Past mass migrations were sparked by geopolitical circumstances rather than natural disasters, O'Neil said.

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From pharaohs to the medieval Vikings, swift and sometimes violent changes in weather patterns sparked mass migrations and technological innovations like irrigation.

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It also highlights the consequences of climate-related mass migrations.

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Hence, the great westward migrations throughout our history, not to mention the mass movement of African-Americans from the South to the North and, more recently, that of Latinos throughout the country.

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