Ozone in the stratosphere blocks deadly ultraviolet rays from the sun,

Ozone in the stratosphere blocks deadly ultraviolet rays from the sun, but chlorofluorocarbons (CFCs) in aerosols and other products have thinned this protective layer. - CFCs in aerosols and other products have thinned the layer of Ozone, which blocks harmful ultraviolet rays from the sun. (We can expect more ultraviolet rays reaching the earth, as a result)

Evidence of this is the ozone hole that forms over the South Pole every Antarctic spring as temperatures drop below –78°C, the temperature at which ozone depletion occurs. - This statement provides evidence for the previous statement that CFCs have thinned the Ozone layer. The evidence is a hole in the ozone layer. The hole forms over the South Pole as temperatures drop below a certain level i.e. - 78°C. This is the temperature at which ozone depletion occurs. (Such low temperature together with CFCs causes depletion of the ozone layer)

Measurements of the ozone hole taken at various times this spring show that, compared with the same times the previous year, its area diminished by four million square kilometers. - This statement suggests that the ozone layer has recovered since the measurements show that the area of the ozone hole has diminished.

Nevertheless, scientists have not concluded that the ozone layer is recovering. - This statement presents a contrast to the previous statement. Even though the measurements suggest that the ozone layer has recovered, scientists have not concluded so.


Why do scientists not conclude that the ozone layer is recovering even though the area of the hole has diminished?

Do the scientists believe the measurement taken? If the scientists believe that the measurements are not reliable or understate the size of the hole, they wouldn’t conclude that the ozone layer is recovering.

Was there anything else other than a recovery that could explain the diminished size of the hole? If this year was exceptional in a way that the hole became smaller than it usually would be, then it will be premature to conclude that the ozone layer is recovering.


The question stem asks us to find a reason for the scientists' reaction to the measurement. We have already discussed two reasons. Of course, as always, there could be others as well. Just that we were able to come up with two.


A. The ozone hole has steadily grown in size every year for the past decade except this year.
Incorrect.The option means that the ozone hole has steadily grown for nine years before this year. However, this year, it has diminished. Just because it has grown for nine years before diminishing last year, does it provide a reason to support that the ozone layer is not recovering? No.

Think about it. Whether a person declined in performance for one year or ten years before his performance improved last year shouldn’t help us decide whether the person has improved in the ‘last year’. If his performance has improved, he has improved compared to last year, regardless of his track record before that.

B. The length of time that the ozone hole persists fluctuates from year to year.
Incorrect.What we need to pay attention to here is that this option talks about ‘the length of time’, not ‘the size of the hole’. If this option had stated that the size of the hole fluctuates from year to year, it would have been able to explain why scientists haven’t concluded the recovery of the ozone layer from the diminished size of the hole. If the size of the hole fluctuates from year to year, then this year’s decrease in size of the hole could just be a fluctuation and not an indication of the recovery of the ozone layer.

However, the option talks about ‘the length of time’, which has no impact on the discussion at hand.

C. As a result of international treaties, CFCs have been completely banned for several years.
Incorrect.This option is in the opposite direction. If the CFCs have been completely banned and, as given in the passage, there has been a decrease in the size of the hole in the ozone layer, the scientists should have concluded a recovery. Rather than explaining why scientists have not concluded a recovery, this option gives a reason to conclude that there is a recovery underway.

If the option had stated that CFCs continue to be used as before, then this option could have explained why scientists have not concluded a recovery. Because in such a case, scientists would be wary of concluding a recovery on the basis of certain measurements since they see that the cause of the ozone layer depletion continues to be there.

D. Weather patterns allowed unusual amounts of warm air to mix into the polar regions this year.
Correct.It’s easy to reject this option. It talks about neither the ozone layer nor the CFCs. Thus, if you have a habit of rejecting options just because they mention or do not mention certain words, hard questions are going to remain elusive to you.

One can understand this option only if one pays attention to this part of the passage: “ –78°C, the temperature at which ozone depletion occurs”. For the ozone depletion to occur, temperatures need to be below a certain level. Now, if an ‘unusual’ amount of warm air mixes into the polar regions, the temperatures at the polar regions are going to be higher than usual. Thus, the hole in the ozone layer is going to be smaller than usual, not because the ozone layer is recovering but because of the unusual amounts of warm air. Thus, this option gives us a reason why the scientists have not concluded a recovery of the ozone layer.

E. Human-made CFCs retain their ability to destroy ozone molecules for seventy-five to one hundred years.
Incorrect.Here, some of us may think that since CFCs retain the ability to destroy ozone molecules for a long time, we shouldn’t expect a recovery soon. If we think so, we are overlooking the fact that we have seen a reduction in the size of the hole. If these CFCs retain their ability to destroy the ozone molecules for a long time and there are still the same number of CFCs out there, we shouldn’t expect a reduction in the size of the hole.

However, given that there is a reduction, either there is a decrease in the number of CFCs or there is some other positive factor that helps the ozone layer. In either case, the ozone layer has recovered. So, this option doesn’t give us a reason to explain why scientists have not concluded a recovery of the ozone layer.

SC Notes: The use of ‘this’ in the second sentence of the passage without a noun following it. ‘this’ refers to the entire first sentence. This usage is correct.


If you have any doubts regarding any part of this solution, please feel free to ask.