Lecture 3 - Mass Balance

Glacial Systems: Mass Balance

Drop-in Sessions

  • Friday 5-6 PM seminar room for questions about the module or coursework.
  • Available for questions related to module content, coursework, or general assistance.

Reading List

  • Resource list on Moodle.
  • Textbooks provide different perspectives; find your favorite and use it for definitions and examples.
  • Academic journals are the best source for current research.
  • Use journal reference lists to find more recent or relevant research.

Introduction to Mass Balance

  • Mass balance: process by which glaciers grow and shrink.
  • Focus on inputs (accumulation) and outputs (ablation) of ice.
  • Plots of surface mass balance for glaciers and the Greenland ice sheet.
  • High accumulation rates in Southeast Greenland and low rates in the North.

Accumulation (Gain of Mass)

  • Snowfall: Primary input.
  • Avalanches: Snow and rock, more common in mountain regions.
  • Rime: Phase conversion from water vapor directly to ice (tiny proportion).
  • Freezing Rain: Rain freezes directly onto the ice surface (important for accumulation).
  • Important: Rain that does not freeze immediately causes melting (ablation).
  • Blowing Snow: Redistribution of snow across the glacier surface.

Ablation (Loss of Mass)

  • Surface Melting: Primary form of ablation in lower elevations.
  • Avalanches: Ice and snow off the glacier.
  • Carving: Ice breaks off into water bodies (fjords, lakes, oceans).
    • Warm water undercuts the ice, destabilizing and forming icebergs.
    • Significant mass loss for marine-terminating glaciers.
  • Evaporation: Water lost from the surface.
  • Sublimation: Ice converts directly to water vapor (tiny proportion).
    • Critical to consider all factors for accurate mass balance calculations.

Mass Balance Equation

  • Mass balance is the difference between accumulation and ablation.
  • Expressed as: Mass\ Balance = Accumulation - Ablation
  • Positive mass balance: Accumulation > Ablation (glacier gains mass).
  • Negative mass balance: Ablation > Accumulation (glacier loses mass).
  • Expressed as depth of water equivalent per area (e.g., millimeters of water equivalent per meter squared).
    • Facilitates understanding of sea level rise.

Spatial Zones on Glaciers

  • Zone of Accumulation: Upper reaches, accumulation > ablation (positive mass balance).
  • Zone of Ablation: Lower reaches, ablation > accumulation (negative mass balance).

Temporal Variations in Mass Balance

  • Accumulation varies seasonally, with more in winter.
  • Graph of mass balance over a year: ablation at the bottom, accumulation at the top.
  • Winter: High accumulation, little ablation.
  • Summer: High ablation, little accumulation.
  • Winter Balance: Mass balance over winter (positive).
  • Summer Balance: Mass balance over summer (negative).
  • Net Balance: Total change in mass over the year (difference between winter and summer balance).

Glacier Accumulation Types

  • Winter Accumulation: Most accumulation in winter, most ablation in summer.
    • Examples: Alps, Scandinavia, western Himalaya, New Zealand.
  • Summer Accumulation: Accumulation and ablation both maximal in summer due to monsoons.
    • Example: Himalaya (monsoon brings snow at high elevations, melt at lower elevations).
  • Year-Round Ablation: Melting occurs continuously, accumulation during wet season.
    • Example: Inner tropics (few glaciers remain).
  • Dominant type: Winter accumulation.

Equilibrium Line Altitude (ELA)

  • The point where net accumulation equals net ablation.
  • Theoretical line that is not visible on glaciers.
  • ELA moves across the glacier and varies year to year.
  • Historical ELA can indicate past climate effects.

Mass Balance and Elevation

  • Graph of mass balance versus elevation.
  • ELA is where net balance is zero.
  • ELA shifts:
    • Lower ELA: More of the glacier is in the accumulation area (positive mass balance).
    • Higher ELA: More of the glacier is in the ablation zone (negative mass balance).
    • No ELA: Entire glacier in the ablation zone.
  • Accumulation and ablation zones are not fixed and vary year by year.

Mass Balance Gradients

  • Comparing mass balance gradients between glaciers.
  • The steepness of the mass balance gradient varies.
  • Example: Nigardsbreen (steep gradient) vs. Devon Ice Cap (shallow gradient).
  • Different rates of accumulation and ablation at similar elevations.

Factors Affecting Mass Balance Gradients

  • Temperature with Altitude: Warmer temperatures (especially higher up) lead to more mass loss.
  • Cloud Cover: Less ablation with more cloud cover.
  • Humidity: Affects mass gain and loss.
  • Proximity to Rock Walls: Shading can reduce melting; rock wall radiation can increase melting.
  • Debris Cover: Thin debris layers enhance melting; thick debris layers insulate and reduce melting.
  • Snowfall with Altitude.
  • Distance from Moisture Source: Affects accumulation rates (e.g., Southeast Greenland).
  • Blowing Snow: Mass redistribution.

Area-Altitude Relationship

  • The shape of the glacier impacts mass balance.
  • Shifting ELA has a bigger impact on glaciers with larger accumulation areas.
  • Glaciers with wide upper areas are more sensitive to ELA shifts.

Marine and Lake Terminating Glaciers

  • Iceberg carving is significant for glaciers protruding into water.
  • Ice shelf collapse increases ice discharge into the ocean.
  • Ice shelves buffer the flow of inland ice.

Debris-Covered Glaciers

  • Surface layer of rocks or sediment of varying thicknesses.
  • Thin Debris: Enhances albedo, increases surface melt.
  • Thick Debris (> cm): Insulates ice, reduces melting.
  • Debris-covered glaciers extend to lower elevations.
  • Mass loss does not happen primarily at the terminus.
  • Example: Khumbu Glacier, Nepal.
  • Thin debris in upper areas increases melting.
  • Surface lowering is highest just below the icefall due to thin debris.
  • Reversal of Mass Balance Gradients: The highest rates of ablation occur where the debris layer is thinnest.

Measuring Mass Balance: Ablation Stakes

  • Common field method.
  • Drill or hammer stakes into the ice.
  • Measure change in stake height over time (seasonal or annual).
  • Flaw: Requires many stakes to cover the glacier surface.
  • Scaling up is necessary (e.g., by contour range or elevation).

Stake Data Analysis

  • Collect data in spring and summer to plot winter and summer mass balance.
  • Calculate cumulative mass balance to observe trends.
  • Example: 12 meters of ice thickness loss in 30 years.

Photogrammetric Methods

  • Calculate glacier area changes over time using photographs.
  • Uses satellite imagery for high resolution and regular measurements.
  • Compare area changes and stake measurements to estimate mass loss.
  • Elevation models can be used to calculate height changes.

Franz Josef Glacier Example

  • Mapping mass balance spatially.
  • Observed thickening of the glacier tongue after mass gain in the accumulation area.
  • Demonstrates the transfer of mass down-glacier.

Hydrologic Method

  • Measures inputs (precipitation) and outputs (runoff) to calculate mass change.
  • Uses meteorological stations and gauging stations.
  • Also accounts for evaporation.
  • Flaw: Water is stored in ice masses and runoff doesn't immediately flow off the glacier.

Measuring Ice Sheet Mass Balance

  • Difficult to use stake methods due to scale.

Three Main Techniques:

1. Input-Output Method
  • Estimates inputs (snowfall) and outputs (runoff).
  • Uses climate models for accumulation estimates.
  • Considers ice velocity to estimate mass loss from ice discharge.
  • Satellites with radar sensors are used for ice velocity data.
2. Satellite Altimetry
  • Measures surface elevation change from space.
  • Uses satellites like CryoSat-2.
  • Can know the area of ice and work out the mass loss if we know the surface elevation change.
3. Satellite Gravimetry
  • Uses changes in the Earth's gravity fields to estimate changes in mass.
  • Measures crustal rebound due to ice melt.
  • GRACE satellite is used.
  • Enabled by new satellite technology.

Input-Output Method Details

  • Regional climate models estimate snowfall inputs.
  • Ice flow velocity data is used to determine ice discharge rates.
  • Interferometric Synthetic Aperture Radar (INSA) calculates ice velocity accurately.

Satellite Altimetry Details

  • Measures height change over a given area.
  • Example: Cryosat-2 provides coverage of the continent.
  • Measures elevation change and translates to mass balance.

Pros and Cons of Different Methods

  • Different levels of detail are given by different methods. The trends will generally be the same, however.

Mass Balance Trends Over Time

  • Plots show mass change over time.
  • Massive negative mass balances in Greenland and Antarctica contributing to sea level rise.

Method Selection

  • It depends on the location and the exact measurement you want to make, depending on scales.

Summary

  • We went over what Mass Balance is, what affects them, and what the techniques are to measure mass balance.