It should be "historic low." Historic and historical are both adjectives. Historic describes something that is so noteworthy that it is noted, or will be noted, in history. For example, "The rainfall during this summer's drought is an historic low." The rainfall, in this example, has been so low, and is so out of the ordinary, that it will be noted in history as a particularly dry summer. Sometimes phrases like this also carry the connotation that, referring to the previous example, the rainfall is the lowest, or one of the lowest in historical record. Therefor it is "historic" because, being the lowest ever, it will be noted in history. "HIstorical" refers more broadly to something that has to do with history. For example, historical figures are important people from history. Historical markers are signs that describe events that took place in history. Historical buildings are old buildings that have interest to historians.