IMAT 2012 Q14 [Assumption | Global Warming]

Global warming is threatening the survival of California’s redwoods. These trees benefit from coastal fog which is captured by the trees, causing water to drip onto the soil and therefore watering them. Since fog is now 30 percent less frequent than it was 50 years ago the trees will not have this source of water and are therefore likely to begin to die out.

Which one of the following must be assumed in the above argument?

A. Global warming is to blame for the reduction in coastal fog.
B. The levels of fog will continue to decline.
C. Other trees will not be able to thrive in these conditions.
D. Redwoods in other areas of the world are being similarly affected.
E. Rainfall has also reduced over the past 50 years.


  1. Identify the question type. Although not phrased in the typical Cambridge format (“What is the assumption in the argument…”) we can easily identify that it is asking for an assumption.

  2. Strategy Tool Kit for solving assumptions

  3. Find the conclusions

  4. Find the reasons

  5. Find the unwritten link that the conclusion relies on

  6. Discard and decide (use negation test if needed)

A If we tried to reverse this statement, then the argument would be invalid because global warming would not be the cause of the reduction of fog and therefore all of the reasons explaining what is affecting the Californian redwoods would have to be the result of another cause. The first statement could be alluding to something else not mentioned if this assumption is untrue. The text focuses on the effects of this loss of fog, so we must assume that global warming is causing this and as a result threatening the survival of the California redwood. Therefore A is the answer.

B Although likely, it does not talk about the future of whether or not the decrease in fog will continue. It only talks about the trees because this current reduction in fog over the past 50 years has already caused enough damage. Regardless of whether or not the fog continues to decline, the damage has already been done. Therefore B is not the correct answer.

C There is no mention of any other trees, therefore this is an irrelevant answer. It is talking solely about the Californian Redwood trees, therefore we cannot generalize this argument to other trees. They may survive or they may not, we cannot know. Therefore C is incorrect.

D This again is not relevant. We are talking about how global warming is specifically affecting this one species of trees, we cannot generalize it to other types of Redwood. Therefore D is incorrect.

E The rainfall amounts are irrelevant because the fog is what is providing the water here and what is less frequent. Assuming rainfall was also reduced would actually weaken the argument because we would not be able to tell if the fog was having the effect, the rain was, or both. Therefore E is incorrect.