The rapid melting of the Arctic ice pack and climate upheaval are transforming the ecosystems of the far north. In particular, the ice shelf in north-east Greenland, an area hitherto seen as a carbon sink, is becoming problematic. The reduction of the ice at the end of the summer could lead to CO2 emissions and alter the flow of carbon between the ocean and the atmosphere.
by Laurie Henry
Cover photo: The Zachariæ Isstrøm glacier in north-east Greenland lost its platform in 2003. Romain Millan-Ige-Cnrs
A recent study has just been published in Biogeosciences, by researchers from the Arctic Research Centre at the University of Aarhus and the Centre for Earth Observation Science at the University of Manitoba. It looked at the CO2 levels observed during an exceptional melting period in 2017, and highlights a hitherto underestimated seasonal phenomenon that raises crucial questions about the future role of this Arctic region in the context of a changing climate.
A balance disrupted by global warming
The continental shelf of north-east Greenland plays a crucial role in carbon storage. Within the global thermohaline circulation, it acts as a transfer route for water from the Arctic to the North Atlantic. Its strategic location allows cold water from the poles, rich in dissolved carbon, to flow southwards through the depths following the East Greenland Current. The water transported through the Labrador Sea undergoes significant vertical mixing, resulting in the formation of intermediate and deep water masses. This deep mixing process allows the absorbed carbon to remain trapped for decades, even centuries, in the global ocean network.
But it seems that global warming is disrupting this mechanism in the Arctic Ocean. The increase in water temperature and the reduction in the associated ice cover seem to be gradually disrupting this plunge of cold water and the burial of carbon to the depths.
A ground-breaking study in extreme conditions
The study by Willcox and colleagues (E. Willcox et al., 2024) documents this worrying phenomenon, which takes place during the Arctic summer and releases substantial quantities of gas stored under the ice. The researchers’ observations were carried out over a previously unexplored latitude between 75°N and 79°N. They show that the concentration of CO2 in the water increases as the ice melts. This potential release of CO2 is linked in particular to changes in biological processes and variations in primary productivity under the ice, which fluctuates according to nutrient inputs and light intensity.
The data for this study was collected during two oceanographic research campaigns conducted in August and September 2017, at a time when the pack ice had exceptionally retreated. This unusual melt gave scientists unprecedented access to the ice shelf in north-east Greenland, which is rarely explored due to the persistent ice cover. They were then able to deploy specialised sensors called CTDs (Conductivity, Temperature and Depth) and monitor the temperature, salinity and depth of the mixed layer of the shelf waters – the latter being the zone where the surface water interacts with the atmosphere and where gas exchanges are concentrated.
(a) Overview of carbon chemistry and samples on the north-east Greenland platform. The arrows show the main currents: white for water of Arctic origin, red for Atlantic water, black for the Greenland countercurrent, purple for the Greenland gyre, orange for the East Greenland current. (b) Sources of total alkalinity (TA) in the Arctic Ocean, including Arctic rivers, sea ice, melted snow and water from the Pacific via the Bering Strait. The study area is indicated by a red rectangle, and the source areas by a dotted white outline. Willcox et al, 2024
At the same time, detailed chemical analyses were carried out to measure dissolved inorganic carbon (DIC) and total alkalinity (TA), two elements needed to understand the marine carbon cycle. DIC represents the quantity of carbon present in dissolved form in the water, while TA reflects the water’s capacity to neutralise acids, a factor that influences the retention or release of CO2. The study looked in particular at variations in the fugacity of CO2 (fCO2), an indicator of the partial pressure of CO2 that makes it possible to estimate whether this zone acts as a sink absorbing atmosphericCO2 or as a source releasing it.
The variations in the parameters measured have also provided valuable insights into the complex interactions between the cold polar waters and the warmer Atlantic waters, contributing to a complete picture of this region and an identification of the changes induced by the reduction in ice cover.
The Northeast Greenland Shelf, a potential late-summer CO2 source to the atmosphere
The results of the study reveal particularly high levels of CO2 in the surface waters of the platform, exceeding regional atmospheric estimates with fCO2values averaging 410.49 µatm up to 10 September. During this initial period, the fCO2 in the mixed layer often exceeded the predicted atmospheric pressure, signalling a potential release ofCO2into the atmosphere. However, after this date, median fCO2 values fell to around 367.89 µatm, indicating a seasonal or geographical change.
This dynamic variation in CO2 levels reflects a possible change in conditions on the platform, probably influenced by the gradual drop in surface temperatures and the increase in ice cover towards the end of the summer. The researchers were thus able to observe a shift in the way the platform functions during this transition period, from CO2 source to sink.
Surface conditions (sea ice fraction and surface temperature) and mean mixed layer temperature, divided into four sampling periods. Surface temperature and ice fraction data provided by the Meteorological Office UK (2019). E. Willcox et al, 2024
In deeper waters, an inverse relationship was noted betweenCO2 concentration and apparent oxygen utilisation (AOU), with lower fCO2 levels associated with areas of high dissolved oxygen concentration. This pattern indicates that biological respiration could play a significant role in CO2 release at depth, where AOU exceeds 100%. Variations in temperature and salinity between the water masses from the polar and Atlantic currents also created marked differences in the results, although there was no clear correlation with ice cover.
Towards a new climate dynamic
The findings of this study indicate that the north-east Greenland shelf could become a source of CO2 in late summer, a phenomenon that is potentially amplified during periods of low ice cover.
Without the ice barrier, surface and deep waters can mix more easily, releasing accumulatedCO2into the atmosphere. This process transforms the regional dynamics of carbon, from potential storage in the depths of the ocean to direct release into the air. This in turn contributes to global warming, with the CO2 released helping to melt the Arctic ice.
The rise in temperature of the Atlantic waters entering the Arctic region also reinforces this trend by increasing the stratification of the water layers, which prevents more even mixing and contributes to the accumulation of CO2 near the surface. The variability of the parameters observed, influenced by phenomena such as eddies and meltwater, suggests that smaller-scale processes need to be better understood in order to anticipate the impact of this transformation on the climate. These implications underline the importance of increased and continuous monitoring in this area, in order to better understand its potential influence on the global carbon cycle and future climate models.
Source : Willcox, E., Lemes, M., Juul-Pedersen, T., Sejr, M. K., Holding, J. M., and Rysgaard, S.: “The Northeast Greenland Shelf as a potential late-summer CO2 source to the atmosphere”, Biogeosciences, 21, 4037–4050