Along U.S. coastlines, how much and how fast sea level rises will not just depend on global trends; it will also be affected by changes in ocean circulation, land elevation, and the rotation and the gravitational field of Earth, which are affected by how much land ice melts, and where.
The primary concern related to ocean circulation is the potential slowing of the Atlantic Ocean Meridional Overturning Circulation (AMOC). An AMOC slowdown would affect poleward heat transport, regional climate, sea level rise along the East Coast of the United States, and the overall response of the Earth’s climate system to human-induced change.34,178,179,180,181
The AMOC moves warm, salty water from lower latitudes poleward along the surface to the northern Atlantic. This aspect of the AMOC is also known as the Gulf Stream. In the northern Atlantic, the water cools, sinks, and returns southward as deep waters. AMOC strength is controlled by the rate of sinking within the North Atlantic, which is in turn affected by the rate of heat loss from the ocean to the atmosphere. As the atmosphere warms, surface waters entering the North Atlantic may release less heat and become diluted by increased freshwater melt from Greenland and Northern Hemisphere glaciers. Both of these factors would slow the rate of sinking and weaken the entire AMOC.
Though observational data have been insufficient to determine if a long-term slowdown in the AMOC began during the 20th century,31,182 one recent study quantifies a 15% weakening since the mid-20th century183 and another, a weakening over the last 150 years.184 Over the next few decades, however, it is very likely that the AMOC will weaken. Under the lower RCP4.5 scenario, climate model simulations suggest the AMOC might ultimately stabilize, though bias-corrected simulations continue to show a long-term risk.180 Under the higher RCP8.5 scenario, projections suggest the AMOC would continue to weaken throughout the century, increasing the probability of an AMOC shutdown (see Box 2.4).26,180,185
For almost all future global average sea level rise scenarios of the Interagency Sea Level Rise Taskforce,76 relative sea level rise is projected to be greater than the global average along the coastlines of the U.S. Northeast and the western Gulf of Mexico due to the effects of ocean circulation changes and sinking land. In addition, with the exception of Alaska, almost all U.S. coastlines are projected to experience higher-than-average sea level rise in response to Antarctic ice loss. Higher global average sea level rise scenarios imply higher levels of Antarctic ice loss; under higher scenarios, then, it is likely that sea level rise along all U.S. coastlines, except Alaska, would be greater than the global average. Along portions of the Alaska coast, especially its southern coastline, relative sea levels are dropping as land uplifts in response to glacial isostatic adjustment (the ongoing movement of land that was once burdened by ice-age glaciers) and retreat of the Alaska glaciers over the last several decades. Future rise amounts are projected to be less than along other U.S. coastlines due to continued uplift and other effects stemming from past and future glacier shrinkage.
Due to sea level rise, daily tidal flooding events capable of causing minor damage to infrastructure have already become 5 to 10 times more frequent since the 1960s in several U.S. coastal cities, and flooding rates are accelerating in over 25 Atlantic and Gulf Coast cities.186,187,188 For much of the U.S. Atlantic coastline, a local sea level rise of 1.0 to 2.3 feet (0.3 to 0.7 m) would be sufficient to turn nuisance high tide events into major destructive floods.189 Coastal risks may be further exacerbated as sea level rise increases the frequency and extent of extreme coastal flooding and erosion associated with U.S. coastal storms, such as hurricanes and nor’easters. For instance, the projected increase in the intensity of hurricanes in the North Atlantic could increase the probability of extreme flooding along most U.S. Atlantic and Gulf Coast states beyond what would be projected based on relative sea level rise alone—although it is important to note that this risk could be either offset or amplified by other factors, such as changes in storm frequency or tracks (e.g., Knutson et al. 2013, 2015170,190).