Arctic Paleotemperature and Sea Ice: The last 10,000 Years
The Holocene interglacial period (11,700 years ago to present) represents the most recent stable warm period, which began after the transitional period of deglaciation (19,000-11,700 years ago). Holocene records of Arctic sea ice extent indicate considerable spatial and temporal variability over the last 10,000 years, including seasonally ice-free periods in various regions from 6,000 to 10,000 years ago when solar insolation in the Arctic was strongest (Polyak et al., 2010; Müller et al., 2012). For many parts of the Arctic, the modern (prior to anthropogenic forcing) perennial sea-ice cover did not fully develop until the latter half of the Holocene, about 5,000 years ago (Darby et al., 2006).
Rapid sediment accumulation in Arctic marginal seas allows for a detailed, higher-resolution history of sea ice variability to be obtained for the Holocene. High-resolution IP25 biomarker (
Table 1) records from the Chukchi and East Siberian seas indicate that sea ice concentrations were controlled not only by decreasing insolation, but also by the fraction of Pacific water delivered via the Bering Strait (Stein et al., 2017; Polyak et al., 2016). Records of IP25 from the Fram Strait (Müller et al., 2012), the Laptev Sea (Horner et al., 2016) and the Canadian Arctic Archipelago (Vare et al., 2009) agree with the long-term trends recorded in the Chukchi and Eastern Siberian seas, demonstrating that there was a circum-Arctic Holocene expansion of sea ice in the marginal seas. Lithologic, geochemical and microfossil records from the Fram Strait also indicate regional variations in sea ice related to varying influx of warm Atlantic waters into the Arctic basin (e.g., Werner et al., 2013).
A particularly high-resolution sea ice history has been established for the last 1,450 years using a network of terrestrial (tree ring and lake sediment) and ice-core records located around the margins of the Arctic (Kinnard et al., 2011). This network yields a history of summer sea ice minima during the pre- and post-industrial periods. Results indicate a pronounced decline in summer sea ice beginning in the 20th century with exceptionally low ice extent recorded since the mid-1990s (
Fig. 3).
While several episodes of reduced and expanded sea ice extent occur in association with climate anomalies such as the Medieval Climate Warm Period (AD 800-1300) and the Little Ice Age (AD 1450-1850), the extent and pace of the modern decline in sea ice is outside of the range of natural variability and unprecedented in the 1450-year reconstruction (Kinnard et al., 2011). The authors conclude that the pre-industrial sea ice climate signal (prior to anthropogenic forcing) was driven by a combination of changes in atmospheric temperature and circulation as well as variations in the inflow of warm saline Atlantic-sourced water masses through the Fram Strait. The significant post-industrial sea ice decline occurs in concert with significant atmospheric and ocean warming driven by an exponential increase in atmospheric CO2 (
Fig. 3). High-resolution temperature proxy records from Arctic regions indicate that the modern rate of increasing surface air temperature has not been observed in at least the last 2000 years (McKay and Kaufman, 2014). Further, the Arctic Ocean has experienced intensified warming due to the increasing inflow of relatively warm Atlantic Waters from the Norwegian Sea into the Arctic basin (
Fig. 3; Spielhagen et al., 2011). Using both ecology and geochemistry of foraminifera from a Fram Strait sediment core, scientists report a 2° C increase in inflowing water temperature since 1900.
Paleoceanographic Perspectives on Arctic Ocean Change
For the last 6000 years the world has been cooling, and the ice has been constant over the north pole. Periods of reduced and expanded ice were recorded in the LIA and MWP. However, at no time has the increase in surface temperature been what we are seeing today.