1.3: Components of the Earth System

The following provides a brief overview of Earth's four major "spheres" as well as three additional components of the Earth system. In total, these are the:

• Exosphere
• Atmosphere
• Hydrosphere
• Cryosphere
• Lithosphere
• Biosphere
• Anthroposphere

Exosphere – Space Environment

Figure \(\PageIndex{1}\): The exosphere, or the space environment in which our planet resides, has many effects on our planet. (ESA)

The exosphere is the space environment. Thought of from the perspective of the Earth, it is the location of our planet in the solar system, around our particular star, as well as the location of our solar system within the Milky Way galaxy, and so forth. All of the energy that powers the other systems on our planet, with the exception of the geosphere, comes from the Sun – outside our planet. This solar radiation, ranging from gamma radiation through long-wave radio waves, is critical for the normal operation of the biosphere, atmosphere, and hydrosphere. The exosphere is also the source of dangerous radiation in the form of galactic cosmic rays and solar particle events. Ultimately, the exosphere is hostile to life, but paradoxically critical to life as well.

Atmosphere – Gaseous Earth (Troposphere, Greenhouse Gases, etc)

The atmosphere is the gaseous envelope that surrounds our planet. Our current atmosphere can be thought of as the third atmosphere our planet has had. The primordial atmosphere of the Hadean (a widely used term referring to the earliest 500 - 700 million years of Earth's history) and outgassing processes eventually gave way to one dominated by volcanic gases and rich in \(\ce{CO2}\). Photosynthesis evolved around 3.8 Ga, after which the atmosphere increased in oxygen and nitrogen. Since about 600 Ma, the concentration of oxygen in the lower atmosphere (troposphere) has been pretty consistent with what we have today.

Figure \(\PageIndex{2}\): Evolutionary history of the atmosphere over Earth history. (NASA)

Our atmosphere is layered, with the densest portion at the bottom. This bottom layer of the atmosphere, the troposphere, is where most of the action occurs. We refer to much of that activity as weather. It is also in this layer that human activity has the greatest impact. It is affected on a daily basis by the uneven heating of the Earth's surface on land and sea and the rotation of the Earth, creating what is called the planetary boundary layer. As Earth's surface absorbs solar radiation, it heats up, warming the air at the Earth surface. This warm air rises, creating turbulence. Above the troposphere is the stratosphere. In contrast to the troposphere, the temperature increases with elevation in the stratosphere, creating stability and minimizing turbulence. In the stratosphere, exospheric UV radiation is absorbed by photochemically produced ozone and diatomic oxygen. Ozone at this level of the atmosphere is critical for protecting terrestrial life. Human activity, through the release of chlorofluorocarbon compounds contained in hairspray, refrigerants, and more, destroyed stratospheric ozone molecules, resulting in an ozone-depleted region centered around the South Pole that was first measured in 1985. In 1987, the world came together to ratify the Montreal Protocol. This protocol banned these classes of chemicals and prevented stratospheric ozone from deteriorating to an even more dangerous state. These actions are subsequently resulting in a gradual reduction in the extent of the ozone hole.

Above the stratosphere lies the mesosphere (middle atmosphere) and thermosphere. As you ascend upward into the mesosphere, temperatures and pressure decrease. At the thermopause, the lower boundary of the thermosphere, pressure continues to drop but temperatures suddenly begin to rise sharply.

Figure \(\PageIndex{3}\):The layers of our atmosphere also depicted with cloud tops and the penetrating ability of various portions of the electromagnetic spectrum. Visible light, some UV light, Infrared energy, and radio waves are the principle sources that reach the ground. (NASA)

The atmosphere regulates Earth’s climate in several significant ways. The most important of these is the concentration of gases in the troposphere that trap heat and prevent it from escaping to the exosphere. Solar radiation, incoming primarily as UV (8%), shortwave infrared (49%), and visible light (42%) radiation, will warm the surface (land and water) and then be re-radiated as longwave radiation. The longwave radiation gets trapped by greenhouse gases like \(\ce{CO2}\), \(\ce{CH4}\), and \(\ce{NO_x}\). All of these gases are increasing in concentration due to human activity. This heat thus gets trapped in the atmosphere, which has a much lower specific heat capacity than water. Generally, this means that the hydrosphere ends up absorbing this heat in all of its uneven nature around the planet, leading to increases in evaporation in some places and precipitation in others. The trapped heat also generally warms the oceans and land surfaces, leading to higher ocean and land temperatures.

The uneven heating of the atmosphere is important for normal weather patterns. As identified in the figure below, there are three atmospheric convection cells between the equator and each of Earth's poles. Each cell has a rising limb with warm moist air and descending limb with cool dry air. These cells play critical roles in atmospheric heat and moisture transfers. The rotation of the Earth produces the Coriolis Effect, which deflects the movement of air in these cells in an easterly direction at the poles (Polar Cell), westerly direction in the mid-latitudes (Ferrell Cell), and again in an easterly direction in the tropics (Hadley Cell). We will learn more about this in Chapter 4.

Figure \(\PageIndex{4}\): Atmospheric circulation patterns at equinox position (Spring or Autumn). Note the positions of Hadley Cells through Polar Cells by latitude and their deflection due to the Coriolis Effect. (NASA)

These latitudinal variations drive our weather and lead to important gradients in the biosphere, called Latitudinal Biodiversity Gradients (LBG). Generally, biodiversity decreases from the tropics to the poles. This change in biodiversity is directly related to precipitation patterns and temperature changes with latitude...today. It is important to note here that today’s LBG is not the norm for Earth’s past. In fact, approximately 50 Ma, it is very likely that the temperate regions of the planet were the most diverse. This is because it was so much hotter in the tropics.

Hydrosphere – Earth’s Water (Fresh, Marine, Ground, etc.)

The hydrosphere includes all water on earth, even that in the atmosphere as water vapor and clouds. It also consists of the oceans and freshwater in groundwater, lakes, rivers, and streams. While it is considered separately here, the cryosphere (e.g., glacial ice) is a critical element of the hydrosphere as it provides a significant storage sink for water.

Freshwater and marine systems have many similarities as well as many differences. The key feature of all water in the hydrosphere is its ability to regulate heat in various ways.

Figure \(\PageIndex{5}\): The hydrologic cycle and hydrosphere. (USGS graphic)

Water has a high heat capacity (specific heat). That is, it will absorb a great deal of heat energy before it gets hot itself. It also requires considerable energy to convert liquid water to water vapor (i.e., the latent heat of evaporation). Thus, when water evaporates, it absorbs a significant amount of heat from the surrounding environment. This heat is stored in the water vapor and is not released until the vapor condenses back into liquid. When water vapor condenses into liquid water, it releases the stored latent heat of vaporization back into the atmosphere. This release of heat warms the surrounding air. Because of these properties, water is a critical regulator of energy on the planet. 1 kg of water has to absorb 4,184 J of energy for its temperature to rise 1 \(^{\circ}\)C. By comparison with rock, in this case, only 385 J are needed to increase the temperature of 1 kg of copper one degree Celsius. Not only does the heat capacity of water make fish happy, it also means that excess heat from the atmosphere, the Sun, or the land during the day can be absorbed by water with little effect. Ultimately, while the hydrosphere is driven by the exchange of heat itself (the hydrologic cycle), it also regulates the planet's temperature in much the same way as sweat helps regulate your body temperature during a workout.

Another critical heat-regulating feature of the hydrosphere is ocean currents. Whether the continents are combined into a single supercontinent or split into what we have today, warm surface waters move and exchange with cool nutrient-rich bottom waters. This exchange, or convection, produces currents. These rivers in the ocean move heat around the planet. Temperature is not the only physical property in play when it comes to marine currents, as the salinity of marine water can vary enough to produce density-driven currents. The combined thermohaline circulation patterns in the ocean regulate our climate.

Cryosphere – Frozen portion of the hydrosphere

The cryosphere is the solid portion of the hydrosphere. It is worth discussing separately from the hydrosphere because of the particular importance of ice in exchanges of heat and light between Earth’s systems. Like water, ice has a heat regulation effect.

Figure \(\PageIndex{6}\): A view of the Antarctic cryosphere (NASA)

Polar ice caps provide an important counterweight to the extreme heat experienced in the mid-latitudes. The temperature differences between these regions drives the movement of air in the atmosphere, forming atmospheric convection cells (warm air rising and cool air descending) that, while deflected by the Earth’s rotation and Coriolis Effect, drive our weather. Polar ice is also a critical water storage sink. As it melts, sea level rises and isostatic adjustment of continents occurs.

Figure \(\PageIndex{7}\): Earth’s Cryosphere (NASA)

The Earth has not always had polar ice. Just 40 Ma ago, the planet's climate was so warm that there was no ice at the poles. With the rising of the Himalayas and Andes came increased erosion and chemical weathering of silicate minerals. As mountains rise, this form of weathering results in a net depletion of carbon dioxide from the atmosphere and deposits the byproduct of that carbon into the oceans as bicarbonate ions. The removal of carbon dioxide from the atmosphere results in a general cooling of the planet. This eventually resulted in ice storage on land over Antarctica, as a continental glacier. Later, ice would form over the northern polar regions in the sea.

The polar ice caps are only part of the story of the cryosphere. The ebb and flow of continental glaciers in places like Greenland and valley glaciers all over the world provide us with important visual thermometers for global temperature changes.

As glaciers and sea ice retreat, there are changes in the color distribution on the Earth’s surface. Albedo, a measure of this, is highest on ice and lowest on ocean water. When sunlight hits ice, it reflects nearly 100% of the energy. When sunlight hits water, water acts similar to a blackbody (an object that absorbs all radiation it receives). As the climate warms, ice recedes, and the climate warms more. We will return to this dynamic later in the textbook.

Lithosphere/Geosphere – Solid Earth (Rock, including molten rock)

The geosphere makes up the solid portion of the planet. If you have taken a course in Physical Geology, you know all about the rocks and minerals that make up the various layers of our planet from the inner core outward to the crust. Chapter 2 in this book will introduce you to the minerals and rocks of the geosphere.

Unique among the spheres, the geosphere produces its own energy. There are three sources. Radioactive decay of unstable elements within our planet’s interior produces a great deal of heat energy. Another heat source comes from remnant primordial heat still yet to dissipate from the planet’s formation. Finally, frictional heating plays a role, descending material rubs against other materials. Earth is hottest near the center and is cooler at the surface. This collective heat leads to the geothermal gradient, the increase in temperature as you descend further into the Earth. It also drives plate tectonics and resulting volcanism, earthquakes, and natural hazards.

Figure \(\PageIndex{8}\): Cross-section of the Earth showing its main divisions and their approximate contributions to Earth’s total internal heat flow to the surface, and the dominant heat transport mechanisms within the Earth (Wikimedia, Bkilli\(_1\))

Because of this independent heat energy, the solid Earth is much more dynamic than any other terrestrial planet we know of. Mercury, Venus, and Mars are very different places with differing amounts of interior activity. Some natural satellites in our solar system likely also produce their own interior heat. Examples may include Saturn’s natural satellite Titan. Earth, because of plate tectonics, is a much more active planet and one where the rocks are constantly interacting with and having an effect on the liquid and gaseous portions of the planet.

Figure \(\PageIndex{9}\): Crustal dynamics in the Earth’s geosphere. Red triangles indicate volcanoes and yellow dots indicate earthquake epicenters. Blue lines are plate boundaries. (NASA SVS)

It is hard to pin down the most important geosphere elements for driving climatic change on the Earth. Volcanism is very likely most important, due to the input of gases like \(\ce{SO2}\) into the atmosphere, which can have a cooling effect due to its ability to reflect solar radiation back to space. There is also airborne dust, sometimes referred to as loess and measured as a component of particular matter. This airborne dust has the ability to absorb heat and heat the atmosphere, causing warming in places where dust is moved from elsewhere.

Biosphere – Life

Without a tectonically active geosphere, an atmosphere with the right concentration of gases, and a hydrosphere made up so heavily of liquid water, there would likely be little to no biosphere on Earth. Unique among planets in our solar system and perhaps anywhere, Earth contains an amazing array of life. The origins of life on Earth are still somewhat elusive, but we do know that over time it has evolved to live in balance with the other spheres to the point where because the biosphere is such a critical influence on the other spheres, it can be considered on its own terms. Being located in the habitable zone (Goldilocks Zone) of a G-type star has its perks. Is Earth unique in this? A number of exoplanets have been identified that are located in the habitable zones around their stars, but as yet, no extraterrestrial life has been confirmed.

The video below provides a sense of life on Earth. You are viewing the ebb and flow of life forms in the oceans through the lens of chlorophyll from space, with data collected by the MODIS sensor. This data was collected by NASA between 2002 and 2010 and displays visually the intricate link between photosynthetic life and seasonality.

The biosphere plays a very important role in the climate, particularly when it comes to the regulation of greenhouse gases. Of course, the decomposition of organic matter is a major source of \(\ce{CH4}\) and \(\ce{NO_x}\) gases, aside from what humans contribute. But, the real effect is the planet-wide exchange of \(\ce{CO2}\) and \(\ce{O2}\) that is so critical to regulating temperature and providing breathable oxygen.

Check out the supercomputer simulation of \(\ce{CO2}\), offered by NASA Goddard below. The red concentrations mostly represent \(\ce{CO2}\), but you will see as the animation and narration progress that, during the southern hemisphere summer, \(\ce{CO}\) concentrations from fires in Africa and South American increase significantly. As the animation progresses, note the role that the biosphere plays in the annual fluctuations of \(\ce{CO2}\) in various places around the planet, but particularly in the Amazon and Congo basins.

3.8 Ba ago, the biosphere began to alter the Earth’s atmosphere for its own purposes. The evolution of photosynthesis and the subsequent Great Oxygenation Event have left a long legacy that lives on today, in the form of algae and other plants that provide the base of the trophic levels so critical to our existence. Today, the biosphere is once again altering the Earth’s atmosphere, but through the actions of a single species.

Anthroposphere – Human portion of the biosphere

Figure \(\PageIndex{10}\): Earth at night (NASA)

The anthroposphere has been suggested to account for human interactions with Earth's spheres. Humans are a force of nature. No single species in the history of life can claim such a mantle. Yet, humans are a part of the biosphere. Because of our outsized impact, it is useful and perhaps even appropriate to consider the human element of the planet independently from the other spheres. Thus, the anthroposphere. The human impact on the planet goes well beyond altering the atmosphere and climate, leading some scientists to argue that the Earth has entered a new geological epoch. However, the Subcommission on Quaternary Stratigraphy of the International Commission on Stratigraphy uniformly rejected this proposal in March 2024.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Which of the Earth's spheres encompasses frozen water?

a. Hydrosphere

b. Cryosphere

c. Lithosphere

Answer

b. Cryosphere

Exercise \(\PageIndex{2}\)

Which of the Earth's spheres do you see interacting here?

a. Biosphere, Atmosphere, Hydrosphere

b. Biosphere, Anthroposphere, Lithosphere

c. Biosphere, Lithosphere, Atmosphere

Answer

c. Biosphere, Lithosphere, Atmosphere