Lisa A. Miller

In ocean carbon budget assessments, sea ice is still treated as an impermeable barrier, rather than a dynamic interface mediating CO₂ exchange between ocean and atmosphere. We compiled more than 6000 chamber-based air–ice CO₂ flux measurements from the Arctic and Southern Oceans between 2003 and 2021, spanning diverse ice, snow, and seasonal conditions. These data show that sea ice releases CO₂ in winter and absorbs it in summer, with summer uptake offsetting winter emissions. On an annual basis, sea ice represents a small net CO₂ source of +4 Tg C yr⁻¹ in the Arctic and +2 Tg C yr⁻¹ in the Southern Ocean, challenging earlier views of a major sink. Although these fluxes are negligible at basin scales relative to open-ocean uptake, sea ice exchanges gases even in cold winter conditions, with implications for small-scale processes and other trace gases, underscoring the need for sustained, process-resolving observations.

In Earth system models and ocean carbon budget assessments, sea-ice is still treated as an impermeable barrier rather than a dynamic interface mediating CO₂ exchange between the ocean and atmosphere. To quantify its contribution to the polar ocean carbon budget, we compiled over 6000 chamber-based air–ice CO2 flux collected across the Arctic and Southern Ocean between 2003 and 2021. This unprecedented database spans diverse sea-ice, snow, and seasonal conditions. Despite spatiotemporal limitations, this database enables a comprehensive reassessment of sea-ice’s role in the carbon budgets of polar oceans. Sea ice acts as a CO₂ source in winter and a sink in summer, as summer drawdowns offset most winter emissions. On an annual basis, sea-ice represents a small net CO₂ source of +4 Tg C yr⁻¹ in the Arctic and +2 Tg C yr⁻¹ in the Southern Ocean, refuting previous hypotheses of a major sea-ice carbon sink. Although these net fluxes remain negligible at basin scales relative to concurrent open-ocean uptake, our results highlight that sea-ice exchanges gases even in cold winter conditions. These exchanges may have critical implications for short-term, small-scale processes and for other trace gases, underscoring the need for continuous, process-resolving flux observations.

The Canadian Arctic is warming four times faster than the global average, yet the impact of this perturbation on the marine carbon cycle remains unknown. Dissolved inorganic carbon (DIC) stable isotope (δ¹³C) and radiocarbon (Δ¹⁴C) values are powerful tools for tracing water mass transport, residence times and carbon cycling. While the hydrography of the Canadian Arctic Archipelago (CAA) is well documented, few DIC δ¹³C and Δ¹⁴C values exist for the region. Here, we present new DIC δ¹³C and Δ¹⁴C depth profiles from 19 stations across the CAA sampled in 2021 and place them into the context of five recently published Baffin Bay values. CAA DIC δ¹³C and Δ¹⁴C values ranged from −0.68‰ to +1.86‰, and −90.7 to +49.5‰, respectively. Several negative DIC Δ¹⁴C values (−44.7‰ and −51.9‰) were observed near the Mackenzie River, indicating riverine permafrost carbon is actively incorporated into the nearshore DIC pool. “Bomb” DIC Δ¹⁴C values in the Kitikmeot Sea were attributed to enhanced tidal mixing and heterotrophy together with high regional water mass residence times. A comparison of historical DIC Δ¹⁴C depth profiles from 2009 to 2021 reveals significant dilution of “bomb” ¹⁴C and minor contributions (2.1%–4.4%) of fossil anthropogenic CO₂ within Pacific Summer Water (PSW), Pacific Winter Water (PWW) and Atlantic Fram Strait Water (ATL_FS) in the Beaufort Sea. Finally, the contrast between deep Beaufort Sea and Baffin Bay DIC δ¹³C and Δ¹⁴C values reveal differences in residence time and carbon sources in the two regions.

This study investigates the biogeochemical drivers of aragonite saturation state (Ω_Ar) in Baffin Bay, with a focus on the relatively undersampled west Greenland shelf. Our findings reveal two main depth‐dependant processes controlling the spatial distribution of Ω_Ar in Baffin Bay; within the upper 200 m, lower Ω_Ar coincides with increasing fractions of Arctic‐outflow waters, while below 200 m organic matter respiration decreases Ω_Ar. A temporal analysis comparing historical measurements from 1997 and 2004 with our 2019 data set reveals a significant decrease in the Ω_Ar of Arctic‐outflow waters, coinciding with reduced total alkalinity (TA). However, no discernible anthropogenic ocean acidification signal is identified. Significant Arctic water fractions (20%–40%) are found to be present on the west Greenland shelf, associated with reduced TA and Ω_Ar. A numerical modeling simulation incorporating a passive tracer demonstrates that periodic changes in wind direction lead to a switch from onshore to offshore Ekman transport along the Baffin Island current, transporting Arctic waters toward the west Greenland shelf. This challenges the conventional understanding of Baffin Bay's circulation and underscores the need for further research on the region's physical oceanography. Based on salinity‐TA relationships, surface waters on the west Greenland shelf have a significantly lower meteoric TA end‐member compared to waters of the Baffin Island Current in western Baffin Bay. The low eastern TA freshwater end‐member agrees well with recent glacial meltwater TA measurements, suggesting that glacial meltwater is the main freshwater source to surface waters on the west Greenland shelf.

Utilizing an 11‐year (2011–2021) data set, we report surface‐ocean pCO₂ in Baffin Bay during the open‐water season (June–October) to establish a baseline understanding of surface seawater carbon dynamics in this region. We found pCO₂ was strongly undersaturated (70–130 μatm below saturation, depending on year), albeit with substantial regional variability. Though temperature was found to be a generally strong control of pCO₂, sea‐ice dynamics and other non‐thermal drivers controlled pCO₂ at certain times and in certain regions. The Baffin Island Current region (western Baffin Bay) experienced relatively high pCO₂ during instances of high sea‐ice cover, usually in early spring. Average pCO₂ was comparatively lower in the West Greenland Current region (eastern Baffin Bay) which is ice‐free substantially longer (by 3–4 months) and where temperature acted as the dominant control to pCO₂. With respect to temporal variations, June had the lowest pCO₂ of the open‐water season, coinciding with active sea‐ice melt. Surface‐ocean pCO₂ then increased month‐to‐month through July and August due to warming and decreased meltwater dilution, increasing again into September before stabilizing into October. Generally, non‐thermal controls acted to decrease pCO₂ during mid‐summer (possibly primary production, sea‐ice melt, and circulation), but acted to increase pCO₂ during early fall (vertical mixing). Despite spatial and temporal variation over the open‐water season, persistent undersaturation suggests that Baffin Bay is a potentially strong CO₂ sink, even in comparison to other uptake regions across the western Arctic.

Polar oceans and sea ice cover 15% of the Earth’s ocean surface, and the environment is changing rapidly at both poles. Improving knowledge on the interactions between the atmospheric and oceanic realms in the polar regions, a Surface Ocean–Lower Atmosphere Study (SOLAS) project key focus, is essential to understanding the Earth system in the context of climate change. However, our ability to monitor the pace and magnitude of changes in the polar regions and evaluate their impacts for the rest of the globe is limited by both remoteness and sea-ice coverage. Sea ice not only supports biological activity and mediates gas and aerosol exchange but can also hinder some in-situ and remote sensing observations. While satellite remote sensing provides the baseline climate record for sea-ice properties and extent, these techniques cannot provide key variables within and below sea ice. Recent robotics, modeling, and in-situ measurement advances have opened new possibilities for understanding the ocean–sea ice–atmosphere system, but critical knowledge gaps remain. Seasonal and long-term observations are clearly lacking across all variables and phases. Observational and modeling efforts across the sea-ice, ocean, and atmospheric domains must be better linked to achieve a system-level understanding of polar ocean and sea-ice environments. As polar oceans are warming and sea ice is becoming thinner and more ephemeral than before, dramatic changes over a suite of physicochemical and biogeochemical processes are expected, if not already underway. These changes in sea-ice and ocean conditions will affect atmospheric processes by modifying the production of aerosols, aerosol precursors, reactive halogens and oxidants, and the exchange of greenhouse gases. Quantifying which processes will be enhanced or reduced by climate change calls for tailored monitoring programs for high-latitude ocean environments. Open questions in this coupled system will be best resolved by leveraging ongoing international and multidisciplinary programs, such as efforts led by SOLAS, to link research across the ocean–sea ice–atmosphere interface.

The water mass assembly of Nares Strait is variable, owing to fluctuating wind forcings over the central Arctic Basin, and irregular northward flows from the West Greenland Current (WGC) in Baffin Bay. Here we characterize the physico‐chemical properties of the water masses entering Nares Strait in August 2014. We employ an extended optimum multi‐parameter (OMP) analysis to estimate the mixing fractions of predefined source water masses, and to distinguish the role of physical and biological processes in governing the distribution of dissolved inorganic carbon (DIC) in Nares Strait. We show the first documented evidence of Siberian shelf waters arriving in Nares Strait, along with a diluted upper halocline layer of partial Pacific‐origin. These mixed‐origin water masses appear to play an important role in driving a modest phytoplankton bloom in Kane Basin, leading to decreased surface pCO₂ concentrations in Nares Strait. Although inorganic nitrogen was already limited near the surface in northern Nares Strait, the rather shallow upper halocline layer and the shoaling bathymetry in Kane Basin facilitated upwelling of nutrients to the surface. Our observations suggest that the positioning of the Transpolar Drift, and hence the balance of Atlantic and Pacific water delivered to Nares Strait, may play an important role in regional biological productivity and carbon uptake from the atmosphere. We also observed water masses from the WGC transported as far north as Kane Basin, contributing to relatively high pCO₂ and low pH in the intermediate and deep water column of southern Nares Strait and northern Baffin Bay.

The water mass assembly of Nares Strait is variable, owing to fluctuating wind forcings over the Arctic Basins, and irregular northward flows from the West Greenland Current (WGC) in Baffin Bay. Here we characterize the physico-chemical properties of the water masses entering Nares Strait in August 2014, and we employ an extended optimum multi-parameter (OMP) water mass analysis to estimate the mixing fractions of predefined source water masses, and to distinguish the role of physical and biological processes in governing the distribution of dissolved inorganic carbon (DIC) in Nares Strait. We show the first documented evidence of Siberian shelf waters in Nares Strait, along with a diluted upper halocline layer of partial Pacific-origin. These mixed-origin water masses appear to play an important role in driving a modest phytoplankton bloom in Kane Basin, leading to decreased surface p CO ₂ concentrations in Nares Strait. Although inorganic nitrogen was already limited in the surface mixed layer in northern Nares Strait, the unique properties of mixed Atlantic-Pacific water facilitated upwelling and nutrient supply to the surface. These observations suggest that the positioning of the Transpolar Drift, and hence the balance of Atlantic and Pacific water delivered to Nares Strait, is likely to play an important role in regional biological productivity and carbon uptake from the atmosphere. We also observed water masses from the WGC transported as far north as Kane Basin, contributing to relatively high p CO ₂ and low pH in the intermediate and deep water column of southern Nares Strait and northern Baffin Bay.

The ocean absorbs anthropogenic carbon, slowing atmospheric CO₂ increase but driving ocean acidification. Long‐term changes in the carbon system are typically assessed from single‐point time series or from hydrographic sections spaced by decades. Using higher resolution observations (1–3 year⁻¹) from the Line P time series, we investigate processes modulating trends in the carbon system of the northeast subarctic Pacific. Dissolved inorganic carbon (DIC) and apparent oxygen utilization (AOU) from 1990 to 2019 reveal substantial trends over most of the upper water column along the 1,500 km coastal to open ocean transect. At the surface, an increasing trend in salinity‐normalized DIC (sDIC₃₃) (+0.5 ± 0.4 μmol kg⁻¹ yr⁻¹) is associated with a decrease in pH (0.01–0.02 decade⁻¹) and a decrease in aragonite saturation state (0.04–0.08 decade⁻¹). These observed trends are driven by anthropogenic CO₂ uptake, partially offset by trends in surface salinity or temperature. Stratification associated with recent marine heat waves appears to have caused anomalously low surface pCO₂. sDIC₃₃ trends of similar magnitude were found below the seasonal thermocline on the 26.7–26.8 isopycnals (150–300 m), which are ventilated in the western Pacific. Roughly, a third (20%–50%) of the subsurface sDIC₃₃ trend is driven by increased remineralization, likely caused by long‐term decreases in ventilation in the western Pacific. Bidecadal oscillations in the ventilation of the 26.7–26.8 isopycnals arising from the Lunar Nodal Cycle cause oscillations in sDIC₃₃ and AOU at the offshore end of our transect. We trace the oscillations to alternating periods of higher anthropogenic carbon uptake or higher carbon remineralization.

Geoengineering strategies to slow sea ice melting would affect not only Earth's climate but also the biology and chemistry of the oceans, atmosphere, and ice.