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 official answer key for the Ocean Acidification passage, so you can check your responses against the correct answers for each question type covered in this test.
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
Here, we have provided a similar practice passage with the same type of questions for you to solve, along with the answers and explanations. So, try to solve the passage first, and only check the key afterward to see why each answer is correct.
Want to understand the complete IELTS test before practising Reading passages? Check theIELTS 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
You've now gone through the official Ocean Acidification passage and its answer key above. The paraphrased passage further down uses the same facts in different words, and the questions below are built to match that paraphrased version, so you get a genuine shot at practising rather than just recognising answers you've already seen.
Go through each question slowly before settling on an answer. Some line up closely with the wording used in the paraphrased passage, but a few require you to match an idea expressed differently, so pay attention to meaning rather than hunting for an exact phrase.
Questions 1–6: Sentence Completion
There are six sentences below, each with a gap. Choose NO MORE THAN THREE WORDS AND/OR A NUMBER from the passage for each answer.
Fabry noticed pteropod shells dissolving once the water in her jars became more __________.
Roughly a third of the CO2 released since 1750 has been absorbed by the __________.
Rocks on land and __________ both release carbonates that help offset ocean acidity.
After the acidification event 55 million years ago, the oceans took over __________ to recover.
A Royal Society report warns that damaged reefs could cost fishing and tourism __________ a year.
Coral skeletons are made of a form of calcium carbonate known as __________.
Questions 7–11: True / False / Not Given
It is now time to decide whether each statement below agrees with the information in the paraphrased passage. Write:
TRUE if the statement agrees with the information
FALSE if the statement contradicts the information
NOT GIVEN if there is no information on this
Pteropods move through water using a flapping motion similar to a butterfly.
Scientists are certain that increased cloud formation will offset global warming.
More acidic water leads microbes to produce more volatile organic compounds.
These compounds can travel into the atmosphere and contribute to cloud formation.
Extra cloud cover might, in theory, have a slight cooling effect.
Question 12: Multiple Choice
There is one multiple-choice question left to test how well you've understood the passage's overall tone rather than a single fact. Choose the correct letter A, B, C or D.
Which statement best reflects the tone of the paragraph describing the possible link between acidification and cooler temperatures?
A. It confirms that acidification will definitely reduce global warming. B. It offers a tentative, unproven possibility rather than a firm claim. C. It dismisses the idea of any benefit from rising CO2. D. It argues fishing and tourism will be unaffected either way.
Ocean Acidification Reading Answers with Explanation
Once you've attempted the questions above, use this key to check your responses and see exactly where each answer traces back to in the paraphrased passage. The explanations walk through why each answer fits, and flag a few spots where it's easy to mix up similar details.
Answers 1–6: Sentence Completion
Here is where you can check your responses for the first six questions against the paraphrased passage.
Q: Fabry noticed pteropod shells dissolving once the water in her jars became more __________. A: acidic Location: Paragraph A Explanation: The paraphrased passage says the shells began dissolving as the water grew more acidic, directly linking the chemical change to the damage Fabry observed.
Q: Roughly a third of the CO2 released since 1750 has been absorbed by the __________. A: oceans Location: Paragraph C Explanation: The paragraph states the oceans have absorbed close to a third of the CO2 released since 1750, so "oceans" is the subject doing the absorbing.
Q: Rocks on land and __________ both release carbonates that help offset ocean acidity. A: (ocean) sediment(s) Location: Paragraph D Explanation: Carbonates are said to come from two sources, rocks on land and sediment on the ocean floor, so sediment is the second source needed here.
Q: After the acidification event 55 million years ago, the oceans took over __________ to recover. A: 100,000 years Location: Paragraph E Explanation: The passage specifies recovery took well over 100,000 years, distinct from the 55-million-year figure that marks when the event happened, not how long recovery took.
Q: A Royal Society report warns that damaged reefs could cost fishing and tourism __________ a year. A: billions of dollars Location: Paragraph F Explanation: The Royal Society report is the source cited for the financial risk, naming losses in the billions for fishing and tourism.
Q: Coral skeletons are made of a form of calcium carbonate known as __________. A: aragonite Location: Paragraph G Explanation: The passage names aragonite specifically as the type of calcium carbonate that makes coral skeletons vulnerable.
Answers 7–11: True / False / Not Given
Below is where each of the True/False/Not Given statements can be checked against the passage, one at a time.
Q: Pteropods move through water using a flapping motion similar to a butterfly. A: TRUE Location: Paragraph A Explanation: The passage compares the pteropods' movement directly to a butterfly's flapping motion.
Q: Scientists are certain that increased cloud formation will offset global warming. A: NOT GIVEN Location: Paragraph H Explanation: The passage calls this a speculative possibility, not a confirmed outcome, so there's no basis for calling it certain.
Q: More acidic water leads microbes to produce more volatile organic compounds. A: TRUE Location: Paragraph H Explanation: The passage states microbes produce greater quantities of these compounds under more acidic conditions.
Q: These compounds can travel into the atmosphere and contribute to cloud formation. A: TRUE Location: Paragraph H Explanation: The compounds are described as escaping into the atmosphere and potentially aiding cloud formation.
Q: Extra cloud cover might, in theory, have a slight cooling effect. A: TRUE Location: Paragraph H Explanation: More clouds are linked to a possible slight cooling effect, though the passage frames this as theoretical rather than settled.
Answer 12: Multiple Choice
Here is the explanation for the last question, which tests the overall tone of the passage rather than one isolated detail.
Q: Which statement best reflects the tone of the paragraph describing the possible link between acidification and cooler temperatures? A: B. It offers a tentative, unproven possibility rather than a firm claim. Location: Paragraph H Explanation: The paragraph opens by calling this a "possible silver lining" that "remains speculative," which signals a tentative tone rather than a firm conclusion. Option A overstates the certainty, while C and D don't match what the paragraph actually argues.
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:
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.
What is the Ocean Acidification IELTS Reading passage about?
The passage explains how rising CO2 levels are making the oceans more acidic and what that means for marine life. It follows biologist Victoria Fabry's discovery about dissolving pteropod shells, then widens out to cover coral reefs, extinction events, and possible side effects like cloud formation. It's a good example of a cause-and-effect science passage.
How many questions are in the Ocean Acidification Reading Test?
There are 13 questions in total. They're split across Short Answer Questions, Flow-chart Completion, and one Multiple Choice question, each with its own instructions and word limit.
What are the Ocean Acidification IELTS reading answers?
The full answer key is listed at the top of this article, covering all 13 questions. Each answer also comes with an explanation below, showing exactly where in the passage it's supported.
What question types appear in Ocean Acidification?
This passage uses Short Answer Questions for numbers 1 to 7, Flow-chart Completion for numbers 8 to 12, and Multiple Choice for question 13. Each format asks you to locate information slightly differently, so it helps to know the pattern before you start.
How difficult is the Ocean Acidification reading passage?
The passage isn't especially long, but it packs in several scientific names, dates, and close-together numbers that can confuse you if you read too fast. Careful attention to which number matches which detail makes a real difference here.
How can I improve my score using this passage?
Try answering all 13 questions without looking at the answer key first. Once you're done, check the Ocean Acidification reading answers with explanation section and figure out whether your mistakes came from missing a keyword, misreading a number, or picking a distractor option.
What should I focus on while reading this passage?
Pay close attention to the numbers and dates, since several of them sit close together and refer to different things. Also track how paragraph H builds a chain of cause and effect, since five questions come from that one paragraph alone.
How can I practise the Ocean Acidification passage effectively?
Read the passage once at a normal pace, then attempt all the questions without pausing to check answers. After you finish, go through the academic reading Ocean Acidification answers and explanations, and mark down which question types gave you the most trouble.
What is the word limit for the Ocean Acidification questions?
Questions 1 to 12 both ask for NO MORE THAN THREE WORDS AND/OR A NUMBER. Question 13 asks you to pick a letter from A to D, so there's no word limit to worry about there.
How can the Ocean Acidification reading answers with explanation help me?
The explanations show exactly which sentence supports each answer, so you're not just memorising a key. This is especially useful for the flow-chart questions, since seeing how one answer leads into the next helps you understand the cause-and-effect structure IELTS often tests.
Is this the same passage used in Achieve IELTS Test 4?
Yes, this Ocean Acidification passage matches the one used in Achieve IELTS Test 4 reading answers, along with several other IELTS practice test collections. The wording and question set stay consistent across most versions you'll find online.
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