Firn and firn aquifers

By Isabelle Wicks

What is firn?

Firn is a material that is halfway between snow and glacier ice. It is defined as snow that has remained on a glacier for at least one year without being melted during the summer. It is not completely solid like ice, but it is also not light and fluffy like snow. Instead, firn has a texture and appearance much like wet sugar.

Unlike snowflakes, firn grains are not hexagonal, and they contain much less air content.

Firn is a store of meltwater

Due to having some air space between the individual grains, firn can act like a sponge and store water. When snow melts on the surface of the glacier, the water produced by melting (known as meltwater) can travel down into the air space of the firn and stay there1,2. The snow above, which is a good insulator due to the air trapped between snowflakes, protects the firn layer from cold conditions and means that this meltwater can remain as a liquid2. However, this does not always occur and meltwater can also refreeze, forming layers of ice within the firn3.

Saturated firn on a small cold glacier on James Ross Island, Antarctic Peninsula

How does firn form?

When snow falls onto a glacier, it settles, much like it does on the roof of a house. If the snow lasts for at least one year and does not melt in summer, then it begins to transform into firn. The weight of new snow on top of the year-old snow begins to compact it, slowly squeezing air out and changing the shape of the snowflakes into pointy grains of snow1.

Under enough pressure, these pointy snow grains are forced into closer and closer contact with one another, forming a mesh of grains with an uneven but connected air space5, known as firn.

his graphic illustrates the process of glacial ice formation and approximate percentages of air (by volume) in each step. — Credit: Department of Geography and Environmental Science/Hunter College

Typically, firn will exist for 100-300 years before being transformed into glacial ice, but in some very cold and dry regions of Antarctica, such as at the South Pole, firn can exist for up to 2000 years before transforming into glacial ice4!

Where is firn found?

Firn covers ~99% of the Antarctic Ice Sheet surface6 and ~90% of the Greenland Ice Sheet surface7, and is also found on the surface of glaciers, beneath fresh snow. The thickness of the firn layer varies depending on its location, due to different amounts of melt and snowfall; along the coastlines of Antarctica and Greenland firn can be as shallow as several metres, and towards the centres of these ice sheets, firn can be over 100 m thick4!

Why is firn important?

Firn helps to regulate the contribution of ice sheets to sea level rise by holding meltwater within its air space, either as liquid water or refrozen into ice1,3. Around 45% of meltwater in Greenland and almost all Antarctic meltwater refreezes in firn1,3, stopping it from entering the oceans and contributing to rising sea levels. As air temperatures rise, meltwater is increasingly running off, leading to more mass loss and increased contribution to sea level rise8.

However, the weight of meltwater and ice within the firn can also increase the pressure on the glacial ice beneath, or it can be rapidly released into large cracks in the glacier, known as crevasses, causing the cracks to widen and deepen5, which can lead to large areas breaking off from the ice shelf. Understanding what firn is and how it evolves through time is critical in helping to understand how Antarctica and Greenland will change as the atmosphere warms.

Further reading

About the Author

Isabella Wicks
Isabelle Wicks

This article was written by Isabelle Wicks, a PhD student at the Centre for Polar Observation and Modelling at Northumbria University.

I am a PhD researcher at Northumbria University, working with the Centre for Polar Observation and Modelling. I am interested in glacier and ice shelf dynamics, especially their interactions with meltwater and climate. My research focusses on using the MONARCHS model to understand how surface melt on Antarctic ice shelves impacts their stability, and how (near-)surface hydrological networks, including firn aquifers and meltwater storage, develop through time and space. I am also passionate about scientific outreach and getting the next generation of women and girls interested in and excited about polar sciences.

References

[1] Amory, C., Buizert, C., Buzzard, S., Case, E., Clerx, N., Culberg, R., Datta, R. T., Dey, R., Drews, R., Dunmire, D., Eayrs, C., Hansen, N., Humbert, A., Kaitheri, A., Keegan, K., Kuipers Munneke, P., Lenaerts, J. T. M., Lhermitte, S., Mair, D., … Wouters, B. (2024). Firn on ice sheets. Nature Reviews Earth and Environment, 5(2), 79–99. https://doi.org/10.1038/s43017-023-00507-9

[2] Horlings, A. N., Christianson, K., & Miège, C. (2022). Expansion of Firn Aquifers in Southeast Greenland. Journal of Geophysical Research: Earth Surface, 127(10). https://doi.org/10.1029/2022JF006753

[3] Brils, M., Munneke, P. K., Jullien, N., Tedstone, A. J., Machguth, H., van de Berg, W. J., & van den Broeke, M. R. (2024). Climatic Drivers of Ice Slabs and Firn Aquifers in Greenland. Geophysical Research Letters, 51(3). https://doi.org/10.1029/2023GL106613

[4] van den Broeke, M. (2008). Depth and density of the Antarctic firn layer. Arctic, Antarctic, and Alpine Research, 40(2), 432–438. https://doi.org/10.1657/1523-0430(07-021)[BROEKE]2.0.CO;2

[5] Munneke, P. K., Ligtenberg, S. R. M., van den Broeke, M. R., & Vaughan, D. G. (2014). Firn air depletion as a precursor of Antarctic ice-shelf collapse. Journal of Glaciology, 60(220), 205–214. https://doi.org/10.3189/2014JoG13J183

[6] Verjans, V., Leeson, A. A., McMillan, M., Stevens, C. M., van Wessem, J. M., van de Berg, W. J., et al. (2021). Uncertainty in East Antarctic firn thickness constrained using a model ensemble approach. Geophysical Research Letters, 48, e2020GL092060. https://doi.org/10.1029/2020GL092060

[7] Noël, B., Lenaerts, J. T. M., Lipscomb, W. H., Thayer-Calder, K. & van den Broeke, M. R. Peak refreezing in the Greenland firn layer under future warming scenarios. Nat. Commun. 13, 6870 (2022).

[8] Noël, B., et al. (2017). “A tipping point in refreezing accelerates mass loss of Greenland’s glaciers and ice caps.” Nature Communications 8(1): 14730.

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