Carbon monoxide levels in the atmosphere grew by enough of an increased percentage during the twentieth century that it began to trap heat radiating from the Earth, and it caused the average surface temperature to rise.

The correct answer is:
Levels of atmospheric carbon monoxide increased sufficiently during the twentieth century to begin trapping heat radiating from the Earth, causing the average surface temperature to rise.

I agree, this is the best option in terms of grammar.
However I think this option distorts the meaning of the original sentence. Although I agree "grew by enough of an increased percentage" is wordy and infact has elements that are redundant, I think the element "percentage" is important. The author's intention might be to indicate that the increase in level by a certain percentage is the issue and not the fact that it has increased sufficiently (although it is inherently implied!). Do you agree? Thanks!

Will you please post the original source? Thanks!

_________________
Jamie Nelson
ManhattanGMAT Instructor

The source is MGMAT SC question bank :-)

Thanks for the source!

Let's look at this two options for describing what is going on:
A) grew by enough of an increased percentage . . . that it
correct answer) levels increased sufficiently . . . to begin . . .

I think that the correct answer implies that whatever tipping point was needed to cause this effect actually did take place. I don't think that we need to specify that it had a particular "percentage". If the "levels" did increase, then that implies an increased percentage. For example, if one year the level was 10, then 15 the next year, then 20 the following year, there was a percentage increase, and in fact any time some number increases there is a percentage increase. We don't need to explicitly state that fact. This answer also contains the word "it" which has no antecedent.

On SC when you see grammar or construction flaws go with those first, THEN go with meaning. A is so poorly worded and redundant that it would have to go, so you wouldn't need to worry about this issue. I hope this helps.

_________________
Jamie Nelson
ManhattanGMAT Instructor