A new research paper published this week has raised the specter of accelerated global warming due to a weakening of the North Atlantic Ocean’s circulation system, which is not only expected to continue to weaken in the coming decades but which could impact global surface temperature levels.
Global surface temperatures have been the focus of a tremendous level of attention over recent decades, due to both their steady rise between 1975 and 1998, as well as a subsequent slowing to growth of around 15 years which has been dubbed by many as an “hiatus.” Those 15 years ended abruptly, however, and in the past 5 years we have experienced the four warmest years on record, serving to demonstrate (to those willing to listen) that understanding the planet’s climate is not a simple task.
The complexity of our planet’s climate patterns is highlighted viscerally in a new research paper published this week in the journal Nature by Xianyao Chen from the Ocean University of China and Qingdao National Laboratory of Marine Science and Technology, and Ka-Kit Tung from the Department of Applied Mathematics at the University of Washington, Seattle. Their research is further highlighted in a Nature article penned by Gerard D. McCarthy and Peter W. Thorne, both from the Irish Climate Analysis and Research Units (ICARUS) of Department of Geography at Maynooth University, Ireland.
The new paper, Global surface warming enhanced by weak Atlantic overturning circulation, focuses on the Atlantic Meridional Overturning Circulation (AMOC) — a major current of the Atlantic Ocean that consists of a northward flow of warm, salty water through the upper layers of the Ocean which leads into a southward flow of colder water into the deeper layers of the Atlantic Ocean. According to Chen and Tung, the AMOC is able to explain changes in rates of global warming given its ability to take heat from the surface of the Ocean and store it in the deeper parts of the Atlantic. Conversely, McCarthy and Thorne describe the role of the AMOC thus:
“The connection between the AMOC and variations in the heat content of the subpolar North Atlantic Ocean has long been acknowledged. The AMOC transports heat northwards to the subpolar North Atlantic and to the Greenland, Iceland and Norwegian Seas. There, through a range of processes, deep water is formed that moves as a southward cold flow. This conveyor belt of northward-flowing, warm, shallow water and southward-flowing, cold, deep water defines the AMOC.”
In the most simple terms, however, if the role of the AMOC was absent from the climate, “surface temperatures could be 5 °C cooler in the subpolar North Atlantic Ocean and up to 10 °C cooler in the Norwegian Sea.”
The AMOC does not run at the same speed and intensity, however, and as such a strong AMOC is typically associated with warming across the Northern Hemisphere — an association that is backed up by paleoclimatology studies (the study of climate changes over Earth’s history) that show warmer periods coincided with a strong or vigorous AMOC and colder periods coincided with a weaker AMOC.
The new study by Chen and Tung, however, presents a different way of looking at the role of the AMOC. Specifically, when taking into account the increased levels of atmospheric greenhouse gases, they suggest that climate mechanisms of the past might not necessarily provide a suitable gauge of how they would act in the present or the future. Specifically, Chen and Tung argue that half of the heat that stems from the ever-increasing atmospheric greenhouse gas levels is being stored in the deep waters of the North Atlantic by the AMOC, and that these levels are increasing and reducing global surface warming (as seen below).
Chen and Tung further show that a cycle of increasing and then decreasing AMOC that spanned the 1940s to the mid-1970s coincided with a period that saw global warming slow, which was then followed by a period of weak AMOC that coincided with rapid warming from the mid-1970s to late-1990s, and another increase in AMOC strength between the late-1990s to 2005 saw a decrease in warming that led to the oft-highlighted “hiatus.”
According to McCarthy and Thorne, the Atlantic Ocean was not a natural suspect to explain the hiatus — in fact, the Pacific Ocean was deemed at the time to be a likely culprit considering its surface temperatures did not increase. The work of Chen and Tung, according to McCarthy and Thorne, “now bring focus to the North Atlantic” and “suggests that the warm surface temperatures there were indicative of an increasing AMOC and that the associated increase in ocean heat uptake played a key part in the ‘hiatus’.”
It’s worth noting that, as with much of climate science, scientists don’t have access to the perfect data-sets they might wish. Chen and Tung were forced to use proxies for AMOC strength because no direct observations exist of the necessary timescale for their research. Further, only four observatories currently exist that measure the AMOC across the full width of the Atlantic, and these observatories would need to be maintained for decades to create the necessary long-term data-sets. Subsequently, this research is preliminary and representative of the need for greater focus on the AMOC and its role in impacting the planet’s atmospheric climate. Chen and Tung also explain that much work needs to be done to understand how the AMOC affects surface temperatures in other oceans and on different timescales. For example, they highlight the potential role of the mammoth Southern Ocean in heat uptake in the period since 2005 which could be part of a see-saw pattern of alternating heat uptake between the North Atlantic and Southern Oceans.