The denialosphere is all atwitter over a recent Josh Willis paper, which found no evidence for a present weakening of the Atlantic Meridional Overturning Circulation (AMOC), presenting its findings as though they contradict mainstream climate science and conflating the latter with the Hollywood disaster flick The Day After Tomorrow.
Fact vs. Fiction
Below are simulated depictions of the global thermohaline circulation from NASA, NOAA, and a cheesy Hollywood movie. Can you guess which is which?
Abrupt climate change (though nothing like in the movie) due to a slowdown/collapse of the AMOC is a genuine climatic phenomenon. A collapse of the AMOC within years or decades due to global warming, which plunges the North Atlantic into an ice age is decidedly not, however.
What does mainstream climate science say?
A fairly standard benchmark for assessing claims of what is considered to be “mainstream” climate science is its treatment in the most recent Intergovernmental Panel on Climate Change’s Assessment Report.
On whether or not the AMOC is currently (as of 2007) weakening-
AR4 WG1 Box 5.1, Has the Meridional Overturning Circulation in the Atlantic Changed?:
In summary, it is very likely that up to the end of the 20th century the MOC was changing significantly at interannual to decadal time scales. Given the above evidence from components of the MOC as well as uncertainties in the observational records, over the modern instrumental record no coherent evidence for a trend in the mean strength of the MOC has been found.
On whether or not we can expect an abrupt collapse of the AMOC this century-
AR4 WG1 10.3.4, Changes in the Atlantic Meridional Overturning Circulation:
Taken together, it is very likely that the MOC, based on currently available simulations, will decrease, perhaps associated with a significant reduction in Labrador Sea Water formation, but very unlikely that the MOC will undergo an abrupt transition during the course of the 21st century. At this stage, it is too early to assess the likelihood of an abrupt change of the MOC beyond the end of the 21st century, but the possibility cannot be excluded (see Box 10.1). The few available simulations with models of different complexity instead suggest a centennial slowdown.
Likewise, the U.S. Climate Change Science Program’s Synthesis and Assessment Report on Abrupt Climate Change found:
- 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.
- 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.
Current Observations and the Consequences of Future Slowdown
In a follow up post, I plan on looking at Willis’s latest paper in the context of the last few years of observational studies of potential AMOC slowdown, and what a slowdown or even a collapse could mean in terms of modern climate change.