Increasing concentrations of carbon dioxide (CO2) in the atmosphere

Question

Increasing concentrations of carbon dioxide (CO2) in the atmosphere are making the oceans more acidic—in other words, lowering the pH of ocean water. Worldwide, the average pH of the upper ocean—the top 1,000 m—has declined 0.12, to 8.1, between 1750 and 2010. If humans continue to emit greenhouse gases at current rates, atmospheric CO2 concentrations will reach 500 parts per million (ppm) by 2050 and 800 ppm by 2100, by which time the pH of the upper ocean could drop to 7.8 or 7.7. The deep ocean—below 4,000 m—had a pH of 8.2 in 2010. At that depth, acidity is relatively stable and not expected to increase as quickly, as is shown in the graph labeled Long-Term Graph.

The base of the ocean food web is phytoplankton, which, like plants, use sunlight for photosynthesis and thus can live only in the upper ocean. For many types of phytoplankton, acidification makes it harder to absorb iron, a vital nutrient. Research suggests that a 0.3 decline in pH reduces phytoplankton iron consumption by about 15 percent, slowing photosynthesis and impacting growth and reproduction. Comparable changes in the past correlated with massive extinctions of sea life.

chetan2u · Accepted Answer

Q.1. Assuming that current trends will continue, for each of the following, select Projected if it is a long-term projection of the information provided, and otherwise select Not projected.

Let us check the relevant information.

(a) Decreased iron absorption by phytoplankton will contribute to increased extinctions of sea life.
The above info is directly available from lines -  Research suggests that a 0.3 decline in pH reduces phytoplankton iron consumption by about 15 percent, slowing photosynthesis and impacting growth and reproduction. Comparable changes in the past correlated with massive extinctions of sea life. 
So, ' Projected '.

(b) Increased iron absorption by phytoplankton will contribute to decreased oceanic CO2 concentrations.
'Increased iron absorption by phytoplankton' has not been linked to 'decreased oceanic CO2 concentrations' anywhere in the para.
So, ' Not Projected'.

(c) Increased atmospheric CO2 concentrations will contribute to decreased ocean water pH
The above info is again directly from the first few lines in the first para. - 'Increasing concentrations of carbon dioxide (CO2) in the atmosphere are making the oceans more acidic—in other words, lowering the pH of ocean water'.
So, ' Projected '.

Q.2. Assuming that current trends will continue, which one of the following projections about the year 2500 most accurately reflects the information in the passage and Long-Term Graph?
Since the para speaks about Upper and deep ocean, let us check the trend in these two regions

Upper Ocean : It is the top 1000 m. The pH in 2010 was 8.1 and, in 2500, is likely to be affected by an amount of -0.5 at 1000 m to -0.8 at top surface as per the long term graph. So, the pH should be anywhere between 8.1-0.8 and 8.1-0.5 or 7.3 and 7.6
(a) The pH of the upper ocean will be less than 7.0:  The entire range is not a part. Hence, not true. 
(b) The pH of the upper ocean will be greater than 7.5:  The range 7.3 to 7.5 is not a part. Hence, not true. 
(e) As compared to 2010, the amount of iron consumed by phytoplankton will have decreased by more than 30 percent: A decline of 0.3 results in decrease by 15%, so for a decrease of 30%, we are looking at a decline of more than 0.3*2 or 0.6 pH, but the range is 0.5 to 0.8.:  Need not be true

Deep Ocean : It is at depth of 4000 m and more. The pH in 2010 was 8.2 and, in 2500, is likely to be affected by an amount of -0.2 over pre-1750 levels as per the long term graph. So, the pH should be about 8.2-0.1 or 8.1, as -0.1 out of -0.2 is already catered in 2010 readings.
(c) The pH of the deep ocean will be 7.8 or less:  Not true 
(d) The pH of the deep ocean will be greater than 7.9:  True

@ akshay.sachdeva,  nolounta , please look at the solution and get back if any query.
 kirallight , there may be an issue in the last portion. We are not looking at 2010, but comparison of 2500 to 2010.

GMATNinja · Answer

The final paragraph tell us that acidification makes it harder for phytoplankton to absorb iron, a VITAL nutrient. How much harder? Well, we don't know for sure, but research suggests that a 0.3 decline in pH reduces phytoplankton iron consumption by about 15 percent, slowing photosynthesis and impacting growth and reproduction.

So we have a 15% decrease in consumption of a vital nutrient. Maybe that's not so bad? Well, on more than one occasion in the past ("changes" is plural), a 0.3 decline in pH has correlated with a massive extinction of sea life.

The evidence paints a very clear picture: pH drops 0.3, consumption of a vital nutrient drops by about 15%, growth and reproduction at the base of the food chain are affected, and there are massive extinctions of sea life.

Now, does this PROVE that the extinctions were caused by the declines in pH? Of course not. All we have is the data, which shows a consistent pattern -- a pattern that makes perfect sense given everything we know about phytoplankton. So if pH is expected to drop by 0.3 or more in the future, then we would certainly PROJECT a similar outcome: extinctions of sea life.

Remember, a projection is not a guarantee. When a business releases its projected fourth-quarter earnings, those are just educated guesses based on the available data. Is it POSSIBLE for a projection to be wrong? Of course, but based on the data and information in the tabs, those extinctions are indeed projected to occur.

I hope that helps!

PReciSioN · Answer

GMATNinja  ,  KarishmaB

In Question-1, why is statement 1 "Decreased iron absorption by phytoplankton will contribute to increased extinctions of sea life."  Projected?

The relevant portion from passage is " Research suggests that a 0.3 decline in pH reduces phytoplankton iron consumption by about 15 percent, slowing photosynthesis and impacting growth and reproduction. Comparable changes in the past correlated with massive extinctions of sea life."

The passage says that comparable changes in the past "CORRELATED" with massive extinctions of sea life. This does not mean that these extinctions were necessarily caused by reduced phytoplankton iron consumption. What's more, even if it happened in the past, is it really reasonable to project that this "correlation" will happen again in the future? Maybe all the species which could get extinct because of these changes, already became extinct.

zlishz · Answer

What is the type of the Long term graph called and how to read it?  chetan2u

chetan2u · Answer

zlishz

These graphs are Area type.

The graph conrains three variables, but in normal graph we have a constarint of depicting only two variables, one on x-axis and one on y-axis. Therefore, the third variable, which is variation in ph value here, is given by the areas.

Now you have to read/relate all three variables at any point. So at any point, drop perpendiculars to x-axis to get dept of ocean and to y-axis to get the years and the point itself would give you the difference in ph.

So, each point will give you at a certain time what was the difference in ph value as compared to a certain relative time being spoken of at various depths.

sayan640 · Answer

MartyMurray   KarishmaB  Will you be kind enough to explain this question , specially how to interpret the second graph ?

KarishmaB · Answer

2%20-%20Long%20term%20graph.png The second graph is an area graph. It shows a particular characteristic - in this case "change in pH of water relative to pre 1750" as it changes with water depth and year. The change in pH of water at the surface of the ocean was -0.1 till 2000. At a depth of 1 km, the change in pH of water was -0.1 till 2100. At a depth of 5 km, the change will be -0.1 till about 2425. After that the pH will change by -0.2 till 2600. The change in pH at the surface of the ocean will be -0.2 till 2025. In the next 13 years, the change will increase to -0.3 (all relative to pre 1750 times) and by 2050, the change will be -0.4 and so on. So if pH of the etnire ocean was 8.2 before 1750, from 1750 to 2000, the pH of the surface water was 8.1. From 2000 to 2025, the pH of surface water was 8.0. From 2025 to 2038 the pH of surface water was 7.9 etc.

Elite097 · Answer

chetan2u   KarishmaB  as mentioned in your explanation:
 "Deep Ocean: It is at depth of 4000 m and more. The pH in 2010 was 8.2 and, in 2500, is likely to be affected by an amount of -0.2 as per the long term graph. So, the pH should be about 8.2-0.2 or 8.0.
(c) The pH of the deep ocean will be 7.8 or less: Not true
(d) The pH of the deep ocean will be greater than 7.9: True"

-0.2 is change from pre 1750. Since it is already reduced by 0.1 in 2010, hence by 2500 it will decrease furthr by 0.1 (total 0.2 decrease from pre 1750) so ph level by 2500 will be 8.2-0.1=8.1 and not 8. Not sure why you have subtracted 0.2 pls clarify. I do understand the solution will coincidentally still be same but pls clarify the doubt

chetan2u · Answer

Yes, you are correct. -0.2 is relative to pre-1750 levels, and relative to 2010, there will be a drop of -0.2-(-0.1) or -0.1. Kudos

iluvsenf · Answer

Altough I got both questions right, I want to give you some takeaways for this type question.

1) When reading the passage, make some cause-effect notes with arrows to quickly summarize the passage. For example for the first paragraph you could write CO2 atmosphere   -> Ocean more acidic -> pH  . This make answering the first question easy and straight forward.

2) If you read the second questions stem carefully, you would notice that it only requires you to combine the information of the passage AND the long term graph, NOT including information of the Acidificaition Tab. This will save you time.
Further combining means that the information should be present in both Tabs, so we can quickly eliminate choice E because it does not talk about the change in pH at all.

siddharth_ · Answer

Although I answered the questions correctly but I took 7 minutes to answer these 2 questions. Is it normal?
   akshay.sachdeva

ToddlerandGMAT · Answer

Hello ,

"Worldwide, the average pH of the upper ocean—the top 1,000 m—has declined 0.12, to 8.1, between 1750 and 2010" --> I didnt quite understand this line on Average pH levels is declined but rather it shows it increased to 8.1 right?

chetan2u   Bunuel   MartyMurray   KarishmaB

KarishmaB · Answer

Average pH levels are declining.

Given: Worldwide, the average pH of the upper ocean—the top 1,000 m—has declined 0.12, to 8.1, between 1750 and 2010.

This means the pH of upper ocean has declined by 0.12 and become 8.1. This means it was 8.22 and has come down to 8.1 i.e. the upper ocean has become more acidic. Lower the pH, higher the acidity

Natansha · Answer

Hi  chetan2u

I had a doubt in the below. We have this as you mentioned:

Upper Ocean : It is the top 1000 m. The pH in 2010 was 8.1 and, in 2500, is likely to be affected by an amount of -0.5 at 1000 m to -0.8 at top surface as per the long term graph. So, the pH should be anywhere between 8.1-0.8 and 8.1-0.5 or 7.3 and 7.6

Deep Ocean : It is at depth of 4000 m and more. The pH in 2010 was 8.2 and, in 2500, is likely to be affected by an amount of -0.2 over pre-1750 levels as per the long term graph. So, the pH should be about 8.2-0.1 or 8.1,  as -0.1 out of -0.2 is already catered in 2010 readings .

The 0.1 which is already catered in 2010 readings, why have we not considered it in Upper Ocean but only in Deep Ocean?

cheshire · Answer

Unless I'm misreading the graph, I agree that the range should account for the starting acidity in 1750, which would shift the range ~1 in the upper ocean. I don't think this changes the answer though.

egmat · Answer

Question 2

This question tests your ability to extrapolate trends from passage data - a classic GMAT RC skill that many students find challenging.

Step 1: Identify the Key Information 
From the passage, we know:

Upper ocean pH declined from ~8.22 to 8.1 (1750-2010) =  0.12  decline in 260 years 
 By 2100, upper ocean pH could drop to 7.7-7.8 = another  0.3-0.4  decline in just 90 years 
 Deep ocean (below 4,000m) had pH of 8.2 in 2010 and is "relatively stable" 
 The passage explicitly states deep ocean acidity is "not expected to increase as quickly"

Step 2: Analyze the Acceleration Pattern 
Notice the acidification is accelerating:

1750-2010:  \frac{0.12}{260}  = ~0.00046 pH units per year
2010-2100:  \frac{0.35}{90}  = ~0.0039 pH units per year (about 8x faster!)

Step 3: Project to 2500 
For the upper ocean (from 2100 to 2500 = 400 years):
If pH is 7.7-7.8 in 2100 and acidification continues accelerating, the pH would likely drop well below 7.0 by 2500.

For the deep ocean:
Given it's "relatively stable" and changes much more slowly than the upper ocean, from a 2010 baseline of 8.2, even over 490 years it would likely remain above 7.9.

Step 4: Evaluate Answer Choices 
Looking at our projections:
(A) Upper ocean < 7.0 - Possible given acceleration
(B) Upper ocean > 7.5 - Unlikely given current trends
(C) Deep ocean ≤ 7.8 - Contradicts "relatively stable" nature
(D) Deep ocean > 7.9 - Consistent with slow change rate
(E) Iron consumption decrease > 30% - Would require detailed calculation

The key insight is recognizing that the deep ocean's "relatively stable" nature makes  Answer D  the most defensible projection.

Want to master the complete framework for handling RC projection questions and see the graph analysis that confirms this answer?  Check out the  step-by-step solution on  Neuron by e-GMAT  to learn the systematic approach for extrapolating trends in RC passages. You'll also discover how to quickly eliminate trap answers in projection questions. Access detailed solutions for official questions to build consistent accuracy on  Neuron .

user84525084572 · Answer

I also struggled quite a bit with Question 1.

It is very important to look carefully at the exact wording in both the question and the text.

In my opinion, the most important part is in the question stem: “Assuming that current trends will continue.”
This means a projection must be based on ongoing trends, not just historical events.

The text says:
“Comparable changes   in the past correlated with massive extinctions of sea life.”

The problem is that this refers to past correlation, not a current trend. Just because two things were correlated in the past does not mean the same pattern is happening now or will continue in the future.

Therefore, we cannot logically project that:
“Decreased iron absorption by phytoplankton will contribute to increased extinctions of sea life.”

So the correct answer should be “Not projected.”

GMATNinja  Am I missing something, or is the official solution incorrect?

Pikachuu · Answer

Hi @ user84525084572 , I think you are following the constraints much more strictly than required.

Please consider the following 3 indications which silently signal us to use approximation: 
 1. The prompts use the words "projected" & "not projected" which hint at estimation. 
 2. The 1st prompt uses the word " contribute " which again means the anything from 1% to 100% contribution. 
 3. The last line of the passage uses the word "correlated". And Correlation is always loose, unlike a direct proportionality.

Hit kudos if it makes sense.

minionbaba · Answer

This is a  contour graph / heat-map style graph . The key skill is learning how to read the  3 dimensions  hidden inside a 2D picture.

This graph is showing:
 
  Year  → horizontal axis  
  Ocean depth  → vertical axis  
  Change in pH (ΔpH)  → represented by the curved contour regions

The critical part on how to read such graphs is that:

The numbers in the graph are  NOT yearly changes . They are the TOTAL cumulative change in pH relative to the pre-1750 level.

i.e. each number already represents the full accumulated drop from the  original  baseline.

	The pH of the upper ocean will be less than 7.0.
	The pH of the upper ocean will be greater than 7.5.
	The pH of the deep ocean will be 7.8 or less.
	The pH of the deep ocean will be greater than 7.9.
	As compared to 2010, the amount of iron consumed by phytoplankton will have decreased by more than 30 percent.

Increasing concentrations of carbon dioxide (CO2) in the atmosphere

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Projected	Not Projected
		Decreased iron absorption by phytoplankton will contribute to increased extinctions of sea life.
		Increased iron absorption by phytoplankton will contribute to decreased oceanic CO2 concentrations.
		Increased atmospheric CO2 concentrations will contribute to decreased ocean water pH

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Increasing concentrations of carbon dioxide (CO2) in the atmosphere

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