34 Chapter 34: Feedback loops

Anastasia Chouvalova

Human activity releases carbon dioxide and methane, two of the most important greenhouse gases, into the atmosphere in several ways. The primary mechanism that releases carbon dioxide is the burning of fossil fuels, such as gasoline, coal, and natural gas. Deforestation, cement manufacture, animal agriculture, the clearing of land, and the burning of forests are other human activities that release carbon dioxide. Methane (CH4) is produced when bacteria break down organic matter under anaerobic conditions. Anaerobic conditions can happen when organic matter is trapped underwater (such as in rice paddies) or in the intestines of herbivores. Methane can also be released from natural gas fields and the decomposition of animal and plant material that occurs in landfills. Another source of methane is the melting of clathrates. Clathrates are frozen chunks of ice and methane found at the bottom of the ocean. When water warms, these chunks of ice melt and methane is released. As the ocean’s water temperature increases, the rate at which clathrates melt is increasing, releasing even more methane. This leads to increased levels of methane in the atmosphere, which further accelerates the rate of global warming. This is an example of the positive feedback loop that is leading to the rapid rate of increase of global temperatures.

Reading Question #1

What is a positive feedback loop?

A. An event in which the product of a reaction slows down that reaction, leading to greater proximity to a balance or set point.

B. An event in which the product of a reaction amplifies that reaction, leading to further deviation from a balance or set point.

C. An event in which the substrate of a reaction stabilizes that reaction.

D. An event in which the product of a reaction stabilizes that reaction.


In class, we will discuss other feedback loops related to climate change with a focus on permafrost and algal blooms. Below you will find introductions to these concepts, although you have briefly read about them in previous chapters in Unit 4.


An important component of the cryosphere (i.e., an Earth system containing frozen water) is permafrost, or frozen ground. The majority of permafrost is found in the Arctic (Figure 33.1), though the small area of land in Antarctica that is not covered by ice sheets also has permafrost. Regions containing permafrost are often categorized by the quantity of total area of the region that is frozen. Continuous permafrost, as its name implies is nearly entirely frozen with 90-100% of the land area of the region containing permafrost. Discontinuous permafrost, as the name implies, is not a single, solid sheet of frozen land (less than 90% of the total area is frozen), and can be sporadic (10-50% frozen area) or isolated (less than 10% frozen area).

Permafrost currently covers 24% of the exposed land in the high latitudes of Northern and Southern Hemispheres. Climate change scientists are carefully watching the impact of global warming on the seasonally and permanently frozen soil of the Arctic and subarctic.


Figurer 33.1 (Credit: National Snow and Ice Data Center)

As with other components of the cryosphere, permafrost is particularly sensitive to warming. One mechanism for tracking changes in permafrost extent is to measure the depth of the active layer, or the unfrozen soil that overlays permafrost. The Circumpolar Active Layer Monitoring (CALM) program tracks soil temperature and active layer depth in over 200 permafrost sites in both hemispheres.

The fate of permafrost in the future is dire. You may not think much about if you live outside the Arctic regions where permafrost is common. Scientists predict that 40 percent of the world’s permafrost could thaw if temperatures rise 2oC (3.6oF) due to global warming. Thawing of the permfrost could allow carbon that has been stored for thousands of years to be released into the atmosphere fueling additional warming. This would increase the respiratory activity of soil organisms, further increasing the release of CO2 in Arctic ecosystems. This represents a concerning feedback loop by which warming caused by atmospheric greenhouse gases leads to thawing of permafrost, which releases more greenhouse gases into the atmosphere, exacerbating warming trends and thawing of permafrost.

In addition, thawing results in land subsidence and mass movement because of the destabilized soil profile – this will lead to sinkholes, mudslides, and collapse of infrastructure. Buildings and infrastruce-like roads, landing strips, and pipeleines, upon which residents depend have been constructed to account for the expansion and contraction that takes place in the active layer of permafrost. Rising temperature due to climate change threatens structures built under a different permafrost freeze-thaw regime.

The Arctic has experienced a signicant rise in air temperature over the last few decades and the permafrost that undelies much of the surface is undergoing substantial changes. Continuous permafrost on Alaska’s North Slope has warmed 2.2o-3.9o C (4o – 7o F) over the last century making it more susceptible to erosion and mass movement. Some places in Alaska have subsided by 4.6 meters (15ft) due to thawing of the permanently frozen subsurface. Accompanied by rising sea level, Alaskan coastal communities near the Arctic Ocean and Bering Sea are being threatened.

Link to Learning

View this video to learn more about permafrost.

Reading Question #2

Which of the following are projected consequences of melting permafrost? Select all that apply.

A. Buildings built on permafrost may be compromised.

B. Reduced release of greenhouse gases into the atmosphere

C. Stabilization of carbon in the lithosphere

D. Northern communities may be more vulnerable to landslides and moving land masses.

Introduction to eutrophication

Eutrophication occurs when excess nutrients are introduced into a body of water. This process increases the rate of supply of organic matter in an ecosystem and stimulates aquatic plant growth. At normal levels, these nutrients feed the growth of organisms called cyanobacteria or algae. But with too many nutrients, cyanobacteria grow out of control. Excess algae block the sunlight needed by bottom-dwelling plants and lead to a decrease in oxygen in the water and consequently leads to negative outcomes.

Eutrophication occurs naturally but anthropogenic activities such as industrial effluent and runoff of fertilizers rich in nitrogen and phosphorus contribute heavily to eutrophication events. When supplied with an excess of nutrients, the algae can grow out of control. This event is known as an “algal bloom,” and disrupts the balance of the ecosystem. As described above, the increased growth blocks the availability of sunlight to benthic organisms and other plants and organisms in the photic zone. The overgrowth of algae eventually begins to die off and is broken down by microbes that consume oxygen during the decomposition process. This creates a hypoxic environment and decreases oxygen availability in the water to other organisms.

Some of the negative effects of this excessive algae production, or algal blooms, are:

  • The production of dangerous toxins that can kill animals and people
  • The creation of “dead zones” (low oxygen hypoxic zones, or no oxygen anoxic zones) in the ocean
  • An increase in treatment costs for cleaning water
  • Harm to industries and communities that rely on the affected watershed
Figure 33.6 The Eutrophication Process and how it leads to hypoxia. Source: https://www.wri.org/our-work/project/eutrophication-and-hypoxia/about-eutrophication

Link to Learning

This link directs you to a video made by Khan Academy that explains the process of eutrophication and how dead zones are formed.

Reading Question #3

What is the main anthropogenic cause for eutrophication?

A. Agricultural runoff

B. Depositing too many algae organisms into water bodies

C. Humans feeding the algae organisms too much

D. Increased levels of minerals in the water


Sources of Eutrophication

Point source

A point source pollution is one that is directly identifiable and can be traced back to a singular distinguishable source. Factories and sewage treatment plants are the most common types of point sources that cause eutrophication. Some factories discharge their waste, called effluent, directly into a water body from sewage pipes. Unregulated discharge of effluent can cause severe damage to human health and the environment. The consequences of unregulated discharge include water pollution, unsafe drinking water, and restricted recreational activities. The sewage dump can deposit nutrients in streams that can be carried out to sea and cause eutrophication events. Symptoms caused by exposure to algal toxins in drinking water can include nausea, vomiting, and throat irritation. When water is consumed in sufficient quantities, the toxins can affect the liver and nervous system. This can also indirectly affect the economy because of the loss of working days due to such health problems.

Non-point source

Non-point source pollution is pollution where the origin is less specified and more diffuse. Non-point source pollution is difficult to remedy as the source cannot be pinpointed. Agricultural runoff is the largest non-point source cause of pollution leading to eutrophication in the Delta. More than 200 million pounds of pesticides are applied to California farms every year which are washed into the delta. Water runoff over landscapes with excess fertilizer can pick up nutrients and carry them out to bodies of water. Urban runoff is also considered a non-point source of pollution affecting eutrophication.

Figure 33.7 Source: Gulf of Mexico Hypoxia https://gulfhypoxia.net/about-hypoxia/

As plant and animal biomass increase, species diversity decreases and the affected area will become overpopulated by phytoplankton feeding off the increased algae. This will also change the dominant biota in the region.

Turbidity is the clouding of water due to sediment. It can be caused by excessive phytoplankton, algae growth, urban runoff, or sediments from erosion. These suspended particles, in addition to making the water look dirty, also help promote the toxins in water as heavy metals and toxic organic compounds can attach easily to the suspended sediment. These suspended particles also absorb heat from the sun, making turbid waters warmer. This also reduces the oxygen content in the water, as more oxygen is dissolved in colder waters. The suspended particles also scatter light, decreasing the photosynthetic activity of plants and algae, which results in a positive feedback loop for decreasing oxygen even more. Some biological impacts include: fish eggs and larvae will be covered and suffocated, and gills will become clogged and damaged. Thus, turbidity is a plausible and extremely harmful effect of eutrophication.

Reading Question #4

Researchers visiting a a lake in northern Texas have noticed an algal bloom and have determined that a nearby farm had inappropriate overuse of a fertilizer. Which of the following statements is true? Select all that apply.

A. This is a non-anthropogenic cause of pollution.

B. This is an anthropogenic cause of pollution.

C. This is a point source of pollution.

D. This is a non-point source of pollution.


Eutrophication can lead to hypoxia in the water column. Hypoxia event occurs when there is low oxygen level in the water. This incident is a consequence of eutrophication due to an excess of nutrient input (nitrogen and phosphorus) in the water that stimulates the growth of phytoplankton and consequently affects fishes and other organisms. Human activities have increased the rate of eutrophication through point source and non-point discharge of nutrients such as nitrogen and phosphorus.

Algal blooms

Ever been told to only eat shellfish during the months that have the letter “R”, (September-April)? Well this rule is actually pretty important for keeping the health of people safe and to allow for many species of shellfish to repopulate. But why are the other months of the year not safe for people to eat shellfish? In short its because of the algae that grow during this time of year and as ocean temperatures rise. During these specific months of warmer weather, billions upon billions of these microorganisms start to take over our oceans and can have many consequences for us.

Figure 33.3 “Multicellular Green Algae” by Frank Fox [CC by 2.0]

Before going into what red tide is or how the populations of these microorganisms seem to be increasing significantly as oceans warm up, lets take a closer look at algae. Most species of algae are single-celled organisms but some species can be multi-cellular as seen in the photo above. Algae are autotrophs, meaning they use photosynthesis as their means of producing energy for themselves. Though similar to plants in the way they are both producers, algae have no stems or leaves and are more closely related to other groups of protists. Habitats for algae include any bodies of water including fresh and salt water, or have extreme external environment factors. There are few cases where they have been found on land such as rocks, trees, hot springs, etc… Species of algae have been well documented to be able to survive many harsh environments and have been on earth far longer than most living organisms to this day. They contributed to the Earth being able to house life by producing oxygen through photosynthesis. Overall Algae species are very tough and can survive in a wide range of environments, which can be seen as both a positive and negative situation.

The red tide occurs when the algae from algal blooms becomes so numerous that it discolors the water. It is also sometime referred to as a Harmful Algal Bloom or “HAB”. This is where the name “red tide”comes from. Some key factors involved in red tides forming are warm ocean surface temperatures, low salinity, high nutrient content, calm seas, and rain followed by sunny days during the summer months. Some effects of the red tide are that it could deplete the oxygen in the water and/or release toxins into the water. The toxins in the water could have negative effects on the health of humans and animals exposed to them. There are three types of algae that can release these harmful toxins, they are Alexandrium fundyense, Alexandrium catenella and Karenia brevis.

Figure 33.4 “Red Tides in Isahaya Bay, Japan / Algal Bloom in Pelee Island, Ontario” by Marufish and Tom Archer [CC by 2.0]

What is important to recognize about “Red tides” and Algal blooms is that it isn’t always obvious that algae growth is there. They are not always a red color. The photos above show two examples of Algal blooms from two very different parts of the world, yet both species are considered “Red Tide” and harmful to some shellfish and animals that eat the shellfish. Algae alone is not an issue and even during the time of the year where there seems to be an excess amount of growth, this is a natural occurrence. What becomes a problem or what classifies as a “Red Tide” are the algae that release toxins in the air and water when they grow. Very few algae species can produce this toxin but when a large enough group forms on shores it can have a negative effect on both the marine environment and humans. The toxins produced can often affect the respiratory and nervous systems of all life forms. Thus when smaller marine animals feed on the algae, the trophic level above them can become poisoned as well. Paralytic Shellfish Poisoning is typically found along the Pacific and Atlantic coasts of the United States and Canada. It can cause paralysis and in extreme cases death. Some of the toxins that cause Paralytic Shellfish Poisoning can be 1,000 times more potent than cyanide. Diarrhetic Shellfish Poisoning is another example of a harmful effect from eating contaminated shellfish. It is caused by Okadaic acid, which is produced by several species of dinoflagellates, and is usually non-deadly to humans. Small amounts of the okadaic acid usually do not have any harmful effects and only become an issue when large amounts are consumed. Amnesic Shellfish Poisoning is the third common poisoning that humans will get from eating contaminated shellfish. It can be life threatening and cause both gastrointestinal and neurological disorders. These disorders are caused by domoic acid. After an incident in Canada in 1987 where 4 people died from Amnesic Shellfish Poisoning, the levels of domoic acid in shellfish are now being monitored.

Figure 33.5 “Toxic Algae Bloom in Lake Erie” by NASA Earth Observatory Under Public Domain [CC by 2.0].

Algal Blooms can have serious effects on corals. Red algae, brown algae, and green algae are a few examples of macro-algae that can have a very negative effect on corals. They do this by outcompeting, overgrowing and eventually replacing sea-grasses and coral reef habitats. According to some research that is being done, harmful tropical algal blooms are increasing in frequency and intensity. This can have a significant impact on coral reefs.

Notable Red Tides:

1844: First recorded case off the Florida Gulf Coast.

1972: Red tides killed 3 children and hospitalized 20 adults in Papua New Guinea.

2005: The Canadian red tide was discovered to have come further south than it has in years prior by the ship (R/V) Oceanus, closing shellfish beds in Maine and Massachusetts. Authorities were also alerted as far south as Montauk to check their beds. The experts who discovered the reproductive cysts in the seabed warned of a possible spread to Long Island in the future. This halted the area’s fishing and shellfish industry.

2013: In January, a red tide occurred on the West Coast Sea of Sabah in the Malaysian Borneo. There were two fatalities reported after they consumed a shellfish that had been contaminated with the red tide toxin.

2015: In September, a red tide bloom occurred in the Gulf of Mexico, affecting Padre Island National Seashore along North Padre Island and South Padre Island in Texas.

Scientists have been able to help control the spread of the effects of harmful algal blooms by developing new technology to help track them better. Tracking these harmful algal blooms could help prevent people from eating contaminated shellfish and knowing which areas will be most effected by them. Some examples of the technology that can help with monitoring them are better and more advanced satellite imagery. Also the development of an antidote to the toxins produced is another way to reduce the harmful effects. Even though they are a natural occurrence, what is alarming some scientists is that they may start to last longer and occur more often during the year as ocean temperatures and CO2 levels rise.

Reading Question #5

What is a large concern with red tides? Select all that apply.

A. Neurological consequences.

B. Algae releasing toxins into the water.

C. Algae extinction.

D. Jeopardization of coral reef ecosystems.



Adapted from Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-introduction








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