According to the United Nations, 40% of the world's population lives within 100 km of a coast, meaning that close to three billion people could be impacted by changes in sea level. Coastal communities are centers of economic, social, and cultural development; they also provide significant ecological and environmental services. Global Mean Sea Level (GMSL) is increasing at about 3.3 millimeters per year (mm/y) and is already having catastrophic effects in coastal communities through flooding, erosion, and storm-related hazards.
Thermal expansion and the addition of fresh water to oceans from glacier and ice sheet melt are causing a rise in GMSL. As the atmosphere warms, much of its heat gets absorbed by the ocean, causing the water to expand. More than 90% of warming over the past 50 years has occurred in the ocean. Along with this thermal expansion, land-based glaciers and ice sheets are melting. Greenland is losing about 289 gigatons (Gt) of ice per year and Antarctica about 132 Gt. To put this in perspective, the largest animal on Earth, a blue whale, weighs about 330,000 pounds or 165 tons; each year Earth loses the equivalent in ice of about 2.5 billion blue whales.