A recent study by Portmann and colleagues (Observational constraints project a ~50% AMOC weakening by the end of this century – PMC) warns of yet another impending “climate disaster.” The study predicts that the Atlantic Meridional Overturning Circulation (AMOC), a key component of thermohaline circulation, will weaken approximately 50 percent by the end of the century.
If true or grounded in solid evidence, it would be quite alarming because the AMOC drives warm water from the tropics and subtropics into the North Atlantic and Arctic. This immense “heat pump” moderates the climates of western Europe to a large degree, keeping it much warmer than inland counterparts at comparable latitudes. Further north, an energized AMOC has brought enough warmth to the Arctic to trigger a phenomenon known as Arctic Amplification.
Arctic Amplification has resulted in the Arctic warming twice as fast as the tropics and seven times as fast as Antarctica since the late 1970s. A 50 percent reduction in the AMOC intensity would reverse the North Atlantic/Arctic warming, ushering in a protracted period of regional cooling. The energy and agricultural systems of the northern tropics would be negatively impacted, posing serious threats to the economic security of the region. Crops would fail, diseases would become rife, and deaths related to cold temperatures — already more of a problem than deaths from heat — would increase.
When you read through Portmann’s article, a huge red flag waves: The model ensembles employed in the study are founded on the questionable assumption that anthropogenic greenhouse warming is the main driver of the AMOC slowdown. This assumption is false. The climate models referenced in the study, as is common with climate models in general, underestimate or exclude other critical drivers of the climate system. These include solar irradiance, cloud coverage, water vapor concentrations, and geothermal heating of the oceans. All these drivers are known to change over time and interact in a non-linear fashion that is exceedingly difficult to model over the long term.
In addition to poorly accounting for these other drivers, the “CO2 is the climate control knob” hypothesis is fraught with other difficulties. For starters, it is known that at current levels, the atmosphere is nearly saturated with regard to infrared absorption by CO2. Additional CO2 loading will yield diminishing returns into the future as the warming decays in asymptotic fashion. This is established science that many climate alarmists ignore, and this critical oversight significantly inflates warming estimates. A study by Roy Spencer shows that the globe has warmed “43 percent less than that produced by computerized climate models.” By all definitions, this is an “epic failure” of the climate modeling community.
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Finally, this is just one study, and its findings are disputed by multiple other studies published in recent years, some of which have found that the AMOC has been relatively stable over the past 60 years, not declining in strength, and others citing data that suggest the AMOC has actually gained strength recently. The Portman et al. study fails to address these findings, focusing strictly on model outcomes.
This veritable “house of cards” reminds me of a fanciful narrative from a college philosophy course that posits three travelers are perilously shipwrecked on a desert island. One is a physicist, one a biologist, and the third is a climate modeler. Unexpectedly, a can of tuna washes ashore from the shipwreck, bringing hope to the marooned trio.
The physicist looks at the surrounding palm trees and says, “If we can climb to the top of the tallest tree and drop it towards the ground, it will achieve terminal velocity, and the force with which it strikes the ground may exceed the tensile strength of the can, exposing its contents.” The biologist looks at the can and says, “Once opened, we can harvest enough protein to sustain us in a semi-starved state for approximately two weeks.” The physicist and the biologist look to the climate modeler, and he unabashedly proclaims, “Assuming we had a can opener ….”
The “CO2 is the primary driver” assumption is the proverbial can opener for all these modeling products: It doesn’t exist, or at least doesn’t exert the influence claimed for it. Real-world data provide no evidence that the AMOC faces a near-term collapse. Policy responses, if any, should be based on reality, not climate model outputs based on flawed assumptions.
Arthur Viterito, Ph.D., retired as a Professor of Geography, College of Southern Maryland and serves as a Policy Advisor to The Heartland Institute. H. Sterling Burnett, Ph.D., is the director of the Arthur B. Robinson Center on Climate and Environmental Policy at The Heartland Institute, a public policy research institute based in Illinois.
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