1. C-Change in GEES Changing Permafrost Environments
Session Three: Permafrost Development

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These slides were last updated in February 2010 2

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4. Three Key Topics Surface Energy Balance
Determines the temperature of the ground surface.
Principally controlled by climate.
Ground Thermal Regime
Change in temperature with depth is controlled by the surface temperature, the properties of the subsurface materials and the duration of any surface temperature changes.
Geothermal gradient determines permafrost thickness.
Surface Boundary Layer
Changing nature of the surface complicates the relationship between climate and permafrost thickness. 4

5. Surface boundary conditions
Related variable
Surface boundary conditions
MEAN ANNUAL
AIR TEMP.
CLIMATE
GROUND
SURFACE
TEMP.
Surface energy
balance
Complex
relationship
Ground
thermal
properties
(+time)
GROUND
THERMAL
REGIME
MEAN ANNUAL
GROUND TEMP.
PERMAFROST
THICKNESS 5

6. 1. Surface Energy Balance
Fig 1.2 IPCC (2001) IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of theIntergovernmental Panel on Climate Change [Houghton, J.T.,Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp 6

7. 1. Surface Energy Balance
Temperature of the ground surface is determined by:
Balance of incoming vs. outgoing radiation
Short-wave and long-wave.
Sensible heat exchange (e.g. cold air mass above a ground surface will cause it to cool).
Latent heat exchange (related to phase changes)
Condensation (will cause ground to warm).
Evaporation (will cause ground to cool).
Midnight sun, Prudhoe Bay, Alaska (R I Waller)
If balance is negative, the surface temperature will drop.
If balance is positive, the surface temperature will rise. 7

8. Energy exchange at the ground surface during a sunny summer’s day.8

9. Energy exchange at the ground surface during a clear night (5am).9

10. Q* = QLE + QH + QG Heat Balance Equation
Q* = Net radiation (largely insolation)
QLE = Latent heat flux
QH = Sensible heat flux
QG = Ground heat flux 10

11. Q* = QLE + QH + QG Heat Balance Equation e.g. Summer’s day
Drives evaporation
Net radiation (shortwave & longwave)
Energy flux into the ground
Ground surface heats the air (convection)
e.g. Summer’s day 11

12. 2. Ground Thermal Regime Ground surface OºC isotherm MAGT (1) (2)
Winter
Summer
Permafrost
thickness
(3)
Refers to the vertical change in temperature with depth.
Changes in the gradient (and therefore permafrost thickness) driven by:
Climate or ground surface changes (affects MAGT).
Variations in the geothermal heat flux.
3 Thermal conductivity of the subsurface materials. 12

13. Subsurface Temperature Changes
Drop in surface temp.
Temperature changes at the ground surface influence subsurface temperatures by altering the ground thermal regime.
0ºC
Permafrost thickens
Ground thermal regime “pulled across” by shift in surface temperature. 13

14. Permafrost thins Permafrost thickens
Theoretical responses to changes in surface temperature (T1 to T2) through time (d1 to d2).
Figure From: Harris, S.A The Permafrost Environment. Croom Helm, Beckenham (p.25).
© Croom Helm Publishers 14

15. Thermal Conductivity 0°C Low conductivity High conductivity
Permafrost thickness
MAGT
Thermal conductivity of the substrate determines:
the rapidity of substrate temperature changes.
the slope of the geothermal gradient.
E.g. High conductivity =
rapid change in ground temperatures.
shallow geothermal gradient – temp. changes extend to a deeper level. 15

16. Table of average thermal conductivities for a variety of materials
…whilst snow is a poor conductor (insulator)
e.g. rocks are good conductors of heat
Table From: French, H.M The Periglacial Environment (3rd ed.). Wiley & Sons, Chichester(p.87).
© Wiley and Sons 16

17. From: Terzaghi, K. (1952) ‘Permafrost’ in From theory to practice in soil mechanics: Selections from the writings of Karl Terzaghi, pp New York and London: John Wiley.
In reality, the subsurface is likely to comprise a number of units with different thermal conductivities, each influencing the geothermal gradient. 17

18. Duration and Depth
Changes in surface energy balance and ground surface temperature occur at a variety of timescales.
Duration of temperature change dictates the depth to which the temperature change extends:
Diurnal changes: few centimetres.
Seasonal changes: up to m.
Longer-term changes: up to around 1 km.
SYR Figure 2-3 IPCC, 2001: Climate Change 2001: Synthesis Report. A Contribution of Working Groups I, II, and III to the Third Assessment Report of the Integovernmental Panel on Climate Change [Watson, R.T. and the Core Writing Team (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, NY, USA, 398 pp 18

19. Seasonal Fluctuations
Seasonal temperature fluctuations influence ground temperatures in the upper few tens of metres.
MAGT measured at the point at which seasonal temperature variations cease.
Williams, P.J. & Smith, M.W The Frozen Earth. Studies in Polar Research. CUP, Cambridge. Fig 1.6 p.12
Reproduced with the permission of Cambridge University Press 19

20. Based on data from: Carson, J. E. and Moses, H
Based on data from: Carson, J.E. and Moses, H. (1963) ‘The annual and diurnal heat exchange cycles in upper layers of soil’ Journal of Applied Meteorology
Seasonal temperature changes at the ground surface are gradually damped and delayed with increasing depth (become zero at the MAGT).
Figure From: Williams, P.J. & Smith, M.W The Frozen Earth. Studies in Polar Research. CUP, Cambridge. 20

21. Long-term fluctuations
Really thick sequences of permafrost require thousands of years to form.
Termed relict permafrost as they relate to colder past climate regimes (usually Pleistocene).
E.g. permafrost up to 700 m in thickness in the western Canadian Arctic reflects persistently low temperatures and non-glacial conditions over the last 40,000 years.
Modelling suggests MAGT must have dropped to a sustained -18°C to produce such a thickness of permafrost. 21

22. Theoretical model illustrating the gradual downward propagation of a stepwise temperature change through time.
From Molochuskin, E.N. (1973) ‘The effect of thermal abrasion on the temperature of the permafrost in the coastal zone of the Laptev Sea. Proceedings of the Second International Conference on Permafrost. Yakutsk, USSR, USSR Contribution. Pp Washington D.C.: National Academy of Sciences. Fig. 2 22

23. 3. The Surface Boundary Layer
Relationship between climate and permafrost thickness is strongly influenced by a series of boundary layer conditions. E.g.
Relief and aspect.
Vegetation.
Snow cover
Presence of water bodies and drainage.
Fire.
Determine local microclimate, MAGT and permafrost thickness.
Reindeer moss on the floor of a forest in northern Finland (R I Waller) 23

24. Boundary or Buffer Layer
Conceptual model describing
the complex relationship
between climate (atmosphere) and permafrost (geothermal regime).
Based on: Luthin, J.N and Guymon, G.L. (1974) ‘Soil moisture-vegetation-temperature relationships in Central Alaska’ Journal of Hydrology, 23, 24

25. Relief & Aspect
Relief and aspect affect the receipt of incoming solar radiation and the depth and duration of snow cover.
Pole-facing slopes:
Lower receipt of insolation.
Lower MAGT.
Thicker permafrost.
Thinner active layer (shorter duration).
…and vice-versa… 25

26. Vegetation
Can influence the surface energy balance in a variety of ways:
Insulates ground surface – reduces magnitude of ground surface temperature changes.
Can also influence other variables - e.g. vegetation traps snow which enhances insulating effect.
Influence depends on type of vegetation: e.g. scrub traps snow, trees intercept snow in their canopies.
The tree line marking the boundary between tundra and boreal forest, northern Finland (R.I. Waller). 26

27. Influence of Vegetation
Ground thermal regime beneath tundra and forest in the western Canadian Arctic.
Tundra offers less insulation at the ground surface - subsurface temperature changes are greater and extend deeper.
Figure From: Rouse, W.R. (1984) ‘Microclimate of Arctic tree line. 2: Soil microclimate of tundra and forest. Water Resources Research, 20, 1, 27

28. Snow cover One of the most important factors.
In general, insulates ground from the extreme winter temperatures.
BUT - timing and duration of snowfall are critical:
autumn snow will delay freezing.
spring snow will delay thaw.
Variations in snowpack thickness can cause widespread local changes in permafrost and active layer thickness. 28

29. Fire & Water Fire Many fires started by lightning in the Boreal Zone.
Effect of fire on underlying permafrost depends upon duration of fire and thermal conductivity of ground.
Greatest effect occurs due to vegetation change and the resulting change in the surface energy balance.
Water
Supremely effective at buffering adjacent ground from temperature fluctuations (high heat capacity).
Saturation of substrate and state of water also influences its thermal conductivity: dry - good insulator; wet & frozen - highly conductive. 29

30. Influence of Water
Numerical simulation of the geothermal disturbances caused by lakes at the ground surface (A-E).
Figure From: French, H.M The Periglacial Environment (3rd ed.). Wiley & Sons, Chichester (p.91).
© Wiley and Sons 30

31. Significance... A relatively slight surface disturbance can induce a
disproportionately large
response... 31

32. Lecture Summary
Changes to the ground thermal regime and permafrost thickness are driven primarily by changes in the surface energy balance.
The surface energy balance is principally determined by climate, although its influence is modified by a complex series of additional factors.
The ground thermal regime is driven by the surface energy balance, the geothermal heat flux and the thermal conductivity of the substrate.
Changes to the surface conditions can dramatically affect permafrost and active layer thicknesses. 32

33. References
French, H.M The Periglacial Environment (3rd ed.). Wiley & Sons, Chichester. Harris, S.A The Permafrost Environment. Croom Helm, Beckenham. Rouse, W.R ‘Microclimate of Arctic tree line. 2: Soil microclimate of tundra and forest. Water Resources Research, 20, 1, Luthin, J.N and Guymon, G.L ‘Soil moisture-vegetation-temperature relationships in Central Alaska’ Journal of Hydrology, 23, Williams, P.J. & Smith, M.W The Frozen Earth. Studies in Polar Research. CUP, Cambridge.

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35. Item Metadata Author Dr Richard Waller Stephen Whitfield
Institute – Owner
Keele University, School of Physical and Geographical Sciences
Title
Permafrost Development PowerPoint Presentation
Date Created
March 2010
Description
Part Three of Changing Permafrost Environments
Educational Level 3
Keywords (Primary keywords – UKOER & GEESOER)
UKOER, GEESOER, surface, energy, thermal, conductivity, heat, balance, snow cover, vegetation
Creative Commons License
Attribution-Non-Commercial-Share Alike 2.0 UK: England & Wales