Early start of 20th century Arctic sea ice decline

Although Arctic-wide warming during the 20th century is well documented, little is known about the response of sea-ice to abrupt warming and it is unclear when the sea-ice decline started. Data coverage in this region is highly restricted, with observation-based satellite data only available since the 1970s, too short to accurately calibrate climate models.

Limited observational records therefore hamper the assessment of long-term changes in sea-ice, leading to large uncertainties in predictions of its future evolution under global warming. In the absence of instrumental data, natural archives of environmental changes, so-called proxies can be used to extend climate data further back in time.

In this study published in Geology, Steffen Hetzinger and colleagues present the first annually resolved 200-year record of past sea-ice variability from High Arctic Svalbard (79.9°N) using a newly developed in situ proxy from long-lived encrusting coralline algae. Annual growth and Mg/Ca ratios in this photosynthesizing benthic marine plant are strongly dependent on light availability on the shallow seafloor, recording the duration of seasonal sea-ice cover.

This proxy opens up a new possibility to study past sea-ice variability, and unlike previously available reconstructions from mainly land-based archives, it provides an annually resolved direct in situ proxy from the surface ocean.

Due to the limited availability of instrumental data, current research largely focuses on sea-ice decline since the late 20th century. The results of this study provide evidence for an earlier start of Arctic sea-ice decline at the beginning of the 20th century, not captured by shorter observational records and land-based reconstructions.

The algae also show that lowest sea-ice values within the past 200 years occurred from the 1980s to the early 2000s. These results may help reduce the large uncertainties that exist among ocean model simulations, providing a new approach for the detection and verification of long-term Arctic sea-ice changes.