Summary

Many rivers flow into the Arctic Ocean north of the Arctic Circle – including the Lena in Siberia and the Mackenzie River in Canada. The deltas of these large and small rivers store large amounts of carbon, which is bound there in frozen soils and sediments. Climate change, however, is destabilising the deltas from the ocean and land side and also from the air. For the first time, an international team led by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in collaboration with the GFZ Helmholtz Centre for Geosciences, has now provided figures demonstrating the significance of this long neglected and highly vulnerable permafrost region between land and sea. Their study has been published in the journal Nature Communications. According to the study, Arctic river deltas store 57.5 gigatonnes of carbon on only one per cent of the global permafrost surface. This equals around 5 per cent of the permafrost soil carbon stored in that region. A better understanding of this region, which is so vitally important for the Arctic carbon cycle, is therefore urgently needed. 

Background: The effects of climate change in Arctic permafrost regions 

The permafrost region covers around a quarter of the land area in the northern hemisphere and stores vast amounts of organic carbon in the form of dead plant remains. While this Arctic freezer remained largely stable for many millennia, rising global temperatures are causing the permafrost to thaw. Soil microorganisms then become active over large areas, decompose the organic material and release more carbon into the atmosphere in the form of CO₂ and methane.

“Consequently, thawing permafrost could potentially intensify climate change. That's why researchers around the world have been working for years to understand the permafrost system in detail, to precisely quantify the carbon it contains and the relevant degradation processes, in order to ultimately create reliable future forecasts using numerical models,” explains Guido Grosse, Head of the Permafrost Research Section at the Alfred Wegener Institute. 

The role of Arctic river deltas in carbon sequestration 

“There is one area, however, that has been somewhat neglected so far: namely the deltas of the many small and large Arctic rivers,” adds Grosse. “It is precisely in these river mouths where traditionally a great deal of carbon supplied by the rivers draining the northern permafrost region is being deposited in soils and sediments that become permafrost over the long term – however, it is this border area between ocean and land that is now under massive pressure from several sides. The sea ice is retreating, the sea level is rising, the land is sinking, while the permafrost is thawing, the thawing season is lengthening, and the river waters and soils are getting warmer. All these factors come together in the already very dynamic Arctic deltas and destabilise a balance that was maintained for millennia.”

[Text: Slightly edited press release by AWI]

New study based on a significantly expanded dataset 

The international research team led by first author of the study and AWI postdoc Matthias Fuchs, who is now continuing his research at the University of Colorado Boulder, USA, has now compiled all available data on the carbon content of Arctic deltas for the first time and calculated the size of the reservoir. The origins of this project date back at least seven years, to when Fuchs was still a postdoctoral researcher in the research group led by second author Torsten Sachs at the GFZ.

“Up to now, the number of studies on Arctic deltas has been very limited,” as Matthias Fuchs reports. “The few publications focussed mainly on a few sampling locations in the mega-deltas of the large rivers Lena in Siberia and Mackenzie in Canada. We have now compiled a wealth of newly published and partly unpublished data from more than 1,600 soil samples from 17 Arctic deltas. Compared to the previously published studies, the number of soil cores analysed has almost tripled.”

The findings highlight the huge significance of Arctic river deltas 

As a result, the huge significance of the Arctic river deltas as a carbon storage hotspot under increasing pressures becomes clear. According to the study team's calculations, the deltas store 57.5 gigatonnes of carbon over an area of almost 100,000 km² (the size of South Korea and twice the size of Lower Saxony). By comparison: The annual increase in carbon in the atmosphere due to human activity stands at around 4.5 gigatonnes – meaning that around 5 per cent of global permafrost carbon is bound in the deltas on "only" 1 per cent of their surface area. "The tremendous importance becomes even clearer if you factor in all of the Earth's soils," explains Guido Grosse. “Then the Arctic deltas bind around 2 per cent of all soil carbon on just 0.08 per cent of the global land area. This makes it clear that the Arctic deltas are currently a particularly critical element in the global carbon cycle. They store a comparatively large amount of carbon in a small area and are overly exposed to the consequences of climate change in several ways. In order to make precise forecasts, we will therefore have to focus more strongly on the estuaries of the large and small Arctic rivers in future research.”
 

Original publication:

Matthias Fuchs, Torsten Sachs, Loeka L. Jongejans, Jens Strauss, Gustaf Hugelius, Gerald V. Frost, Benjamin M. Jones, Steven V. Kokelj, Lars Kutzbach, Ingmar Nitze, Pier Paul Overduin, Juri Palmtag, Chien-Lu Ping, Oleg S. Pokrovsky, Elizaveta Rivkina, Alexandra Runge, Lutz Schirrmeister, Georg Schwamborn, Matthias B. Siewert, Claire Treat, Alexandra Veremeeva, Sebastian Zubrzycki, Guido Grosse: Large stocks of permafrost soil organic carbon and nitrogen in Arctic river deltas; Nature Communications (2026). 
DOI: 10.1038/s41467-026-73092-2