Though temperatures were warmer during the last interglacial period, the greenhouse effect of atmospheric C02 is now melting more Arctic sea ice.
The prognosis for summer Arctic sea-ice loss over the next few decades is worse than it was 125,000 years ago during the last interglacial period, despite the fact temperatures were higher then.
That is the sobering conclusion reached by researchers at the Alfred-Wegener Institute in Germany. Their findings are published in the journal Nature Communications.
A team led by paleoclimatologist Rüdiger Stein combined sediment core data and climate models to estimate historical ice levels in the Arctic Ocean, in light of evidence-weighted predictions that such ice may disappear during the northern summer in the next 50 to 100 years.
Even though “the high latitudes were significantly warmer than today” during the interglacial period, the scientists state, “sea ice existed in the central Arctic Ocean during summer”.
The key climatic difference between then and now, they found, was the amount of carbon dioxide in the atmosphere. Sediment cores revealed that during the interglacial period, atmospheric CO2 levels were about 290 parts per million (ppm). Now the level is about 400 ppm.
Modelling used by the Intergovernmental Panel on Climate Change predicts CO2 levels will increase to 500 ppm over the next few decades. Plugging these values into their models, Stein and colleagues predict sea ice in the central zone of the Arctic Ocean will decline rapidly over the next several decades and disappear completely in about 250 years.
Rising CO2 levels add layers of complexity to Arctic climate models because it alters the feedback mechanisms that operate between ice, sunlight and seawater.
Wide, thick ice sheets reduce the biological fecundity of the waters beneath them but also have a strong albedo effect – reflecting sunlight back into atmosphere and thus limiting the energy absorbed by the ocean.
As CO2 levels increase, the ice sheets become less robust, shrinking and thinning. The albedo effect is proportionately reduced, potentially boosting ocean biomass through increasing light and heat.
The researchers write that sea ice, because it is involved in several key climate feedbacks including ice-albedo feedback and cloud-radiation feedback, “plays a substantial role in the global climate system variability, known as polar amplification”.
To assess sea-ice thickness more than 100,000 years ago, Stein and his team used biomarkers – specifically, signatures of a species of algae that exists beneath moderate sea ice sheets.
The absence of algal traces at points in the sediment cores is clear evidence of periods when the ice sheet was too thick for the species to inhabit the location.
The researchers concede their modeling and the ice core evidence contain inconsistencies that limit the predictive power of their findings. However, they term the paper an “important groundtruthing” exercise. They call for more detailed data and more sophisticated models that examine, in particular, “external forcings and related internal feedback mechanisms” to further improve results.
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