L20 Flashcards
What is permafrost?
- Ground frozen for at least two consecutive years
- 24% of land in northern hemisphere
- Mostly high latitudes but some alpine areas too
- Permafrost stores carbon as peat and soil organic matter & methane
- 1300 Gt of carbon stored in permafrost, more than twice as much C than the atmosphere
- Permafrost is thawing - there are carbon release issues with this
- Permafrost zones (continuous, discontinuous and sporadic)
- Permafrost thaws but never melts - eg includes rocks
How much of land in the Northern hemisphere is permafrost?
24%
How much carbon is stored in permafrost?
1300 Gt - more than twice as much C than the atmosphere
The active layer
- Surface that thaws each summer (re-freezes in winter)
-Permafrost thaw from climate warming
-Active layer gets progressively deeper
= active layer thickening/ deepening- Permafrost is below the active layer
- Permafrost is thawing due to warmer temperatures, more permafrost is becoming part of the active layer
- Exposes more carbon to decay previously preserved in the permafrost
- Positive feedback to climate (not good)
5 Ecosystem controls of active layer thickness
- Evergreen trees (vs deciduous)
- Moss cover (winter vs summer heat transfer)
- Shrub/ understorey cover (shields irradiance)
- Organic (peaty) soils
- Dry soil
Evergreen trees (vs deciduous)
- Winter canopy snow trapping
- Stopping snow insulation and help to keep the ground cool
Moss promotion
- Protects permafrost
- Moss is dry in summer so insulates and keeps summer heat away
- It is wet in winter so conducts heat and transfers heat out of the ground to the atmosphere
Dry soil
- Drainage
- Precipitation vs transpiration
Large scale ecological relationships
- Walker et al 2003
-Transect through northern alaska
-Thaw depth does not get deeper
-North- south changes ecosystem characteristics that affects permafrost
-Larger and leafier southern plants shades and keeps ground dry, so protects permafrost and counters the warmer temperature
-No relationship between active layer depth and temperature
-Demonstrates ecosystem characteristics importance as they counter the warmer climate to protect the permafrost
Ecosystem controls on ALT
-Tree canopy leaf area
-Understorey leaf area
-Understorey height
-Moss depth
-Soil organic layer depth
-Soil moisture at surface
-Soil moisture at depth
-% cover of plant species
Birch ALT
- Large
Large spruce ALT
- Small
What increases ALT?
Burning - this is bad for permafrost
Why is burning bad for permafrost?
-Heat of fire is little effect, main effect is canopy and moss loss so no protection
A thicker moss layer result in…?
A shallower ALT
A thicker organic layer results in…?
A shallower ALT
Which is more important for ALT, moss thickness or organic matter thickness?
Moss
Tree canopy LAI
- Of some importance
- Only surface moisture increases with fire
Interaction of ecosystem characteristics
-Steep sites have least influence of organic thickness
- shallower sites organic thickness is more important
- Good drainage on steeper slopes probably lessens importance of insulating organic layer
- Tree canopy leaf area has a much lesser influence in drier soils
-Shading of irradiance less important when you have dry soils that insulate well
Methane flux and permafrost thaw
- Subsidence of the ground when ice thaws
- This can then be filled with water and create a wetland
Thermokarst Wetlands produce high methane, little of this comes from old C formerly locked in permafrost
Methane emissions is thought to be ¼ of permafrost associated global warming
Methane emissions increase with biological activity
-Similar amounts of methane emitted from centre of wetland vs outer of wetland (new)
- Long term capacity to keep emitting methane
- Consistent emissions
Methane emissions account for what % of permafrost global warming?
1/4
Methane dating experiment
-Deepest methane is thousands of years old
- Methane has been made from permafrost carbon
- Permafrost melting must supply carbon
Surface methane
- Isn’t dominated by old carbon
- Only 8% of carbon in surface methane is from old deep stores in permafrost
- Wetlands do not release large amounts of carbon than then contribute to methane emissions
- Methane arises from recent carbon not long term carbon stores