Weakening Atlantic Meridional Overturning Circulation
Scientists expect the Atlantic Meridional Overturning Circulation (AMOC)—the major basinwide ocean current responsible for circulation in the Atlantic and certain global weather patterns—to weaken over the course of the 21st century due to the continued influx of cool freshwater from the melting Greenland ice sheet into the North Atlantic.
The unprecedented changes in Arctic sea ice as well as ice sheets and glaciers in Antarctica and Greenland pose a series of threats that intersect with many other climate change impacts. These changes contribute to sea level rise, play critical roles in global weather formation, and are a pivotal force in determining ocean currents.
One such current affected by ice sheet and glacier melt is the Atlantic Meridional Overturning Circulation, responsible for basin wide water circulation in the Atlantic and certain global weather patterns. Studies of past climates—known as paleoclimate reconstructions—have confirmed that cooling in the North Atlantic in the past has weakened the AMOC. The strength of the AMOC affects rainfall patterns close to the equator, including weather patterns as widespread as the American, African and Asian monsoons.
Global trends and projections
According to the IPCC, scientists have yet to identify observational evidence of a trend in the AMOC, based on the decade-long record of the complete AMOC and longer records of individual AMOC components.
The IPCC projects, however, that it is, “very likely that the Atlantic Meridional Overturning Circulation (AMOC) will weaken over the 21st century,” relative to 1850-1900 values. The best estimate projections range from a reduction of 11 to 34 percent, depending on future emissions levels. The IPCC goes on to state, “it is likely that there will be some decline in the AMOC by about 2050.”
The increased transport of freshwater into the North Atlantic and decreased density of the ocean surface layer in the North Atlantic “could act to reduce deep ocean convection there and contribute to a near-term reduction of strength of Atlantic Meridional Ocean Circulation (AMOC). However, the strength of the AMOC can also be modulated by changes in temperature, such as those from changing radiative flux.”
AMOC tipping points
“For an abrupt transition of the AMOC to occur, the sensitivity of the AMOC to forcing would have to be far greater than seen in current models, or would require meltwater flux from the Greenland ice sheet greatly exceeding even the highest of current projections. Although neither possibility can be excluded entirely, it is unlikely that the AMOC will collapse beyond the end of the 21st century for the scenarios considered, but a collapse beyond the 21st century for large sustained warming cannot be excluded.”