The Atlantic Meridional Overturning Circulation (AMOC) is a massive, looped belt of water that connects the world’s oceans, carrying warm waters north and cold waters south. The system not only transports heat and nutrients but also affects key weather patterns like the Indian and African monsoons.
Climate models suggest potentially devastating consequences if this system were weakened or lost, including more shifts in precipitation patterns, extreme weather, and changes to regional sea levels. Scientists believe that climate change has the potential to switch AMOC “on” and “off”; however, they aren’t yet certain what conditions would flip that switch.
In its most recent report, the Intergovernmental Panel on Climate Change stated that although AMOC could weaken as a result of human-induced global warming, it is unlikely to collapse entirely in the 21st century. That report did not include factors such as the melting of the Greenland Ice Sheet, however, which has accelerated over the past several decades, or the basic uncertainty surrounding AMOC’s stability. Now Bakker et al. have included those factors in eight state-of-the-art climate models, projecting what is likely to happen under two separate climate scenarios up to and beyond the 21st century.
In one scenario, anthropogenic carbon dioxide (CO2) emissions peak around 2040, then decline. Under these conditions, AMOC does not collapse entirely but weakens by about 18% by roughly 2090–2100, the scientists report.
In the second, “business as usual,” scenario, emissions continue to rise at their current rate throughout the 21st century. In their models, the researchers found that by 2100, the system will lose about 37% of its strength. By 2290–2300, AMOC will diminish by approximately 75% and will have a 44% chance of collapsing entirely, the scientists report.
Although the latter scenario may sound bleak, it suggests that aggressive action to reduce CO2 emissions could have a profound impact, according to the authors. Indeed, the likelihood of full collapse stays significantly smaller if global warming is limited to less than 5 K, they say. (Geophysical Research Letters, doi:10.1002/2016GL070457, 2016)
—Emily Underwood, Freelance Writer