Only 0.5 percent of the world’s population lives in the Arctic, a remote region where extreme conditions prevail and daily life revolves around ice, both glacial ice and sea-ice. And as global warming continues to cause rising atmospheric and ocean temperatures, that all-important sea ice is increasingly melting away.

Cambridge Bay, Nunavut

There are two reasons why disappearing sea ice in the Arctic matter to someone living in a large urban centre in the U.S. or India or China: Because most of the world’s biggest cities—including New York, Mumbai, and Shanghai—are located on coastlines and melting glacial ice sheets, and subsequent sea-level rise will threaten...