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Transcript
Summary of Research on
Climate Change Feedbacks
in the Arctic
Erica Betts
April 01, 2008
Issues relating to climate change in
the Arctic

Thawing of permafrost
– Increased carbon emissions (sink to source)

Boreal forests move northward
– Surface albedo change
– respiration rate change (tundra to forest vegetation
change)
– change in energy flux (vegetation changes)
Soil subsidence due to melting of ice in
permafrost soils
 Changing snow cover

– Change in surface hydrology
– Change in surface temperatures
Introduction
Arctic
 Permafrost
 Boreal Forest
 Peatlands

Arctic
Arctic tundra soils contain 14% of the global soil
carbon pool.
 Significant heterogeneity in CO2 release patterns
as a result of plant species differences (root
respiration, litter quality for decomposition, etc.)
as well as soil variations (moisture content, size
of active layer, etc.)
 Need for further studies relating below ground
respiration patterns to vegetation patterns
 Better understanding of winter and summer
variations in respiration

Arctic tundra: regional study
Water vapor and CO2 exchange measured by eddy
covariance method in 24 ecosystems along the Arctic
coast
 Variations in ecosystem exchange across the region
controlled by differences in net uptake of CO2 due to
photosynthesis rather than by differences in ecosystem
respiration
 Daytime CO2 related mainly to differences in LAI,
nighttime CO2 efflux related to LAI and soil moisture
 Temperature had no effect on regional patterns of
respiration during the growing season
 Water vapor and CO2 fluxes poorly coupled because
water vapor exchange largely determined by evaporation
from mosses and CO2 exchange controlled by vascular
plant activity

Permafrost
Permafrost (permanently frozen ground) is
a large carbon reservoir rarely
incorporated into analysis of changes in
global carbon reservoirs
 Yedoma is a type of permafrost comprised
of 2-5% carbon and 50-90% ice. Covers
more than 1 million km2 of Siberia and
Central Alaska

Ground Temperature Profile
Permafrost


Estimate that carbon reservoir in frozen yedoma to be
~500Gt, another ~400Gt in non-yedoma permafrost,
and 50-70 Gt in peatlands
Organic matter in Yedoma decomposes quickly when
thawed resulting in respiration rates of 10-40 g
carbon/m3 initially and then 0.5-5g carbon/m3 per day
over several years
– If these rates are sustained, most carbon will be released within
a century
– Carbon in frozen yedoma is preserved for tens of thousands of
years

Yedoma carbon is decomposed by microbes under
anaerobic conditions – this produces methane
– During a lake freeze/thaw cycle associated with migration,
~30% of yedoma carbon is decomposed by microbes and
converted to methane
Permafrost
Permafrost carbon is depleted in
radiocarbon (14C)
 Methane, CO2, and DOC have radiocarbon
age reflecting the time when the yedoma
was formed in the glacial age
 This differentiates the permafrost carbon
signal from other reservoirs

Boreal Forest
Boreal forest
Boreal forest region occupies 12-14 million
km2 or 10% of vegetated surface of the
globe
 Boreal forest region dominates terrestrial
interactions with the Earth’s climate north
of 50°N
 Global warming expected to be most
pronounced in the high latitudes of the
northern hemisphere

Boreal forests
Account for ~20% of the world’s reactive soil
carbon pool
 Has been estimated that understory could
contribute more than 1/3 of forest
photosynthesis
 Predominant understory type are mosses which
insulate the soil (reduce soil temp.), absorb
atmospheric nutrients and decompose very
slowly
 Climate warming likely to reduce productivity of
mosses

Schuur and Trumbore

Second paper detailed current research to determine
rates of respiration of various components of the boreal
forest ecosystem
– Specifically difference in autotrophic and heterotrophic
respiration
Difference between net ecosystem uptake of C
(photosynthesis) and release of C (respiration) = NEE
(determines if ecosystem is a source or sink)
 Difficulty in separating components of an ecosystem and
determining rates of respiration
 This paper looks at separating plant and microbial
respiration from total soil respiration

Methodology and Results

Different sources (microbes vs. plant roots) are expected
to respire C with differing isotopic signatures
– Measurements should show differing concentrations of 14C as a
result of utilizing different stores of carbon.
– Residence time is largest factor determining 14C concentration
in carbon pool.

Expected that root respiration would have 14C isotope
values similar to atmospheric concentrations – not the
case
– Carbon fueling root respiration is up to several years old


Heterotrophic respiration pools from carbon stores
averaging a residence time of 10 years
Heterotrophic respiration accounts for over half of total
soil respiration
Peatlands




Peatland ecosystems cover 25-30% of boreal forest
region globally
Southernmost occurrence of permafrost is restricted to
peatlands
Records show increases in near-surface permafrost
temperatures over past several decades in response to
changing air temperatures and snow cover – this has
triggered widespread permafrost degradation
Recent study by Turetsky et al. 2007 found that loss of
surface permafrost in peatlands increases net carbon
storage as peat accumulation is increased
– CH4 emissions increase and in around 70 years, offset carbon
sink in terms of radiative forcing
Effects of Permafrost Melt on
Infrastructure
Barrow, Alaska
Effects of
Permafrost Melt
on
Infrastructure
Snow






Landscape patterns of tundra snow cover develop from
wind distribution of snow rather than from spatial
variability in precipitation
Blowing snow can cause 10-50% of the snowfall to be
returned to atmosphere by sublimation of the windblown snow particles
Increased vegetation (larger vegetation types) results in
greater snow depth accumulation resulting in increased
runoff from snow melt (w/no increase in precipitation)
Change in surface albedo and energy exchange
Increased snow thickness increases ground
temperatures and decreases conductive heat flow to the
atmosphere
Shorter periods of snow cover (longer snow-free periods)
lead to more solar radiation absorbed by land surface
and longer growing season
Questions?