One of the most important questions in the geophysical sciences is whether the system of currents of the North Atlantic, collectively referred to as the Atlantic Meridional Overturning Circulation, or AMOC, has strengthened or weakened in recent years.
Opinions vary widely, but there’s a strong consensus among climate modelers that the AMOC has weakened in the past and will continue to weaken going forward. A slowing AMOC could have profound impacts on the global climate, ranging from cooling in Northern Europe and the Arctic to accelerated trade winds in the Pacific.
By contrast, recent empirical studies have challenged the “weakening” hypothesis. Getting the argument “right” is critically important, as a strengthened AMOC would increase Arctic temperatures. This would result in diminished ice cover in the Arctic Ocean, permafrost retreat, expansion of the boreal forests, and shrinkage of the Eurasian and North American tundra. Three recent studies clearly illustrate this critical epistemological divide.
In a 2018 modeling study by Caesar, et al., (Observed fingerprint of a weakening Atlantic Ocean overturning circulation | Nature), the authors conclude that the AMOC has been weakening since the mid-twentieth century. To quote directly from the study:
“This weakening is revealed by a characteristic spatial and seasonal sea-surface temperature ‘fingerprint’—consisting of a pattern of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region—and is calibrated through an ensemble of model simulations from the CMIP5 project. We find this fingerprint both in a high-resolution climate model in response to increasing atmospheric carbon dioxide concentrations, and in the temperature trends observed since the late nineteenth century. The pattern can be explained by a slowdown in the AMOC and reduced northward heat transport….”
By contrast, a 2021 empirical study by Oziel, et al. (Faster Atlantic currents drive poleward expansion of temperate phytoplankton in the Arctic Ocean | Nature Communications) largely contradicts this finding. In their research of North Atlantic currents, the authors demonstrate that the Northern Branch of the Gulf Stream has increased in speed by as much as a factor of two since 1995. Using phytoplankton and satellite data, they conclude:
“The Arctic marine biome, shrinking with increasing temperature and receding sea-ice cover, is tightly connected to lower latitudes through the North Atlantic. By flowing northward through the European Arctic Corridor (the main Arctic gateway where 80% of in- and outflow takes place), the North Atlantic Waters transport most of the ocean heat, but also nutrients and planktonic organisms toward the Arctic Ocean. Using satellite-derived altimetry observations, we reveal an increase, up to two-fold, in North Atlantic current surface velocities over the last 24 years.”
A third study, which combines modeled results with a wide array of empirical inputs, corroborates the Oziel et al. findings. As the authors state in Nordic Seas Heat Loss, Atlantic Inflow, and Arctic Sea Ice Cover Over the Last Century – Smedsrud – 2022 – Reviews of Geophysics – Wiley Online Library:
“The Arctic Ocean, including the Nordic and Barents Seas, has warmed since the 1970s. This warming is congruent with increased ocean heat transport and sea ice loss and has contributed to the retreat of marine-terminating glaciers on Greenland.”
A press release of this study by the Bjerknes Center for Climate Research (The Gulf Stream has increased steadily over the last century (uib.no)) states:
“The heat transport into the Nordic Seas has increased steadily in volume and temperature over the last century…”
The Bjerknes press release goes on to say:
“It was surprising to find such consistent results that show a steady increase, which entails that the Gulf Stream’s extension into the Nordic Seas has strengthened…. With the surprising volume increase, the total heat transport has increased … 30 percent.”
Of note in these remarks is that the researchers (there were 17 on the team) were “surprised” by the strengthening. Undoubtedly, this is a reaction to the large number of modeling studies that have concluded otherwise. Clearly, climate science has become deeply committed to numerical modeling with diminishing regard for empirical inputs.
Rooted in the long-standing traditions of the scientific method, we should never ignore the value of observation. Furthermore, we must stop being so reliant on “tuned” algorithmic constructs of our world, as their products have been used to craft far-reaching policies affecting how we value and use our resources. Empiricism should always be the touchstone of climate inquiry. The modelers may think differently on this, but the temperate-loving phytoplankton that have recently migrated to the Arctic would surely beg to differ.