Research Arctic Sea Ice Loss Likely to Be Reversible

New research by Till Wagner and Ian Eisenman, scientists at Scripps Institution of Oceanography, UC San Diego, resolves a long-running debate over irreversible Arctic sea ice loss.

Ever since the striking record minimum Arctic sea ice extent in 2007, the ominous scenario of a sea ice tipping point has been a fixture in the public debate surrounding man-made climate change and a contingency for which Arctic-bordering countries have prepared.

For decades, scientists have been concerned about such a point of no return, beyond which sea ice loss is irreversible. This concern was supported by mathematical models of the key physical processes (known as process models) that were believed to drive sea ice changes. The process models forecasted that increased global warming would push the Arctic into an unstoppable cascade of melting that ceases only when the ocean becomes ice-free.

Implications of a permanently ice-free Arctic for the environment and for national and economic security are significant, driving deep interest in predictive capabilities in the region.

Wagner and Eisenman’s research was co-funded by the Office of Naval Research (ONR) and by the National Science Foundation. It supports the goals of the Navy’s U.S. Arctic Roadmap, which calls for an assessment of changes in the Arctic Ocean to clarify the national security challenges for future naval operations as this strategic region becomes increasingly accessible.

“The Navy has broad interest in the evolution of the Arctic,” said the ONR’s Frank Herr. “Sea ice dynamics are a critical component of the changing environmental picture. Our physical models lack important details on the processes controlling ice formation and melting, thus ONR is conducting a series of experimental efforts on sea ice, open water processes, acoustics, and circulation.”

During the past several years, scientists using global climate models (GCMs) that are more complex than process models found sea ice loss in response to rising greenhouse gases in their computer simulations is actually reversible when greenhouse levels are reduced.

“It wasn’t clear whether the simpler process models were missing an essential element, or whether GCMs were getting something wrong,” said Wagner, the lead author of the study. “And as a result, it wasn’t clear whether or not a tipping point was a real threat.”

Wagner and Eisenman resolve this discrepancy in the study in an upcoming Journal of Climate article,  “How Climate Model Complexity Influences Sea Ice Stability.”

They created a model that bridged the gap between the process models and the GCMs, and they used it to determine what caused sea ice tipping points to occur in some models but not in others.

“We found that two key physical processes, which were often overlooked in previous process models, were actually essential for accurately describing whether sea ice loss is reversible,” said Eisenman, a professor of climate dynamics at Scripps Oceanography. “One relates to how heat moves from the tropics to the poles and the other is associated with the seasonal cycle. None of the relevant previous process modeling studies had included both of these factors, which led them to spuriously identify a tipping point that did not correspond to the real world.”

“Our results show that the basis for a sea ice tipping point doesn’t hold up when these additional processes are considered,” said Wagner. “In other words, no tipping point is likely to devour what’s left of the Arctic summer sea ice. So if global warming does soon melt all the Arctic sea ice, at least we can expect to get it back if we somehow manage to cool the planet back down again.”

Record lows in Arctic sea ice extent are making frequent headlines in recent years. The change in albedo when sea ice is replaced by open water introduces a nonlinearity that has sparked an ongoing debate about the stability of the Arctic sea ice cover and the possibility of Arctic “tipping points”.

Previous studies identified instabilities for a shrinking ice cover in two types of idealized climate models: (i) annual-mean latitudinally-varying diffusive energy balance models (EBMs) and (ii) seasonally-varying single-column models (SCMs). The instabilities in these low-order models stand in contrast with results from comprehensive global climate models (GCMs), which typically do not simulate any such instability.

To help bridge the gap between low-order models and GCMs, we develop an idealized model that includes both latitudinal and seasonal variations. The model reduces to a standard EBM or SCM as limiting cases in the parameter space, thus reconciling the two previous lines of research. We find that the stability of the ice cover vastly increases with the inclusion of spatial communication via meridional heat transport or a seasonal cycle in solar forcing, being most stable when both are included.

If the associated parameters are set to values that correspond to the current climate, the ice retreat is reversible and there is no instability when the climate is warmed. The two parameters have to be reduced by at least a factor of 3 for instability to occur. This implies that the sea ice cover may be substantially more stable than has been suggested in previous idealized modeling studies.

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