Reconstructing centuries of Arctic climate and sea ice conditions using annually-banded coralline algae

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Rapid Arctic warming during the past decades has contributed to a dramatic decline of sea-ice extent and thickness at a pace greater than simulated by climate models. If current trends persist, projections suggest that Arctic summer sea-ice may disappear within the next decades. However, the uncertainties in sea-ice modelling are large as long-term sea-ice variability is poorly understood due to short instrumental and satellite records, generally not extending beyond the late 1970s. In order to accurately model and therefore project future evolution of Arctic climate, it is essential to have a better understanding of year-to-year temperature and sea-ice variability during the past centuries, particularly prior to the industrial revolution when humans started impacting climate and ocean chemistry by the emission of fossil fuels. Such reconstructions are typically achieved by using proxy archives, such as tree rings or ice cores on land. Until recently though, no such proxy archive was known from the Subarctic and Arctic Ocean. In a breakthrough study my research group has now discovered that long-lived coralline algal buildups on the shallow sea floor can be used to reconstruct Arctic climate and ice variability during the past 650 years. During times of extensive sea ice cover little light reaches the seafloor resulting in low annual growth rates of the photosynthetic algae. In contrast, when sea ice cover is low annual algal growth rates increase. By measuring algal growth and geochemical composition year-by-year into the past sea ice cover and temperatures can be reconstructed. Coralline algal buildups are widespread throughout the entire Arctic Ocean where they can live for centuries attached to the shallow seafloor while forming annual growth bands in their calcitic skeletons.