Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] superior Impact on Upper Midwest Regional Carbon Balance

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



Why talk about Why talk about Lake Superior?Lake 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



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



Regional view: CHEAS-y lakeRegional view: CHEAS-y lake

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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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Regional view: Land-air fluxRegional view: Land-air flux

• Net regional land flux likely a small sink





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



Climate change view: Ice coverClimate change view: Ice cover

• Declining trends in mean ice cover

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Climate Change view: TemperatureClimate Change view: Temperature

• Water temperature trend tracking air temperature



What do we What do we know about know about

Lake Superior’s Lake Superior’s carbon cycle?carbon cycle?



Bottom-up scaling: ObservationsBottom-up scaling: Observations

• Atmos. flux is ~3 Tg yr-1 = 35-140 gC m-2 yr-1





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



Top-down scaling: ObservationsTop-down scaling: Observations

• [CO2] Air flowing over lake > [CO2] over land





Top-down scaling: PotentialTop-down scaling: Potential

• Potential exists for constraining flux with regional observations of CO2

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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



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



The Way The Way Forward: Forward:

Modeling Lake Modeling Lake SuperiorSuperior



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



Computation domain: 2kmComputation domain: 2km



Progress: Thermal forcingProgress: Thermal forcing

• Compares well to AVHRR SST

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Progress: CirculationProgress: Circulation

Beletsky et al 1999

Our model



Challenges: Vertical mixingChallenges: Vertical mixing

• Sharp gradients at thermocline difficult to capture

Obs.

10km

2km

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th [

m]



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



Challenges: VariabilityChallenges: Variability

• Lots of interannual biogeochemical variability– e.g., Annual avg. dissolved organic carbon (DOC)

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Challenges: VariabilityChallenges: Variability

• Spatial, too.



Challenges: Forcing/observationsChallenges: Forcing/observations

• Many observations are sparse



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



CyCLeS Project (NSF)

http://atlantic.aos.wisc.edu/~superior

[email protected]



1973 observation1973 observation

June, 1973 observation by Niebauer et al 1977 (Chen et al 2001)

Thermo-bar











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Documents

Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] superior Impact on Upper Midwest Regional Carbon Balance