TYPICAL ERROR By the year 2100 so I have heard sea levels are likely to have risen at least a metre.
CORRECTION By the year 2100, so I have heard, sea levels are likely to have risen at least a metre.
EXPLANATION A parenthetical phrase is one such as so I have heard, which is not really part of the sentence into which it has been inserted and which could be left out. It is a comment that is outside or alongside (para-) the main idea (-thesis) of the sentence. As such, it should be marked off from the rest of the sentence by commas, brackets or dashes - depending on how dramatically the parenthetical idea interrupts the flow of the main sentence.