When doing sentence correction, we should take a look at the initial sentence and try to find the errors that we know GMAT test writers are instructed to insert. In this case we read:

Biologists worry (present)
that the Arctic ecosystem will unravel quickly, (future)
citing the fact that the average temperature within the Arctic circle is (present)
already two degrees warmer than ten years ago.

What's wrong with this sentence? Well, it seems to be saying that the average temperature is warmer than two years ago. Yet a temperature cannot be warmer than a year. A temperature can be warmer than another temperature. It seems that we should be comparing the temperature now to that of two years ago.

Answers A, B, and E contain this error. Accordingly, we are down to C vs. D.

Answer choice D is highly suspect. Since the entire sentence above is in the present and future, it seems highly unlikely that we will need the past perfect. Generally speaking, we only use the past perfect when we are talking about the past and then need to talk about something that happened before that. For example:

I realized that I had forgotten my keys when I tried to open the car door.

Accordingly, C is the best answer.