Where Freshwater Meets the Fjord: Researching Carbon in a Changing Arctic

We sat down with Henry Henson, a PhD student at Aarhus University, whose path led from an early love of nature to studying the frontlines of climate change in the Arctic. Henry works with both Aarhus University’s Arctic Research Centre and the Greenland Climate Research Centre in Nuuk, exploring how Greenland’s coastal oceans absorb CO₂ and how a warming, freshening Arctic is transforming these fragile ecosystems. His story offers a glimpse into the challenges and significance of Arctic research.

🌊What first inspired you to study CO₂ uptake in the oceans around Greenland?

I’ve been fascinated by icy environments and water for as long as I can remember. Growing up near the Great Lakes, I loved snow, ice, and large bodies of water and often wondered what was happening beneath the surface. I only saw the ocean for the first time when I was 13 years old, but being around water all my life made it feel seem somehow familiar.

During my master’s, I spent a semester at the Greenland Institute of Natural Resources in Nuuk. That experience shifted my focus from freshwater systems to the ocean, and a course on air-sea exchange of greenhouse gases sparked my interest in CO₂ uptake. Now, during my PhD, I get to teach that very course that first introduced me to this field — a real full-circle moment. Working in Greenland wasn’t always my plan, but the Arctic’s extreme environments, the communities that depend on the ocean, and the unique environmental and societal dynamics made it a natural place to focus my research.

🌊Why is the Greenland coastal ocean such a special – and important – place for understanding climate change?

Arctic coastal environments, including Greenland, are important for understanding climate change for several reasons. The ocean absorbs roughly 25% of the carbon dioxide we emit each year, playing a crucial role in the global climate cycle. Much of this CO₂ enters the ocean in polar regions where cold waters and biologically productive conditions, like abundant phytoplankton, promote high CO₂ uptake.

At the same time, these polar regions are changing faster than anywhere else: the Arctic is warming rapidly and receiving increasing amounts of freshwater from the Greenland ice sheet, Arctic rivers, and changing precipitation patterns. This mix of high carbon uptake, rapid warming, and freshening makes Greenland’s coastal ocean a key place to study. My PhD focuses on understanding how increasing freshwater runoff affects carbon cycling in this vulnerable region.

🌊Can you describe what a typical day in the field looked like for you during your measurement campaigns? What’s one memorable or unexpected moment from your fieldwork?

Fieldwork in the Arctic is highly dependent on weather, so flexibility is key. One of the main ways we quantify fluxes of CO₂ is by measuring the gradient between the ocean and the atmosphere. Most days, we stay at a research station or town and take a small boat into the fjord with 2-3 colleagues to collect water samples and measurements. While I focus on the carbon system, we all help each other with tasks like measuring chlorophyll, sampling zooplankton, or recording physical oceanography data.

I always think its fascinating when I am on fieldwork and can witness freshwater meeting the ocean. As somewhat of an unconventional oceanographer, I often head ashore in order to collect river samples. These interfaces are super interesting because you can physically see the mixing firsthand. Still, the most unforgettable day was in the summer of 2023 in Young Sound, northeast Greenland, when a polar bear wandered near our station and later swam past us in a small sampling boat. Right after it had passed, my colleague Carl cracked his tooth in half by biting into a stale cookie. Most of our food at the station is a few years out of date, so it turned out our old treats were more dangerous than the bear.

🌊There are still large differences in estimates of how much CO₂ Greenland’s coastal waters absorb. What makes it so challenging to pin down an exact number?

Measuring CO₂ fluxes in Greenland’s coastal waters is challenging. One method we use called eddy covariance, requires towers close to the ocean and only works reliably when the wind blows from the sea, meaning a lot of data gets filtered out. When we compare these results to CO₂ flux estimates from the flux gradient method, they often don’t match. Part of the problem is that many equations used to calculate fluxes—like gas transfer velocities or carbonate system parameters— were developed for non-Arctic or open-ocean conditions, making them less accurate in cold, freshwater-environments like along Arctic coastlines.

Spatial and seasonal variability adds further uncertainty (Henson et al., 2024). Each fjord differs in freshwater input, biological activity, and circulation, and most measurements are sparse and focused on summer, leaving gaps for other seasons. Year-round coverage across all fjords would be ideal but is currently unrealistic.

🌊How can your findings help improve predictions about the future of our climate? What are some of the most exciting or surprising discoveries you made during your PhD?

Our findings can improve climate predictions because the Arctic is becoming rapidly becoming both warmer and “fresher.” My PhD focuses on how this freshwater runoff affects carbon cycling—processes not well represented in current global models—helping us better predict a future Arctic ocean is less saline and that may be ice-free.

One of the most exciting discoveries we have made is how glacier meltwater amplifies the impact biogeochemical processes (Henson et al., 2025). Glacier runoff has very low alkalinity, which dilutes the ocean’s buffering capacity. This means processes like photosynthesis, bacterial respiration, and mineral dissolution have a greater influence on the CO₂ system. In some fjords we study, photosynthesis has double the impact on pCO₂ compared in the open ocean, due to this enhanced sensitivity caused by freshwater.

🌊What’s the next big question in polar ocean research you’d want to answer, and where do you see your work heading?

I’m fascinated by the big-picture connections—how freshwater, circulation, biology, and chemistry all interact in coastal systems. These environments link terrestrial, freshwater, and marine ecosystems, so breaking down research silos and collaborating across disciplines is key. I’m also curious about the human dimension, like how ocean acidification affects Arctic food webs and local communities’ food security.

Looking ahead, my plan is to continue this research in a postdoc, focusing on how fresher, more ice-free Arctic coastal oceans impact carbon cycling and ecosystems. I also hope to keep teaching, including the course that first inspired me, and expand my work in science communication to share these findings beyond academia.

Further Reading:

Henson, H. C. et al. Resolving heterogeneity in co2 uptake potential in the greenland coastal ocean. J. Geophys. Res. Biogeosci. 129, e2024JG008246 (2024).

Henson, H.C., Puts, I.C., Sejr, M.K. et al. Glacial meltwater increases coastal carbon dioxide uptake and sensitivity to biogeochemical change. Commun Earth Environ 6, 687 (2025). 

Interviewed and edited by M. Keppens