A recent article in Nature Communications warns of a forthcoming collapse of the Atlantic Meridional Overturning Circulation (AMOC), with a transition most likely to occur between 2025-2095. Other scientists, though, say it is questionable whether a collapse could occur this century. However, if a collapse were to happen to the AMOC, there could be global ramifications, including abrupt cooling across large parts of the northern hemisphere, changes in tropical rainfall, and non-linear changes in sea-level rise in the North Atlantic Ocean.
The AMOC is a complex system of ocean currents that distribute heat throughout the Atlantic by transporting warmer waters north and cooler waters south. In this way, the AMOC, which is part of a global “conveyor belt,” helps regulate Earth’s climate; and through its impact on climate, the AMOC affects patterns of drought, flooding, disease, and mass migrations. Scientists say an AMOC shutdown may have occurred previously during the transition out of the last ice age about 14,500 years ago.
Oceanus spoke with three WHOI scientists — physical oceanographers Nicholas Foukal and Jiayan Yang and paleoceanographer Sophie Hines—to get their take on what is happening with this critical ocean circulation system, find out how they’re studying it, and learn about how changes in the AMOC could affect global climate change and ultimately all of us.
Based on your work, do you feel the AMOC is close to reaching a tipping point?
Foukal: I don’t know if the AMOC is reaching a tipping point, but this discussion is a good reminder that we should distinguish between an AMOC weakening that is predicted by the Intergovernmental Panel on Climate Change (IPCC) models to occur over the next century and has very strong scientific consensus, and a collapse that could happen over the course of a few years but is more speculative at this point. Though both would be catastrophic, a collapse would provide very little warning and likely lead to many consequences that we do not fully understand at this point. We have a better idea of what a weakening would entail: Europe would cool, the North Atlantic storm track would strengthen – which affects the entire eastern seaboard of the U.S. as well as Europe – and precipitation patterns over equatorial regions would shift southward. These effects would directly impact agriculture, food prices, transportation, construction, disease, immigration, and political stability. One of the most scientifically robust results at this point is that the AMOC is strongly connected to precipitation over the Sahel, a region that just experienced its seventh political coup in the past three years. I don’t think this is a coincidence—climate variability is directly linked to economic impacts and political stability. This recent Nature Communications article is an important step forward on this topic because it provides a proposed timeline for an AMOC collapse by mid-century based on historical ocean temperature measurements, but it does not provide information on why the AMOC would collapse then, if at all. The lack of information on this topic does not mean the AMOC will not collapse, just that we do not know enough at this point to say one way or the other. Uncertainty on a topic of this magnitude should be worrisome enough.
Yang: I don’t know whether the AMOC is near a tipping point or not. The AMOC has a lot of variability, it changes from year to year and from decade to decade, and a lot of that change is related to natural climate variability. If AMOC variability this year is less than last year, it’s probably not due to the AMOC slowing down but rather might be year-to-year change. If there is variability along a longer time scale, perhaps 30 or 50 years, that could indicate a slowdown. The AMOC is a complex system with many processes that can affect it. For instance, we need to understand how bathymetry affects the AMOC’s response to the atmosphere so that we can predict future AMOC changes.
To have better AMOC predictions for societal benefit, we need to have robust climate science and a sufficient understanding of the dynamics of the climate system. However, if climate models predict that we are near a tipping point and that the AMOC will shut down, we should pay attention to that. Even if we don’t yet have the observational data sufficient to prove whether this is a shutdown or not, we should not wait for another 50 years to collect the data—because that might be already too late.
Hines: I’m not strongly in one camp or another as to whether the AMOC is coming to a tipping point. The challenge is that we don’t have very long observational records. Because the AMOC is such an important part of Earth’s climate system, it’s so important to understand what is driving the AMOC, how it is changing today, and how it has changed in the past. Reconstructions of climate in the past indicate that you can have pretty rapid and large changes in climate, and we want to understand what drives those tipping points or where they occur, so we can be prepared.