Climate Change - Permafrost Notes

Permafrost

Covers approximately 20% of the Earth's terrestrial surface.
The ground must remain at or below 0°C (32°F) for at least two consecutive years.
Permafrost can be several thousands of years old.

Active Layer: The top layer of permafrost that thaws and refreezes annually.
Permafrost: Ground that remains frozen all year round.

Contains accumulated carbon from dead plants and animals, dating back 2,000 to 43,000 years.
Short growing/decomposing seasons lead to the accumulation of organic matter.
Permafrost contains approximately twice as much carbon as the Earth's atmosphere.
The active layer thaws for a brief period each summer.

Decomposition of organic carbon from thawing permafrost soils releases carbon into the atmosphere.
This release is considered a potentially critical feedback on climate change.
Increasing temperatures and longer summer seasons cause the active layer to thaw deeper and for a longer duration.

Recent IPCC report (2019) indicates methane will constitute a small proportion of carbon released from permafrost.
Methane could contribute 40-70% of the total warming impact from permafrost emissions due to its higher warming potential.
- Dry Soils: Microbes produce carbon dioxide as they decompose organic carbon (respiration): C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O
- Waterlogged Soils: Permafrost thaw leads to waterlogged soil due to the underlying frozen soil preventing drainage, resulting in anaerobic conditions.
- Anaerobic Conditions: Absence of oxygen leads to decomposition via microbial processes, producing methane (anaerobic respiration): C6H{12}O6 \rightarrow 3CH4 + 3CO_2
- The amount of methane released depends on the drainage capacity of meltwater; deeper thawing in warmer climates affects this.

IPCC Special Report 2019 projects permafrost area changes out to 2100.
90% loss of permafrost area by 2300 is projected under the RCP8.5 (high emissions) scenario and 29% loss under RCP4.5 (medium emissions).
Much of the long-term loss is expected by 2100, even in lower emission scenarios.

Warmer conditions may stimulate plant growth in permafrost areas, potentially offsetting some or all of the gradual carbon losses, at least during this century.
However, in areas where shrubs grow, the active layer thaws earlier and deeper, leading to increased permafrost thawing.

Current IPCC predictions do not fully account for pulse disturbances.
High confidence exists that fire and abrupt thaw will accelerate permafrost change relative to climate effects alone, especially if disturbance rates increase.
Common pulse disturbances include:
- Wildfires
- Thaw Slumps
- Thermokarst Lakes

Ice within permafrost acts as cement, holding the ground together.
Warming can destabilize ice on slopes and coasts, leading to landslides.
Self-reinforcing feedbacks can expand the slump area for years or decades until the ground stabilizes.
Material stored deep in the soil becomes exposed, leading to decomposition and carbon release.

Water has a greater heat capacity than land, making these lakes efficient at thawing surrounding and underlying permafrost.
As ice melts, the land surface slumps (subsides), creating depressions that fill with water.
Merged ponds have even greater thawing capacity.

Sediments in thermokarst lakes have been found to thaw as much as 15 meters deep within half a century, compared to typical tundra soil thaw rates of tens of centimeters.
Ponds that no longer freeze to the bottom are known as thermokarst lakes.
Thawing conditions prevail year-round, even in winter.
In winter, methane released from the lake bottom gets trapped as bubbles in the surface ice.

A recent study suggests that abrupt thermokarst thaw more than doubles the previously estimated carbon release from gradual thaw alone.
Monitoring the dynamics and modeling the influence of abrupt thaw on the climate has been a major challenge due to the vast abundance and small size of these features.

Decomposition of organic carbon from thawing permafrost soils and the resulting release of carbon to the atmosphere are considered a potentially critical feedback on climate change.
Anaerobic conditions in waterlogged parts of the active layer lead to the release of methane.
Methane emissions can contribute a large proportion of the total warming impact due to its higher warming potential.
Increased plant growth in thawing permafrost areas might offset some of the carbon released by thawing; however, increased thawing of permafrost underneath increased vegetation in turn might offset this effect; the net effect remains a subject of research.
Common pulse disturbances, such as wildfires, thaw slumps, and thermokarst lakes, could more than double the previously estimated carbon release from gradual thaw alone.