

What Powers the Earth System?

The Earth system is powered by energy from two sources. The Sun drives external processes that occur in the atmosphere, in the hydrosphere, and at Earth’s surface. Weather and climate, ocean circulation, and erosional processes are driven by energy from the Sun. Earth’s interior is the second source of energy. Heat remaining from the planet’s formation, as well as heat that is continuously generated by radioactive decay, powers the internal processes that produce volcanoes, earthquakes, and mountains.

• atmosphere: The gaseous portion of a planet, the planet’s envelope of air; one of the traditional subdivisions of Earth’s physical environment.

• biosphere: The totality of life-forms on Earth.

• lithosphere: The rocky outer layer of Earth that lies beneath the atmosphere and ocean.

• system: A collection of numerous interacting parts, or subsystems, that form a complex whole. The hydrologic cycle is one example.

• Earth’s physical environment is traditionally divided into three major parts: Earth’s gaseous envelope, called the atmosphere; the water portion of our planet, called the hydrosphere; and the solid Earth, called the lithosphere. A fourth Earth sphere is the biosphere, the totality of life on Earth.

• Although each of Earth’s four spheres can be studied separately, they are all related in a complex and continuously interacting whole that is called the Earth system.

• Earth system science uses an interdisciplinary approach to integrate the knowledge of several academic fields in the study of our planet and its global environmental problems.

• The two sources of energy that power the Earth system are (1) the Sun, which drives the external processes that occur in the atmosphere, hydrosphere, and at Earth’s surface, and (2) heat from Earth’s interior that powers the internal processes that produce volcanoes, earthquakes, and mountains.