In the United States, nearly 30 percent of people live in coastal areas, where sea level rise plays a role in flooding, shoreline erosion, and hazards from storms. Globally, 8 of the world’s 10 largest cities are near a coast.

Rising seas threaten roads, bridges, subways, water supplies, oil and gas wells, power plants, sewage treatment plants, landfills—in short, almost everything we rely on in our communities. Sea level rise also threatens important ecosystems. Birds and other wildlife that rely on low-lying habitats will find it harder to survive as these habitats disappear. Sea turtles, which rely on beaches to lay their eggs, will face new challenges as beaches disappear and physical barriers like sea walls prevent them from reaching the ocean to nest.

Currently, global sea levels are rising at a rate about twice as fast as it did in the 20th century. This increase is mainly caused by thermal expansion of the ocean as it warms (water expands when it heats) and by increased melting of glaciers and ice sheets. These changes are occurring because the planet is warming from human activities.

Scientists can’t say exactly how high the seas will rise this century, but they know that if humans continue on our current course of greenhouse gas emissions—which would cause even more warming of the atmosphere and ocean—the pace of sea level rise will dramatically accelerate. Sea levels could eventually rise by more than a meter, and that is very bad news for millions of people around the globe, especially in coastal regions.