Understanding Past Changes in Southern Ocean Sea Ice – Eos

Southern Ocean sea ice plays a fundamental role in regulating key Earth system processes, including nutrient cycling, air-sea gas exchange, ocean heat transport, bottom water formation, and stabilization of ice shelves along the Antarctic Ice Sheet margin. Scientists anticipate changes in the areal distribution and seasonal extent of sea ice in response to continued Southern Ocean warming. Thus, to simulate future climate accurately using Earth system models, it is important to understand the complex interactions between sea ice and the climate system.

Studying past changes in Antarctic sea ice enables scientists to investigate these interactions under the very different climatic conditions prevalent in the past, providing excellent case studies against which to test models. However, the most detailed and high-resolution Southern Ocean sea ice reconstructions span only the Last Glacial Maximum and Holocene periods. The lack of data encompassing a full glacial-interglacial cycle greatly limits our ability to understand the role of sea ice in the large-scale reorganizations of ocean and atmosphere circulation, ice sheet configuration, and carbon cycling that occurred across the glaciation and deglaciation periods.

This knowledge gap is the target of the Cycles of Sea-Ice Dynamics in the Earth system (C-SIDE) working group, who held their inaugural workshop last October. Thirty-two participants represented 11 countries. Participants’ complementary expertise encompassed research areas from proxy-based sea ice reconstructions to Earth system modeling. The 2.5-day program began by examining existing sea ice proxy data, including information from marine microfossils, organic biomarkers, and ice core chemical constituents. Participants presented exciting new data that will help researchers improve the temporal resolution and spatial coverage of proxy records—an ongoing aspiration in the community.

Other key challenges discussed included constraining past variability in the seasonal cycle of sea ice, which affects bottom water formation, water mass composition and distribution, primary productivity, and carbon sequestration (Figure 1). In addition, participants discussed the growing need for ground truthing, process-based understanding, and standardization of proxies. Participants considered how multiproxy data sets can provide complementary evidence of sea ice change and agreed on a framework for data sharing and assimilation.

Principal feedbacks and interactions between Antarctic sea ice and the ocean, biosphere, atmosphere, and cryosphere.
Fig. 1. Principal feedbacks and interactions between Antarctic sea ice and the ocean, biosphere, atmosphere, and cryosphere. Credit: Claire Allen

The C-SIDE group also aims to quantify the interaction of sea ice with the wider climate system using geochemical tracers of related processes and Earth system models. The group heard how models capable of long, transient simulations provide a dynamic framework that can be used to identify how physical climate phenomena and biogeochemical processes are linked to Antarctic sea ice changes. At present, the mechanisms responsible for sea ice formation during past and present climates differ between models, adding complexity to our understanding of the downstream influence on the deep-ocean state and circulation changes. To help with future model evaluation, participants resolved to synthesize and share sea ice and sea surface temperature simulations from Paleoclimate Modelling Intercomparison Project (PMIP) phase 3 and PMIP4 (when it becomes available).

The next C-SIDE workshop is planned for 29–31 August 2019 in Sydney, Australia. It will focus on synthesizing and reconciling compiled sea ice proxy records and model simulations across the complete glacial cycle.

We thank Past Global Changes (PAGES) and Simon Fraser University, Canada, for financial support.

—Molly Patterson, Department of Geological Sciences, Binghamton University, N.Y.; Rachael Rhodes ([email protected]), Department of Geography and Environmental Sciences, Northumbria University, Newcastle-upon-Tyne, U.K.; and Claire Allen, British Antarctic Survey, Cambridge, U.K.