­Earth scientist: Mount Kilimanjaro in Tanzania has lost 90 percent of

­Earth scientist: Mount Kilimanjaro in Tanzania has lost 90 percent of its ice cover over the past century. But this loss cannot be due to global warming. Recent data shows that temperatures of the air surrounding the mountain’s glaciers never rise above freezing, so the glaciers cannot be melting. The loss must be explained by sublimation, or direct evaporation of the ice, and by not enough snowfall to replenish it.

The argument works as follows.

It begins with background information:

Mount Kilimanjaro in Tanzania has lost 90 percent of its ice cover over the past century.

Then, comes the main conclusion:

this loss cannot be due to global warming

Next comes factual evidence:

Recent data shows that temperatures of the air surrounding the mountain’s glaciers never rise above freezing

Finally, we have two intermediate conclusions:

the glaciers cannot be melting

The loss must be explained by sublimation, or direct evaporation of the ice, and by not enough snowfall to replenish it.

So, the factual evidence supports the intermediate conclusions, which in turn support the main conclusion that the loss cannot be due to global warming.

Which of the following is an assumption the earth scientist’s argument requires?

This is an Assumption question. So, the correct answer will be state something that must be true for the argument to work. In other words, the correct answer will be information necessary for the evidence to support the conclusion.

A. If temperatures near Mount Kilimanjaro’s glaciers often rose above freezing, global warming would be the most likely explanation of the ice cover’s disappearance.

The conclusion of the argument is basically that global warming is not the explanation for the ice cover's disappearance.

So, the argument does not require assuming that, if things were different, global warming would be the explanation.

Eliminate

B. Annual snowfall on Mount Kilimanjaro has declined by at least 90 percent over the past century.

This choice is tempting because the passage says, "Mount Kilimanjaro in Tanzania has lost 90 percent of its ice cover over the past century," and "The loss must be explained by sublimation, or direct evaporation of the ice, and by not enough snowfall to replenish it."

At the same time, the argument does not require this choice because it doesn't have to be the case that the reason Mount Kilimanjaro has lost 90 percent of its ice cover is that annual snowfall has declined by 90 percent. After all, the ice cover could have decreased by 90 percent over time by declining a little each year, in other words, possibly because annual snowfall has decreased by a small percentage.

Eliminate.

C. The sublimation and lack of snowfall referred to cannot be due to global warming.

As we saw in our above analysis of the structure of the argument, the main conclusion is supported by the intermediate conclusion "The loss must be explained by sublimation, or direct evaporation of the ice, and by not enough snowfall to replenish it."

So, the argument jumps from "The loss must be explained by sublimation ... and by not enough snowfall," to the main conclusion "this loss cannot be due to global warming."

We see that the argument relies on the assumption stated by this choice for that jump to work. After all, if the sublimation and lack of snowfall CAN be due to global warming, then the argument doesn't work since, in that case, the intermediate conclusion that the decrease in ice cover is the result of sublimation and lack of snowfall doesn't mean the loss cannot be due to global warming.

Keep.

D. It has not been clearly demonstrated that global warming has had any effect on Mount Kilimanjaro.

This choice is tempting because the conclusion is "this loss (of ice on Mount Kilimanjaro) cannot be due to global warming." So, we might think the support for the conclusion requires this choice.

However, the truth is that this choice goes farther than what must be assumed for the argument to work. After all, the argument doesn't require assuming that global warming has not had "any effect" on Mount Kilimanjaro. Global warming could have had some effect and still not be the cause of the 90 percent decrease in ice cover.

Eliminate.

E. Sublimation and a lack of snowfall could fully explain the loss of ice cover on at least some mountains other than Kilimanjaro.

We don't need to assume that sublimation and a lack of snowfall explain the loss of ice cover on other mountains to conclude that they explain the loss of ice cover on Kilimanjaro.

Eliminate.
 
Correct answer: C