AP Environmental Science

## Learning Outcomes

- Define **natural sources of atmospheric carbon dioxide** within the carbon cycle
- Identify the main natural processes that release **CO2** to the atmosphere
- Explain how respiration, decomposition, ocean release, and volcanism add carbon dioxide
- Distinguish clearly between **sources** and **sinks** of carbon
- Explain why natural CO2 sources do not by themselves account for the modern long-term rise in atmospheric CO2
- Avoid common exam mistakes, especially the idea that volcanoes are the main cause of current CO2 increases

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand natural sources of atmospheric carbon dioxide, with a focus on the following syllabus points:

- Know that **CO2** is naturally emitted and absorbed as part of the **carbon cycle**
- Identify major natural CO2 sources, including respiration, decomposition, ocean release, volcanic outgassing, and natural wildfires
- Explain how **cellular respiration** returns carbon from living organisms to the atmosphere
- Understand that **decomposition** releases carbon from dead organic matter, often as CO2
- Explain that the ocean is both a **source** and a **sink** of CO2, depending on conditions
- Know that warmer ocean water holds less dissolved CO2, so warming can increase outgassing
- Understand that volcanic emissions are real but much smaller than modern human emissions
- Distinguish natural carbon cycling from **anthropogenic CO2 emissions**, which drive the current long-term increase

## 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. Name four natural sources of atmospheric carbon dioxide.
2. How does cellular respiration add CO2 to the atmosphere?
3. Why can the ocean act as both a carbon sink and a carbon source?
4. Why is it incorrect to say that volcanoes are the main cause of the modern rise in atmospheric CO2?
5. What is the difference between decomposition and soil respiration?

## Introduction

Atmospheric carbon dioxide is often discussed in the context of global warming, but for AP Environmental Science you must first understand that CO2 is a normal part of Earth’s systems. Long before humans burned coal, oil, and natural gas on a large scale, carbon dioxide was already moving constantly among the atmosphere, biosphere, hydrosphere, and geosphere.

The key exam idea is balance. Natural sources release CO2 into the atmosphere, while natural sinks remove it. For long periods, those flows were close enough to balance that atmospheric CO2 did not keep rising in the way it does today. That is why AP questions often ask you to identify natural sources of CO2 and then compare them with the modern human disturbance of the carbon cycle.

> **Key Term: carbon cycle**
> The movement of carbon among the atmosphere, oceans, soils, rocks, and living organisms through physical, chemical, and biological processes.

### Natural CO2 sources as part of the carbon cycle

A natural source of atmospheric CO2 is any naturally occurring process that releases more carbon dioxide to the air than it removes, without direct human action. The main natural sources you should know are **cellular respiration**, **decomposition**, **soil respiration**, **ocean-atmosphere exchange**, **volcanic outgassing**, and in some contexts **natural wildfires**.

> **Key Term: source**
> In environmental science, a source is a process or reservoir that releases more of a substance than it absorbs.
>
> **Key Term: sink**
> A sink is a process or reservoir that absorbs or stores more of a substance than it releases.

Students often lose marks because they list examples without showing how they work. On APES, “identify” may only need a list, but “explain” requires a process. For example, saying “respiration” is not enough if the prompt asks how carbon enters the atmosphere. You need to state that organisms break down organic molecules for energy and release CO2 as a product.

Natural sources matter because they show that atmospheric CO2 is not purely a human-made gas. However, this does not weaken the case for modern climate change. The exam expects you to hold both ideas at once:

- CO2 has important natural sources
- the current long-term increase is mainly caused by human activity

That distinction is central.

> **Test Tip:** If a question asks for a **natural source** of CO2, do not give **photosynthesis** or **ocean uptake**. Those are sinks, not sources.

### Biological sources: respiration, decomposition, and soil respiration

The most constant natural CO2 releases come from life itself. Organisms do not just store carbon; they also return it to the atmosphere.

**Cellular respiration** occurs in plants, animals, fungi, protists, and many microbes. Organisms break down glucose and other organic molecules to release usable energy. In that process, carbon that was once fixed into biomass is released as CO2. This is why forests are not only carbon sinks through photosynthesis; all the organisms in forests also respire.

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

This can confuse students because plants both remove and release CO2. During photosynthesis, plants take in CO2. During respiration, they release it. Over the full growing season, many plants are still net carbon sinks, but on short timescales they do both.

**Decomposition** is the return of carbon from dead organisms and wastes to the environment. Bacteria and fungi break down leaf litter, dead wood, dead animals, and other organic material. If oxygen is present, much of that carbon is released as CO2. If oxygen is absent, some may instead be released as methane, but for this topic the key idea is that decomposition is a natural atmospheric CO2 source.

> **Key Term: decomposition**
> The breakdown of dead organic material by decomposers such as bacteria and fungi, releasing nutrients and carbon back to the environment.

A closely related term is **soil respiration**. This is often tested because students confuse it with decomposition. Soil respiration includes both:

- respiration by plant roots
- respiration by soil organisms, especially microbes decomposing organic matter

So decomposition is part of soil respiration, but soil respiration is broader.

> **Key Term: soil respiration**
> The release of CO2 from soil, mainly from plant roots and soil microbes.

Here is the cause-and-effect chain students should be able to explain:

1. Plants fix carbon during photosynthesis.
2. Carbon moves into leaves, stems, roots, and food webs.
3. Organisms respire and release some of that carbon as CO2.
4. When organisms die, decomposers break down the organic matter.
5. Soil microbes and roots continue releasing CO2 from soils.

This is why soils can be major short-term sources of atmospheric CO2 even while natural systemssss store large total amounts of carbon.

A seasonal example helps. In the Northern Hemisphere, atmospheric CO2 often drops in spring and summer because photosynthesis increases. In autumn and winter, photosynthesis decreases, but respiration and decomposition continue, so atmospheric CO2 rises again. That annual rise-and-fall pattern reflects natural biological exchange.

> **Test Tip:** If asked to distinguish **decomposition** from **soil respiration**, say that decomposition is the breakdown of dead matter, while soil respiration includes both decomposition by microbes and respiration by living roots.

### Physical and geologic sources: ocean outgassing and volcanic emissions

Not all natural CO2 comes from living things. Some comes from physical exchange with water and from geologic processes deep within Earth.

The **ocean** is one of the largest carbon reservoirs on Earth. Carbon dioxide constantly moves between seawater and the atmosphere. This exchange goes in both directions. In some places and under some conditions, the ocean absorbs CO2. In others, it releases CO2 back to the air.

> **Key Term: ocean-atmosphere exchange**
> The two-way movement of carbon dioxide between seawater and the atmosphere.

Why does the ocean sometimes release CO2? A key rule is that **warmer water holds less dissolved gas than colder water**. As seawater warms, its ability to keep CO2 dissolved decreases, so some CO2 escapes to the atmosphere. This release is called **ocean outgassing**.

> **Key Term: ocean outgassing**
> The release of dissolved carbon dioxide from seawater into the atmosphere.

Ocean circulation also matters. Deep waters can store large amounts of dissolved carbon. When those deeper waters rise toward the surface through **upwelling**, some of that dissolved CO2 can be released into the air, especially if surface conditions favor outgassing. So temperature and circulation both affect whether a stretch of ocean behaves as a source or a sink.

A common mistake is to describe the ocean as only a sink. That is wrong. On the AP exam, better wording is:

- the ocean is a major carbon sink overall
- but some ocean regions also act as natural CO2 sources

The second major non-biological natural source is **volcanic outgassing**. Volcanoes and other geologic vents release carbon dioxide from Earth’s interior. Over geologic time, this is an important part of the carbon cycle because it moves carbon from the geosphere into the atmosphere.

> **Key Term: volcanic outgassing**
> The release of carbon dioxide and other gases from volcanoes and geologic vents.

Volcanic CO2 matters for long-term Earth history, but students must not overstate its present role. A very common misconception is that volcanoes emit more CO2 than humans. They do not. Official scientific summaries state that modern volcanic emissions are far smaller than human emissions. Volcanoes are a real natural source, but not the main driver of the modern CO2 rise.

This distinction is good exam material because it combines factual recall with evaluation. If a multiple-choice option says modern atmospheric CO2 is rising mainly because of volcanoes, eliminate it.

You may also see **natural wildfires** listed as a natural CO2 source. This is acceptable in APES framing, as long as the context is careful. Fire combusts plant biomass, quickly releasing stored carbon to the atmosphere as CO2. However, many modern fire patterns also have human influences through land management and climate change, so use wildfire examples only when the question clearly asks for natural causes.

### Why natural sources do not explain the modern long-term CO2 rise

This is the most tested distinction in this subtopic. Natural sources of CO2 are large, real, and constant. So why are they not the main explanation for the steady modern increase in atmospheric CO2?

Because natural sources are paired with natural sinks.

Photosynthesis removes atmospheric CO2. Oceans absorb CO2. Soils, forests, peat, and sediments store carbon. Before major human disruption, natural emissions and natural removals were much closer to equilibrium over long periods. Seasonal and short-term fluctuations still happened, but there was no comparable long-term upward trend like the one recorded since the Industrial Revolution.

The modern problem is that human activity adds extra CO2 on top of the natural cycle. Burning fossil fuels releases carbon that had been stored underground for millions of years. Deforestation reduces carbon sinks and often releases stored carbon as biomass is burned or decomposes. Natural sinks still absorb a substantial share of this added CO2, but not enough to keep atmospheric concentrations stable.

This is the logic chain you should be able to write in an FRQ:

1. Natural sources such as respiration, decomposition, ocean release, and volcanoes emit CO2.
2. Natural sinks such as photosynthesis and ocean uptake remove CO2.
3. These natural fluxes were roughly balanced over long timescales.
4. Human activities now add extra CO2 beyond that natural balance.
5. Therefore atmospheric CO2 rises over time.

You do not need exact numbers unless the question provides them, but relative scale matters. For example, volcanic emissions are much smaller than anthropogenic emissions, so “volcanoes caused the modern rise” is not a supported conclusion.

Another important point is timescale. Natural fluxes can be large and still not produce a long-term rise if removal is also large. Students sometimes assume “large source” automatically means “cause of increase.” That is incorrect. An increase happens when **inputs exceed outputs over time**.

A simple analogy: a bathtub can have a lot of water entering from the tap and a lot leaving through the drain. The water level only rises if more enters than leaves. The atmosphere works similarly.

> **Test Tip:** In free-response questions, use the phrase “**natural sources and natural sinks were roughly balanced**” to explain why natural CO2 sources alone do not account for the recent steady increase.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Natural sources of atmospheric CO2 are normal parts of the carbon cycle
- Major natural CO2 sources include cellular respiration, decomposition, soil respiration, ocean outgassing, volcanic outgassing, and natural wildfires
- Cellular respiration releases CO2 when organisms break down organic molecules for energy
- Decomposition returns carbon from dead organic matter to the atmosphere, often as CO2
- Soil respiration includes both root respiration and microbial activity in soils
- The ocean is both a carbon sink and a carbon source depending on temperature, circulation, and concentration gradients
- Warmer water holds less dissolved CO2, so warming can increase ocean outgassing
- Volcanic emissions are a real natural CO2 source but are much smaller than modern human emissions
- Photosynthesis and ocean uptake are carbon sinks, not sources
- Natural sources do not explain the modern long-term CO2 rise because natural sources and sinks were closer to balance before major human disruption
- Human activities add extra CO2 beyond the natural balance, causing atmospheric concentrations to rise
- A large natural flux does not by itself explain an increase; what matters is whether inputs exceed outputs over time

## Key Terms and Concepts

- carbon cycle
- source
- sink
- cellular respiration
- decomposition
- soil respiration
- ocean-atmosphere exchange
- ocean outgassing
- volcanic outgassing


Atmospheric CO2 and Particulates - Natural sources of carbon dioxide