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Earth's Systems / Water cycle / Snow, ice and meltwater storage / Glacier and ice-sheet mass balance and flow

Glacier and ice-sheet mass balance and flow

How snow accumulation, surface melt, ice flow, iceberg calving and ocean melting set the mass balance of glaciers and the Greenland and Antarctic ice sheets, with the melt-elevation and marine ice-sheet feedbacks that can make loss self-sustaining.

Boundary: where this process starts

Starts with: Above the equilibrium line more snow falls than melts and compacts into ice; below it melting and sublimation remove mass (ablation zone)

Boundary: where it ends

Ends after: Faster glacier flow and iceberg discharge (49.7%), rising from 46 to 87 Gt/yr of imbalance; Reduced surface mass balance: more meltwater runoff (50.3%; and 84% of the extra loss after 2009)

PARTICIPANTSRESOURCESIce sheetsOceanGlaciersIce shelvesAtmosphereIceMeltwaterHeatWaterImpact +4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.50%Impact -4 of 5. Click for details and evidence.50%Impact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.↻ reinforces · weak · 0.2Impact +4 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.55%Impact -4 of 5. Click for details and evidence.45%Impact -4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.↻ reinforces · moderate · 0.3Impact -3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -2 of 5. Click for details and evidence.Above the equilibrium line more snow falls than melts and compacts into ice; below it melting and sublimation remove mass (ablation zone): makes IcemakesAbove the equilibrium line more snow falls than melts and compacts into ice; below it melting and sublimation remove mass (ablation zone): uses up Wateruses upIce deforms under its own weight and slides on its bed; ice streams and outlet glaciers drain the ice sheets, with tributaries reaching 1000 km inland: uses IceMass balance = accumulation minus runoff, iceberg discharge and basal melt; in the mid-1990s Greenland's discharge of about 411 Gt/yr matched its surface mass b: uses IceMass balance = accumulation minus runoff, iceberg discharge and basal melt; in the mid-1990s Greenland's discharge of about 411 Gt/yr matched its surface mass b: uses MeltwaterHow Greenland lost 3,902 Gt of ice in 1992-2018: uses IceReduced surface mass balance: more meltwater runoff (50.3%; and 84% of the extra loss after 2009): uses IceReduced surface mass balance: more meltwater runoff (50.3%; and 84% of the extra loss after 2009): makes MeltwatermakesFaster glacier flow and iceberg discharge (49.7%), rising from 46 to 87 Gt/yr of imbalance: uses IceMelt-elevation feedback: as the surface lowers into warmer air it melts faster, which beyond a threshold becomes self-sustaining: uses IceMelt-elevation feedback: as the surface lowers into warmer air it melts faster, which beyond a threshold becomes self-sustaining: uses HeatHow Antarctic ice shelves lose their ice (about 2,400 Gt/yr): uses IceOcean melting from below (about 55%): half of it from 10 small warm-cavity shelves in the Amundsen-Bellingshausen sector covering 8% of shelf area: uses IceOcean melting from below (about 55%): half of it from 10 small warm-cavity shelves in the Amundsen-Bellingshausen sector covering 8% of shelf area: uses HeatIceberg calving (about 45%): uses IceIce shelves buttress the grounded ice; where they thin or lose their front, outflow speeds up (Amundsen and Bellingshausen shelves have almost no passive ice to: uses IceMarine ice-sheet instability: on beds that deepen inland, grounding-line flux rises steeply with thickness, so retreat can self-sustain (West Antarctica): uses IcemakesIce: makesuses upWater: uses upusesIce: usesusesIce: usesusesMeltwater: usesusesIce: usesusesIce: usesmakesMeltwater: makesusesIce: usesusesIce: usesusesHeat: usesusesIce: usesusesIce: usesusesHeat: usesusesIce: usesusesIce: usesusesIce: usesStart: Glacier andice-sheet mass balanceand flowSTARTStart: Glacier and ice-sheet mass balance and flowEnd: Glacier andice-sheet mass balanceand flow ↗1ENDEnd: Glacier and ice-sheet mass balance and flowAbove the equilibriumline more snow fallsthan melts and compac…📄2Above the equilibrium line more snow falls than melts and compacts into ice; below it melting and sublimation remove mass (ablation zone)Ice deforms under itsown weight and slideson its bed; ice strea…📄2Ice deforms under its own weight and slides on its bed; ice streams and outlet glaciers drain the ice sheets, with tributaries reaching 1000 km inlandMass balance =accumulation minusrunoff, icebe…📄2Mass balance = accumulation minus runoff, iceberg discharge and basal melt; in the mid-1990s Greenland's discharge of about 411 Gt/yr matched its surface mass bHow Greenland lost3,902 Gt of ice in1992-2018📄1How Greenland lost 3,902 Gt of ice in 1992-2018Reduced surface massbalance: more meltwaterrunoff (50.3%; and 8…📄2Reduced surface mass balance: more meltwater runoff (50.3%; and 84% of the extra loss after 2009)Faster glacier flow andiceberg discharge(49.7%), rising from …📄1Faster glacier flow and iceberg discharge (49.7%), rising from 46 to 87 Gt/yr of imbalanceMelt-elevationfeedback: as thesurface lowers in…📄1Melt-elevation feedback: as the surface lowers into warmer air it melts faster, which beyond a threshold becomes self-sustainingHow Antarctic iceshelves lose their ice(about 2,400 Gt/yr)📄2How Antarctic ice shelves lose their ice (about 2,400 Gt/yr)Ocean melting frombelow (about 55%): halfof it from 10 sma…📄1Ocean melting from below (about 55%): half of it from 10 small warm-cavity shelves in the Amundsen-Bellingshausen sector covering 8% of shelf areaIceberg calving (about45%)📄2Iceberg calving (about 45%)Ice shelves buttressthe grounded ice; wherethey thin or lose the…📄2Ice shelves buttress the grounded ice; where they thin or lose their front, outflow speeds up (Amundsen and Bellingshausen shelves have almost no passive ice toMarine ice-sheetinstability: on bedsthat deepen inlan…📄1Marine ice-sheet instability: on beds that deepen inland, grounding-line flux rises steeply with thickness, so retreat can self-sustain (West Antarctica)
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
Participant
Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
Resource
Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
Step
One thing that happens in a process. Every step has at least one participant and carries evidence.
Decision fork
A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
Loop
A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
Size of impact
Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
Evidence
The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
Boundary
Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
Subprocess
A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
Description
A plain-language summary of what a process is and what it covers.
Hypothesis
A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.

Every step, with its evidence

The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.

Above the equilibrium line more snow falls than melts and compacts into ice; below it melting and sublimation remove mass (ablation zone)
GlaciersIce sheetsSnowpackWaterIce
Evidence (2) · added by Briangold-Claude
  • NSIDC, Glaciers overview, https://nsidc.org/learn/parts-cryosphere/glaciers: At higher elevations more snow falls than melts, adding to the glacier's mass; the surplus of built-up ice flows downhill to lower elevations where it is lost. (confidence 0.9)
  • van den Broeke et al. 2016, On the recent contribution of the Greenland ice sheet to sea level change, The Cryosphere 10:1933-1946, https://tc.copernicus.org/articles/10/1933/2016/: Surface mass balance is the net of accumulation and runoff and averaged about 418 Gt/yr for Greenland in 1958-1995. (confidence 0.85)
Ice deforms under its own weight and slides on its bed; ice streams and outlet glaciers drain the ice sheets, with tributaries reaching 1000 km inland
Ice sheetsGlaciersIce streamsIce
Evidence (2) · added by Briangold-Claude
  • NSIDC, Ice sheets overview, https://nsidc.org/learn/parts-cryosphere/ice-sheets: Ice sheets are constantly in motion, spreading out under their own weight in broad domes. (confidence 0.9)
  • Bamber, Vaughan & Joughin 2000, Widespread complex flow in the interior of the Antarctic Ice Sheet, Science 287:1248-1250, https://doi.org/10.1126/science.287.5456.1248: Each major Antarctic drainage basin is fed by complex systems of tributaries that penetrate up to 1000 km from the grounding line into the interior. (confidence 0.9)
Mass balance = accumulation minus runoff, iceberg discharge and basal melt; in the mid-1990s Greenland's discharge of about 411 Gt/yr matched its surface mass b
Ice sheetsGlaciersIceMeltwater
Evidence (2) · added by Briangold-Claude
  • van den Broeke et al. 2016, On the recent contribution of the Greenland ice sheet to sea level change, The Cryosphere 10:1933-1946, https://tc.copernicus.org/articles/10/1933/2016/: The 1995 discharge of 411 Gt/yr and the 1958-1995 average surface mass balance of 418 Gt/yr are similar, indicating an ice sheet in approximate balance; after the mid-1990s SMB decreased while discharge rose. (confidence 0.9)
  • Shepherd et al. (IMBIE) 2020, Mass balance of the Greenland Ice Sheet from 1992 to 2018, Nature 579:233-239, https://doi.org/10.1038/s41586-019-1855-2: The ice sheet was close to balance in the 1990s, with losses rising since. (confidence 0.85)
⑂ How Greenland lost 3,902 Gt of ice in 1992-2018
Ice sheetsAtmosphereOceanIce
Evidence (1) · added by Briangold-Claude
  • Shepherd et al. (IMBIE) 2020, Mass balance of the Greenland Ice Sheet from 1992 to 2018, Nature 579:233-239, https://doi.org/10.1038/s41586-019-1855-2: Greenland lost 3,902 +/- 342 Gt between 1992 and 2018; 50.3% from reduced surface mass balance (runoff) and 49.7% from increased glacier discharge. (confidence 0.9)
Reduced surface mass balance: more meltwater runoff (50.3%; and 84% of the extra loss after 2009)
Ice sheetsMeltwaterIceMeltwater
Evidence (2) · added by Briangold-Claude
  • Shepherd et al. (IMBIE) 2020, Mass balance of the Greenland Ice Sheet from 1992 to 2018, Nature 579:233-239, https://doi.org/10.1038/s41586-019-1855-2: Reduced SMB drove 1,964 Gt (50.3%) of the loss through increased runoff. (confidence 0.9)
  • Enderlin et al. 2014, An improved mass budget for the Greenland ice sheet, Geophys. Res. Lett. 41:866-872, https://doi.org/10.1002/2013GL059010: 84% of the increase in mass loss after 2009 was due to increased surface runoff; discharge's share of loss fell from 58% before 2005 to 32% in 2009-2012. (confidence 0.9)
Faster glacier flow and iceberg discharge (49.7%), rising from 46 to 87 Gt/yr of imbalance
Ice sheetsGlaciersOceanIce
Evidence (1) · added by Briangold-Claude
  • Shepherd et al. (IMBIE) 2020, Mass balance of the Greenland Ice Sheet from 1992 to 2018, Nature 579:233-239, https://doi.org/10.1038/s41586-019-1855-2: 1,938 Gt (49.7%) was due to glacier dynamical imbalance, which rose from 46 Gt/yr in the 1990s to 87 Gt/yr since. (confidence 0.9)
Melt-elevation feedback: as the surface lowers into warmer air it melts faster, which beyond a threshold becomes self-sustaining
Ice sheetsAtmosphereHeatIce
Evidence (1) · added by Briangold-Claude
  • Levermann & Winkelmann 2016, A simple equation for the melt elevation feedback of ice sheets, The Cryosphere 10:1799-1807, https://tc.copernicus.org/articles/10/1799/2016/: Beyond a critical temperature threshold a meltdown of the Greenland Ice Sheet is induced by the self-enforcing feedback between lowering surface elevation and increasing surface melt. (confidence 0.85)
⑂ How Antarctic ice shelves lose their ice (about 2,400 Gt/yr)
Ice shelvesOceanIce
Evidence (2) · added by Briangold-Claude
  • Rignot et al. 2013, Ice-shelf melting around Antarctica, Science 341:266-270, https://doi.org/10.1126/science.1235798: Basal melt of 1325 +/- 235 Gt/yr and calving of 1089 +/- 139 Gt/yr. (confidence 0.9)
  • Depoorter et al. 2013, Calving fluxes and basal melt rates of Antarctic ice shelves, Nature 502:89-92, https://doi.org/10.1038/nature12567: Total calving flux 1,321 Gt/yr and basal mass balance -1,454 Gt/yr. (confidence 0.85)
Ocean melting from below (about 55%): half of it from 10 small warm-cavity shelves in the Amundsen-Bellingshausen sector covering 8% of shelf area
Ice shelvesOceanIceHeat
Evidence (1) · added by Briangold-Claude
  • Rignot et al. 2013, Ice-shelf melting around Antarctica, Science 341:266-270, https://doi.org/10.1126/science.1235798: Basal melt 1325 Gt/yr exceeds calving; half of the meltwater comes from ten small warm-cavity Southeast Pacific ice shelves occupying 8% of the area. (confidence 0.9)
Iceberg calving (about 45%)
Ice shelvesOceanIce
Evidence (2) · added by Briangold-Claude
  • Rignot et al. 2013, Ice-shelf melting around Antarctica, Science 341:266-270, https://doi.org/10.1126/science.1235798: Calving flux 1089 Gt/yr. (confidence 0.9)
  • Depoorter et al. 2013, Calving fluxes and basal melt rates of Antarctic ice shelves, Nature 502:89-92, https://doi.org/10.1038/nature12567: Calving 1,321 Gt/yr, about 34% less than earlier iceberg-tracking estimates. (confidence 0.85)
Ice shelves buttress the grounded ice; where they thin or lose their front, outflow speeds up (Amundsen and Bellingshausen shelves have almost no passive ice to
Ice shelvesIce sheetsIce
Evidence (2) · added by Briangold-Claude
  • Furst et al. 2016, The safety band of Antarctic ice shelves, Nature Climate Change 6:479-482, https://www.nature.com/articles/nclimate2912: Ice shelves in the Amundsen and Bellingshausen seas have limited or almost no passive portion, so further front retreat yields important dynamic consequences; upstream ice rests on a backward-sloping bed. (confidence 0.85)
  • Depoorter et al. 2013, Calving fluxes and basal melt rates of Antarctic ice shelves, Nature 502:89-92, https://doi.org/10.1038/nature12567: Basal mass loss correlates with surface elevation change of ice shelves. (confidence 0.75)
Marine ice-sheet instability: on beds that deepen inland, grounding-line flux rises steeply with thickness, so retreat can self-sustain (West Antarctica)
Ice sheetsOceanIce
Evidence (1) · added by Briangold-Claude
  • Schoof 2007, Ice sheet grounding line dynamics: Steady states, stability, and hysteresis, J. Geophys. Res. 112:F03S28, https://doi.org/10.1029/2006JF000664: Ice flux through the grounding line increases sharply with ice thickness; steady grounding lines cannot be stable on reverse bed slopes, and marine ice sheets with overdeepened beds show hysteresis. (confidence 0.9)