Recommended resources
The freely available resources this lesson will direct you to.
Hello! Welcome to the second lesson in our module on the Global Carbon Cycle.
Introduction
In our previous lesson, we created a static "map" of the Earth's carbon, quantifying the major reservoirs or stocks. We learned that the lithosphere and oceans hold vast quantities of carbon, while the atmosphere—the reservoir most critical for climate—is comparatively small.
Today, we bring that map to life. We will shift our focus from the stocks to the fluxes—the flows of carbon between these reservoirs. Your background in economics and systems analysis, where understanding stocks and flows is fundamental, will be particularly useful here. We will examine the massive natural exchanges that have kept the system in a dynamic equilibrium for millennia and then see how human activities have introduced a new, disruptive flux.
Our learning outcome is to: Compare the magnitudes of the main natural fluxes of carbon (e.g., photosynthesis/respiration, ocean-atmosphere exchange) with anthropogenic emissions.
By the end of this lesson, you will have a clear, quantitative understanding of why the relatively small anthropogenic flux has such a profound impact on the entire Earth system.
1. What is a Carbon Flux?
Before we dive into the numbers, let's establish a clear definition. A carbon flux is simply the rate of carbon exchange between different reservoirs.
To get a formal definition and introduce the units we'll be using, please read this short introduction.
This resource from Energy Education provides a concise definition of a carbon flux and the standard unit of measurement.
Please read the section titled 'Carbon flux' down to the end of the first paragraph.
As the text states, we measure these fluxes in gigatons of carbon per year (GtC/yr). This allows us to directly compare the annual movement of carbon with the total stock in each reservoir (measured in GtC), which we established in the last lesson.
2. The Natural Carbon Fluxes
For thousands of years before the industrial revolution, the carbon cycle was in a state of dynamic equilibrium. This doesn't mean carbon wasn't moving; in fact, enormous quantities were being exchanged continuously. However, the fluxes in and out of the atmosphere were balanced. Let's examine the two primary natural exchanges.
This next section from the same 'Carbon flux' article provides the approximate magnitudes of the two largest natural exchanges.
Read the two bullet points under the 'Natural Exchanges' subheading, which describe the land-atmosphere and ocean-atmosphere fluxes.
From this reading, we can identify two massive, naturally balanced cycles:
- Land-Atmosphere Flux: Plants absorb CO₂ from the atmosphere through photosynthesis, and release it back through respiration and decomposition. The gross flux is huge, on the order of ~120 GtC/yr in each direction.
- Ocean-Atmosphere Flux: CO₂ dissolves into the ocean surface and is also released back into the atmosphere (outgassing). This exchange is also very large, on the order of ~90 GtC/yr in each direction.
The key takeaway is that for each of these natural pathways, the amount of carbon leaving the atmosphere was roughly equal to the amount entering it. The net flux was close to zero.
3. The Anthropogenic Perturbation
This balanced system was disrupted by the onset of the industrial revolution. Humans began extracting carbon that had been locked away in the lithosphere for millions of years (fossil fuels) and burning it, releasing CO₂ into the active carbon cycle.
For a high-level overview of this disruption, please watch the following video.
The carbon cycle is key to understanding climate change
This video from The Economist provides an excellent conceptual overview of the carbon cycle, the idea of 'dynamic equilibrium', and how human activities have knocked it out of balance.
Watch from 0:58 to 3:11. Pay attention to the explanation of the natural equilibrium and how the addition of fossil fuel carbon acts as a new, unbalanced input.
The video highlights the crucial point: unlike the natural fluxes, the anthropogenic flux is largely a one-way street. It adds new carbon to the atmosphere-ocean-land system without a corresponding natural removal process on the same timescale.
Now, let's get the precise, up-to-date numbers for these anthropogenic fluxes.
This text from the McKinley Ocean Carbon Group at the University of Wisconsin-Madison provides the latest figures from the Global Carbon Budget, breaking down anthropogenic emissions by source.
Please read the following three short sections: Start at the heading 'Humans add CO2 to the Atmosphere...' and read the first paragraph. Read the first paragraph under the 'Fossil Fuels' section. Read the first paragraph under the 'Land Use' section. Focus on the quantities of carbon emitted from fossil fuels and land-use change for the most recent decade (2014-2023).
From this reading, we can quantify the two main anthropogenic sources for the 2014-2023 period:
- Fossil Fuel Combustion: ~9.7 GtC/yr
- Land-Use Change (primarily deforestation): ~1.1 GtC/yr
This gives a total anthropogenic flux of ~10.8 GtC/yr into the atmosphere.
4. Synthesis: Comparing the Fluxes
We now have all the pieces to address our learning outcome. The diagram below, from the 2023 Global Carbon Budget, visualizes the entire system. It shows the reservoirs (stocks, in black text), the pre-industrial natural fluxes (grey arrows), and the average anthropogenic perturbation for 2014-2023 (red arrows).
Take a moment to study this diagram carefully. Use it to compare the magnitude of the natural fluxes (grey arrows) with the anthropogenic ones (red arrows).
Let's summarize the comparison in a table, using the figures from the diagram:
| Flux Description | Flux Type | Magnitude (GtC/yr) | Net Effect (Pre-industrial) |
|---|---|---|---|
| Photosynthesis | Natural (Atmosphere to Land) | ~110 | \multirow{2}{*}{~0} |
| Respiration & Decomposition | Natural (Land to Atmosphere) | ~110 | |
| Ocean-Atmosphere Exchange | Natural (Gross fluxes) | ~80 (in each direction) | ~0 |
| Fossil Fuel Emissions | Anthropogenic | +9.4 | \multirow{2}{*}{+10.9} |
| Land-Use Change Emissions | Anthropogenic | +1.5 |
This comparison reveals the central paradox of the carbon cycle problem:
The anthropogenic flux (~11 GtC/yr) is tiny compared to the gross natural fluxes (~110 GtC/yr and ~80 GtC/yr). However, the natural fluxes are balanced, whereas the anthropogenic flux is a net addition.
It's this net addition that perturbs the system. Think of it like a finely balanced scale. The weights on each side are enormous (the natural fluxes), but they are equal. Adding a much smaller weight to just one side (the anthropogenic flux) is enough to tip the entire scale.
5. The System's Response: The Sinks
So, where does this extra ~11 GtC of carbon go each year? Does it all stay in the atmosphere?
Look at the diagram again. The red arrows show that the natural system is responding to the higher concentration of CO₂ in the atmosphere. The ocean and the land are now absorbing more carbon than they release. They have become net sinks.
- Ocean Sink: The ocean is absorbing a net 2.7 GtC/yr.
- Land Sink: The land biosphere is absorbing a net 3.4 GtC/yr.
The total anthropogenic emission is 9.4 + 1.5 = 10.9 GtC/yr.
The total uptake by natural sinks is 2.7 + 3.4 = 6.1 GtC/yr.
The remainder is what accumulates in the atmosphere: 10.9 - 6.1 = 4.8 GtC/yr. This is the "Atmospheric growth" value shown in the diagram.
This means that natural sinks are currently absorbing over half (6.1 / 10.9 ≈ 56%) of our annual emissions. Without this service provided by the land and oceans, the rate of climate change would be more than double what it is today.
Conclusion
In this lesson, we moved from a static view of carbon stocks to a dynamic view of carbon fluxes. We quantified the massive natural exchanges and compared them to the smaller, but system-disrupting, anthropogenic emissions.
Key Takeaways:
- The main natural carbon fluxes (land-atmosphere and ocean-atmosphere) are enormous, moving around 200 GtC in and out of the atmosphere each year.
- Prior to the industrial era, these natural fluxes were balanced, resulting in a stable atmospheric CO₂ concentration.
- Anthropogenic emissions from fossil fuels and land-use change, totaling about 11 GtC/yr, represent a net addition of carbon to the active cycle.
- While this anthropogenic flux is small compared to the gross natural fluxes, its one-way nature has disrupted the system's equilibrium.
- In response, the land and ocean have become net sinks, absorbing over half of our annual emissions. The remaining portion, nearly 5 GtC/yr, accumulates in the atmosphere, driving global warming.
In the next lesson, we will delve deeper into the largest anthropogenic flux. You will learn how to perform a 'back-of-the-envelope' calculation of global CO₂ emissions from fossil fuel combustion using global energy consumption data, connecting these abstract gigaton figures to the real-world activities that produce them.