Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] superior Impact on Upper Midwest...
Transcript of Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] superior Impact on Upper Midwest...
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Impact on Upper Midwest Regional Carbon Balance
Carbon Cycling in Lake SuperiorCarbon Cycling in Lake Superior
Ankur R Desai1, Galen A McKinley1, Noel Urban2, Chin Wu3
With support from: Nazan Atilla1, Nobuaki Kimura1, Val Bennington1, and Marek Uliasz4
Funding from NSF Carbon-Water1 Dept of Atmospheric & Oceanic Sciences, University of Wisconsin-Madison
2 Dept of Civil and Environmental Engineering, Michigan Technological University
3 Dept of Civil and Environmental Engineering, University of Wisconsin-Madison
4 Dept of Atmospheric Sciences, Colorado State University
AGU Fall Meeting 2007 B41F-03
December 13, 2007
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Why talk about Why talk about Lake Superior?Lake Superior?
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Global viewGlobal view
• Not so important at short time scales on global scale
• But: Great Lakes have been globally important long term sinks of carbon in sediments
• On short term, many lakes in general are sources of carbon (recycling of terrestrial input)
• Important freshwater source (Great Lakes = >20% of world’s non-frozen freshwater)
• More ocean-like than lake-like in physical and biogeochemical processes
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Regional viewRegional view
• Small catchment, low productivity, large area• A regional annual net flux of same order as
terrestrial carbon sink/source?• Strongly influences regional tracer
concentrations– Not likely to affect flux tower footprints
• Role of water bodies and wetlands not well studied in observing, modeling, and predicting regional carbon exchange
• Lakes are indicators of long-term regional climate change and carbon cycling
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Regional view: CHEAS-y lakeRegional view: CHEAS-y lake
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Regional view: Rel. contribution to Regional view: Rel. contribution to WLEF tracer source areaWLEF tracer source area
• Land: 85.4%
• Lake Superior: 9.5%
• Lake Michigan: 1.8%
• Other water: 3.1%
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Regional view: Land-air fluxRegional view: Land-air flux
• Net regional land flux likely a small sink
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Climate change viewClimate change view
• Significant changes in temperature and precipitation expected in upper Midwest over next 100 years
• Recent activity to quantify climate change effects on regional land carbon cycle as part of NACP and MCI efforts
• Limited work on Lake Superior
• Trends in ice cover, lake temperature and lake levels have been noted
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Climate change view: Ice coverClimate change view: Ice cover
• Declining trends in mean ice cover
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Percent Ice Cover
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Climate Change view: TemperatureClimate Change view: Temperature
• Water temperature trend tracking air temperature
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
What do we What do we know about know about
Lake Superior’s Lake Superior’s carbon cycle?carbon cycle?
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Bottom-up scaling: ObservationsBottom-up scaling: Observations
• Atmos. flux is ~3 Tg yr-1 = 35-140 gC m-2 yr-1
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Bottom-up scaling: IssuesBottom-up scaling: Issues
• DOC flux too low to support net demand
• Urban et al (2005) JGR• Implies fast pool: DOC residence time 8 years
– vs. hydrologic residence time of 170 years
Inputs 2.4-2.7 Tg
Erosion 0.02
River 0.4-0.9
Precip 0.02-0.1
Photosynthesis 2.0-6.7
Outputs 13.2-83.1 Tg
Outflow 0.1
Resp. 13-81
Burial 0.45
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Top-down scaling: ObservationsTop-down scaling: Observations
• [CO2] Air flowing over lake > [CO2] over land
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Top-down scaling: PotentialTop-down scaling: Potential
• Potential exists for constraining flux with regional observations of CO2
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Top-down scaling: IssuesTop-down scaling: Issues
• pCO2 is supersaturated with respect to atmosphere in obs made in Apr and Aug ‘07– Some individual measurements are below in
summer, suggesting drawdown by algae– More obs needed over seasonal cycle
• Simple boundary layer budget tracer study suggests summer 2007 efflux: 4-14 gC m-2 d-1
– Analysis requires modeling of stable marine boundary layer
– Much larger than traditional air-sea pCO2 exchange calculation
– Requires significant respiration in water column
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
UpshotUpshot
• On annual and decadal timescales, Lake Superior is possibly a source of CO2 to the atmosphere
• This source could be on the order of magnitude as the terrestrial regional flux
• Regional carbon budgets have to take lakes into account
• What’s missing: Full biogeochemical accounting/modeling to understand and predict variability in Lake Superior carbon cycle
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
The Way The Way Forward: Forward:
Modeling Lake Modeling Lake SuperiorSuperior
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
An oceanic lakeAn oceanic lake
• CyCLeS: Cycling of Carbon in Lake Superior• Adapt the MIT-GCM ocean model to simulate
physical and biogeochemical environment of Lake Superior– 10km and 2km resolution models
• Physical model of temperature, circulation– Mostly implemented
• Biogeochemical model of trace nutrients and air-sea exchange– In progress
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Computation domain: 2kmComputation domain: 2km
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Progress: Thermal forcingProgress: Thermal forcing
• Compares well to AVHRR SST
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Progress: CirculationProgress: Circulation
Beletsky et al 1999
Our model
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Challenges: Vertical mixingChallenges: Vertical mixing
• Sharp gradients at thermocline difficult to capture
Obs.
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Challenges: Thermal barsChallenges: Thermal bars
• Thermal bars typically develop in spring nearshallow coastal areas
• Both a modeling challenge (resolution) and of interest for biogeochemical cycling
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Challenges: VariabilityChallenges: Variability
• Lots of interannual biogeochemical variability– e.g., Annual avg. dissolved organic carbon (DOC)
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Challenges: VariabilityChallenges: Variability
• Spatial, too.
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Challenges: Forcing/observationsChallenges: Forcing/observations
• Many observations are sparse
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
ConclusionConclusion• In some respects, the Great Lakes are harder to
model than ocean basin!– Many challenges remain, biogeochemical modeling
in progress– Multiple top-down & bottom-up constraints needed
• Unlike small lakes, where terrestrial input dominates, in Lake Superior, internal processes dominate interannual variability in CO2 fluxes
• Great Lakes are significant players in regional carbon budgets and have potential to offset land carbon uptake
• Regional climate changes likely to significantly affect land & water carbon cycles in Midwest
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
CyCLeS Project (NSF)
http://atlantic.aos.wisc.edu/~superior
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
1973 observation1973 observation
June, 1973 observation by Niebauer et al 1977 (Chen et al 2001)
Thermo-bar
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
QuickTime™ and a decompressor
are needed to see this picture.
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior
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Ankur R Desai, UW-MadisonAGU Fall 2007 B41F-03
[email protected]://atlantic.aos.wisc.edu/~superior