Even a small amount of gases have / has a large effect.
This sentence is an exerpt from a reading passage, which is talking about CO2 in global warming. It says CO2 accounts for only 0.038% in the atmosphere, and then the example sentence 'Even a small amount of gases have / has a large effect.' appears.
Which is the most appropriate answer?
If gases is the subject, and it is plural, the correct answer would be 'have'.
If the subject is a small amount, then the answer would be 'has'.
What do we see as the subject here? Can both be the subject?