Planet Earth/1b. Earth System Science: Gaia or Medea?
Earth as a Puddle
edit“Imagine a puddle waking up one morning and thinking, ‘This is an interesting world I find myself in — an interesting hole I find myself in — fits me rather neatly, doesn’t it? In fact it fits me staggeringly well, must have been made to have me in it!’ This is such a powerful idea that as the sun rises in the sky and the air heats up and as, gradually, the puddle gets smaller and smaller, frantically hanging on to the notion that everything’s going to be alright, because this world was meant to have him in it, was built to have him in it; so the moment he disappears catches him rather by surprise. I think this may be something we need to be on the watch out for.”
In December 1968, Astronaut William A. Anders on Apollo 8 took a picture of the Earth rising above the Moon’s horizon. It captured how small Earth is when viewed from space, and this image suddenly had a strange effect on humanity. Our planet, our home, is a rather small, insignificant place when viewed from the great distance of outer space. Our lives are collectively from the perspective of Earth looking outward, but with this picture, taken above the surface of the Moon looking back at us, we realized that our planet is really just a small place in the universe.
Earth system science was born during this time in history, in the 1960s, when the exploration of the moon and other planets, allowed us to turn the cameras back on Earth and study it from afar. Earth system science is the scientific study of Earth’s component parts and how these components —the solid rocks, liquid oceans, growing life-forms, and gaseous atmosphere—function, interact, and evolve, and how these interactions change over long timescales. The goal of Earth system science is to develop the ability to predict how and when those changes will occur from naturally occurring events, as well as in response to human activity. Using the metaphor of Douglas Adams’ salient puddle above, we don’t want to get surprised if our little puddle starts to dry up!
A system is a set of things working together as parts of a mechanism or an interconnecting network, and Earth system science is interested in how these mechanisms work in unison with each other. Scientists interested in these global questions simplify their study into global box models. Global box models are analogies that can be used to help visualize how matter and energy move and change across an entire planet from one place or state to another.
For example, the global hydrological cycle can be illustrated by a simple box model, where there are three boxes, representing the ocean, the atmosphere, and lakes and rivers. Water evaporates from the ocean into the atmosphere where it forms clouds. Clouds in the atmosphere rain or snow on the surface of the ocean and land filling rivers and lakes (and other sources of fresh water), which eventually drain into the ocean. Arrows between each of the boxes indicate the direction that water moves between these categories. Flux is the rate at which matter moves from one box into another, which can change depending on the amount of energy. Flux is a rate, which means that it is calculated as a unit over time, in the case of water, this could be determined as volume of water that it rains or snows per year.
There are three types of systems that can be modeled. Isolated systems, in which energy and matter cannot enter into the model from the outside. Closed systems, in which energy, but not matter, can enter into the model, and open systems, in which both energy and matter can enter into the model from elsewhere.
Global Earth systems are regarded as closed systems since the amount of matter entering Earth from outer space is a tiny fraction of the total matter that makes up the Earth. In contrast, the amount of energy from outer space, in the form of sunlight, is large. Earth is largely open to energy entering the system and closed to matter. (Of course, there are rare exceptions to this when meteorites from outer space strike the Earth.)
In our global hydrological cycle, if our box model was isolated, allowing no energy and no matter to enter the system, there would be no incoming energy for the process of evaporation, and the flux rate between the ocean and atmosphere would decrease to zero. In isolated systems with no exchange of energy and matter, over time they will slow down and eventually stop functioning, even if they have an internal energy source. We will explore why this happens when we discuss energy. If the box model is open, such as if ice-covered comets frequently hit the Earth from outer space, there would be a net increase in the total amount of water in the model, or if water was able to escape into outer space from the atmosphere, there would be a net decrease in the total amount of water in the model over time. So it is important to determine if the model is truly closed or open to both matter and energy.
In box models we also want to explore all possible places where water can be stored, for example, water on land might go underground to form groundwater and enter into spaces beneath Earth’s surface, hence we might add an additional box to represent groundwater and its interaction with surface water. We might want to distinguish water locked up in ice and snow by adding another box to represent frozen water resources. You can begin to see how a simple model can, over time, become more complex as we consider all the types of interactions and sources that may exist on the planet.
A reservoir is a term used to describe a box which represents a very large abundance of matter or energy relative to other boxes. For example, the world’s ocean is a reservoir of water because most of the water is found in the world’s oceans. A reservoir is relative and can change if the amount of energy or matter in the source decreases in relation to other sources. For example, if solar energy from the sun increased and the oceans boiled and dried away, the atmosphere would become the major reservoir of water for the planet since the portion of water locked in the atmosphere would be more than found in the ocean. In a box model, a reservoir is called a sink when more matter is entering the reservoir than is leaving it, while a reservoir is called a source if more matter leaves the box than is entering it. Reservoirs are increasing in size when they are a sink and decreasing in size when they are a source.
Sequestration is a term used when a source becomes isolated and the flux between boxes is a very slow rate of exchange. Groundwater, which represents a source of water isolated from the ocean and atmosphere, can be considered an example of sequestration. Matter and energy which is sequestrated have very long residence times, the length of time energy and matter reside in these boxes.
Residence times can be very short, such as a few hours when water from the ocean evaporates, then falls back down into the ocean as rain; or very long, such as a few thousand years when water is locked up in ice sheets and even millions of years underground. Matter that is sequestered is locked up for millions of years, such that it is taken out of the system.
An example of sequestration is an earth system box model of salt (NaCl), or sodium chloride. Rocks weather in the rain, resulting in the dissolution of sodium and chloride, which are transported to the ocean dissolved in water. The ocean is a reservoir of salt since salt will accumulate over time by the process of the continued weathering of the land. Edmund Halley (who predicted a comet’s return and was later posthumously named after him) proposed in 1715 that the amount of salt in the oceans is related to the age of the Earth, and he suggested that salt has been increasing in the world’s ocean over time, which will become saltier and saltier into the future. However, this idea was proved false when scientists determined that the world’s oceans have maintained a similar salt content over its history. There had to be a mechanism to remove salt from ocean water. The ocean loses salt through the evaporation of shallow seas and landlocked water. The salt left behind from the evaporation of the water in these regions is buried under sediments and becomes sequestered underground. The flux of incoming salt into the ocean from weathering is similar to the flux leaving the ocean by the process of evaporated salt being buried. This buried salt will remain underground for millions of years. The salt cycle is at an equilibrium, as the oceans maintain a fairly persistent rate of salinity. The sequestration of evaporated salt is an important mechanism that removes salt from the ocean. Scientists begin to wonder if Earth exhibits similar mechanisms that maintain an equilibrium through a process of feedbacks.
Equilibrium is a state in which opposing feedbacks are balanced and conditions remain stable. To illustrate this, imagine a classroom which is climate controlled with a thermostat. When the temperature in the room is above 75 degrees Fahrenheit, the air conditioner turns on, when the temperature in the room is below 65 degrees Fahrenheit, the heater turns on. The temperature within the classroom will most of the time be at equilibrium between 65 and 75 degrees Fahrenheit, as the heater and air conditioner are opposing forces that keep the room in a comfortable temperature range. Imagine now that the room becomes filled with students, which increase the temperature in the room when the room reaches 75 degrees Fahrenheit, the air conditioner turns on, cooling the room. The air conditioner is a negative feedback. A negative feedback is where there is an opposing force that reduces fluctuations in a system. In this example, the increase in the heat of the students in the room is opposed by the cooling of the air conditioner system turning on.
Imagine that a classmate plays a practical joke, and switches the thermostat. When the temperature in the room is above 75 degrees Fahrenheit, the heater turns on, when the temperature in the room is below 65 degrees Fahrenheit, the air conditioner will turn on. With this arrangement, when students enter the classroom and the temperature slowly reaches 75 degrees Fahrenheit, the heater turns on! The heater is a positive force in the same direction as the heat produced by the students entering the room. A positive feedback is where there are two forces that join together in the same direction, which leads to instability of a system over time. The classroom will get hotter and hotter, even if the students leave the room, the classroom will remain hot, since there is no opposing force to turn on the air conditioner. It likely will never drop down to 65 degrees Fahrenheit, with the heater turned on. Positive feedbacks are sometimes referred to as vicious cycles. The tipping point in our example is 75 degrees Fahrenheit, when the positive feedback (the heater) turned on, resulting in the instability of the system and leading to a very miserable hot classroom experience. Tipping points are to be avoided if there are systems in place with positive feedbacks.
Gaia or Medea?
editOne of the most important discussions within Earth system science is whether the Earth exhibits mostly negative feedbacks or positive feedbacks and how well regulated are these conditions that we find on Earth today. The two hypotheses are named after two figures in Greek mythology, Gaia the Goddess of Earth, and Medea lover of Jason, who murdered her own children. The Gaia hypothesis maintains that the global Earth system maintains an equilibrium or long-term stability through various negative feedbacks that oppose destabilization of the planet. The Medea hypothesis maintains that the global Earth system does not maintain a stable equilibrium resulting in frequent episodes of catastrophic events. From a geological point of view, the Gaia hypothesis predicts Uniformitarianism, that past geological processes through time have mostly remained continuous and uniform, while the Medea hypothesis predicts Catastrophism, where most past geological processes are the result of sudden, short-lived, and violent events.
This dichotomy of the optimistic view of the Gaia hypothesis and pessimistic Medea hypothesis is a simplification. In reality the true Earth system likely exhibits both types of negative feedbacks and positive feedbacks that interact in complex ways.
Imagine a classroom, now equipped with two thermostats, a normal negative feedback that turns on the air conditioner when the room gets above 75 degrees, and a malfunctioning positive feedback that turns on the heater when the room gets above 80 degrees. Assuming that the classroom begins with a temperature of 70 degrees, and each student who enters the classroom raises the temperature by 1 degree. The air conditioning will turn on when 5 students enter the classroom. This air conditioner is weak and only lowers the temperature by 1 degree every 10 minutes.
The classroom will maintain an equilibrium temperature as long as the rate of students entering the room is below 10 students per 100 minutes. For example, if 7 students entered the classroom at the same time, the temperature would rise by 7 degrees, to 77 degrees—turning on the air conditioner when it crossed 75 degrees and taking 20 minutes to lower the temperature back down to 75 degrees. Another 4 students could enter the classroom, raising the temperature to 79 degrees, turning on the air conditioner and lowering the temperature down to 75 degrees in 40 minutes.
However, if 12 students enter the room at the same time, the temperature will rise to 82 degrees, turning on both the air conditioner at 75 degrees and heater at 80 degrees, the heater warms the room faster (+2 degrees every 10 minutes) than the air conditioner is able to cool the room (−1 degree every 10 minutes). This positive feedback will cause the classroom to increase in temperature until it is a hot oven because the net temperature will increase +1 degree every 10 minutes. The tipping point was when the 12 students entered the room all at once, which set off this vicious cycle of a positive feedback. If the rate of students entering the classroom remains low, the room temperature will remain stable, and it appears to be governed by the Gaia hypothesis. However, if the rate of students entering the classroom is fast, the temperature could become unstable, as governed by the Medea hypothesis.
In determining the mechanisms of how Earth systems play out over time, we also need to be aware of the fallacy of the salient puddle at the beginning of this chapter—a puddle that believes it is perfectly tailored to the environment it finds itself in. The Gaia hypothesis views the Earth as a perfectly working system that is able to adjust to changes and maintain an equilibrium state. This is similar to the salient puddle believing that it fits perfectly within the environment that it finds itself within. In contrast, the Medea hypothesis views that there will inevitably be an event that dries up the puddle, and that the puddle is not at equilibrium under a warm sun.
Who Came Up with These Ideas?
editThe Gaia hypothesis, has a longer pedigree, and was first formulated in the 1970s by James Lovelock and Lynn Margulis. Initially called the Earth feedback hypothesis, the name Gaia was proposed by the writer William Golding, the author of Lord of the Flies and close neighborhood friend of Lovelock. Lovelock was an expert on air quality and respiratory diseases in England, but he took up the study of the Earth’s sulfur cycle, noting negative feedbacks that appeared to regulate cloud cover. In the 1970s, he had been invited to work on the Viking Missions to Mars by NASA to evaluate the Martian atmosphere for the possible presence of life. Lovelock suggested that if the Viking lander found significant oxygen in the Martian atmosphere it would be indicative of life existing there. Instead the Viking lander found that the Martian atmosphere was 96% carbon dioxide, similar to the atmosphere of Venus. Working with Lynn Margulis, the two formulated a hypothesis that there were natural negative feedback systems on Earth that kept both oxygen and carbon dioxide in the atmosphere within a low range, with photosynthesizing plants and microbes taking in carbon dioxide and producing oxygen while animals take in oxygen and produce carbon dioxide. Life on Earth appeared to keep the atmosphere stable in relation to these two types of gases in the atmosphere. Without life, carbon dioxide remained high in the atmosphere of Mars and Venus.
The Medea hypothesis is a newer and a more frightening idea, first proposed by Peter Ward, an American paleontologist in a book published in 2009 (The Medea Hypothesis: Is Life on Earth Ultimately Self-Destructive?). Ward started out as a marine biologist, but a traumatic diving accident that left his diving partner dead pushed him to study marine organisms found in rocks rather than deep underwater. Ward became interested in fossil ammonites along the coast of Europe, which flourished in the oceans of the Mesozoic Era, during the age of the dinosaurs, but had become extinct with the dinosaurs, 66 million years ago. Ward studied ammonites and other fossils, and he became fascinated with mass extinction events that have occurred in Earth’s history. He became keenly interested in the Permian-Triassic extinction event in South Africa, which divides the Paleozoic Era (ancient life) with the Mesozoic Era (middle life), the great time divisions of Earth’s history. This extinction event was one of the worst, called colloquially the Great Dying, and it appears to have be caused by an imbalance of too much carbon dioxide in the atmosphere. Ward saw these episodes of mass extinction events in the rock record as evidence of when the Earth system became out of balance and resulted in catastrophic change. Ward, with Donald Brownlee, postulated that Earth’s atmosphere millions of years in the future would lose all its carbon dioxide as tectonic and volcanic activity on Earth ceases, resulting in no new sources of carbon dioxide released into the atmosphere. Carbon dioxide would become sequestered underground as photosynthesizing plants and microbes die and are buried, and there would be no new carbon dioxide emitted from volcanoes. As a result, less and less carbon dioxide would be available in the atmosphere, ultimately dooming the planet with the inevitable extinction of all photosynthesizing life forms.
Neither of the advocates of the two hypotheses view the Earth system exclusively governed by either hypothesis, but a mix of both negative and positive feedbacks working on a global scale over long time intervals. Another way to frame the Gaia and Medea hypotheses is to ask whether the global Earth system behaves mostly under negative or positive feedbacks. Of course, the goal of this class is to determine how you can avoid positive feedback loops that would result in catastrophic change to your planet while keeping your planet balanced with negative feedback loops and remain a habitable planet for future generations.
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