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  • Review Article
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Firn on ice sheets

Nature Reviews Earth & Environment volume 5, pages 79–99 (2024)Cite this article

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A Publisher Correction to this article was published on 12 February 2024

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Abstract

Most of the Greenland and Antarctic ice sheets are covered with firn — the transitional material between snow and glacial ice. Firn is vital for understanding ice-sheet mass balance and hydrology, and palaeoclimate. In this Review, we synthesize knowledge of firn, including its formation, observation, modelling and relevance to ice sheets. The refreezing of meltwater in the pore space of firn currently prevents 50% of meltwater in Greenland from running off into the ocean and protects Antarctic ice shelves from catastrophic collapse. Continued atmospheric warming could inhibit future protection against mass loss. For example, warming in Greenland has already contributed to a 5% reduction in firn pore space since 1980. All projections of future firn change suggest that surface meltwater will have an increasing impact on firn, with melt occurring tens to hundreds of kilometres further inland in Greenland, and more extensively on Antarctic ice shelves. Although progress in observation and modelling techniques has led to a well-established understanding of firn, the large uncertainties associated with meltwater percolation processes (refreezing, ice-layer formation and storage) must be reduced further. A tighter integration of modelling components (firn, atmosphere and ice-sheet models) will also be needed to better simulate ice-sheet responses to anthropogenic warming and to quantify future sea-level rise.

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Fig. 1: Schematic representation of important ice-sheet features.
Fig. 2: Current metrics for firn on the Antarctic and Greenland ice sheets.
Fig. 3: Effects of meltwater on firn structure.
Fig. 4: Firn facies and features on the polar ice sheets.
Fig. 5: Modelling and observational techniques.
Fig. 6: Future metrics for firn on the Antarctic and Greenland ice sheets.

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Acknowledgements

This Review is the result of the International Firn Symposium held online in June 2022, which was intentionally collaborative and inclusive in its setup. No travel or participation expenses were required. Junior and senior scientists were paired for writing the sections. As such, this paper is written as a team effort without a specified author order. We thank all symposium participants for their contributions and discussions. We also thank K. Poinar for her feedback on the representation of hydrological features in Fig. 1.

Author information

Authors and Affiliations

  1. Institut des Géosciences de l’Environnement, University Grenoble Alpes, CNRS, Grenoble, France

    Charles Amory

  2. College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR, USA

    Christo Buizert

  3. School of Earth and Environmental Sciences, Cardiff University, Cardiff, UK

    Sammie Buzzard

  4. Department of Geography and Environmental Sciences, Northumbria University, Newcastle upon Tyne, UK

    Sammie Buzzard

  5. Department of Earth and Environmental Sciences, Columbia University, NY, USA

    Elizabeth Case

  6. Department of Geosciences, Université de Fribourg, Fribourg, Switzerland

    Nicole Clerx

  7. Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA

    Riley Culberg

  8. Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, CO, USA

    Rajashree Tri Datta, Devon Dunmire, Jan T. M. Lenaerts, Megan Thompson-Munson & Nander Wever

  9. National Centre for Polar and Ocean Research (NCPOR), Ministry of Earth Sciences, Vasco da Gama, Goa, India

    Rahul Dey

  10. Department of Geosciences, University of Tübingen, Tübingen, Germany

    Reinhard Drews & Falk M. Oraschewski

  11. Center for Global Sea Level Change, New York University Abu Dhabi, Abu Dhabi, UAE

    Clare Eayrs

  12. Department of Glacial Environment Research, Korea Polar Research Institute, Incheon, Republic of Korea

    Clare Eayrs

  13. Geodesy and Earth Observation, DTU Space, Technical University of Denmark, Kongens Lyngby, Denmark

    Nicolaj Hansen & Sebastian B. Simonsen

  14. Danish Meteorological Institute, Copenhagen, Denmark

    Nicolaj Hansen

  15. Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany

    Angelika Humbert

  16. Faculty of Geosciences, University of Bremen, Bremen, Germany

    Angelika Humbert

  17. Observatoire de la Côte d’Azur, Université Côte d’Azur, CNRS, Valbonne, France

    Athul Kaitheri

  18. Department of Geological Sciences and Engineering, University of Nevada, Reno, USA

    Kaitlin Keegan & Ian McDowell

  19. Institute for Marine and Atmospheric Research (IMAU), Utrecht University, Utrecht, The Netherlands

    Peter Kuipers Munneke

  20. Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium

    Stef Lhermitte

  21. Department of Geoscience and Remote Sensing, Delft University of Technology (TUDelft), Delft, The Netherlands

    Stef Lhermitte, Sophie de Roda Husman & Bert Wouters

  22. School of Environmental Sciences, University of Liverpool, Liverpool, UK

    Doug Mair

  23. Graduate Program of Hydrologic Sciences, University of Nevada, Reno, NV, USA

    Ian McDowell

  24. Department of Geology, University at Buffalo, Buffalo, NY, USA

    Jessica Mejia

  25. Thayer School of Engineering, Dartmouth College, Hanover, NH, USA

    Colin R. Meyer

  26. Scott Polar Research Institute, University of Cambridge, Cambridge, UK

    Elizabeth Morris

  27. Ice Dynamics and Palaeoclimate, British Antarctic Survey, Cambridge, UK

    Dorothea Moser & Elizabeth R. Thomas

  28. Department of Earth Sciences, University of Cambridge, Cambridge, UK

    Dorothea Moser

  29. Institut Terre et Environnement de Strasbourg (ITES), Université de Strasbourg, Strasbourg, France

    Emma Pearce

  30. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    Nicole-Jeanne Schlegel

  31. NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA

    Nicole-Jeanne Schlegel

  32. Institute for Mechanics, TU Darmstadt, Darmstadt, Germany

    Timm Schultz

  33. Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD, USA

    C. Max Stevens

  34. Cryospheric Sciences Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA

    C. Max Stevens

  35. Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado Boulder, Boulder, CO, USA

    Megan Thompson-Munson

  36. WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland

    Nander Wever

Consortia

The Firn Symposium team

  • Charles Amory
  • , Christo Buizert
  • , Sammie Buzzard
  • , Elizabeth Case
  • , Nicole Clerx
  • , Riley Culberg
  • , Rajashree Tri Datta
  • , Rahul Dey
  • , Reinhard Drews
  • , Devon Dunmire
  • , Clare Eayrs
  • , Nicolaj Hansen
  • , Angelika Humbert
  • , Athul Kaitheri
  • , Kaitlin Keegan
  • , Peter Kuipers Munneke
  • , Jan T. M. Lenaerts
  • , Stef Lhermitte
  • , Doug Mair
  • , Ian McDowell
  • , Jessica Mejia
  • , Colin R. Meyer
  • , Elizabeth Morris
  • , Dorothea Moser
  • , Falk M. Oraschewski
  • , Emma Pearce
  • , Sophie de Roda Husman
  • , Nicole-Jeanne Schlegel
  • , Timm Schultz
  • , Sebastian B. Simonsen
  • , C. Max Stevens
  • , Elizabeth R. Thomas
  • , Megan Thompson-Munson
  • , Nander Wever
  •  & Bert Wouters

Contributions

P.K.M. led the Introduction. C.A., N.H. and R.T.D. led ‘Atmospheric forcing’. I.McD. and C.M.S. led ‘Firn properties’. R.C. and C.M. led ‘Firn hydrology’. R.De., D.Ma., D.Mo., E.R.T., R.Dr., E.P., S.S., S.deR.H., C.B., C.E. and K.K. led ‘Firn observations’. A.H., E.M., F.M.O. and T.S. led ‘Modelling dry firn densification’. S.B. and J.M. led ‘Modelling wet firn and firn hydrology’. C.B. led ‘Modelling chemical tracer transport’. C.A. and R.T.D. led ‘The changing ice-sheet and ice-shelf firn’. E.C., D.D., S.L., N.-J.S., M.T.-M., N.W. and B.W. led the ‘Summary and future perspectives’. N.C. and A.K. contributed to earlier drafts of the manuscript. Figs. 1 and 3 were created by R.T.D., Figs. 2 and 6 by C.A., Fig. 4 by R.C., and Fig. 5 by S.deR.H. J.T.M.L. and N.W. coordinated the International Firn Symposium that resulted in this Review. R.T.D, P.K.M. and N.W. coordinated the paper writing and did the final editing.

Corresponding author

Correspondence to Nander Wever.

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Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Reviews Earth & Environment thanks Baptiste Vandecrux, Vincent Verjans and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Glossary

Ablation zone

An ice-sheet region with negative yearly local surface mass balance.

Accumulation

The increase in mass at the surface of the ice sheet caused by precipitation, or the deposition of wind-transported snow.

Atmospheric blocking

The presence of large-scale, nearly stationary pressure anomalies generally driven by high pressure in the atmosphere, which disrupt the mean storm track and can cause persistent weather (especially temperature extremes) in a large area, lasting several days or even weeks.

Backscatter modelling

Modelling the interaction (reflection, refraction and scattering) of electromagnetic waves with the ice-sheet surface.

Blue-ice areas

Areas with sufficient negative surface mass balance to expose the bare glacial ice.

Bucket models

Simple description of liquid water flow in models, in which capillarity is represented by a threshold of liquid water content that must be reached in a layer before water moves downward to the layer below.

Buried lake

Also known as subsurface lakes. Liquid water body in the firn, formed when a surface lake freezes over and gets buried by snowfall. To be distinguished from aquifers, which denote saturation of the pore space in firn.

Capillary barrier

Strong contrast in capillarity between two snow layers owing to contrasting snow microstructure properties, which inhibits the downward percolation of water.

Coffee-can method

A method to measure firn compaction by anchoring a string or pole at the bottom of a borehole and measuring the surface-height change relative to a reference point on the string or pole. So named because the original anchors were coffee cans.

Darcy’s law

A relationship between flow rate and pressure drop, governed by the permeability of the medium and the viscosity of the fluid that describes flow through a fully saturated porous medium.

Deep firn cores

Firn cores reaching deeper than approximately 15 m, such that elaborate equipment is often required to extract the core.

Dry snow zone

Area of the firn that remains dry throughout the year.

Elastic properties

Material properties that define the response of a material to the application of a force, for example by deformation or compression.

Empirical densification laws

Densification laws that use parameters derived from observed depth–density profiles.

Firn air content

A measure of the amount of air-filled pore space in the firn, computed as the volume integral over the porosity.

Firn aquifers

Areas inside the firn with full saturation of meltwater, which remains liquid throughout winter until the start of the next melt season.

Fractionation

Separation of molecules with different isotopic compositions caused by pore close-off, temperature differences or the presence of a temperature gradient.

Glacial ice

Part of the ice sheet where the pores in the matrix are closed off all the way to the bedrock (typical density >830 kg m–3).

Metamorphism

Changes in the microstructure of firn caused by vapour transport, the presence and flow of liquid water, and variations in pressure within the snow or firn cover.

Percolation zone

Area of the firn with sufficient meltwater production or rainwater input to trigger downward water flow.

Permeability

In the context of firn, it indicates the ability of liquids and gases to move through the ice matrix.

Permittivity

A measure of a material’s ability to interact with, and become polarized by, an applied electric field, thereby governing the transmission, reflection and absorption of electromagnetic waves in the material.

Pore close-off

State of the ice matrix at which the firn air becomes occluded into closed, isolated bubbles and the individual pores are no longer connected and thus cannot exchange gases, chemicals or liquid water.

Preferential flow

Inhomogeneous water flow in firn caused by microstructural features such as vertical pipes, small-scale spatial variability in hydraulic properties, or flow fingering resulting from instabilities in the wetting front.

Runoff

Liquid water leaving the firn column, firn layer or ice sheet, depending on the context. Surface runoff refers specifically to water leaving the firn at the surface.

Runoff area

Area of the firn in which at least some of the meltwater produced will leave the firn layer through surface runoff or drainage through the englacial drainage system in the same melt season it was produced.

Sastrugi

Surface snow bedform, which is widespread in environments dominated by drifting snow, characterized by elongated ridges of wind-packed snow that form owing to snow erosional processes carving into wind-packed snow.

Semi-empirical

Empirical approach that satisfies the principles of continuum mechanics, such as balance equations and material theory.

Shallow firn cores

Firn cores up to approximately 15 m depth. Typically obtained using hand drilling or only lightweight equipment.

Sintering

The formation and growth of bonds between snow particles.

Slug tests

Test to determine the horizontal hydraulic conductivity of a saturated medium by removing, adding or displacing water and monitoring the water level while equilibrium conditions return.

Slush fields

Areas of the firn where meltwater can be observed at the surface, suggesting a high degree of water saturation.

Strain softening

Reduction of a material’s viscosity with increasing strain as it is deformed.

Super-resolution downscaling

A technique typically used in machine learning to construct high-resolution images from low-resolution images.

Wetting front

Separation between uniformly wet and dry firn.

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The Firn Symposium team. Firn on ice sheets. Nat Rev Earth Environ 5, 79–99 (2024). https://doi.org/10.1038/s43017-023-00507-9

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