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boreal forest_permafrost

boreal forest_permafrost - Summary of Research on Climate...

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Unformatted text preview: 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 Boreal forests move northward Increased carbon emissions (sink to source) 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 Peatlands Boreal Forest 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 25% carbon and 5090% 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 nonyedoma permafrost, and 5070 Gt in peatlands Organic matter in Yedoma decomposes quickly when thawed resulting in respiration rates of 1040 g carbon/m3 initially and then 0.55g 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 1214 million km2 or 10% of vegetated surface of the globe Boreal forest region dominates terrestrial interactions with the Earth's climate north of 50N 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 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 Specifically difference in autotrophic and heterotrophic 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 2530% of boreal forest region globally Southernmost occurrence of permafrost is restricted to peatlands Records show increases in nearsurface 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 1050% 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 snowfree periods) lead to more solar radiation absorbed by land surface and longer growing season Questions? ...
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