I was asked what the difference between need and necessity was by a non native speaker. It was in the context of the name of an article to do with global warming, i.e "The need/necessity for....". I was completely stumped and googling it only produced speculative answers.
In my mind a necessity is an absolute requirement, for example food and water is a necessity for survival. I would have said that a need is a requirement but maybe not an absolute one. For example, there is a need for an umbrella when walking outside in the rain.
Could you please give a definitive answer, if there is one?
EDIT - Attempting to address Edwin Ashworth's need for signs of research.
Need - Necessity, requirement.
Necessity - Constraint or determination by some external force; an instance of this.
In Fleischer's 1804 book "English Synonymous Or the Difference Between Words Esteemed Synonymous in the English Language: Useful to All who Would Either Write and Speak with Propriety and Elegance", need and necessity are given by
Need and necessity relates less to the situation of life than the other three words [in the context of poverty, indigence and want]; but more to the relief we expect or the remedy we seek, with this difference between the two that need seems less pressing than necessity.
although this is both contextual and dated. I am struggling to find other reliable sources, hence my post on this website.