AP Environmental Science

## Learning Outcomes

- Understand how carbon moves among the atmosphere, biosphere, hydrosphere, and geosphere
- Identify the main carbon reservoirs and distinguish them from carbon fluxes
- Explain how photosynthesis, respiration, decomposition, burial, sedimentation, and combustion transfer carbon
- Compare short-term carbon cycling with long-term carbon cycling
- Describe how ocean uptake of carbon dioxide changes seawater chemistry
- Apply residence time, source, sink, and sequestration to AP-style questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand carbon movement between reservoirs, with a focus on the following syllabus points:

- Know that carbon cycles among the **atmosphere**, **biosphere**, **hydrosphere**, and **geosphere**
- Identify the difference between a **carbon reservoir** and a **carbon flux**
- Explain how **photosynthesis** moves carbon from the atmosphere to biomass
- Explain how **cellular respiration**, **decomposition**, and **combustion** return carbon to the atmosphere
- Understand how **burial**, **sedimentation**, and shell formation move carbon into long-term geologic storage
- Explain atmosphere-ocean carbon exchange and why the ocean is a major carbon reservoir
- Understand that increased ocean uptake of atmospheric carbon dioxide causes **ocean acidification**
- Compare rapid biological transfers with much slower geologic transfers

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between a carbon reservoir and a carbon flux?
2. How does carbon move from the atmosphere into the biosphere?
3. Name two processes that move carbon from the biosphere back to the atmosphere.
4. How does carbon enter long-term geologic storage?
5. Why can the ocean both absorb carbon dioxide and release it?

## Introduction

The carbon cycle is one of the most tested biogeochemical cycles in AP Environmental Science because it connects natural systemss, climate, ocean chemistry, and human impacts. Students often memorize a diagram but then struggle when asked to trace a specific pathway, such as how carbon moves from fossil fuels to the atmosphere, or from seawater to marine sediments.

For this topic, the main task is not just naming parts of the cycle. You need to explain **where** carbon is stored, **how** it moves, and **why** some transfers happen quickly while others take thousands or millions of years. That distinction helps with multiple-choice questions, graph interpretation, and free-response explanations.

> **Key Term: carbon cycle**
> The movement of carbon among the atmosphere, biosphere, hydrosphere, and geosphere through biological, chemical, and geologic processes.

### Carbon reservoirs, fluxes, and residence time

To understand carbon movement, start with the idea of storage. Carbon does not stay in one place. It is held in different parts of the Earth system and moves between them at different rates.

> **Key Term: carbon reservoir**
> A storage location where carbon is held for a period of time, such as the atmosphere, oceans, soils, plants, sediments, rocks, or fossil fuels.

The four major APES reservoirs are:

- **Atmosphere**: mainly carbon dioxide, with smaller amounts of methane
- **Biosphere**: living organisms and recently dead organic matter
- **Hydrosphere**: oceans, lakes, and dissolved carbon in water
- **Geosphere**: soils, sediments, sedimentary rock, carbonate rock, and fossil fuels

A common mistake is to confuse the size of a reservoir with the speed of exchange. A very large reservoir does not always exchange carbon rapidly. For example, sedimentary rock stores huge amounts of carbon, but most of that carbon moves very slowly.

> **Key Term: carbon flux**
> The rate at which carbon moves from one reservoir to another.

So a reservoir is the **where**, and a flux is the **how fast**. On APES questions, if you see arrows on a carbon cycle diagram, the boxes are usually reservoirs and the arrows represent fluxes.

Another useful idea is residence time.

> **Key Term: residence time**
> The average time carbon stays in a reservoir before moving out; calculated as reservoir size divided by total flux out.

This matters because atmospheric carbon may cycle in years to centuries, vegetation in years to decades, but geologic carbon can remain stored for millions of years. That is why moving carbon from fossil fuels into the atmosphere is such a major problem: it shifts carbon from a long-term reservoir into a much faster cycle.

> **Test Tip:** If a question asks which reservoir is largest, think **marine sediments and sedimentary rock**. If it asks which exchanges carbon quickly, think **atmosphere, vegetation, surface ocean, and soils**.

### Fast carbon movement: atmosphere, biosphere, and surface waters

The short-term carbon cycle is driven mainly by living organisms and surface processes. These transfers happen over days, seasons, years, or decades rather than geologic time.

The first major pathway is atmosphere to biosphere through photosynthesis. Plants, algae, and phytoplankton take in carbon dioxide and use sunlight to build sugars.

$$
6CO_2 + 6H_2O + \text{sunlight} \rightarrow C_6H_{12}O_6 + 6O_2
$$

This means atmospheric carbon becomes plant biomass. Leaves, stems, roots, wood, and stored sugars all contain that carbon. When herbivores eat plants, the carbon moves through food webs into animal tissue.

> **Key Term: photosynthesis**
> The process by which producers use carbon dioxide, water, and sunlight to make glucose, storing carbon in biomass.

The reverse movement is biosphere to atmosphere through cellular respiration. Plants, animals, and decomposers all break down glucose and release carbon dioxide.

$$
C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}
$$

So even forests that remove carbon by photosynthesis also release some through respiration. What matters is the balance between carbon taken in and carbon released.

> **Key Term: cellular respiration**
> The process by which organisms break down organic molecules and release carbon dioxide, water, and energy.

Decomposition is another major fast transfer. When organisms die, decomposers such as bacteria and fungi break down their remains. In oxygen-rich conditions, much of that carbon returns to the atmosphere as carbon dioxide. In oxygen-poor conditions, such as waterlogged soils, wetlands, or some sediments, decomposition can release methane.

This is why wetlands are interesting in APES. They can store carbon in soils, but anaerobic conditions can also produce methane. Both ideas can be true at once.

So far, the short-term biological pathways are:

- **Atmosphere → Biosphere** by photosynthesis
- **Biosphere → Atmosphere** by respiration
- **Dead biomass → Atmosphere/Soil** by decomposition
- **Plant biomass → Animals** by consumption in food webs

The ocean also participates in this fast cycle. Carbon dioxide dissolves from the air into surface seawater, and some dissolved carbon can later leave the water and return to the air. So the movement is two-way.

> **Key Term: source**
> A part of the carbon cycle that releases more carbon than it absorbs.
>
> **Key Term: sink**
> A part of the carbon cycle that absorbs or stores more carbon than it releases.

A forest can be a sink if photosynthesis exceeds respiration and decomposition. An ocean region can be a sink if more carbon dioxide dissolves in it than escapes from it.

> **Test Tip:** In FRQs, do not just say “carbon moves through the carbon cycle.” Name the process: **photosynthesis**, **respiration**, **decomposition**, **diffusion**, **combustion**, **burial**, or **sedimentation**.

### Long-term carbon movement: soils, sediments, rock, and fossil fuels

The long-term carbon cycle matters because it explains how carbon becomes locked away for very long periods. The main idea is burial. If dead organic matter is not fully decomposed, it can be covered by sediment and stored.

This can happen in peat-forming wetlands, marine sediments, and other low-oxygen environments. Over very long timescales, buried organic matter may be transformed by heat and pressure into coal, oil, or natural gas.

That means carbon that once moved quickly through food webs can become part of a very slow geologic reservoir.

Another important pathway is in the ocean. Marine organisms such as corals, shellfish, and some plankton combine calcium with carbonate to form calcium carbonate shells or skeletons. When these organisms die, their shells sink. This is sedimentation.

> **Key Term: sedimentation**
> The settling and accumulation of particles, including carbon-rich shells and organic matter, in sediments.

If those sediments are buried and compacted over time, they can form limestone and other carbonate rocks. This is one reason sedimentary deposits are such a huge carbon reservoir.

Students often blur sedimentation and burial together. They are related but not identical. Sedimentation is the settling and accumulation step. Burial is the longer-term storage after material is covered and isolated.

Weathering is another slower pathway. Atmospheric carbon dioxide can dissolve in water and participate in chemical reactions that help break down rock. Over long periods, weathering can help move carbon into dissolved forms that eventually reach oceans and contribute to carbonate formation.

Finally, geologic carbon can return to the atmosphere through volcanic outgassing. Carbon stored deep in Earth materials is released as carbon dioxide during volcanic activity. This is a natural source, but on modern timescales it is much smaller than human fossil fuel combustion.

### Ocean-atmosphere exchange and ocean acidification

The ocean is one of the most important carbon reservoirs because carbon dioxide dissolves in water easily. But ocean carbon is not stored mostly as dissolved CO₂ gas. Most dissolved inorganic carbon in seawater is present as bicarbonate.

> **Key Term: dissolved inorganic carbon**
> Carbon dissolved in water mainly as carbon dioxide, bicarbonate, and carbonate ions.

When atmospheric carbon dioxide dissolves in seawater, it reacts with water:

$$
CO_2 + H_2O \rightarrow H_2CO_3
$$

Carbonic acid then contributes hydrogen ions and bicarbonate:

$$
H_2CO_3 \rightarrow H^+ + HCO_3^-
$$

The increase in hydrogen ions lowers pH. That is ocean acidification.

> **Key Term: ocean acidification**
> The decrease in ocean pH caused by increased absorption of atmospheric carbon dioxide.

The chemistry matters because added hydrogen ions also reduce carbonate availability. Carbonate ions are needed by many marine organisms to build calcium carbonate shells and skeletons.

$$
CO_2 + H_2O + CO_3^{2-} \rightleftharpoons 2HCO_3^-
$$

So as atmospheric carbon dioxide rises, seawater generally has:

- more dissolved carbon
- lower pH
- less available carbonate for calcifying organisms

That can weaken coral reef building, shell formation, and some plankton populations. Since plankton are part of marine food webs and carbon cycling, this has wider ecological effects.

A subtle point students miss: the ocean is both a reservoir and a sink, but not always in every place or at every time. Cold water tends to absorb gases more effectively than warm water. If oceans warm, they can become less effective at absorbing carbon dioxide and may release more back to the atmosphere.

This creates a positive feedback:

1. atmospheric CO₂ rises
2. climate warms
3. warmer ocean holds less gas
4. less CO₂ is absorbed, or more is released
5. atmospheric CO₂ rises further

### Human changes to carbon movement between reservoirs

Before strong industrial disruption, major natural carbon fluxes were much closer to balance. Carbon moved rapidly between air, plants, soils, and oceans, but inputs and outputs were more nearly matched.

Human activity changes that balance mainly by accelerating transfer from long-term reservoirs into the atmosphere.

The two biggest pathways are:

- **Fossil fuel combustion**: geosphere → atmosphere
- **Deforestation and land-use change**: biosphere → atmosphere

Combustion rapidly oxidizes carbon in coal, oil, natural gas, and biomass, releasing carbon dioxide. Extraction alone does not move most carbon into the atmosphere; the major transfer occurs when the fuel is burned.

Deforestation does two things at once:

- it releases carbon stored in biomass and often soils
- it reduces future carbon uptake by photosynthesis

So cutting and burning forests is a double impact. Carbon leaves a sink, and the sink itself gets smaller.

Soil disturbance also matters. Deep plowing, land clearing, mining, and drainage of peat soils can increase soil respiration and erosion, releasing stored carbon.

A good APES summary of the human problem is this: humans are moving carbon from **slow reservoirs** into **fast reservoirs** much more quickly than natural long-term removal processes can balance.

> **Key Term: carbon sequestration**
> The long-term removal and storage of carbon in biomass, soils, oceans, or geologic formations.

Sequestration is a process, not a place. Forests, soils, wetlands, deep ocean waters, sediments, and rock formations can all be involved in sequestration.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Carbon moves among the atmosphere, biosphere, hydrosphere, and geosphere
- A carbon reservoir is a storage location, while a carbon flux is the rate of transfer
- Photosynthesis moves carbon from the atmosphere into biomass
- Cellular respiration returns carbon from organisms to the atmosphere
- Decomposition moves carbon from dead matter into soils and back to the atmosphere or methane-producing conditions
- Atmosphere-ocean exchange is two-way and occurs by dissolution and release
- Most ocean dissolved carbon is stored mainly as bicarbonate, not just dissolved carbon dioxide
- Sedimentation and burial move carbon into long-term storage in sediments, fossil fuels, and rock
- Combustion transfers carbon rapidly from fossil fuels to the atmosphere
- Volcanic outgassing returns some geologic carbon to the atmosphere
- Ocean acidification lowers pH and reduces carbonate availability for shell-building organisms
- Human activity shifts carbon from long-term reservoirs into short-term atmospheric storage faster than natural removal can keep up

## Key Terms and Concepts

- carbon cycle
- carbon reservoir
- carbon flux
- residence time
- photosynthesis
- cellular respiration
- source
- sink
- sedimentation
- dissolved inorganic carbon
- ocean acidification
- carbon sequestration


The Carbon Cycle - Carbon movement between reservoirs