“To everything, there is a season.” – Ecclesiastes 3:1 and The Byrds
Every event in the history of our planet, throughout its entire 4.5 Ba history, happened because something forced it to happen. An asteroid once caused an extinction. Extinctions led to novel new evolutionary pathways for origination. Plate tectonics caused continents to move around. In each of these realities, we might be tempted to simply describe them as they have been here, in a linear fashion. A caused B. We could leave it at that. However, is it really that simple?
Contained within our galactic system is a solar system powered an average G type star. “On a mote of dust suspended in a sunbeam” (Carl Sagan) resides our Earth system. Like any system, it is composed of many moving parts, powered by flows of energy and sustained in a state of dynamic equilibrium. Simply put, a system is a naturally occurring group of interacting, interrelated, or interdependent elements that form a complex whole.
Within the Earth system, there are subsystems. These subsystems encompass the space environment (exosphere), gaseous environment (atmosphere), liquid environment (hydrosphere), solid environment (lithosphere and geosphere, hereafter lithosphere), and living environment (biosphere). In some cases, the icy environment (cryosphere) is broken away from the hydrosphere and the anthroposphere (human environment) from the biosphere, for emphasis. Energy, coming from the Sun on one hand and the Earth’s interior on the other, powers these systems. As energy flows through these systems, so do nutrients and elements, through what are called biogeochemical cycles. Examples of these include the carbon and nitrogen cycles.
Energy does not flow through these systems in a linear fashion. Its pathways are complex, taking advantage of the points where systems interact to transfer from one system to another and back again. In order to really gain a grasp of how our planet works today and how it has worked in the past (uniformitarianism), we need to use some of the principles of systems thinking. If we think in terms of systems, we begin to think in terms of cycles, feedbacks, forcing mechanisms, storage sinks, and flows of energy and material.
Remember the extinction caused by an asteroid mentioned above? While the asteroid (from the exosphere) did indeed cause the extinction, it was the effect that asteroid had on the hydrosphere, atmosphere, and lithosphere that led to the destruction that occurred in the biosphere. In systems thinking, the asteroid was a forcing mechanism that put the entire Earth system into a state of disequilibrium, past a tipping point of no return. While certainly some organisms were directly crushed by the impending rock from space, most were killed by the downstream effects as its energy rippled across the interconnected we of systems.
“To everything, there is a season”.
Earth Systems Overview
Events have an effect on the Earth system as a whole, though they usually begin within a particular sphere. For example, let us explore a volcanic eruption as an example. When a volcanic eruption event occurs, what happens?
- Ash and rock are blasted into the atmosphere;
- Gases are emitted into the atmosphere;
- Lava may run downslope;
- Pyroclastic flows, superheated ash, may rush downslope at great velocity;
- Lahars, or mudflows, may result from the rapid melting of snow and ice, bringing large amounts of debris downslope;
- Flora and fauna in the immediate vicinity of the volcano may be killed.
There are other effects associated with volcanic eruptions beyond these. If you were to place the eruption in the very center of this diagram and write some of these interactions along the golden arrows toward a sphere, where would you place them? These are direct effects on each sphere from the event.
These direct effects are not the end of the story. You can see that there are also arrows connecting each sphere to one another. What are some of the indirect interactions (forcings) that result from the eruption? How is the biosphere affected, for example, by how the eruption directly affects the lithosphere (geosphere)? Ultimately, understanding the interconnectedness of the Earth system is critical for gaining a holistic view of how our planet is affected by individual events. In the history of our planet, there are no shortage of such events!
Earth System Forcing Events
|Melting of Polar Ice||Direct - Isostatic adjustment as ice located on land melts||Direct - influx of freshwater into the marine realm affects thermohaline circulation.|
Change in planetary albedo leading to warmer seawater due to the loss of light-colored ice
|Direct - Loss of sea ice leads to warming of oceans and changes in evaporation and weather patterns||Habitat fragmentation of organism ranges in polar regions|
|Stratovolcanic Eruption||Direct - Release of eruptive materials in the vicinity of the mountain and the creation of volcanic strata||Indirect via Atmosphere - influx of sulfur and carbon dioxide may lead to lower water pH||Direct - influx of ash, sulfur and carbon dioxide||Direct - Major effects on life in the vicinity of the volcano; Temporary regional or even global cooling affects some groups of organisms|
|Supervolcano Eruption||Direct - Major new volcanic deposits form over a large area. Vicinity of eruption subject to collapse.||Indirect via Atmosphere - surge of carbon and sulfur dioxide in atmosphere leads to lower pH in marine and freshwater environments.||Direct - Major influxes of volcanic gases and ash for extended periods of time.||Direct - Local, regional, and perhaps global extinctions due to deposits of massive amounts of volcanic material and extended global cooling|
|Deforestation||Indirect via Biosphere - Loss of vegetation leads to more mass wasting events||Indirect via Biosphere - Loss of vegetation affects regional transpiration, water storage, and drainage patterns||Direct - Loss of plant life leads to higher concentrations of carbon dioxide in the atmosphere||Direct - Habitat fragmentation and biodiversity loss. Possible localized or regional extinctions|
|Asteroid Impact||Direct - Major crater formed, ejecta deposits created over large area; high-pressure metamorphism in zone of impact; Seismic energy release||Direct - Vaporization of large amounts of surface and groundwater from a local to global level, depending upon size and composition of impactor||Direct - Frictional heating as impactor enters atmosphere; Heating as hot ejecta is thrown into atmosphere and exosphere that rains back down on Earth; Introduction of impact related particulate matter||Direct - local to global extinction event|
|Earthquake||Direct - Local to regional changes in landscape near fault||Indirect via Geosphere - Possible tsunamis or coastal landslides and turbidity currents||Indirect via Geosphere - Release of dust, debris, and toxic gases in some instances||Direct and Indirect - Depending on the species, building collapse and liquefaction can lead to death; Ecosystem changes are possible|
|Burning of Fossil Fuels||Indirect via Atmosphere - Addition of particulate matter and black carbon to sedimentary deposits; Climate change leads to other downstream effects||Direct - Changes in regional evaporation rates due to warming; Changes in hydrologic cycle||Direct - addition of carbon dioxide warms the troposphere; Changes in weather patterns||Direct - Climate warming leads to biodiversity loss, extinction, etc.|
|Viral Pandemic Among Human Population||Indirect - Social isolation and pause in economic activity leads to lower ambient seismic activity||Indirect - Reduction in pollution; Reduction in habitat disturbance from recreational boating||Indirect - Reduction in air pollution due to changes in economic activity and energy production||Direct - Widespread sickness and mortality among human species|
Earth Systems Interactions
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 – Space Environment
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 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.
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 rise sharply.
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 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, CO2, CH4, and NOx. 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.
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.
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 1ºC. 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.
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.
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 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.
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.
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 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 on on land, with data provided by NASA, over the course of ten years and as seasons come and go during that time. At sea, you can see chlorophyll concentrations increase and decrease seasonally. On land, vegetation fluctuates seasonally as viewed through a normalized vegetation index (NDVI), a combination of red, green, and blue bands of light with a near-Infrared band.
Check out the supercomputer simulation of CO2, offered by NASA Goddard below. The red concentrations mostly represent CO2, but you will see as the animation and narration progress that, during the southern hemisphere summer, 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 CO2 in various places around the planet, but particularly in the Amazon and Congo basins.
Anthroposphere – Human portion of the biosphere
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.
Earth’s Energy Budget
Energy from the Exosphere
The Sun powers our solar system. In addition to the Sun, some planets have a significant source of internal heat. Venus is one such planet, releasing its heat through volcanism. Saturn produces more internal heat than it receives from the Sun, given its great distance from the star it orbits. Earth is also one of these planets. In Earth’s case, the radiation received from the Sun is the primary driver for the entire Earth system, including its climate.
When considering the energy that arrives from the Sun, it is worth studying the image below from NASA. Radiation from the Sun reaches the Earth system in a wide range of wavelengths spanning the entire electromagnetic spectrum. Much of this energy is blocked by the Earth’s atmosphere, allowing only radio waves, visible light, short-wave infrared radiation, and some amount of UV radiation to penetrate to the surface of the planet. This selective admission is important for the functioning of life on Earth as we see it today. But, it has not always been this way. The Earth’s atmosphere has evolved along with life, affecting life’s evolution and also being changed by life itself. The types of radiation that are admitted to the surface have also changed over time.
Energy from the Sun flows through the Earth system in its many forms and powers the hydrologic and the biogeochemical cycles that all make life possible on our planet. Of the radiation that enters the atmosphere, the shortwave (UV/Visible/infrared) radiation is most important. UV radiation gives you “sunburn” and is carcinogenic over time. Your eyes are evolved to detect the violet to red wavelengths of visible light. Infrared radiation is what you experience as heat. Some of this is reflected by clouds, a notoriously difficult factor to model, given their variability. The rest of it hits the Earth’s surface, being absorbed in places land and ocean (low albedo/dark in color) and reflected in other places like ice and snowpack (high albedo/light in color).
After the energy is absorbed, it can be re-radiated in the form of longwave infrared radiation. This is still in the form if heat, but has lower energy (longer wavelength). As this radiation attempts to leave the atmosphere, much of it is absorbed by greenhouse gases, such as water vapor, carbon dioxide, methane, and nitrogen dioxide. This trapping of heat by gases is what is referred to as the greenhouse effect. And, this effect is critical for life on Earth.
Energy from Earth’s Interior
Unlike Mercury and Mars, who have a negative energy balance as they lose more heat over time than they create, Earth’s interior experiences a balance. Since the formation of all of the planets, they have been cooling over time. Whether a planet is able to balance this heat loss with its own heat production is a function of the composition of its interior. In Earth’s case (and for all terrestrial planets to varying degrees), it is producing much of its own interior heat through radioactive decay. The heat from this decay is caused by particles, emitted during decay of unstable isotopes, bouncing off of other particles. These collisions transform this kinetic energy into heat.
This heat is transported outward by conduction, convection, and advection, as seen in the figure below. Plate tectonics is driven by this internal heat, in addition to gravity.
Through plate tectonics, this internal energy teams up with solar energy to help drive the biogeochemical cycling that provides important nutrients that sustain life, including carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Because of this, energy from the Sun is only part of the story of our planet’s geologic past. The combination of these sources of energy is critical for the function of Earth’s systems. You can see examples of some of these important interactions in the figure below.
Geological Sources of Important Elements for Life
|Elemental Nutrient||Primordial Source on Earth||Important Life Processes|
|Carbon||Cosmogenetic, Mantle, Volcanism||Basic element of organic chemistry, important nutrient for photosynthesis of glucose|
|Hydrogen||Cosmogenetic, Mantle, Volcanism||Basic element of organic chemistry, important nutrient for photosynthesis of glucose|
|Nitrogen||Cosmogenetic ammonia, Earth’s atmosphere||Important for the creation of amino acids, enzymes|
|Oxygen||Cosmogenetic, Volcanism, Carbon Dioxide dissociation, Photodissociation in Atmosphere of Water||By-product of photosynthesis, Necessary for respiration and associated oxidation|
|Phosphorus||Phosphides in Earth’s core, Inorganic minerals such as apatite and fluorite, ocean-floor sediments||“Spine” of DNA molecule, important micronutrient that helps store energy in cells via ATP|
|Sulfur||Cosmogenetic, Stored in Earth’s interior, Volcanism||Allows for the synthesis of a greater variety of amino acids, Important nutrient for chemosynthesis|
Earth Systems collide: Events and Interactions
Systems exist in a state of dynamic equilibrium. Equilibrium is a balance. When it is dynamic, it means that when the state of equilibrium changes the system(s) adjust. Consider a seesaw with equal weight on both sides. To balance it, you would place the fulcrum in the center. Put two people on either end and then attempt to balance it and you would find that the position of the fulcrum would need to adjust. The system would need to adjust to the new reality.
Every event that occurs within the Earth system upsets a state of equilibrium. In the chapters in this book that deal with evolution, the term for this was “stasis.” When a volcano erupts, the stasis/equilibrium is changed and a new reality emerges for a time. In most cases, the new equilibrium is not much different than the prior situation. In some cases, a tipping point is reached. In such situations, a radically new equilibrium becomes the reality, eventually, and the overall change to the system is extreme. A good example of such a tipping point in the Earth’s past is the Chicxulub asteroid impact that occurred at the end of the Cretaceous Period, mentioned earlier in the chapter.
However, we need to define some systems thinking terms to help us enable our ability to communicate about events and interactions. In systems parlance, events are known as “forcings.” These are things that force changes to the dynamic equilibrium at the time. Anthropogenic greenhouse emissions are a forcing that today is causing the global temperature to warm.
The events we have been discussing are referred to as forcings in systems parlance. A forcing event is an action that moves a system away from dynamic equilibrium, usually through an initial push within one system. In terms of climate change, anthropogenic additions of greenhouse gases to the atmosphere are forcing the climate to warm through additional trapping of greenhouse gases. This initial forcing, of course, leads to additional effects downstream.
In the Earth’s history, there have been plenty of examples of these. When photosynthesis evolved and led to the Great Oxygenation Event, the climate cooled which eventually led not only to the rapid radiation of photosynthetic organisms, but also the snowball Earth events of the Precambrian. During the late Devonian extinction, it is thought by some that massive expansion of vascular plants on land led to huge influxes of organic matter into the epeiric seas that existed at the time, leading to eutrophic conditions and mass extinction. In some situations, massive flood basalt events (end-Permian, end-Cretaceous) led to massive emissions of sulfur dioxide and carbon dioxide and subsequent climatic change.
In our modern environment as in the Earth’s past, the climate system is pivotal to life processes. The figure above lists an array of anthropogenic (human) forcing factors that influence the climate system. Changes in any one of these will drive the climate to warm or cool. However, a change in one factor could actually lead to a change in another, independent of any warming or cooling. That is the nature of a system – it is not linear. Interconnections matter. Forcing events never happen in isolation, there are always additional direct and indirect repercussions.
Feedbacks are the results of forcings. No matter what the forcing event is, there are always feedbacks that occur as a result. Feedbacks can amplify the initial forcing (positive or amplifying feedback). They can also balance that forcing (negative or balancing feedback). A great modern example of an amplifying feedback occurs as a result of the reduction of polar ice due to global warming. Because polar ice is reduced by warming, more dark-colored water is exposed, reducing the albedo that the ice once had in that area of the ocean. Seawater is much darker in color than ice. More energy is then absorbed into by the water than was by the ice, further warming the water and melting even more ice. Because polar ice also helps moderate the planet’s climate, less ice leads to an amplification of warming.
Amplifying feedback loops tend to move an already off-balance system further away from regaining equilibrium. The image below illustrates the ice-albedo amplifying feedback. Arctic sea ice loss between 1979 and 2012 is shown on the map with areas north of eastern Russia and Alaska shown in false-color to indicate the change in albedo color over time. Dark red areas represent much greater albedo change and, thus, warmer seas. The reduction in white sea ice and snow cover amplifies global warming forced by anthropogenic greenhouse gases.
Balancing (negative) feedback loops provide sustainability in systems. They are the foundations of systems at dynamic equilibrium. In the example to the left, when predator numbers get too high, overkill of prey will lead to a natural decrease in predators over time due to starvation. Likewise, an increase in the birthrate of prey will lead to an increase in the number of predators, until balance is reached once more.
Eventually, balancing feedbacks will work to bring a system to equilibrium once again. Amplifying feedbacks are not always going to happen, by contrast.
Within systems, energy and materials can be stored until it is moved from one part of a system to another. Locations within a system where this occurs are referred to as sinks. Every biogeochemical cycle has sinks. In the carbon cycle, carbon is stored for very long periods of time in the form of limestone or fossil fuels. On the shorter term, it is stored in ocean water, in plants, or in the atmosphere. When a forcing event occurs, carbon may begin flowing from one sink to another. In the case of modern climate change, the carbon is flowing from coal and petroleum products into the atmosphere via combustion.
In the nitrogen cycle, we remove nitrogen from a sink, the atmosphere (Haber-Bosch Process), to create ammonia-based fertilizers that we then spread on crops. This nitrogen moves into storage in plants (a sink), but excess is washed into waterways where it eventually leads to eutrophic conditions as algae blooms out of control, dies, and is digested by microbes that pull the oxygen out of the water column. This kills everything else. But, that nitrogen then becomes a part of the sediment and, eventually, denitrifies back into the air. Throughout this biogeochemical cycling, a molecule of nitrogen can be stored in a wide range of sinks for varying duration.
It is possible for a system to move so far out of its prior balance that, once it regains a sustainable equilibrium, it has a very different look and behavior than it did before. Initial forcings, followed by the actions of amplifying feedbacks can eventually cause a system to tip into an entirely new circumstance.
An excellent natural and historical example of this from the Earth’s past might be the Great Oxygenation Event. Once photosynthesis evolved, organisms using this new energy pathway could harness an abundant resource, sunshine, and radiate far and wide throughout the nascent biosphere. Evolution took on new pathways and the course of life would lead to eukaryotes and multicellular structures and, eventually, to humans. However, not before the Earth system would find itself reacting violently (geologically) to this reduction in atmospheric carbon dioxide and increase in atmospheric oxygen. The result was a series of massive global glaciations as the Earth system struggled to gain a new equilibrium state.
Eventually, this period of global glaciation would subside. Afterward, the Earth that existed prior to these snowball Earth events was gone and the new Earth was much different. A new dynamic equilibrium was established.
If we bring all of these systems characteristics together to model a system, we can create causal loop diagrams. These diagrams allow us to do two important things. First, they are effective tools for analyzing a system in as many directions as possible. You can see that this example, focused on climate change and, particularly, global biogeochemical cycles and climate tipping points, is quite complicated. These diagrams also help determine potential areas of focus for research. Finally, in a modern context we can use these to develop mitigation and management strategies for dealing with Earth system forcing events.
Scales of Measurement: Geography
Earth system forcings (events) happen at different geographic scales. That is, some have global effects while some have more regional or local effects. We will explore some of these below.
There have been many events throughout the Earth’s past that have affected change on a global scale. These tend to be the big events, though they may have begun as small events that cascaded into larger-scale forcings. There are many to choose from, but for this section we will discuss the forcing effects caused by the evolution of vascular plants during the Silurian Period.
Evolution of Vascular Plants
Vascular plants entered the scene on land about 420 Ma. One of the challenges plants would encounter as they evolved to colonize land would be how to get water to move up toward the top of a plant against the downward pull of gravity. When you consider the Giant sequoia or Redwood trees in the Pacific northwest of the United States, you can really tell that plants found a way to master this problem. However, it is also true that there are limits to this. Plants overcame gravity in part by taking advantage of some of water’s unique properties, such as capillary action. The evolution of specific tissues called xylem and phloem was the key development that allowed plants to harness this.
Xylem is a vascular tissue that transports water upward while phloem is a vascular tissue that transports photosynthetic cells to other parts of the plant for food or storage. The evolution of these tissues made it possible for plants to stand upright and, thus, colonize land.
Why is this “event” a forcing mechanism? Well, consider the effect that photosynthetic vascular plants have on our atmosphere today. They are not only a critical part of the water cycle because of the transpiration of water through their tissues, but also for the removal of carbon dioxide from the atmosphere. Ultimately, plants’ ability to convert carbon in carbon dioxide into glucose is a critical process for all life on Earth. Once vascular plants evolved, they spread prolifically, as there was a nearly endless array of niches to fill on land. This proliferation of plant material certainly forced changes in the atmosphere. Plant photosynthesis would remove carbon dioxide and replace it with oxygen, leading to a cooling climate globally. Their effect was so impactful through their drawdown of atmospheric CO2 that it likely contributed to the Devonian extinction. They would not be outdone by their non-vascular cousins, who likely had the same impact on the end-Ordovician extinction.
Some Earth system events create interesting regional dynamics. These are still affected by global changes, but the regionalism of impact simply means that global effects manifest themselves in different ways in different locations.
In terms of regional variation within Earth system, there are several examples that are important to understand today and that have applied or may have been different in the Earth’s past. In each of the examples used to discuss region effects, the biosphere will be the focus.
Latitudinal Biodiversity Gradients
Species richness is highest in the tropics and lowest at the poles. In a general sense, this gradient is
directly impacted by seasonality and the current, relatively moderate, climate. In Earth’s past, particularly during times like the late Paleocene, it is thought that the tropics were so hot that biodiversity was lower than at temperate latitudes. Diversity in polar regions was also lower, but the climate was warm enough to support palm trees and alligators above the Arctic Circle. Biodiversity gradients, then, were not only very different then, but have changed throughout Earth’s past. The Biodiversity gradient we see today from the tropics to the poles has not always looked the way it does today.
Why would latitudinal biodiversity gradients change over time? In the image below, from Mannion et al. (2013), one idea is that diverse tropics have not always been the norm. In cooler periods (blue bands), it may be, but during warmer global climates, the tropics may be much more inhospitable. This would lead to a situation where the biosphere may have had bands of high diversity at mid-latitudes that trended downward north to the poles and south to the equator (in the northern hemisphere).
Mathematically, latitudinal biodiversity gradients are governed by a simple relationship:
For any given area, S is the number of species predicted by the formula, per unit area. A is the area of the region of study, calculated in some unit for area. C and Z are constant values. C is linked to z values but is most directly related to the unit being used for area. Z values are the most important factor in the equation and different environmental characteristics can be assigned different z values for the purposes of modeling. A region’s climate is one such characteristic, though other items a wide-ranging as reproductive capacity and life mode can be assigned z values also. The species-area relationship is typically used to describe individual species and not entire ecosystems. However, once you calculate the number of a given species using this formula, knowledge of population ecology and ecosystem dynamics can be used to express a more holistic view of an environment. Generally, as the size of an area increases, so does the number of species.
Because of the climate, a very small area in a tropical region like the Amazon can not only have a dizzying array of species, but a very small area might also serve as the only location where that species can be found! When using the species-area relationship to model species richness as a function of climate, the lack of diversity in polar regions is a predicted result of the modeling. In addition, a single species might cover a very large area, such as spruce forests or Arctic Willow do in portions of the Canadian wilderness.
It is reasonable to assume that mathematical relationships like the species-area relationship not only apply to environments from the Earth’s past as it does today. It should then be possible to model not only course global effects of forcings/events, but to examine regional variations also.
Continental Interiors and Margins
The location on a continent is another important climate regionality. Generally, the interiors of continents have more extreme temperature and moisture differences during cold and warm periods (day/night, summer/winter). Areas along a coastline, by contrast, tend to have much more moderate temperature and moisture variation. This is due to the already discussed elevated specific heat of water and is ability to buffer high temperatures. Continental interiors also tend to have higher elevations. Such locations could be mountainous or high plains. In either case, the average temperature drops as altitude increases. Seasonally, this leads to much greater variation in temperatures between day and night or summer and winter.
Earth circa 500 Ma. Note the much different position of the continents and of sea level.
Throughout Earth’s past, the paleogeography has changed as plate tectonics forced continents into new positions. These forcings (events), though very long term in nature, led to significant changes in ocean current patterns and regional weather patterns. Short-term Earth system events, such as volcanic eruptions, might affect these patterns for a short time regionally or globally, but the return of the earlier dynamic equilibrium can typically be expected. Under average conditions, continental interior regions will have large range between moist and dry or warm or cold periods than their coastal counterparts.
Ultimately, all of these geographic variations lead to different areas experiencing the water cycle differently. The key climatic variables, independent of temperature, for any location that determine the kinds of ecosystems that can exist are evapotranspiration and precipitation. There are myriad natural biomes and climatic zones that can be described by these two variables.
Earth forcings (events) also have local effects that vary widely, including as these affect local climate. While detecting local changes due to particular Earth events in the rock record is usually very challenging or even impossible, it is important to note that we can use modern analogs (uniformitarianism) to hypothesize how a locality might have behaved. Global events, like large asteroid impacts, certainly had global impact and detectable regional impact. Localized impact can be harder to see, but even if we do not see evidence for it, we can know that it happened.
Orographic Effects and Slope Aspect
A good example of local climate effects can be found in the impact that mountains have on local (and regional) weather patterns. On a single northern hemisphere hillslope, the north side of a mountain or hill might be covered in tall pines while the south slope might be a grassland. The temperature and moisture effects of the degree of incident sunlight on a hillslope, referred to as slope aspect, can have a profound effect on the type of ecosystem and communities that it supports.
Likewise, in situations where weather is moving from west to east, the eastern slope of a mountain range will typically experience a lower degree of moisture than the western slopes. This rain shadow effect is produced as orographic uplift causes weather systems to drain their energy and precipitation on one side of the mountain, leaving the leftovers (so to speak) to drop a pittance of their former moisture on the other side. Such local effects may or may not be recorded in the rock record. Once the position of ancient mountain ranges is determined and their latitude defined, it is then possible to craft hypotheses about how weather may have been manifest in a specific area at that time.
Forests can certainly extend over a very large area, but their effect is also very regional to local in terms of climate. Forests are excellent places for moisture to accumulate and carbon to be sequestered. Transpiration of water through leaves is one way that plants transport soil moisture back to the atmosphere, raising the local relative humidity. This has a cooling effect. Forests can then have a similar effect on land as water has on coastlines, producing a more even temperature and moisture profile and reducing the severity of swings for these two variables.
Scales of Measurement: Time
Earth system events and forcing mechanisms also vary across time. Some events are very short in duration but bring with them an intense amount of energy. This leads to massive and rapid change and a completely new dynamic equilibrium, eventually. Some events are much more gradual.
If you stop and think, you can certainly come up with a list of daily environmental “events” that go by without much notice. The Sun rises and sets while the Earth rotates on its axis. Tides come in and go out along coastlines. You sleep, you awaken. Temperatures rise and fall with the Sun. Such events are not always recorded in the rock record, yet we know they played an important role through time.
The nature of the term diurnal itself has variation across Earth’s past. The length of a diurnal cycle today is just under 24 hours. However, this has not always been the case and will not remain this way into the future. Using fossil coral and fine layering in some sedimentary deposits that can be attributed to changes in lunar cycles, it is possible to extrapolate the change in diurnal cycles over time. Every 100 years, the length of a day is increased by 0.0024 seconds.
In the graph above, note that while a day has gotten longer over time, the number of days in a year has decreased. This is because it is assumed that the Earth revolves around the Sun in a fixed amount of time, 365.25 days.
Why does this matter? Many, perhaps most, species today are very much connected to the diurnal cycle. The entire biosphere then is very much linked with this cycle, as it is with seasonal cycles throughout a year.
Annual and longer events
Earth system changes are ideally measured in terms of years. This can be a shorter term event that affects only a given year or two (small volcanic eruption, earthquake, forest fires). Or, important Earth system events can be measured over millions of years. A great example of this is silicate weathering.
When two continents collide, mountains are raised up. We call this orogeny. As you drive those silica-rich rocks into the atmosphere they begin to weather, or break down. This is done through a process called hydrolysis, where weakly acidic rain (carbonic acid due to carbon dioxide) breaks down feldspar minerals in the rocks. In the image below, the role of silicate weathering in carbon regulation is highlighted. The long term effect of building up a mountain range is to force the climate to cool over a long period of time. Chemical weathering by hydrolysis breaks up silicate minerals and pulls carbon dioxide out of the air. Two by-products of the process are free calcium ions (aqueous) and aqueous bicarbonate. These new inputs into nearby basins can lead to a flourishing of carbonate life forms, who use these two substances to build their shells. Over time, they die, and their shells and the carbon they contain become limestone, a very effective sink for carbon storage.
Anomalies (Comparison to past trends/normals)
When analyzing data from any particular time period, modern or ancient, how do we know whether or not it is unusual or typical?
In order to define any variable within a system as complex as the Earth system at any given moment, it is useful to define anomalies. Simply put, an anomaly is a departure from normal behavior. The degree of anomaly can help scientists define the intensity of a forcing event and the state of the system at the time.
A very useful modern example, again taken from climate studies, comes from ground-based NASA surface temperature data, also known as GISSTEMP (Goddard Institute for Space Studies Surface Temperature Analysis). These charts, like the one below, are updated monthly. However, in order to define surface temperatures, the models use a baseline set of data, typically several decades in length, as an average for comparison.
The sea surface temperature anomaly map above comes from satellite data collected by the National Oceanic and Atmospheric Administration. Using the GOES-15 satellite, is is possible to see which localized regions of the oceans are experiencing warmer temperatures than normal. For analyzing the degradation and bleaching of coral reefs, as one useful example, such data is critically important.
Ground-based instrumentation and satellite data
How do we study the Earth system as a whole?
Earth observing systems maintained by NASA and other space agencies around the world still use ground-based data. Such ground-based data measures variables on land and sea, as well as in marginal areas along coastlines. This data is often the highest accuracy data available for areas where it is collected. However, as station distribution tends to be uneven, as does maintenance and reporting, ground-based data has significant resolution challenges. Even still, it is very important for describing and forecasting a wide range of modern environmental variables.
Ground based systems used to observe the Earth system include data collected by a wide spectrum of instrumentation. Some of these are completely automated, such as weather stations or tidal gauges. Others are maintained by a vast army of human volunteers, such as the GLOBE program. Perhaps the most comprehensive system of ground-based observation is maintained and run jointly by NASA, NOAA, EPA, and other governmental agencies.
Some of the challenges of ground-based data are resolved through the use of satellites.
Over the last forty years, a great deal of energy and money has been invested in the development of satellite systems to study the Earth from space. Such systems are very difficult to develop, very expensive, and very difficult to maintain. Despite these and other challenges, satellite observation systems now make up the backbone of data collection on the Earth system.
NASA Earth Observing System (EOS)
Below is an image the outlines the current constellation of satellites. Some of these current satellites are descendants of earlier missions. The LANDSAT program is a great example. LANDSAT 8 is the latest iteration of the program with missions as far out as LANDSAT 10 in planning. LANDSAT 7 is a great example of longevity in an Earth observing mission, as it is still active over 20 years after launch!
Nearly all of the data produced by these missions is in the public domain. In fact, NASA has developed websites that allow non-professional users to access and explore it. A very useful example is the NEO website. Perhaps the best total clearing house for NASA Earth observing data is one that places it in the context of global climate change. NASA’s climate change website provides an excellent dashboard for keeping track of key data related to climate, but also provides links and news about other related missions.
Space Weather Observations
Understanding the dynamics of the Earth system is not complete without exploring space weather – the forcing mechanisms of the exosphere. Dominated by the Sun and its solar output, such phenomena also include monitoring the Earth’s magnetic field, forms of cosmic radiation from the Sun and Milky Way, asteroids, and geomagnetic storm potential. Again, NASA has a number of missions exploring these things. More information about them can be found here.
Kleidon, Axel (2012). How does the Earth system generate and maintain thermodynamic disequilibrium and what does it imply for the future of the planet? Philosophical Transactions of the Royal Society A. DOI:10.1098/rsta.2011.0316
- 1 Introduction
- 2 Earth Systems Overview
- 3 The Symphony of the Spheres
- 3.1 Exosphere – Space Environment
- 3.2 Atmosphere – Gaseous Earth (Troposphere, Greenhouse Gases, etc)
- 3.3 Hydrosphere – Liquid Earth’s Water (Fresh, Marine, Ground, etc.)
- 3.4 Cryosphere – Frozen portion of the hydrosphere
- 3.5 Lithosphere/Geosphere – Solid Earth (Rock, including molten rock)
- 3.6 Biosphere – Life
- 3.7 Anthroposphere – Human portion of the biosphere
- 4 Earth’s Energy Budget
- 5 Earth Systems collide: Events and Interactions
- 6 Scales of Measurement: Geography
- 7 Scales of Measurement: Time
- 8 Ground-based instrumentation and satellite data
- 9 Further Reading