Article: Iran's climate ranges from arid or semiarid, to subtropical along the Caspian coast and the northern forests. On the northern edge of the country (the Caspian coastal plain) temperatures rarely fall below freezing and the area remains humid for the rest of the year. Summer temperatures rarely exceed 29 °C (84.2 °F). Annual precipitation is 680 mm (26.8 in) in the eastern part of the plain and more than 1,700 mm (66.9 in) in the western part. United Nations Resident Coordinator for Iran Gary Lewis has said that "Water scarcity poses the most severe human security challenge in Iran today".

Now answer this question: According to Gary Lewis of the UN, what is the most pressing human security challenge in Iran?
Water scarcity