One byproduct of rising carbon-dioxide levels is increasing ocean acidity — a phenomenon that scientists have termed an existential threat to marine life. The waters of the Arctic and the far-north Pacific are particularly prone to acidification as a result of several natural factors, so scientists regard the region as the proverbial canary in the coal mine for the rest of the world's oceans. A new study shows that within just fifteen years these waters may be too acidic for a range of marine animals to build and maintain their shells year round.