3.5: 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.

Forcings

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.

Figure \(\PageIndex{1}\): Natural and anthropogenic climate forcing mechanisms. Any one of these forcing mechanisms can have an amplifying or balancing effect on the Earth’s climate, depending upon whether there is an increase or decrease in degree. (EPA graphic)

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

Figure \(\PageIndex{2}\): The ice albedo effect can be a reinforcing feedback for both periods of warming and cooling in the Earth’s climate (Used with permission: Climate Interactive)

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 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.

Figure \(\PageIndex{3}\): Decreasing sea ice and its effect on albedo in the Arctic. (NASA graphic)

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.

Figure \(\PageIndex{4}\): Examples of amplifying (positive) and balancing (negative) feedback loops. Amplifying feedback loops tend to further force a system that is out of balance further out of balance. Balancing feedback loops represent sustainability, in this case, the predator/prey relationship. (Neely and Walters, 2016)

Eventually, balancing feedbacks will work to bring a system to equilibrium once again. Amplifying feedbacks are not always going to happen, by contrast.

Sinks

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.

Figure \(\PageIndex{5}\): Short-term and long-term carbon cycles as they currently exist in the Earth System today. (NASA graphic)

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.

Figure \(\PageIndex{6}\): The nitrogen cycle (Pidwirny and Jones, physicalgeography.net)

Tipping Points

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.

Figure \(\PageIndex{7}\): Global climate tipping points. While these are certainly based upon today’s environment, many of these would have also been tipping points at various times in Earth’s past. (Wikipedia Commons)

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.

Causal Loops

Figure \(\PageIndex{8}\): Causal loop diagram describing climate change tipping points with various feedbacks built in (amplifying and balancing). (Andrew Jones, Climate Interactive)

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.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

An event in one Earth system will have an impact on other systems. When that impact accentuates that effect, what kind of feedback is that?

a. Balancing Feedback

b. Reinforcing Feedback

Answer

b. Reinforcing Feedback