As climate change increasingly becomes a fact of daily life, the health hazards of rising sea levels, catastrophic storms, water and food shortages, respiratory and vector-borne diseases, and temperature extremes are coming into sharper focus. Military leaders are warning that climate change could result in a refugee crisis of “unimaginable scale,” and some experts believe that a direct line can already be drawn to geopolitical events, such as the Syrian war and the rise of Boko Haram in Nigeria. Understanding the health consequences of these events, and especially their cumulative impact, is a crucial step toward action.