Sentence:  Only a radical change in the nature of the population in the central city would be likely to destroy this preference - and we must now turn our attention to the question of whether such a change , gloomily foreseen by so many urban diagnosticians , <WSD>is</WSD> actually upon us . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)