The North Water (NOW) polynya, also known as Pikialasorsuaq ('the great upwelling' in Greenlandic), is the largest and most productive polynya in the Northern Hemisphere. Located in northern Baffin Bay, this annually recurring ice-free area plays a crucial role in sustaining the world's northernmost Inuit communities and several keystone Arctic species. The ice-free waters facilitate an early and extensive phytoplankton bloom, forming the base of a rich food web that supports Arctic cod, seabirds (particularly the little auk), and marine mammals such as narwhal, beluga, walrus, and polar bear. This ecosystem is vital to the hunting and fishing economies of the Inuit. The NOW's ecological significance is widely recognized, leading to its identification by the World Conservation Union (IUCN) as one of the most important marine areas in the Arctic and a proposed UNESCO Natural Marine World Heritage Site. The NOW's formation depends on the stability of a winter ice arch across the southern Kane Basin, which prevents the inflow of multiyear sea ice from the Arctic Ocean. A stable ice arch, combined with northerly winds and ocean currents, removes newly formed sea ice, leading to a deep mixed layer and high nutrient availability for the spring phytoplankton bloom. The entrainment of deeper, warmer Atlantic water further contributes to this productivity. Diatoms, including open-water, marginal ice zone, and sea-ice (sympagic) taxa, are the primary producers, with a significant portion of their production reaching the seafloor sediments. The little auk, the most abundant seabird in the North Atlantic, relies heavily on the NOW, with over 80% of its global breeding population depending on the polynya's abundant copepods. These birds transport vast quantities of marine-derived nutrients (MDN) to land through their guano, transforming coastal landscapes into green oases. Therefore, the NOW's productivity directly impacts the little auk population and the surrounding terrestrial ecosystems. The strategic location of the NOW, at the narrowest point between Canada and Greenland, also marks a crucial gateway for prehistoric human migrations into Greenland. The region is of critical significance to present-day Inuit communities, and changes in the NOW ecosystem due to sea-ice conditions represent a significant climate change hazard. This study aims to investigate the vulnerability of this crucial ecosystem and its associated human communities to climate change.

Existing research highlights the dependence of Inuit communities on the North Water polynya, documenting their millennial-scale reliance on its resources for sustenance and cultural practices. Studies have also shown the connection between the polynya’s productivity and its physical conditions, including sea-ice dynamics and water circulation patterns. The importance of the ice arch in Kane Basin in maintaining the polynya's open-water state has been established through various studies using satellite imagery and oceanographic models. These studies have shown increasing instability in the ice arch in recent decades, causing concerns about the future of the polynya. The ecological role of keystone species like little auks has been examined, highlighting their importance in nutrient cycling and ecosystem functioning. However, there has been a lack of long-term records to fully assess the vulnerability of the NOW ecosystem to climate change, prompting the need for this retrospective study. The intersection of the IPCC’s framework of exposure, hazard and vulnerability is used here to analyse the climate risk presented to this region.

This study employs a retrospective approach to assess the vulnerability of the North Water polynya to climate change by reconstructing long-term trends in sea ice, primary production, and little auk colony dynamics. The researchers analyzed two sediment cores: a marine core spanning the past ~4000 years and a lacustrine core covering the last ~6000 years. The marine core, retrieved from Smith Sound, provides data on marine primary production (quantified through diatom valves and *Chaetoceros* resting spores), and sea-ice dynamics (using IP25 and HBI III, specific molecular biomarkers indicative of sea ice and marginal ice zone conditions respectively). The lacustrine core, from Annikitsoq lake on the Cape York Peninsula, was used to infer little auk colony dynamics. Multiple proxies were employed to track the birds' presence and abundance, including δ¹⁵N (stable nitrogen isotope), Cadmium to Titanium ratios (Cd:Ti), cholesterol and β-sitosterol concentrations, and diatom assemblage composition changes. The change in diatom assemblages from oligotrophic to acidophilic species in the lake sediments after ~4200 cal yrs b2k strongly indicates the impact of the little auk colony on lake chemistry. These sediment records were chronologically constrained using radiocarbon (¹⁴C) and ²¹⁰Pb/¹³⁷Cs dating, with age-depth models constructed using the BACON software. Various analytical techniques were used, including computed tomography (CT) scanning, loss on ignition (LOI) measurements, X-ray fluorescence (XRF) spectroscopy, gas chromatography-mass spectrometry (GC-MS), and optical microscopy for diatom analysis. Statistical methods, such as Monte Carlo simulations and generalized additive models (GAMs), were used to analyze the data and identify significant changes over time. Principal component analysis (PCA) was also used to identify underlying patterns within the multiple proxies from each core. Finally, modern sea ice concentration data from satellite observations were compared with the historical data. The study meticulously accounts for various uncertainties associated with dating and measurements, utilizing Bayesian approaches to provide robust interpretations.

The study's results reveal a strong correlation between climate variability, ecosystem productivity, and human settlements in the North Water region. The marine sediment core indicates a highly productive polynya from approximately 4400–4200 cal yrs b2k, coinciding with the arrival of the first humans in Greenland, as evidenced by archaeological records and genetic studies. This period showed high diatom fluxes, indicating high primary production, alongside high IP25 fluxes, representing active polynya with seasonal sea ice formation. The minimal HBI III fluxes indicate reduced influence of marginal ice zone conditions at the core site. The lacustrine core reveals a marked transition at around 4400–4200 cal yrs b2k, corresponding to the arrival of little auks at a nearby colony. This is evidenced by a sharp increase in organic matter content, changes in diatom assemblages (shift towards acidophilic species due to acidification by bird guano), and a rise in δ¹⁵N and cholesterol concentrations. Both records showed relative stability of the polynya and little auk population between 4200 and 2700 cal yrs b2k. However, between approximately 2700 and 2200 cal yrs b2k, both records show a decline in little auk abundance, followed by an apparent abandonment of the colony around 2300 cal yrs b2k. This coincides with a significant drop in marine primary production, suggesting polynya instability and contraction. This period of polynya instability, lasting until about 800 cal yrs b2k, also corresponds with a long period (approximately 2200-1200 cal yrs b2k) when Greenland was uninhabited by humans. Between 800 and 100 cal yrs b2k, cooler temperatures (Little Ice Age) and more stable polynya conditions are indicated, correlating with the recovery of the little auk colony and the arrival of the Thule Culture, ancestors of modern Inuit. Analysis of climate indices (Arctic Oscillation, North Atlantic Oscillation, and Atlantic Multidecadal Variability) suggest a strong influence of atmospheric and oceanic patterns on polynya dynamics. Positive AO conditions, which increase cyclonic activity, correlate with polynya instability and reduced productivity, while predominantly positive modes of the AO (and NAO) correlate with the period of human abandonment of Greenland. Finally, analysis of modern sea ice concentration data shows increasing polynya instability in recent decades, with earlier ice arch break-up and enhanced sea ice export from the Arctic Ocean. This trend supports the long-term findings suggesting future risk to the unique ecosystem and the reliance of Inuit culture upon it.

The findings of this study strongly suggest that the North Water polynya's productivity and stability are highly sensitive to climate fluctuations. The observed correlations between past periods of reduced productivity and human abandonment of Greenland underscore the significance of the polynya for both ecosystem health and human societies. The long-term perspective offered by this study provides compelling evidence of the vulnerability of this unique ecosystem to climate change. The unprecedented rate of modern warming, particularly in the Arctic, suggests that the future collapse of the NOW ecosystem is a serious risk. This threat not only impacts the biodiversity of the region but also poses a significant challenge to the livelihoods and cultural practices of the Inuit communities who depend on the polynya's resources. The study highlights the complex interplay between atmospheric and oceanic processes, sea-ice dynamics, and ecological responses. Understanding these intricate relationships is crucial for developing effective conservation and adaptation strategies. The findings emphasize the importance of ecosystem-based management approaches, taking into account the social and ecological dimensions of climate change vulnerability.

This study provides a long-term perspective on the vulnerability of the North Water polynya ecosystem to climate change. The strong correlation between past periods of polynya instability and human abandonment of Greenland highlights the potential for future ecosystem collapse due to ongoing climate warming and sea ice decline. The research underscores the need for collaborative efforts involving governments, indigenous organizations, and local communities to implement effective monitoring, conservation, and adaptation measures to protect this critical ecosystem and the livelihoods of the Inuit who depend on it. Further research should focus on refining predictive models, improving understanding of the complex interactions within the NOW ecosystem, and exploring innovative adaptation strategies for Inuit communities.

While this study provides valuable insights into the long-term dynamics of the North Water polynya, certain limitations should be acknowledged. The proxies used to reconstruct past conditions have inherent uncertainties, and the interpretation of these proxies relies on established relationships that may not fully capture the complexity of the system. The temporal resolution of the sediment records, while providing a valuable long-term perspective, may not capture rapid changes that might occur within shorter timeframes. The study primarily focuses on a specific geographical area of the polynya and might not fully represent the heterogeneity of the entire ecosystem.