Atlantic Circulation: Holding Steady or Heading for a Slowdown?

The Debate Over Declining Currents

The Atlantic Meridional Overturning Circulation (AMOC), a vital system of ocean currents including the Gulf Stream, plays a crucial role in regulating global climate by transporting heat to the Northern Hemisphere. Scientists agree that climate change poses a threat to this delicate system, potentially leading to a weakening of these currents. The question of whether this decline has already begun, however, is a subject of intense debate.

Recent research offers a surprising perspective. A study published in Nature Communications suggests the AMOC has remained remarkably stable over the past six decades. This finding hints at a greater resilience to warming than previously anticipated. However, this conclusion has sparked controversy, as other studies paint a different picture, some even suggesting the AMOC is weaker now than at any point in the last millennium.

As Maya Ben-Yami, a climate researcher at the Technical University of Munich, puts it, "whether or not the AMOC has weakened is still an open question."

Unraveling the Mysteries of the AMOC

Direct observations of the AMOC's strength are limited, reaching back only to 2004. To investigate long-term trends, researchers rely on indirect clues, or "fingerprints," like sea surface temperature, salinity, and data from marine sediments. The challenge is that these fingerprints don't always tell the same story.

The recent study employed a novel approach, utilizing cutting-edge climate models and focusing on "sea-air heat flux," the exchange of heat between the ocean and the atmosphere. The models revealed no decline in heat transfer over the last 60 years, leading the researchers to conclude that the AMOC has held steady.

"Our paper says that the Atlantic overturning has not declined yet,” explains study co-author Nicholas Foukal, an assistant scientist at the Woods Hole Oceanographic Institution. "That doesn't say anything about its future, but it doesn't appear the anticipated changes have occurred yet."

A Sea of Uncertainty

Not everyone is convinced. Critics argue that the reliability of sea-air heat flux data is questionable, as it's derived from potentially flawed observations. Factors like air temperature, wind speed, and cloud cover, all difficult to measure accurately, can significantly influence heat transfer.

Stefan Rahmstorf, an oceanographer at the Potsdam Institute for Climate Impact Research, expresses skepticism, stating, "Therefore, these surface heat loss data are rather inaccurate and in my view not so well suited to reconstruct the AMOC time evolution.”

A New Perspective

Despite the uncertainties, some experts see value in the new research. David Thornalley, a professor at University College London, notes that the focus on air-sea heat fluxes offers "useful new insight." He points out that the idea of little change in the AMOC over recent decades isn't entirely novel. His own research suggests a weakening occurred in the 1800s, followed by a period of relative stability.

The Future of the AMOC

While the past behavior of the AMOC remains a topic of ongoing investigation, the future trajectory appears less ambiguous. The scientific consensus overwhelmingly points toward a slowdown in Atlantic circulation due to climate change, even with significant reductions in greenhouse gas emissions.

As Foukal concludes, "it's almost unanimous at this point that the Atlantic overturning will slow in the future."