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In this study, we reconstruct three-century of sea ice cover history in the Chukchi Sea from the downcore profile of total organic carbon (TOC) and biomarker proxies, namely the Ice Proxy with 25 carbon atoms (IP _25 ), the di- and tri-unsaturated highly branched isoprenoid (HBI II and HBI III) and two phytosterols (brassicasterol and dinosterol) in three sediment cores from the northern, eastern and southern Chukchi Sea reflecting different sea ice conditions. Our data indicate higher IP _25 values in the eastern site and lowest ones in the northern Chukchi Sea site that are consistent with the modern sea ice distribution. They also underline the predominance of sympagic over pelagic production except at the southern site where pelagic production depicts a sharp increased over the last decades. We present a new approach improving the linear relationship between P _III IP _25 (P _B IP _25 ) and satellite-derived spring (summer) sea ice concentrations (SIC) to advance sea ice reconstructions across the Arctic Ocean. This method results in better assessment of PIP _25 derived SIC and reconstruction of past seasonal sea ice conditions. They indicate marginal sea ice conditions at the three sites until 1950s–1960s followed by a reduction of seasonal sea ice as captured by P _B IP _25 index.

Abstract Highly branched isoprenoids (HBIs) in marine sediments have emerged as promising semi‐quantitative proxies to reconstruct seasonal sea ice in polar oceans. In this work, we examine the distribution of sympagic HBIs (IP25 and HBI‐II), pelagic phytoplankton biomarkers (brassicasterol, dinosterol and HBI‐III) as well as terrestrial sterols (campesterol and β‐sitosterol) in the surface sediments of the East Siberian Sea (ESS) to test their reliability as sea‐ice proxies under continental run‐off influence. Our data suggest that dinosterol performs better than brassicasterol to assess sea ice across the ESS shelf, yet the correlation between PDIP25 and spring sea ice is relatively weak but improves when removing sites with salinity

Faced with sea level rise and the intensification of extreme events, human populations living on the coasts are developing responses to address local situations. A synthesis of the literature on responses to coastal adaptation allows us to highlight different adaptation strategies. Here, we analyze these strategies according to the complexity of their implementation, both institutionally and technically. First, we distinguish two opposing paradigms – fighting against rising sea levels or adapting to new climatic conditions; and second, we observe the level of integrated management of the strategies. This typology allows a distinction between four archetypes with the most commonly associated governance modalities for each. We then underline the need for hybrid approaches and adaptation trajectories over time to take into account local socio-cultural, geographical, and climatic conditions as well as to integrate stakeholders in the design and implementation of responses. We show that dynamic and participatory policies can foster collective learning processes and enable the evolution of social values and behaviors. Finally, adaptation policies rely on knowledge and participatory engagement, multi-scalar governance, policy monitoring, and territorial solidarity. These conditions are especially relevant for densely populated areas that will be confronted with sea level rise, thus for coastal cities in particular.

Abstract In recent years, several large outlet glaciers in Greenland lost their floating ice tongue, yet little is known regarding their stability over a longer timescale. Here we compile historical documents to demonstrate a major ice tongue collapse of Kangerlussuaq Glacier between 1932 and 1933. This event resulted in a 9‐km retreat, exceeding any of the glacier's recent major retreat events. Sediment cores from the fjord are used to reconstruct sea surface temperatures and to investigate a potential sedimentological trace of the collapse. During the 1920s, local and regional sea surface temperatures and air temperatures increased rapidly, suggesting a climatic trigger for the collapse. Fjord bathymetry played an important role too, as the (partially) pinned ice tongue retreated off a submarine moraine during the event. This historical analogue of a glacier tongue collapse emphasizes the fragility of remaining ice tongues in North Greenland within a warming climate.

Paleo-climate proxy records documenting sea-ice extent are important sources of information to assess the time of emergence and magnitude of on-going changes in the Arctic Ocean and better predict future climate and environmental evolution in that region. In this study, a suite of geochemical tracers including total organic carbon (TOC), total nitrogen (TN), carbon/nitrogen ratio (C/N), stable isotope composition of organic carbon and nitrogen ( δ ^13 C, δ ^15 N), and phytoplankton biomarkers (highly branched isoprenoids (HBIs) and sterols) were measured in a marine sediment core to document the sea-ice variability in the Chukchi Sea since the beginning of the Industrial Era. The downcore profile of the sea-ice proxy HBIs suggests a transition from extensive sea ice in the late 19th century to Marginal Ice Zone (MIZ) in AD 1930–1990s and then moderate sea-ice cover since 1990s. Rising of all HBI abundances between AD 1865–1875 indicate a transient retreat of summer ice edge off the shelf followed by a return to near-perennial sea ice till 1920–1930 as revealed by the absence of HBIs and brassicasterol. Sea ice retreat occurred again in AD 1920–1930 and followed by colder decades in 1940s–1960s before a sustained decline since the 1990s. The downcore profile of C/N, δ ^13 C of organic matter and sterols indicates a gradual increase of terrigenous inputs accelerating during the most recent decades likely due to enhanced fluvial run-off and sediment transport by sediment-laden sea ice. Concomitantly, increasing δ ^15 N values suggest limited nutrient utilization due to enhanced stratification of the surface ocean caused by increased freshening. The role of the Arctic oscillation (AO), the Pacific decadal oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) are discussed to explore potential drivers of the observed sea-ice changes.

Abstract The early 2000s accelerated ice-mass loss from large outlet glaciers in W and SE Greenland has been linked to warming of the subpolar North Atlantic. To investigate the uniqueness of this event, we extend the record of glacier and ocean changes back 1700 years by analyzing a sediment core from Sermilik Fjord near Helheim Glacier in SE Greenland. We show that multidecadal to centennial increases in alkenone-inferred Atlantic Water SSTs on the shelf occurred at times of reduced solar activity during the Little Ice Age, when the subpolar gyre weakened and shifted westward promoted by atmospheric blocking events. Helheim Glacier responded to many of these episodes with increased calving, but despite earlier multidecadal warming episodes matching the 20th century high SSTs in magnitude, the glacier behaved differently during the 20th century. We suggest the presence of a floating ice tongue since at least 300 AD lasting until 1900 AD followed by elevated 20th century glacier calving due to the loss of the tongue. We attribute this regime shift to 20th century unprecedented low sea-ice occurrence in the East Greenland Current and conclude that properties of this current are important for the stability of the present ice tongues in NE Greenland.