One of the latest efforts to understand how global heating is changing the Atlantic Ocean’s surface and deep currents has concluded that natural variability—not forcing due to human influence—may have dominated the complex current system since 1900.
In the February, 2022, edition of the journal Nature Geoscience, researchers at the University of Maryland Center for Environmental Science urged more detailed study of the notoriously complex Atlantic Meridional Overturning Circulation (AMOC). Now, oceanographer Mojib Latif and his team from the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany are repeating that call in a paper just published in the journal Nature Climate Change.
The latest study describes the AMOC as a “three-dimensional system of current in the Atlantic Ocean with global climatic relevance.”
The February study responded to an August 2021 warning from the Potsdam Institute that the AMOC has become wildly unstable and dangerously weak due to global warming caused by human activity. The authors of the latest study affirm that the Earth’s oceans have taken up more than 90% of the accumulated heat and roughly a third of all CO2 emissions since the dawn of the industrial age, leading to clearly measurable and devastating impacts like marine heat waves, sea level rise, and ocean acidification.
Multiple climate models “predict substantial AMOC slowing if atmospheric GHG concentrations continue to rise unabatedly,” and those changes “would drive major climatic impacts such as shifting rainfall patterns on land, accelerating regional sea level rise, and reducing oceanic CO2 uptake,” they write. But it isn’t easy to confirm that the Atlantic circulation is actually slowing, partly because the ocean possesses such “large thermal and dynamical inertia.”
It is also extremely difficult to directly observe ocean circulation patterns in the North Atlantic, and proxies like sea surface temperature are “subject to large uncertainties,” the scientists say. Based on the available data, the GEOMAR study attributes localized sea surface cooling in the North Atlantic since 1900 to natural AMOC variability—not, as had been hypothesized, to a global heating-induced breakdown in the AMOC’s capacity to transfer heat.
While maintaining that an impact on the AMOC from human-induced warming “cannot be reliably estimated” from sea surface temperature readings, and that “a linear and direct relationship between radiative forcing and AMOC may not exist,” the authors call for “systematic and sustained in-situ AMOC observation systems to detect with high confidence externally forced AMOC slowing.”