Should you use an article in front of noun

● It is the temperature that has the biggest impact on food. I think "the" in this sentence is fine.but,using "it" as a subject here is weird for me,because "it" is used to refer to the noun previously mentioned.In this sentence,I think using "this" is more preferable.

● There is a correlation between the degree of decomposition and time of death. This sentence is fine as well.And it must have "the" here because the word "degree" is singular.If you have a singular noun in your sentence,It must have an article,demonstrative adjective before.but,If you have a plural noun,That is not necessary.