The Atlantic meridional overturning circulation (AMOC) stretches thousands of miles. Image credit: NOAA
A system of Atlantic Ocean currents that plays a crucial role in regulating Earth’s climate has long been predicted to significantly weaken over time due to global warming—thereby bringing about major shifts in regional sea level and temperatures.
However, a new analysis by an international team of researchers shows that while climate change will cause this system of ocean currents—the Atlantic meridional overturning circulation (the AMOC)—to weaken, this is likely to occur less dramatically than current climate model projections have suggested.
The study, which included researchers from New York University, Caltech, the University of Washington, and China’s Laoshan Laboratory and appears in the journal Nature Geoscience, sheds light on a long-standing and previously unexplained feature of climate models: the link between the present-day and future strength of the AMOC.
“Our results imply that, rather than a substantial decline, the AMOC is more likely to experience a limited decline over the 21st century—still some weakening, but less drastic than previous projections suggest,” says Dave Bonan, a postdoctoral researcher at the University of Washington who led the study as a doctoral student at Caltech.
“This study is our attempt to bridge the gap between understanding and predicting AMOC changes with simple physical principles and real-world data,” adds Laure Zanna, a professor at NYU’s Courant Institute of Mathematical Sciences and NYU’s Center for Data Science.
In addition to transporting heat from the Southern to the Northern Hemisphere, the AMOC modulates regional weather—from the mild summers in Europe to the monsoon seasons in Africa and India. In predicting the AMOC’s near-collapse, existing climate models have predicted that such a weakening would have far-reaching consequences, including changes in regional sea level rise and major shifts in regional climate, such as colder conditions in northern Europe and drier weather in parts of the Amazon and West Africa.
In the new Nature Geoscience paper, Bonan, Zanna, and their colleagues developed a simplified physical model based on fundamental principles of ocean circulation—specifically, how density differences and the depth of the overturning circulation are related.
In doing so, the scientists offered a better gauge of the AMOC’s present-day strength, which then yielded a more precise accounting of its future strength.
Broadly speaking, climate models that simulate a stronger present-day AMOC tend to project greater weakening under climate change. The researchers found that this relationship stems from estimates of the current depth of the AMOC. More specifically, estimates of a stronger present-day AMOC typically model greater depths and allow changes in surface water temperature and salinity properties—caused by global warming and freshwater input—to penetrate deeper into the ocean and, as a result, drive greater weakening in the future.
Therefore, a climate model with a stronger and deeper AMOC is less resilient to surface changes and experiences proportionally more AMOC weakening than one with a shallower current. Climate models with a shallower present-day AMOC, as calculated in the Nature Geoscience paper, still show weakening under climate change, but to a lesser extent than those with a deeper present-day AMOC.
Using this updated model and advanced methods, they found that the AMOC will weaken by around 18 to 43 percent at the end of the 21st century. The authors acknowledge that their conclusion represents some weakening. However, they add, it does not predict the substantial weakening that the more extreme climate model projections have suggested. In addition, the Nature Geoscience research significantly narrows the range of future AMOC weakening, addressing a long-standing uncertainty in climate science—very recent estimates calculated this weakening to be as low as 20 percent and as high as 80 percent.
The research was conducted in the laboratories of study authors and Caltech professors Tapio Schneider and Andrew Thompson. The study’s other authors are Kyle Armour, a climate scientist at the University of Washington, and Shantong Sun, an oceanographer at Laoshan Laboratory in Qingdao, China.
The research was supported by grants from the National Science Foundation (DGE1745301, OCE-1756956, OCE-1850900, and AGS-1752796), the David and Lucile Packard Foundation, and Schmidt Sciences LLC.