Say and said?

Both are grammatically correct but a) gives the reader an impression that the statement is always true (no time limit). For b) it is just a sentence making reference to what Scientists has said earlier or in the past.

Other example.
We always say: The sun rises from the East and never the sun rose from the East - although the second sentence is grammatically correct, it is a factual statement which is ever true and therefore not limited by time.

同樣是講的意思,只是 tenses不同