3.3: The Symphony of the Spheres

Now that we have this background, it is time to turn our attention to the spheres themselves in more detail. In particular, the sections below will not only describe the four main spheres that have been mentioned so far, but will also add in three additional spheres. 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 in the solar system, around our particular star, 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 a place hostile to life, but paradoxically critical to it also.

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 and outgassing processes eventually gave way to one dominated by volcanic gases and rich in \(\ce{CO2}\). Once photosynthesis evolved around 3.8 Ba, the atmosphere would increasingly contain 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 layer, the troposphere, is where most of the action occurs. It is where all human activity and weather occur. It is really the upper boundary of mountain growth at orogenic belts. It is affected on a daily basis by the uneven heating of the land and sea and the rotation of the Earth, creating what is called the planetary boundary layer. Above the troposphere is the stratosphere. In the stratosphere, exospheric UV radiation is absorbed by photochemically produced ozone. Ozone at this level of the atmosphere is very important for protecting life. Human activity, through the release of chlorofluorocarbon compounds via hairspray, refrigerants, and more had carved a hole in this critical layer. During the late 1980s, the world came together to ratify the Montreal Protocol. This banned these classes of chemicals and prevented the concentration of this important gas from deteriorating to an even more dangerous state.

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 \(\PageIndex{3}\): Layers of the Atmosphere (NASA)

The atmosphere plays a very important role in the Earth’s climate. 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 as shortwave infrared energy, will warm the surface (land and water) and then be re-radiated as longwave radiation. This gets trapped by greenhouse gases like water vapor, \(\ce{CO2}\), \(\ce{CH4}\), and \(\ce{NO_x}\). All of these are gases that are increasing in concentration due to human activity. This heat 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. It also generally warms the oceans and land surfaces, leading to higher ocean and land temperatures.

The uneven heating of the atmosphere is very important for normal weather patterns, however. Rising warm air at the equator and descending cool air at the poles create the foundation for convection cells that circulate air between the equator and poles. 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).

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 not only drive our weather, but they also lead to important gradients in the biosphere, called the latitudinal biodiversity gradients (LDB). Generally, biodiversity decreases from the tropics to the poles. This change in biodiversity is directly related to temperature changes with latitude...today. It is important to note here that today’s LDB is not the norm for Earth’s past. In fact, during the Eocene epoch, 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 – Liquid Earth’s Water (Fresh, Marine, Ground, etc.)

The hydrosphere makes up the liquid envelope of our planet. It consists not only of the oceans, but also all freshwater contained in lakes, rivers, and streams. While it is considered separately here, the cryosphere is a critical element of this sphere also, as it provides a significant storage sink for water.

Freshwater and marine systems have many similarities and differences. However, the key feature of all liquid 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. Because of this, 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 a special native mineral called copper, only 385 J are needed to accomplish the same task. Not only does this property make fish very happy, but it also means that excess heat from the atmosphere, the Sun, or the land during the day can be absorbed by water with very little effect. Ultimately, while the hydrosphere is driven by the exchange of heat itself (the hydrologic cycle), it also regulated the planet in the same way sweat helps regulate your body temperature during a workout.

Another critical heat regulating feature of the hydrosphere are ocean currents. Whether the continents are combined into a single supercontinent or split into what we have today, warm surface waters will 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)

The presence of polar ice caps provides a very important counterweight to the extreme heat experienced in the mid-latitudes. The temperature differences between these regions drive the movement of air in the atmosphere, forming Hadley 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, there was no ice at the poles because the planet’s climate was so warm. With the rising of the Himalayas and Andes came increase silicate weathering. As mountains rise, this form of weathering pulls carbon dioxide out of the atmosphere and deposits the byproduct of that carbon into the oceans as bicarbonate ions. Eventually water began to be stored on land over Antarctica first, 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, it acts nearly as a blackbody, absorbing nearly all of the incoming energy. As the climate warms, ice recedes, and the climate warms more. We will return to this later in the chapter.

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. You can read in great detail about the geosphere in the Earth Materials chapters of this book on rocks and minerals.

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)

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 consider whether the Earth itself has indeed entered a new geological epoch. As of this writing, the Anthropocene Working Group, a subcommittee of the Quaternary Working Group of the International Commission on Stratigraphy has officially voted to recommend several things. First, that the Anthropocene should be treated as a chronostratigraphic unit for the geologic time scale (Series/Epoch level). Second, that the mid-20th century should be the time marker for its start. If ratified by the International Union of the Geosciences, the Holocene Epoch will have ended around 1950 and we will be living, officially, in the Anthropocene.

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