Transcript Document

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 1977 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005
Ankur R Desai, UW-Madison
AGU Fall 2007 B41F-03
[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