ABRUPT CHANGE IN THE ATLANTIC
MERIDIONAL OVERTURNING CIRCULATION
Key Findings
The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the
Earth’s climate system, characterized by a northward flow of warm, salty water in the upper
layers of the Atlantic, and a southward flow of colder water in the deep Atlantic. This ocean
circulation system transports a substantial amount of heat from the Tropics and Southern
Hemisphere toward the North Atlantic, where the heat is transferred to the atmosphere.
Changes in this circulation have a profound impact on the global climate system, as indicated
by paleoclimate records. These include, for example, changes in African and Indian monsoon
rainfall, atmospheric circulation of relevance to hurricanes, and climate over North America
and Western Europe. In this chapter, we have assessed what we know about the AMOC and
the likelihood of future changes in the AMOC in response to increasing greenhouse gases,
including the possibility of abrupt change. We have five primary findings:
- It is very likely that the strength of the
AMOC will decrease over the course of the
21st century in response to increasing greenhouse gases, with a best estimate decrease
of 25–30%.
21st century in response to increasing greenhouse gases, with a best estimate decrease
of 25–30%.
- Even with the projected moderate AMOC
weakening, it is still very likely that on
multidecadal to century time scales a warming trend will occur over most of the
European region downstream of the North Atlantic Current in response to increasing
greenhouse gases, as well as over North America.
multidecadal to century time scales a warming trend will occur over most of the
European region downstream of the North Atlantic Current in response to increasing
greenhouse gases, as well as over North America.
- No current comprehensive climate model
projects that the AMOC will abruptly
weaken or collapse in the 21st century. We therefore conclude that such an event is
very unlikely. Further, an abrupt collapse of the AMOC would require either a
sensitivity of the AMOC to forcing that is far greater than current models suggest
or a forcing that greatly exceeds even the most aggressive of current projections
(such as extremely rapid melting of the Greenland ice sheet). However, we cannot
completely exclude either possibility.
weaken or collapse in the 21st century. We therefore conclude that such an event is
very unlikely. Further, an abrupt collapse of the AMOC would require either a
sensitivity of the AMOC to forcing that is far greater than current models suggest
or a forcing that greatly exceeds even the most aggressive of current projections
(such as extremely rapid melting of the Greenland ice sheet). However, we cannot
completely exclude either possibility.
- We further conclude it is unlikely that the
AMOC will collapse beyond the end of
the 21st century because of global warming, although the possibility cannot be
entirely excluded.
the 21st century because of global warming, although the possibility cannot be
entirely excluded.
- Although our current understanding suggests it
is very unlikely that the AMOC
will collapse in the 21st century, the potential consequences of such an event
could be severe. These would likely include sea level rise around the North
Atlantic of up to 80 centimeters (in addition to what would be expected from
broad-scale warming of the global ocean and changes in land-based ice sheets due
to rising CO2), changes in atmospheric circulation conditions that influence hurricane
activity, a southward shift of tropical rainfall belts with resulting agricultural impacts,
and disruptions to marine ecosystems. The above conclusions depend upon our
understanding of the climate system, and on the ability of current models to simulate
the climate system. However, these models are not perfect, and the uncertainties
associated with these models form important caveats to our conclusions. These
uncertainties argue for a strong research effort to develop the observations,
understanding, and models required to predict more confidently the future evolution
of the AMOC.
will collapse in the 21st century, the potential consequences of such an event
could be severe. These would likely include sea level rise around the North
Atlantic of up to 80 centimeters (in addition to what would be expected from
broad-scale warming of the global ocean and changes in land-based ice sheets due
to rising CO2), changes in atmospheric circulation conditions that influence hurricane
activity, a southward shift of tropical rainfall belts with resulting agricultural impacts,
and disruptions to marine ecosystems. The above conclusions depend upon our
understanding of the climate system, and on the ability of current models to simulate
the climate system. However, these models are not perfect, and the uncertainties
associated with these models form important caveats to our conclusions. These
uncertainties argue for a strong research effort to develop the observations,
understanding, and models required to predict more confidently the future evolution
of the AMOC.
