The Carbon
Cycle
Explore how carbon moves through Earth's atmosphere, biosphere, hydrosphere, and geosphere: the biogeochemical cycle that drives climate and life.
AP Environmental Science
What Makes Carbon Special?
Carbon is the fourth most abundant element in the universe and the backbone of all organic molecules. Its ability to form four covalent bonds allows it to create the complex molecules essential for life: proteins, lipids, carbohydrates, and nucleic acids.
Earth's Carbon Reservoirs
Atmosphere
\(\text{CO}_2\) and \(\text{CH}_4\) gases: ~830 GtC (gigatons of carbon)
Biosphere
Living organisms and dead organic matter: ~2,000 GtC
Geosphere
Fossil fuels and sedimentary rocks: ~66,000,000 GtC
Hydrosphere
Dissolved \(\text{CO}_2\) and carbonates in oceans: ~38,000 GtC
Why the Carbon Cycle Matters
1
Climate regulation: Atmospheric \(\text{CO}_2\) is a greenhouse gas that traps heat. Changes in the carbon cycle directly affect global temperatures.
2
Energy for life: Photosynthesis and cellular respiration are carbon-cycle processes that power virtually all ecosystems.
3
Ocean chemistry: Dissolved \(\text{CO}_2\) forms carbonic acid, affecting ocean pH and marine organisms that build calcium carbonate shells.
4
Human impact: Burning fossil fuels transfers geosphere carbon to the atmosphere far faster than natural sinks can absorb it.
The Carbon Cycle: Overview
Photosynthesis vs. Respiration
Photosynthesis
$$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$$
Plants, algae, and cyanobacteria remove \(\text{CO}_2\) from the atmosphere and convert it into organic carbon (glucose).
Cellular Respiration
$$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$$
All living organisms release \(\text{CO}_2\) back into the atmosphere by breaking down organic molecules for energy.
Fast vs. Slow: Two Carbon Cycles
| Fast (Biological) Cycle | Slow (Geological) Cycle | |
|---|---|---|
| Timescale | Days to decades | Thousands to millions of years |
| Key Processes | Photosynthesis, respiration, decomposition | Weathering, sedimentation, volcanism |
| Carbon Moved | ~120 GtC/year | ~0.1 GtC/year |
| Reservoirs | Atmosphere, biosphere | Geosphere, deep ocean |
| Human Impact | Deforestation, land-use change | Fossil fuel combustion |
One tagged carbon atom and one added carbon pulse tell different stories
Tracking one tagged carbon atom differs from tracking an added carbon pulse. M/F requires a defined stock, gross outflow and mixing assumptions; an added pulse redistributes among reservoirs over multiple timescales.Tagged-atom exchange time differs from persistence of a bulk carbon perturbation.
Tracking one tagged carbon atom differs from tracking an added carbon pulse. M/F requires a defined stock, gross outflow and mixing assumptions; an added pulse redistributes among reservoirs over multiple timescales.The Ocean Carbon Pump
The oceans absorb about 25% of annual human \(\text{CO}_2\) emissions through two major mechanisms:
Physical (Solubility) Pump
\(\text{CO}_2\) dissolves in cold surface water, forming carbonic acid: \(\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3\). Cold water sinks at the poles, carrying dissolved carbon to the deep ocean.
Biological Pump
Phytoplankton use photosynthesis to fix \(\text{CO}_2\) into organic matter. When they die, their remains sink to the deep ocean, transporting carbon downward for centuries.
The Geological Carbon Cycle
Sources vs. Sinks
Carbon Sources
Processes that release carbon into the atmosphere: combustion of fossil fuels, respiration, decomposition, volcanic eruptions, deforestation.
Carbon Sinks
Processes that absorb carbon from the atmosphere: photosynthesis, ocean dissolution, rock weathering, soil organic matter storage.
How to Analyze Carbon Flux
1
Identify reservoirs: List all carbon pools involved (atmosphere, ocean, soil, vegetation, fossil fuels).
2
Map fluxes: Draw arrows between reservoirs and label each with the process (photosynthesis, respiration, combustion, etc.).
3
Quantify rates: Assign flow rates in GtC/year. Compare inputs and outputs for each reservoir.
4
Calculate net change: If \(\text{inputs} > \text{outputs}\), the reservoir is a net sink. If \(\text{outputs} > \text{inputs}\), it is a net source.
5
Evaluate perturbations: Ask: "What happens if this flux increases or decreases?" Trace cascading effects through the system.
Example: Simple Carbon Budget
A forest ecosystem receives 12 GtC/year via photosynthesis, loses 9 GtC/year through respiration, and 1 GtC/year through decomposition. Is this forest a net source or sink?
Example: Ocean Acidification
Since the Industrial Revolution, ocean pH has dropped from 8.21 to 8.10. The relationship between dissolved \(\text{CO}_2\) and pH involves these reactions:
Example: Human Perturbation
Humans emit approximately 10 GtC/year from fossil fuels. Natural sinks absorb about 5.5 GtC/year (oceans: 2.5, land: 3.0). Where does the rest go?
Common Mistakes
✕ Confusing Reservoir and Flux
A reservoir is a pool of stored carbon (GtC). A flux is a rate of movement between reservoirs (GtC/year). Don't mix up stocks and flows.
✕ Ignoring the Ocean
Students often focus only on terrestrial processes. The ocean absorbs ~25% of human \(\text{CO}_2\) emissions and holds 50x more carbon than the atmosphere.
✕ "Plants Fix Everything"
While forests are carbon sinks, they also respire and decompose. A mature forest may be near carbon-neutral. Only growing forests are strong net sinks.
Carbon Cycle Summary
| Process | Direction | Timescale | Example |
|---|---|---|---|
| Photosynthesis | Atmos → Bio | Seconds | Trees, algae |
| Respiration | Bio → Atmos | Seconds | All organisms |
| Decomposition | Bio → Atmos/Soil | Days to years | Bacteria, fungi |
| Ocean Dissolution | Atmos → Ocean | Years | Surface uptake |
| Combustion | Geo → Atmos | Instant | Fossil fuels |
| Weathering | Atmos → Geo | 10⁴-10⁸ yrs | Silicate rocks |
| Volcanism | Geo → Atmos | Episodic | Eruptions |
Question 1 of 5
Which of the following is the largest reservoir of carbon on Earth?
The geosphere holds ~66,000,000 GtC in sedimentary rocks and fossil fuels, dwarfing all other reservoirs combined. The oceans hold ~38,000 GtC, the biosphere ~2,000 GtC, and the atmosphere ~830 GtC.
Question 2 of 5
Photosynthesis and cellular respiration are considered part of which carbon cycle?
Photosynthesis and respiration operate on timescales of seconds to decades, making them part of the fast (biological) carbon cycle. The slow (geological) cycle involves processes like weathering and volcanism over millions of years.
Question 3 of 5
If a forest absorbs 15 GtC/year through photosynthesis and releases 12 GtC/year through respiration and decomposition combined, what is its net carbon status?
Net change = inputs - outputs = \(15 - 12 = +3\) GtC/year. Since inputs exceed outputs, the forest is a net carbon sink, storing 3 GtC more than it releases each year.
Question 4 of 5
Why does burning fossil fuels disrupt the carbon cycle more significantly than burning wood?
Fossil fuels contain carbon that was removed from the atmosphere millions of years ago and stored in the geosphere (slow cycle). Burning them adds "ancient" carbon back to the fast cycle at a rate that natural sinks cannot match. Wood carbon, by contrast, was recently in the atmosphere and cycles on short timescales.
Question 5 of 5
Ocean acidification is primarily caused by which process?
When the ocean absorbs excess \(\text{CO}_2\), it forms carbonic acid (\(\text{H}_2\text{CO}_3\)), which dissociates into \(\text{H}^+\) and \(\text{HCO}_3^-\). The increase in \(\text{H}^+\) ions lowers the pH. This process has caused a ~30% increase in ocean acidity since pre-industrial times.