During the 20th century, global sea level rose 20 cm (8 in.). That number may seem small, but the last time it occurred, it took 6,000 years for seas to rise so much. In the next century, sea levels could rise anywhere from 0.3 to 1 meters (1 to 3 feet), and it is virtually certain sea levels will continue to rise centuries after 2100.
Sea level rise occurs in two major ways. As oceans warm, they take up more space, called thermal expansion. Second, as glaciers melt, they add volume to the oceans. These processes are causing oceans to swell and overtake the coasts.
In Chesapeake Bay, sea levels appear to be rising at 3 to 4 millimeters per year, roughly twice the global average. This is because the land of Chesapeake Bay is also sinking, partly due to a geologic reflex from the end of the last Ice Age and partly ongoing compaction from a bolide impact 35 million years ago. It’s enough to threaten to drown wetlands, which protect our homes from storms, among other ecosystem services. Wetlands can rise upward when facing accelerated sea-level rise by building soil, but will tidal wetlands continue to build new soil fast enough to stay above the tide? SERC ecologists discovered in 2009 that elevated CO2 can help tidal marshes gain elevation, and they are presently studying how this varies with other global changes such as warming, nitrogen pollution and invasive species. Much of their ability to build elevation depends on society. Dams, changes in hydrology and other development jeopardize wetland’s ability to withstand sea level rise.
Other projects investigate how sea-level rise impacts mangroves, the dense, tangled trees lining tropical coasts around the world. Mangroves have evolved to survive with their roots underwater, or propped high in the air. Like temperate wetlands, mangroves buffer against storms, store carbon and offer critical habitat for fish and other species. But as seas rise, even they will be forced to migrate inland or drown.