Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Carbon Cycling in Lake Superior Impact on Upper Midwest Regional Carbon Balance 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-Water 1 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 2007 Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 13, December [email protected] http://atlantic.aos.wisc.edu/~superior Why talk about Lake Superior? Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Global 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-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Regional 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-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Regional view: CHEAS-y lake QuickTime™ and a decompressor are needed to see this picture. Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Regional view: Rel. contribution to WLEF tracer source area 1 • Land: 85.4% 0.8 0.6 0.4 • Lake Superior: 9.5% 0.2 LA N D 0 • Lake Michigan: 1.8% 1 LA K E S U P E R IO R 0.8 0.6 • Other water: 3.1% 0.4 0.2 0 Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Regional view: Land-air flux • Net regional land flux likely a small sink QuickTime™ and a decompressor are needed to see this picture. Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Climate 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-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Climate change view: Ice cover • Declining trends in mean ice cover 100 90 80 Percent Ice Cover 70 60 50 40 30 20 10 0 1973 1975 Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005 [email protected] http://atlantic.aos.wisc.edu/~superior Climate Change view: Temperature • Water temperature trend tracking air temperature Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior What do we know about Lake Superior’s carbon cycle? Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Bottom-up scaling: Observations • Atmos. flux is ~3 Tg yr-1 = 35-140 gC m-2 yr-1 QuickTime™ and a decompressor are needed to see this picture. Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Bottom-up scaling: Issues • DOC flux too low to support net demand 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 • Urban et al (2005) JGR • Implies fast pool: DOC residence time 8 years – vs. hydrologic residence time of 170 years Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Top-down scaling: Observations • [CO2] Air flowing over lake > [CO2] over land QuickTime™ and a decompressor are needed to see this picture. Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Top-down scaling: Potential 6 6 4 4 2 2 C O 2 [p p m ] C O 2 [p p m ] • Potential exists for constraining flux with regional observations of CO2 0 -2 0 -2 -4 -4 -6 -6 -8 -8 5 6 7 8 m on th s Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 9 10 11 5 6 7 8 9 10 11 m on th s [email protected] http://atlantic.aos.wisc.edu/~superior Top-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-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Upshot • 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-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior The Way Forward: Modeling Lake Superior Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior An 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-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Computation domain: 2km Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Progress: Thermal forcing • Compares well to AVHRR SST MAY AUGUST JUNE SEPTEMBER JULY Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 OCTOBER [email protected] http://atlantic.aos.wisc.edu/~superior Progress: Circulation Beletsky et al 1999 Our model Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Challenges: Vertical mixing Depth [m] • Sharp gradients at thermocline difficult to capture Obs. 10km 2km Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Challenges: Thermal bars • Thermal bars typically develop in spring near shallow coastal areas • Both a modeling challenge (resolution) and of interest for biogeochemical cycling Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Challenges: Variability • Lots of interannual biogeochemical variability – e.g., Annual avg. dissolved organic carbon (DOC) 3.5 3 DOC (mgL-1) 2.5 2 1.5 1 0.5 0 1973 Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 1983 1986 1987 1996 1997 2001 2005 [email protected] http://atlantic.aos.wisc.edu/~superior Challenges: Variability • Spatial, too. Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Challenges: Forcing/observations • Many observations are sparse Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Conclusion • 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-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior CyCLeS Project (NSF) http://atlantic.aos.wisc.edu/~superior [email protected] Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior 1973 observation Thermo-bar June, 1973 Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 observation by Niebauer et al 1977 (Chen et al 2001) [email protected] http://atlantic.aos.wisc.edu/~superior QuickTime™ and a decompressor are needed to see this picture. Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior 1973 6 5 DOC (mgL- 4 3 2 1 0 May June July-Aug September October November 1983 6 5 DOC (mgL- 4 3 2 1 0 May Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 June September october [email protected] http://atlantic.aos.wisc.edu/~superior Ankur R Desai, UW-Madison AGU Fall 2007 B41F-03 [email protected] http://atlantic.aos.wisc.edu/~superior