17 Chapter 17: Carbon Cycle and Climate Change
Lisa Limeri and Anastasia Chouvalova
Learning Objectives
By the end of this section, students will be able to…
- Create a diagram, drawing, or model to communicate how carbon is assimilated into organic compounds in plants and relate this to the flow of energy and the synthesis of molecules required for maintenance and growth.
- Explain how the CO2 in “weightless” air is the source of mass in a redwood tree to an 8-year-old.
- Explain how human activities impact climate change by disrupting the global carbon cycle.
The Carbon Cycle
Carbon is the second most abundant element in living organisms. Carbon is present in all organic molecules, and its role in the structure of macromolecules is of primary importance to living organisms. The carbon cycle is most easily studied as two interconnected sub-cycles: one dealing with rapid carbon exchange among living organisms and the other dealing with the long-term cycling of carbon through geologic processes (Figure 17.1).
Living organisms are connected in many ways, even between ecosystems. A good example of this connection is the exchange of carbon between autotrophs and heterotrophs within and between ecosystems by way of atmospheric carbon dioxide. Carbon dioxide is the basic building block that most autotrophs use to build multicarbon, high energy compounds, such as glucose, through photosynthesis. The energy harnessed from the sun is used by these organisms to form the covalent bonds that link carbon atoms together. These chemical bonds thereby store this energy for later use in the process of respiration. Most terrestrial autotrophs obtain their carbon dioxide directly from the atmosphere, while marine autotrophs acquire it in the dissolved form (carbonic acid, H2CO3−). However carbon dioxide is acquired, a by-product of the process is oxygen. The photosynthetic organisms are responsible for depositing approximately 21% oxygen content of the atmosphere that we observe today.
Reading Question #1
The process that converts carbon from gaseous form to organic biomass is called…
A. Respiration
B. Photosynthesis
C. Volcanic eruption
D. Weathering
E. Decomposition
Heterotrophs and autotrophs are partners in biological carbon exchange (especially the primary consumers, largely herbivores). Heterotrophs acquire the high-energy carbon compounds from the autotrophs by consuming them, and breaking them down by respiration to obtain cellular energy, such as ATP. The most efficient type of respiration, aerobic respiration, requires oxygen obtained from the atmosphere or dissolved in water. Thus, there is a constant exchange of oxygen and carbon dioxide between the autotrophs (which need the carbon) and the heterotrophs (which need the oxygen). Gas exchange through the atmosphere and water is one way that the carbon cycle connects all living organisms on Earth.
The movement of carbon through the land, water, and air is complex, and in many cases, it occurs much more slowly geologically than as seen between living organisms. Carbon is stored for long periods in what are known as carbon reservoirs, which include the atmosphere, bodies of liquid water (mostly oceans), ocean sediment, soil, land sediments (including fossil fuels), and the Earth’s interior.
The atmosphere is a major reservoir of carbon in the form of carbon dioxide and is essential to the process of photosynthesis. The level of carbon dioxide in the atmosphere is greatly influenced by the reservoir of carbon in the oceans. The exchange of carbon between the atmosphere and water reservoirs influences how much carbon is found in each location, and each one affects the other reciprocally. Carbon dioxide (CO2) from the atmosphere dissolves in water and combines with water molecules to form carbonic acid, and then it ionizes to carbonate and bicarbonate ions. The equilibrium coefficients are such that more than 90% of the carbon in the ocean is found as bicarbonate ions. Some of these ions combine with seawater calcium to form calcium carbonate (CaCO3), a major component of marine organism shells. These organisms eventually form sediments on the ocean floor. Over geologic time, the calcium carbonate forms limestone, which comprises the largest carbon reservoir on Earth.
Reading Question #2
The reservoir of carbon in the ocean primarily exists in what form?
A. Gaseous carbon dioxide
B. Dissolved carbon dioxide
C. Fossil fuels
D. Bicarbonate ions
On land, carbon is stored in soil as a result of the decomposition of living organisms (by decomposers) or from weathering of terrestrial rock and minerals. This carbon can be leached into the water reservoirs by surface runoff. Deeper underground, on land and at sea, are fossil fuels: the anaerobically decomposed remains of plants that take millions of years to form. Fossil fuels are considered a nonrenewable resource because their use far exceeds their rate of formation. A nonrenewable resource, such as fossil fuel, is either regenerated very slowly or not at all. Another way for carbon to enter the atmosphere is from land (including land beneath the surface of the ocean) by the eruption of volcanoes and other geothermal systems. Carbon sediments from the ocean floor are taken deep within the Earth by the process of subduction: the movement of one tectonic plate beneath another. Carbon is released as carbon dioxide when a volcano erupts or from volcanic hydrothermal vents.
Humans contribute to atmospheric carbon by the burning of fossil fuels and other materials. Since the Industrial Revolution, humans have significantly increased the release of carbon and carbon compounds, which has in turn affected the climate and overall environment. 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. Animal husbandry by humans also increases atmospheric carbon. The large numbers of land animals raised to feed the Earth’s growing population results in increased carbon dioxide levels in the atmosphere due to farming practices and respiration and methane production. 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. This is another example of how human activity indirectly affects biogeochemical cycles in a significant way. Although much of the debate about the future effects of increasing atmospheric carbon on climate change focuses on fossils fuels, scientists take natural processes, such as volcanoes and respiration, into account as they model and predict the future impact of this increase.
Reading Question #3
The carbon stored in fossil fuels originally came from where?
A. Photosynthesis by ancient organisms.
B. Respiration by ancient organisms.
C. Decomposition of ancient organisms.
D. Ancient volcanic eruptions.
Global Climate Change
Scientists who study climate have noted a series of marked changes that have gradually become increasingly evident during the last sixty years. Global climate change is the term used to describe altered global weather patterns, especially a worldwide increase in temperature and resulting changes in the climate, due largely to rising levels of atmospheric carbon dioxide.
The Intergovernmental Panel on Climate Change (IPCC) was created in 1988 by the United Nations Environment Programme and the World Meteorological Organization. It is charged with the task of evaluating and synthesizing the scientific evidence surrounding global climate change. The IPCC uses this information to evaluate current impacts and future risks, in addition to providing policymakers with assessments. These assessments are released about once every every six years. The most recent report, the 6th Assessment, was released starting in 2022 (link). Hundreds of leading scientists from around the world are chosen to author these reports. Over the history of the IPCC, these scientists have reviewed thousands of peer-reviewed, publicly available studies. The scientific consensus is clear: global climate change is real and humans are very likely the cause for this change.
Additionally, the major scientific agencies of the United States, including the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA), also agree that climate change is occurring and that humans are driving it. In 2010, the US National Research Council concluded that “Climate change is occurring, is very likely caused by human activities, and poses significant risks for a broad range of human and natural systems”. Many independent scientific organizations have released similar statements, both in the United States and abroad. This doesn’t necessarily mean that every scientist sees eye to eye on each component of the climate change problem, but broad agreement exists that climate change is happening and is primarily caused by excess greenhouse gases from human activities.
Evidence for Global Climate Change
Since scientists cannot go back in time to directly measure climatic variables, such as average temperature and precipitation, they must instead indirectly measure temperature. To do this, scientists rely on historical evidence of Earth’s past climate. Antarctic ice cores are a key type of such evidence for climate change. These ice cores are samples of polar ice obtained by means of drills that reach thousands of meters into ice sheets or high mountain glaciers. Viewing the ice cores is like traveling backwards through time; the deeper the sample, the earlier the time period. Trapped within the ice are air bubbles and other biological evidence that can reveal temperature and carbon dioxide data. Antarctic ice cores have been collected and analyzed to estimate the temperature of the Earth over the past 400,000 years (Figure 17.2a).
The data represented in Figure 17.3A is an example of historical climate analyses based on ice core data. In 17.3A, the Y-axis represents deviation from the long-term average of temperature on Earth over its history, such that 0°C is the long-term average. Temperatures that are greater than 0°C exceed Earth’s long-term average temperature and temperatures that are less than 0 °C are less than Earth’s average temperature.
The graph in Figure 17.3B shows Earth’s actual temperature (black line) since 1850 and expected climate based on one model that includes only natural factors and another model that includes both natural and human-caused factors. The model that includes human-caused factors matches the data, whereas the model with only natural factors would not have predicted the climate warming.
Past and Current Drivers of Global Climate Change
Because it is not possible to go back in time to directly observe and measure climate, scientists must use indirect evidence to determine the drivers that may be responsible for climate change. The indirect evidence includes data collected using ice cores, boreholes (a narrow shaft bored into the ground), tree rings, glacier lengths, pollen remains, and ocean sediments. The data shows a correlation between the timing of temperature changes and drivers of climate change. Before the Industrial Era (pre-1780), there were three drivers of climate change that were not related to human activity or atmospheric gases: Milankovitch cycles, changes in luminosity (solar intensity), and volcanic activity.
Milankovitch cycles describe the effects of slight changes in the Earth’s orbit on Earth’s climate. The length of the Milankovitch cycles ranges between 19,000 and 100,000 years. In other words, one could expect to see some predictable changes in the Earth’s climate associated with changes in the Earth’s orbit at a minimum of every 19,000 years. Milankovitch cycles occur over much longer time scales than a couple hundred years and the rate of change is dramatically slower than the climate change we are currently experiencing.
The luminosity of the sun refers to the amount of electromagnetic energy radiated from the sun per unit time, and it naturally varies over large time scales. The variation in the sun’s intensity is the second natural factor responsible for climate change. As luminosity increases (or decreases), the Earth’s temperature correspondingly increases (or decreases). Changes in solar intensity have been proposed as one of several possible explanations for past ice ages. However, luminosity has not changed significantly in the past 200 years.
Volcanic eruptions can last a few days, but the solids and gases released during an eruption can influence the climate over a period of a few years, causing short-term climate changes. The gases and solids released by volcanic eruptions can include carbon dioxide, water vapor, sulfur dioxide, hydrogen sulfide, hydrogen, and carbon monoxide. Generally, volcanic eruptions cool the climate. This occurred in 1783 when volcanoes in Iceland erupted and caused the release of large volumes of sulfuric oxide. This led to haze-effect cooling, a global phenomenon that occurs when dust, ash, or other suspended particles block out sunlight and trigger lower global temperatures as a result; haze-effect cooling usually extends for one or more years before dissipating in intensity. In Europe and North America, haze-effect cooling produced some of the lowest average winter temperatures on record in 1783 and 1784.
None of these natural phenomena can explain the current fast rate of climate change that we are currently experiencing.
Greenhouse Effect
Gardeners that live in moderate or cool environments use greenhouses because they trap heat and create an environment that is warmer than outside temperatures. Greenhouses contain glass or plastic that allow visible light from the sun to pass. This light, which is a form of energy, is absorbed by plants, soil, and surfaces and heats them. Some of that heat energy is then radiated outwards in the form of infrared radiation, a different form of energy. Unlike with visible light, the glass of the greenhouse blocks the infrared radiation, thereby trapping the heat energy, causing the temperature within the greenhouse to increase. The same phenomenon happens inside a car on a sunny day. Have you ever noticed how much hotter a car can get compared to the outside temperature? Light energy from the sun passes through the windows and is absorbed by the surfaces in the car such as seats and the dashboard. Those warm surfaces then radiate infrared radiation, which cannot pass through the glass. This trapped infrared energy causes the air temperatures in the car to increase.
When discussing climate, the greenhouse effect is the warming of Earth due to greenhouse gases in the atmosphere (Fig 17.4). Greenhouse gases are probably the most significant drivers of the climate. When heat energy from the sun strikes the Earth, gases known as greenhouse gases trap the heat in the atmosphere, in a similar manner as do the glass panes of a greenhouse keep heat from escaping (Fig 17.4). Approximately half of the radiation from the sun passes through these gases in the atmosphere and strikes the Earth. This radiation is converted into thermal (infrared) radiation on the Earth’s surface, and then a portion of that energy is re-radiated back into the atmosphere. Greenhouse gases, however, reflect much of the thermal energy back to the Earth’s surface (Fig 17.4). The more greenhouse gases there are in the atmosphere, the more thermal energy is reflected back to the Earth’s surface, heating it up and the atmosphere immediately above it. GHGs act like a blanket, making Earth significantly warmer than it would otherwise be. Scientists estimate that average temperature on Earth would be -18ºC without naturally-occurring greenhouse gases.
Being a greenhouse gas is a physical property of certain types of gases; because of their molecular structure they absorb wavelengths of infrared radiation, but are transparent to visible light. Some notable greenhouse gases are water vapor (H2O), carbon dioxide (CO2), and methane (CH4). Carbon dioxide and methane are the most important greenhouse gases directly emitted by human activity. Carbon dioxide (CO2) is the primary greenhouse gas that is contributing to recent global climate change. CO2 is a natural component of the carbon cycle, involved in such activities as photosynthesis, respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, primarily the burning of fossil fuels and changes in land use, release very large amounts of CO2 to the atmosphere, causing its concentration in the atmosphere to rise.
Although this concentration is far less than that of CO2, methane (CH4) is 28 times as potent a greenhouse gas. Methane is produced when bacteria break down organic matter under anaerobic conditions and can be released due to natural or anthropogenic processes. Anaerobic conditions can happen when organic matter is trapped underwater (such as in rice paddies) or in the intestines of herbivores. Anthropogenic causes now account for 60% of total methane release. Examples include agriculture, fossil fuel extraction and transport, mining, landfill use, and burning of forests. Specifically, raising cattle releases methane due to fermentation in their rumens produces methane that is expelled from their GI tract. Methane is more abundant in Earth’s atmosphere now than at any time in at least the past 650,000 years, and methane concentrations increased sharply during most of the 20th century. They are now more than two and-a-half times pre-industrial levels (1.9 ppm), but the rate of increase has slowed considerably in recent decades.
Water vapor is the most abundant greenhouse gas and also the most important in terms of its contribution to the natural greenhouse effect, despite having a short atmospheric lifetime. Some human activities can influence local water vapor levels. However, on a global scale, the concentration of water vapor is controlled by temperature, which influences overall rates of evaporation and precipitation. Therefore, the global concentration of water vapor is not substantially affected by direct human emissions.
Ground-level ozone (O3), which also has a short atmospheric lifetime, is a potent greenhouse gas. Chemical reactions create ozone from emissions of nitrogen oxides and volatile organic compounds from automobiles, power plants, and other industrial and commercial sources in the presence of sunlight. In addition to trapping heat, ozone is a pollutant that can cause respiratory health problems and damage crops and ecosystems.
Reading Question #4
How do greenhouse gases affect temperature on Earth?
A. Greenhouse gases cause more incoming solar radiation to be absorbed in the atmosphere.
B. Greenhouse gases cause heat radiated from earth to pass through the atmosphere more readily.
C. Greenhouse gases cause heat radiated from earth to be trapped within the Earth’s atmosphere.
D. Greenhouse gases generate heat by interacting with other gases in the atmosphere.
Carbon dioxide concentration in the atmosphere over time
Direct evidence supports the relationship between atmospheric concentrations of carbon dioxide and temperature: as carbon dioxide rises, global temperature rises, and vice-versa (Fig 17.5).
Since the beginning of the Industrial Revolution, the concentration of atmospheric carbon dioxide has increased from about 280 ppm to currently over 400 ppm (Fig 17.6).
A longer-term view shows how dramatic this increase is compared to the concentration of carbon dioxide throughout Earth’s history (Fig 17.7). Before the Industrial Revolution, atmospheric carbon dioxide has cycled between 180-300 ppm throughout Earth’s history, and are currently 50% higher than the previous historical peak (Fig 17.7). Historical reconstruction data reveals that it took around 50,000 years for the atmospheric carbon dioxide level to increase from its low minimum concentration to its higher maximum concentration (Fig 17.7). However, beginning only a few centuries ago, atmospheric carbon dioxide concentrations have increased beyond the historical maximum of 300 ppm. The current increases in atmospheric carbon dioxide have happened very quickly—in a matter of hundreds of years rather than thousands of years.
What is the reason for this difference in the rate of change and the amount of increase in carbon dioxide? A key factor that must be recognized when comparing the historical data and the current data is the presence and industrial activities of modern human society; no other driver of climate change has yielded changes in atmospheric carbon dioxide levels at this rate or to this magnitude.
Human activity increases greenhouse gases in the atmosphere
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 (Figure 16.3). The Industrial Revolution, which began around 1750, was characterized by changes in much of human society. Advances in agriculture increased the food supply, which improved the standard of living for people in Europe and the United States. New technologies were invented that provided jobs and cheaper goods. These new technologies were powered using fossil fuels, especially coal. When a fossil fuel is burned, carbon dioxide is released. With the beginning of the Industrial Era, atmospheric carbon dioxide began to rise (Fig 17.7). 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.
Reading Question #5
Which of the following describes the current level of atmospheric carbon dioxide concentration compared to historical levels of atmospheric carbon dioxide (before the Industrial era)?
A. Carbon dioxide concentration is currently lower than it has ever been in Earth’s history.
B. Carbon dioxide concentration is within the historical range.
C. Carbon dioxide concentration is currently matching the the highest historical peak.
D. Carbon dioxide concentration is currently ~50% higher than its highest historical level.
References
Adapted from:
National Oceanic and Atmospheric Administration, National Centers for Environmental Information. (2022). Paleoclimatic Data for the Last 2,000 years. Retrieved from ncer.noaa.gov/
Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-introduction
Fowler, S., Roush, R., & Wise, J. (2013). Concepts of Biology. OpenStax. Retrieved from https://openstax.org/details/books/concepts-biology
Ha, M., and Schleiger, R. (2022). Environmental Science. LibreTexts Biology. Retrieved from https://bio.libretexts.org/Bookshelves/Ecology/Environmental_Science_(Ha_and_Schleiger)/06%3A_Environmental_Impacts/6.03%3A_Climate_Change/6.3.01%3A_The_Greenhouse_Effect_and_Climate_Change
Iredale, L. (2024). Environmental Geology. Retrieved from https://minnstate.pressbooks.pub/environmentalgeology
Schmittner, A., (2019). Introduction to Climate Science. Retrieved from https://open.oregonstate.education/climatechange/chapter/impacts/
