Global warming has a few major effects on the oceans. As water gets warmer, it expands. And as glaciers and ice caps in places like Greenland and Antarctica melt, they add water to the ocean. That all causes sea levels to go up.
Global average sea levels have risen roughly 19 centimeters (7.5 inches) since the 19th century, after 2,000 years of relatively little change. The rate of sea-level rise has continued to increase in recent decades:
Exactly how high sea levels will rise in the future depends on how greenhouse gas emissions rise and how the world warms. This IPCC chart shows future projections under a low-emissions scenario (in blue) and a high-emissions scenario (in red):
Low emissions: We’ve already warmed the planet enough to heat and expand the oceans and lock in some melting of land ice. So even if we do reduce emissions, we can still expect some additional sea-level rise in the decades ahead — possibly half a meter (1.6 feet) by the end of the century, and continuing thereafter.
High emissions: If greenhouse gas emissions keep growing, however, sea-level rise gets even more drastic. The IPCC is currently predicting up to 1 meter of sea-level rise (3.3 feet) by century’s end if emissions keep growing unchecked. And the oceans would continue to rise for centuries thereafter.
Effects: Rising sea levels are expected to increase the risk of flooding, storm surges, and property damage in coastal cities and regions. One 2013 study in Nature Climate Change estimated that average annual losses from flooding in the world’s biggest coastal cities could rise from $6 billion per year today to $1 trillion per year by 2050. Cities could build flood defenses such as levees, pumps, and movable barriers, but at a cost of tens of billions of dollars per year.
Uneven rise: Sea levels also won’t rise evenly everywhere. In some regions the land is actually sinking, due to sediment erosion or freshwater pumping. In other regions, strong wind and ocean currents can warp the waters and affect local sea levels. The melting of the giant ice caps will also have odd gravitational effects.
That’s why, for example, New York and New Jersey could experience sea-level rise that’s 8 inches higher than the global average this century.