The claim in the passage, "Beyond Thirst: The Global Water Crisis" is this: "Left unchecked, the crisis will only worsen."
A claim is an opinion that is usually made at the introductory part of a text. In the second paragraph of the cited passage, the author claimed that there is an existing challenge with water, which left unchecked, will only worsen.
In the body of the passage, he supported this claim with proof.
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