Ocean Acidification Reading Answers

IELTS Reading is one part of the four-part test, alongside Listening, Writing, and Speaking. In this section, you read a passage and answer questions that check whether you actually understood it, not just skimmed it.

Ocean Acidification is one of the passages you'll come across in both practice sets and real tests, and it's a common one to search for when you're looking up Ocean Acidification IELTS reading answers.  

It follows biologist Victoria Fabry, who noticed something odd happening to tiny sea creatures called pteropods in her research jars. From there, the passage widens out into coral reefs, a mass extinction millions of years ago, and a strange possible upside involving clouds.

The passage has ten short paragraphs in total. Between the science and the dates, there's a fair bit to track, but once you know the structure, it gets much easier to follow.

Here's the answer key for quick reference:

Question	Answer	Question Type
1	(small) flaps	Short Answer
2	(their/the) shells	Short Answer
3	(about) 1/3 // a third	Short Answer
4	rocks (on land)	Short Answer
5	(over) 100,000 years	Short Answer
6	fishing and tourism	Short Answer
7	coral(s)	Short Answer
8	microbes	Flow-chart Completion
9	(the) atmosphere	Flow-chart Completion
10	clouds	Flow-chart Completion
11	cooler	Flow-chart Completion
12	global warming	Flow-chart Completion
13	C	Multiple Choice

Before you check the academic reading Ocean Acidification answers below, try the full practice passage first. Once you've attempted it, come back to this key and check your work against the explanations further down, so you understand why each answer is correct and not just what it is.

Want to understand the complete IELTS test before practising Reading passages? Check the IELTS Exam page for the test format, sections, duration, and key details.

Ocean Acidification IELTS Reading Passage for Practice

This IELTS Reading Test passage explains ocean acidification, which is a real environmental issue and not something invented for the exam, and it's the same text used in Achieve IELTS Test 4 reading Ocean Acidification answers and several other practice sets. When CO2 enters seawater, it reacts and forms an acid, and over time, this makes the ocean more acidic than it used to be.

Why does this matter? A lot of sea life depends on stable water chemistry to survive:

• Corals build their skeletons from calcium carbonate.
• Pteropods, tiny sea creatures, build their shells the same way.
• Once the water turns more acidic, these shells and skeletons start to dissolve.

The passage walks through how scientists first noticed this, why it's happening faster than anyone expected, and what it could eventually mean for reefs, fishing, and even the wider climate.

The passage below is paraphrased. So it's not word for word the original. But it keeps the same facts and the same meaning.

Here's a tip before you start. Scan the questions first. Look for numbers, dates, and names. They help you find answers faster later. Now read the passage below. Take your time with it.

Ocean Acidification IELTS Reading Passage for Practice

Paragraph A: A few years ago, biologist Victoria Fabry saw the future of the world’s oceans in a jar. She was aboard a research ship in the North Pacific, carrying out experiments on a species of pteropod – small molluscs with shells up to a centimetre long, which swim in a way that resembles butterfly flight, propelled by small flaps. Something strange was happening in Fabry’s jars. ‘The pteropods were still swimming, but their shells were visibly dissolving,’ says Fabry. She realised that the animals’ respiration had increased the carbon dioxide (CO2) in the jars, which had been scaled for 48 hours, changing the water’s chemistry to a point where the calcium carbonate in the pteropods’ shells had started to dissolve. What Fabry had stumbled on was a hint of ‘the other CO2 problem’.

Paragraph B: It has taken several decades for climate change to be recognised as a serious threat. But another result of our fossil-fuel habit – ocean acidification – has only begun to be researched in the last few years. Its impact could be momentous, says Joanie Kleypas of the National Centre for Atmospheric Research in Boulder Colorado.

Paragraph C: CO2 forms carbonic acid when it dissolves in water, and the oceans are soaking up more and more of it. Recent studies show that the seas have absorbed about a third of all the fossil-fuel carbon released into the atmosphere since the beginning of the Industrial Revolution in the mid-eighteenth century, and they will soak up much more over the next century. Yet until quite recently, many people dismissed the idea that humanity could alter the acidity of the oceans, which cover 71% of the planet’s surface to an average depth of about four kilometres. The ocean’s natural buffering capacity was assumed to be capable of preventing any changes in acidity even with a massive increase in CO2 levels.

Paragraph D: And it is – but only if the increase happens slowly, over hundreds of thousands of years. Over this timescale, the release of carbonates from rocks on land and from ocean sediments can neutralise the dissolved CO2, just like dropping chalk in an acid. Levels of CO2 are now rising so fast that they are overwhelming the oceans’ buffering capacity.

Paragraph E: In 2003 Ken Caldeira of the Carnegie Institution in Stanford and Michael Wickett at the Lawrence Livermore National Laboratory calculated that the absorption of fossil CO2 could make the oceans more acidic over the next few centuries than they have been for 300 million years, with the possible exception of rare catastrophic events. The potential seriousness of the effect was underlined in 2005 by the work of James Zachos of the University of California and his colleagues, who studied one of those rare catastrophic events. They showed that the mass extinction of huge numbers of deep-sea creatures around 55 million years ago was caused by ocean acidification after the release of around 4500 giga-tonnes of carbon. It took over 100,000 years for the oceans to return to their normal state.

Paragraph F: Around the same time as the Zachos paper, the UK’s Royal Society published the first comprehensive report on ocean acidification. It makes grim reading, concluding that ocean acidification is inevitable without drastic cuts in emissions. Marine ecosystems, especially coral reefs, are likely to be affected, with fishing and tourism-based around reefs losing billions of dollars each year. Yet the report also stressed that there is huge uncertainty about the effects on marine life.

Paragraph G: The sea creatures most likely to be affected are those that make their shells or skeletons from calcium carbonate, including tiny plankton and huge corals. Their shells and skeletons do not dissolve only because the upper layers of the oceans are supersaturated with calcium carbonate. Acidification reduces carbonate ion concentrations, making it harder for organisms to build their shells or skeletons. When the water drops below the saturation point, these structures will start to dissolve. Calcium carbonate comes in two different forms, aragonite and calcite, aragonite being more soluble. So organisms with aragonite structures such as corals will be hardest hit.

Paragraph H: So far the picture looks relentlessly gloomy, but could there actually be some positive results from adding so much CO2 to the seas? One intriguing finding, says Ulf Riebesell of the Leibniz Institute of Marine Sciences in Kiel Germany, concerns gases that influence climate. A few experiments suggest that in more acidic conditions, microbes will produce more volatile organic compounds such as dimethyl sulphide, some of which escapes to the atmosphere and causes clouds to develop. More clouds would mean cooler conditions, which could potentially slow global warming.

Paragraph I: Calculating the effect of ocean acidification on people and economies is virtually impossible, but it could be enormous. Take the impact on tropical corals, assuming that warming and other pressures such as pollution do not decimate them first. Reefs protect the shorelines of many countries. Acidification could start eating away at reefs just when they are needed more than ever because of rising sea levels.

Paragraph J: ‘No serious scientist believes the oceans will be devoid of life,’ says Caldeira. ‘Wherever there is light and nutrients, something will live. A likely outcome will be a radical simplification of the ecosystem. Taking this and other scientists’ views into account, it seems clear that acidification will mean the loss of many species so our children will not see the amazingly beautiful things that we can. It is important to tell them to go and see the corals now before it is too late.

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Ocean Acidification Reading Questions

If you're working through Ocean Acidification IELTS reading answers for the first time, know that this passage comes with three question types: Short Answer, Flow-chart Completion, and one Multiple Choice question at the end. Each format asks for something slightly different, so it's worth reading the instructions for every section instead of assuming they're all the same. 

Questions 1–7: Short Answer Questions

Answer the questions below. Choose NO MORE THAN THREE WORDS AND/OR A NUMBER for each answer.

These seven questions ask you to pull specific facts straight out of the passage, things like measurements, causes, and outcomes. The answers usually sit close to a keyword from the question, so once you spot that keyword in the text, the answer is often in the same sentence or the one right after it.

1. What does the pteropod use to move through the water?
2. Which part of the pteropods was being damaged by increased acidification?
3. What proportion of the carbon released over the last 200 years has been taken in by the oceans?
4. Where do carbonates enter the oceans from?
5. How long did the oceans need to recover after the destruction of marine life by acidification 55 million years ago?
6. Which businesses will suffer if reefs are damaged?
7. What type of creatures makes their skeleton out of aragonite?

Questions 8–12: Flow-chart Completion

Complete the flow-chart below. Choose NO MORE THAN THREE WORDS AND/OR A NUMBER from the passage for each answer.

This flow chart traces one specific chain of events from the passage, starting with acidification and ending with a possible effect on global warming. All five blanks come from the same paragraph, and they follow the same order the passage presents them in, so once you find where this section starts in the text, you can usually fill the blanks moving straight down the page.

A Possible Benefit from Increased CO2 Levels in the Sea

Increased ocean acidification

Large quantities of organic compounds made by 8. ________________

Transfer to 9. ______________

1. ________________ are formed
2. ________________ temperatures

Reduction in rate of 12. _______________

Question 13: Multiple Choice

Choose the correct letter A, B, C or D.

This last question asks for something different from the rest. Instead of pulling out one fact, you need to judge the writer's overall stance across the whole passage. Watch out for options that sound confident and dramatic, since IELTS often uses those as distractors even when the passage itself is more cautious.

1. Which of the following statements best summarises the writer's view of the passage?

A. We will have to wait and see if acidification has serious effects.
B. It is clear that acidification will cause huge damage to marine life.
C. It is likely that increased CO2 will change marine ecosystems considerably.
D. The theory that increased CO2 could have positive results is believable.

Ocean Acidification Reading Answers with Explanation

Ocean Acidification reading answers with explanation are only half the job. The real skill IELTS is testing is whether you can trace that answer back to the exact line in the passage that supports it. Below, every answer comes with its location and a full explanation, so you can see the reasoning instead of just matching it against a key.

Answers 1–7: Short Answer Questions

This set tests how fast you can scan for specific details like numbers, causes, and outcomes. Most of these answers sit right next to a keyword from the question, so once you find that keyword in the passage, you're usually one sentence away from the answer.

Q1: What does the pteropod use to move through the water?

A: (small) flaps

Location: Paragraph A

Explanation: The passage describes pteropods as creatures that swim in a way that looks like butterfly flight, and it says this movement is powered by small flaps. That's a direct match for what the question is asking, so there's no need to infer anything here.

Q2: Which part of the pteropods was being damaged by increased acidification?

A: (their/the) shells

Location: Paragraph A

Explanation: Fabry noticed the calcium carbonate in the pteropods' shells had started to dissolve once the water inside her jars changed chemically. The passage names the shells specifically, so that's your answer word for word.

Q3: What proportion of the carbon released over the last 200 years has been taken in by the oceans?

A: (about) 1/3 // a third

Location: Paragraph C

Explanation: The passage says the seas have absorbed about a third of all fossil-fuel carbon released since the mid-eighteenth century. The question rounds that off as roughly 200 years, so don't get confused by the slightly different phrasing. It's talking about the same stretch of time.

Q4: Where do carbonates enter the oceans from?

A: rocks (on land)

Location: Paragraph D

Explanation: The passage mentions two sources here, rocks on land and ocean sediments, both of which release carbonates that help neutralise dissolved CO2. The question only needs one of these, and rocks on land is the answer that's typically accepted.

Q5: How long did the oceans need to recover after the destruction of marine life by acidification 55 million years ago?

A: (over) 100,000 years

Location: Paragraph E

Explanation: This is stated plainly in the closing line of the paragraph, which says it took over 100,000 years for the oceans to return to their normal state. Watch out here, since the same paragraph also mentions 55 million years and 300 million years. Those numbers answer different questions, not this one.

Q6: Which businesses will suffer if reefs are damaged?

A: fishing and tourism

Location: Paragraph F

Explanation: The Royal Society report specifically names fishing and tourism as the industries that stand to lose billions of dollars a year if reefs are damaged. Since the question asks about businesses in the plural, you need both terms in your answer, not just one.

Q7: What type of creatures makes their skeleton out of aragonite?

A: coral(s)

Location: Paragraph G

Explanation: Corals are named as creatures that build shells and skeletons from calcium carbonate, and a bit later in the same paragraph, the passage confirms that aragonite structures like corals will be hit the hardest. You do need to read a little further into the paragraph to connect calcium carbonate with the specific mention of aragonite.

Answers 8–12: Flow-chart Completion

All five of these answers come from one single paragraph, and they follow the exact order the passage lays them out in. Once you locate where this section begins in the text, the rest tends to fall into place moving straight down the paragraph.

Q8: Large quantities of organic compounds made by ________________

A: microbes

Location: Paragraph H

Explanation: The passage says that in more acidic conditions, microbes produce more volatile organic compounds, including one called dimethyl sulphide. Microbes are the subject doing the action here, so that's your answer.

Q9: Transfer to ______________

A: (the) atmosphere

Location: Paragraph H

Explanation: The same sentence that introduces microbes also says some of these compounds escape to the atmosphere. This is simply the next step in the chain the flow-chart is tracking.

Q10: ________________ are formed

A: clouds

Location: Paragraph H

Explanation: Once these compounds reach the atmosphere, the passage says they cause clouds to develop. This answer sits in the same sentence as the previous one, just slightly further along.

Q11: ________________ temperatures

A: cooler

Location: Paragraph H

Explanation: The passage states that more clouds would mean cooler conditions. The word cooler is used exactly as written, so there's no paraphrasing to untangle here.

Q12: Reduction in rate of _______________

A: global warming

Location: Paragraph H

Explanation: The same sentence finishes by saying these cooler conditions could potentially slow global warming. This is the final link in the chain, tying back to the flow-chart's title about a possible benefit from rising CO2.

Answer 13: Multiple Choice

This last question steps back and asks about the writer's overall stance, not one isolated fact. It's a good test of whether you've understood the passage as a whole, not just individual details.

Q13: Which of the following statements best summarises the writer's view of the passage?

A: C. It is likely that increased CO2 will change marine ecosystems considerably.

Location: Paragraphs F, G, and I together

Explanation: Across the passage, the writer lays out real risks to coral reefs and marine ecosystems, but also repeatedly points out how much uncertainty still surrounds the full picture. Option B overstates this by claiming the damage is already clear and certain, which doesn't match the writer's more cautious tone. Option C strikes that same careful balance the writer actually uses, saying change is likely without pretending the outcome is fully settled.

Also Read:
IELTS Syllabus	IELTS Test Format	IELTS Exam Pattern	IELTS Exam Eligibility

How to Approach the Ocean Acidification IELTS Reading Questions?

The academic reading Ocean Acidification answers cover short answers, flow-chart completions, and one multiple-choice question. The passage packs in a lot of numbers, dates, and scientific names close together, so it's easy to grab the wrong detail if you're moving fast.

A clear approach for each question type helps you land on the right part of the passage and work through the Ocean Acidification reading answers without rereading the whole thing twice.

Use Numbers Carefully for Short Answer Questions

Numbers are your best clue here, but this passage has several sitting close together, so check what each one is actually measuring before you commit to an answer.

What to do:

• Spot the number or measurement in the question.
• Read the full sentence around it in the passage, not just the number itself.
• Confirm what that number refers to, since two different figures can sit in the same paragraph.
• Check the word limit before writing your final answer.

For example, paragraph E mentions both 55 million years and over 100,000 years. The first is when the mass extinction happened, and the second is how long the oceans took to recover. Only the second one answers question 5.

Follow the Chain of Events for Flow-chart Completion

Flow-chart questions test whether you can follow a sequence, so treat the blanks as steps rather than isolated facts.

What to do:

• Read the whole flow-chart first so you understand where it starts and ends.
• Find the paragraph in the passage that matches this sequence.
• Fill the blanks in order, since each step usually leads into the next.
• Stick to the word limit given in the instructions.

Questions 8 to 12 all sit inside paragraph H, and once you find where microbes are mentioned, the rest of the chain follows in the same order right down to global warming.

Handle the Multiple Choice Question Carefully

Multiple-choice options are often written to look partly correct, so don't pick the first one that sounds reasonable.

What to do:

• Read the question stem first and work out exactly what it's asking.
• Rule out options that sound too extreme or too certain.
• Compare each remaining option against the actual tone of the passage, not just its content.
• Choose the option that matches the writer's overall stance.

Question 13 is a good example. Option B claims the damage is already clear, but the passage keeps flagging uncertainty, so option C's more careful wording, likely rather than certain, matches the writer's actual tone.

Watch for Paraphrased Wording

IELTS rarely uses the exact same words in the question that appear in the passage.

For example:

• "proportion of carbon" in the question becomes "about a third" in the passage.
• "businesses will suffer" becomes "fishing and tourism... losing billions of dollars."
• "type of creatures" becomes "organisms with aragonite structures."

So look for the same meaning, not just matching words, and use numbers or names nearby to confirm you've found the right spot.

Follow the Word Limit

Both Short Answer and Flow-chart questions ask for NO MORE THAN THREE WORDS AND/OR A NUMBER. Read this limit before you write anything, since going over it means the answer gets marked wrong even if the content is correct.

What to do:

• Check the word limit before you start writing your answer.
• Count your words once you're done, including short words like "the" or "a."
• Keep numbers as numbers unless the passage spells them out.
• Double-check spelling, since a small typo can cost you the mark on an otherwise correct answer.

For example, "hull shape" fits a three-word limit, but "the shape of the hull" doesn't, even though both mean the same thing.

Looking for more IELTS Reading practice? Read these answer guides to practise different passage topics and question types:

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Conclusion

Ocean Acidification reading answers all trace back to one thing: cause and effect. One event leads into the next, and once you can follow that chain, the questions stop feeling scattered and start feeling like one connected story.

Here's what makes this passage click:

• Paragraph H covers the whole chain from microbes to clouds to cooler temperatures.
• Once you spot that link, questions 8 through 12 practically answer themselves in order.
• The passage isn't testing whether you can find a fact. It's testing whether you can see how one fact leads to the next.

Keep going back to the Ocean Acidification reading answers with explanation above until you can point to the answer location yourself, before checking the key. That habit of tracing an answer back to its source is exactly what carries over to every other IELTS Reading passage you'll face on test day.

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