AP Environmental Science

## Learning Outcomes

- Explain why oceans absorb a large share of atmospheric carbon dioxide
- Describe step by step how dissolved carbon dioxide changes seawater chemistry
- Understand how more hydrogen ions lower pH and reduce carbonate ion availability
- Interpret why a small pH drop represents a major chemical change
- Explain why calcifying organisms such as corals, oysters, and plankton are vulnerable
- Distinguish ocean acidification from ocean warming and other ocean changes
- Apply this content to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand how oceans absorb atmospheric carbon dioxide, with a focus on the following syllabus points:

- Know that the ocean absorbs about **30%** of the carbon dioxide released to the atmosphere
- Explain that ocean acidification is caused mainly by atmospheric **CO2** dissolving into seawater
- Identify the main reaction sequence: **CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-**
- Understand that added **H+** lowers pH and also converts **carbonate ions** into **bicarbonate**
- Explain why reduced **carbonate ion** availability makes shell formation harder for marine calcifiers
- Know that average surface ocean pH has fallen by about **0.1 pH units** since the Industrial Revolution, meaning about a **30% increase in acidity**
- Distinguish between “more dissolved inorganic carbon overall” and “less carbonate specifically”
- Understand that seawater usually remains slightly basic even while becoming less basic over time

## 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. Why do oceans absorb atmospheric carbon dioxide in the first place?
2. What happens chemically when carbon dioxide dissolves in seawater?
3. Why does ocean acidification reduce carbonate ion availability?
4. Why are corals, oysters, and calcareous plankton especially vulnerable?
5. Why does a drop of only 0.1 pH units matter so much?

## Introduction

Ocean acidification is one of the clearest links between the atmosphere and the oceans. When humans burn fossil fuels and alter land use, atmospheric carbon dioxide rises. The ocean takes up a large fraction of that carbon dioxide, which slows the buildup in the atmosphere. That might sound helpful, but the absorbed CO2 changes seawater chemistry in ways that create serious problems for marine life.

For AP Environmental Science, you need more than the simple idea that “CO2 lowers pH.” You need the full cause-and-effect chain: more atmospheric CO2 leads to more dissolved CO2 in seawater, which forms carbonic acid, increases hydrogen ion concentration, lowers pH, and reduces the supply of carbonate ions needed by many organisms to build calcium carbonate shells and skeletons.

> **Key Term: ocean acidification**
> The ongoing decrease in seawater pH caused mainly by the absorption of atmospheric carbon dioxide.

## Introduction to the chemistry of ocean CO2 uptake

### Why oceans absorb atmospheric carbon dioxide

The ocean is a major **carbon sink**. Carbon dioxide in the atmosphere and carbon dioxide dissolved in surface seawater are constantly exchanged. If atmospheric CO2 rises, more of it tends to move into the ocean. This is why the oceans absorb roughly **30%** of human-released carbon dioxide.

That uptake matters in two ways. First, it slows how quickly atmospheric CO2 rises. Second, it changes the chemistry of seawater. On AP questions, students often get credit for saying the ocean absorbs CO2, but stronger answers explain that this is part of the global carbon cycle and that the cost of this uptake is acidification.

> **Key Term: carbon sink**
> A reservoir that absorbs and stores more carbon than it releases over a given period.

Since the Industrial Revolution, atmospheric CO2 has risen sharply from about **280 ppm** before industrialization to much higher modern levels. As atmospheric concentration rises, the ocean's dissolved CO2 also rises. This is why ocean acidification is directly tied to human greenhouse gas emissions.

A common mistake is to think ocean acidification is mainly caused by acid rain. Acid rain can affect some waters, especially freshwater systems, but the main driver of ocean acidification is **atmospheric CO2 absorption**.

> **Test Tip:** If an APES question asks for the best long-term solution to ocean acidification, the strongest answer is usually **reducing CO2 emissions**, not just reducing local water pollution.

### What happens when CO2 dissolves in seawater

Once carbon dioxide enters seawater, it does not stay in just one form. It reacts with water and enters a series of reversible reactions:

$$
\mathrm{CO_2 + H_2O \rightleftharpoons H_2CO_3}
$$

$$
\mathrm{H_2CO_3 \rightleftharpoons H^+ + HCO_3^-}
$$

This means dissolved carbon dioxide forms **carbonic acid**, and some of that carbonic acid dissociates into **hydrogen ions** and **bicarbonate ions**.

> **Key Term: carbonic acid**
> A weak acid, H2CO3, formed when carbon dioxide dissolves in water.
>
> **Key Term: hydrogen ion**
> A positively charged ion, H+, whose concentration determines how acidic a solution is.

The rise in hydrogen ions is what lowers pH. That is the acidification step. But for many marine organisms, the more biologically serious issue is what those hydrogen ions do next.

## How acidification reduces carbonate ions

### The crucial shift from carbonate to bicarbonate

Hydrogen ions do not just make the ocean more acidic. They also react with carbonate ions already present in seawater:

$$
\mathrm{H^+ + CO_3^{2-} \rightleftharpoons HCO_3^-}
$$

This means extra hydrogen ions convert **carbonate ions** into **bicarbonate ions**. So even though the total dissolved inorganic carbon in seawater goes up, the amount of carbonate available for shell-building goes down.

That is a distinction students often miss. There is **more carbon in the water overall**, but **less carbonate specifically**.

> **Key Term: carbonate ion**
> The ion CO3^2-, needed by many marine organisms to build calcium carbonate shells and skeletons.
>
> **Key Term: bicarbonate ion**
> The ion HCO3-, the dominant dissolved inorganic carbon form in normal seawater.

In normal seawater, most dissolved inorganic carbon is present as bicarbonate, a smaller portion as carbonate, and a very small portion as dissolved CO2 plus carbonic acid. When more atmospheric CO2 dissolves, bicarbonate increases further, while carbonate declines.

This matters because many marine organisms do not build shells directly from bicarbonate. They rely on carbonate ions combining with calcium ions:

$$
\mathrm{Ca^{2+} + CO_3^{2-} \rightleftharpoons CaCO_3}
$$

> **Key Term: calcium carbonate**
> CaCO3, the compound used by many organisms to build shells and skeletons.

So the full chain is:

- more atmospheric CO2
- more dissolved CO2 in seawater
- more carbonic acid
- more H+
- lower pH
- less CO3^2-
- harder shell formation

That sequence is central to APES.

### Why a small pH drop is a big deal

Students sometimes assume a pH decline of 0.1 is tiny. Chemically, it is not. The pH scale is logarithmic. A drop of about **0.1 pH units** since preindustrial times corresponds to roughly a **30% increase in acidity**, meaning about a 30% increase in hydrogen ion concentration.

This is exactly the kind of quantitative reasoning AP Environmental Science likes to test. A small-looking change on a pH scale can represent a major chemical shift.

For example, if pH falls, it does not mean the ocean has become acidic in the everyday sense below pH 7. Most seawater is still slightly basic. What has happened is that it has become **less basic**. That wording matters.

> **Key Term: pH**
> A logarithmic measure of hydrogen ion concentration; lower pH means greater acidity.
>
> **Test Tip:** Avoid writing “the ocean becomes acidic” unless a question is very general. A more precise APES answer is “the ocean becomes **more acidic** or **less basic** as pH decreases.”

## Biological consequences of lower carbonate availability

### Why shell-building organisms are at risk

Organisms such as corals, clams, oysters, sea urchins, and calcareous plankton need carbonate ions to build and maintain calcium carbonate structures. When carbonate ion concentration falls, calcification becomes more difficult.

This can lead to:

- slower shell or skeleton growth
- weaker shells
- greater energy costs for maintaining shell formation
- increased vulnerability during larval stages
- dissolution if water becomes chemically unfavorable enough

Corals are a major example because coral reefs support extremely high biodiversity and provide habitat, coastal protection, and food resources. If coral calcification slows, the reef structure becomes harder to build and maintain.

Plankton matter too. Students often focus only on corals and shellfish, but many forms of calcareous plankton are part of the base of marine food webs. If these populations are harmed, effects can move upward through the food chain.

This is why ocean acidification can affect human life even if humans never see the chemistry directly. It threatens fisheries, reef habitatss, and food supply for communities that rely heavily on marine protein.

### Saturation state and shell dissolution

A more advanced but useful concept is **aragonite saturation state**. Aragonite is a form of calcium carbonate used by many organisms. Its saturation state depends heavily on carbonate ion concentration.

> **Key Term: aragonite saturation state**
> A measure of how favorable seawater conditions are for forming or maintaining aragonite shells and skeletons.

If saturation state is above 1, shell formation is more favorable. If it falls below 1, dissolution is favored. For APES, you do not need deep chemistry detail, but you should know the basic logic: lower carbonate ion concentration lowers saturation state, making calcification harder and dissolution more likely.

Some ocean regions are naturally closer to this threshold already. Cold, deep waters can be more acidic, so calcifying organisms often survive only above a certain depth. This is linked to the **saturation horizon**, the depth below which calcium carbonate structures are more likely to dissolve.

As acidification progresses, that horizon can move closer to the surface. This means organisms lose safe habitat even if they stay in the same ocean.

## Why acidification varies and why it links to other global changes

### Why some regions are more vulnerable

Ocean acidification is global, but it is not uniform. Cold waters absorb more CO2, so polar regions are especially vulnerable. The Arctic and Southern Ocean are therefore major concern areas.

Coastal waters can also show strong variation because they are influenced not only by atmospheric CO2, but by runoff, respiration, upwelling, and local pollution. In some coastal systems, acidification can be more intense or more variable than in the open ocean.

Upwelling regions can also face strong acidification because deep water brought to the surface may already contain high dissolved CO2. When this combines with additional atmospheric uptake, surface organisms may face stressful chemistry faster.

### Ocean acidification is not the same as ocean warming

Ocean acidification and ocean warming are related because both are driven by rising atmospheric CO2 and broader climate change, but they are not the same process.

- **Ocean warming** is an increase in ocean temperature caused by the buildup of greenhouse gases
- **Ocean acidification** is a drop in ocean pH caused by CO2 dissolving into seawater

They often happen together, which can make impacts worse. For example, coral reefs are hit both by acidification, which reduces calcification, and by warming, which contributes to coral bleaching. Some fish may also show behavioral changes under more acidic conditions, such as reduced ability to detect predators.

That co-occurrence is important in FRQs. If asked to explain marine food webs impacts, strong answers often connect multiple stressors rather than treating acidification in isolation.

### How to structure an APES FRQ answer on this topic

A strong free-response answer usually follows a clean sequence:

1. state that oceans absorb atmospheric CO2
2. show the chemical reaction or describe carbonic acid formation
3. explain that H+ increases and pH decreases
4. explain that H+ reacts with carbonate, lowering CO3^2-
5. connect reduced carbonate to harder shell formation
6. name affected organisms or environment consequences

For example, a compact FRQ sentence might say:

“Atmospheric CO2 dissolves into seawater, forming carbonic acid, which releases H+ ions; those hydrogen ions lower pH and bind with carbonate ions, reducing the carbonate available for corals and shellfish to build calcium carbonate shells.”

That is far better than only writing “more CO2 makes the ocean acidic.”

> **Test Tip:** On FRQs, include at least one chemical step and one biological consequence. Chemistry alone is incomplete; biology alone often misses the mechanism.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Oceans absorb about **30%** of atmospheric carbon dioxide released by human activity
- Ocean acidification is the ongoing decrease in seawater pH caused mainly by atmospheric **CO2** uptake
- Dissolved CO2 reacts with water to form **carbonic acid**
- Carbonic acid releases **hydrogen ions** and **bicarbonate ions**
- More hydrogen ions lower pH and make seawater more acidic
- Hydrogen ions also react with **carbonate ions**, reducing carbonate availability
- Many marine organisms need carbonate ions to form **calcium carbonate** shells and skeletons
- Corals, oysters, clams, sea urchins, and calcareous plankton are especially vulnerable
- Average surface ocean pH has fallen by about **0.1 pH units** since preindustrial times, meaning about a **30% increase in acidity**
- Ocean acidification does not mean the whole ocean is now below pH 7; it usually remains slightly basic but becomes less basic
- Polar and some coastal waters can be especially vulnerable because of colder water, upwelling, and local chemical variation
- The best long-term solution is reducing atmospheric **CO2 emissions**

## Key Terms and Concepts

- ocean acidification
- carbon sink
- carbonic acid
- hydrogen ion
- carbonate ion
- bicarbonate ion
- calcium carbonate
- pH
- aragonite saturation state


Ocean Acidification - Oceans absorb atmospheric carbon dioxide