According to a new study, Arctic fjords could store carbon less effectively in the future as global warming progresses, with potentially far-reaching consequences for the climate.
Arctic fjords may store carbon far less effectively in the future than they do today. This is the conclusion of new research by an international team led by Dr. Jochen Knies from the iC3 Polar Research Hub at UiT The Arctic University of Norway in Tromsø. While climate change could temporarily boost primary production in fjords, their long-term capacity to sequester carbon is likely to decline due to shrinking ice cover, altered nutrient dynamics, and increasing stratification of the water column.
The study focuses on Kongsfjorden on Svalbard, a fjord that is already severely affected by Atlanticification: warm Atlantic water is flowing in, sea ice is retreating, and glaciers are melting. These changes are directly affecting the physical conditions and biological productivity in the fjord.
The research team analyzed geochemical biomarkers in marine sediments to trace how the composition of plankton in Kongsfjorden has changed over the past 14,000 years — from the Younger Dryas, the final cold phase of the last ice age, to the present day.
The findings, published April 25 in Nature Communications Earth & Environment, show that Arctic fjord systems have been sensitive and responsive to past climate changes such as ice loss and temperature rise, with dramatic shifts in the species composition of the phytoplankton community and the fjords’ ability to store carbon.
Carbon sinks in transition
Fjords are considered important carbon sinks: phytoplankton absorbs carbon dioxide during photosynthesis, and some of this organic material sinks to the seabed, where it is stored long-term — a natural mechanism that helps regulate the climate. However, the efficiency of this process depends heavily on local environmental conditions.
During the Younger Dryas, the sea ice cover in the Kongsfjorden was extensive. Only specialized ice algae were able to survive and productivity was low.
At the end of this cold period — around 11,600 years ago — a rapid temperature increase caused the glaciers to retreat. Simultaneously, glacial meltwater carried large quantities of nutrients into the sea, triggering a dramatic shift in the phytoplankton community: diatoms became dominant, primary production surged, and the input of organic carbon into the sediments increased significantly.
However, this effect was only temporary. During the subsequent warm phase — the Holocene Thermal Maximum, roughly 10,000 to 6,000 years ago — continued meltwater input reduced the salinity of surface waters, leading to stronger stratification of the water column and hindering vertical transport.
Fewer nutrients from deeper waters reached the sunlit surface layer, and the biological pump — which transports carbon to the deep ocean via excrement and sinking organic matter — came to a halt. Although surface productivity remained high, significantly less fixed carbon made its way to the seafloor.
“The changes we observe suggest that the future of these fjord ecosystems will depend heavily on how well they adapt to a warmer climate.” says Dr. Knies in a press release from the institute.
Impact on the fjord ecosystem
These historical patterns could repeat themselves again today – but at an accelerated pace. In an email to polarjournal.net, Dr. Knies warns:
“The critical question will be if the water column stratification we have seen during the Holocene Thermal Maximum propagates towards the open ocean and the future ice-free Arctic Ocean including the shelf seas such as the Barents Sea. If this is the case, the impact of reduced carbon storage capacity can be enormous.”
The Barents Sea is currently considered a major carbon sink, as large amounts of organic carbon from the productive upper water layer reach the seafloor. “The Barents Sea today stores plenty of carbon produced in the photic zone. If the Barents Sea will be more stratified due to higher sea surface temperatures, the ocean may loose the capacity since carbon produced in the photic zone will likely be remineralized before it reaches the seafloor.”
Glacier meltwater also plays a key role as a source of nutrients. The disappearance of glaciers has unpredictable effects on nutrient availability in the fjord and could, in the long term, negatively impact the health of fjord ecosystems. If the nutrient supply can no longer be maintained, the ecological balance may be disrupted — with consequences for the food web and the overall productivity of the fjords.
The role of Arctic fjords in carbon storage
Although Arctic fjords cover only a small portion of the total area of the Arctic Ocean, they play a crucial role in carbon storage.
Recent data from Norway highlights just how crucial this carbon storage function is. According to a study published in June 2024 — in which Dr. Knies was also involved — approximately 814 million tons of organic carbon are stored in just the top ten centimeters of sediment on the Norwegian continental shelf. This carbon accumulates at a rate of about six million tons per year. Fjords carved by glaciers are particularly active in this process, accounting for nearly half of the total storage. These marine sediments far surpass the carbon storage capacity of many vegetated coastal ecosystems, such as seagrass beds or salt marshes.
These findings underscore the central role that fjords play in the global carbon cycle — and how important it is to closely monitor their future development. Their storage capacity could decline significantly in a warming Arctic, leaving more carbon dioxide in the atmosphere and potentially contributing to further global warming.