The Physics and Ecology of Carbon Offsets: Case Study of Energy Exchange over Contrasting
Landscapes, a grassland and oak woodland
Dennis Baldocchi
Ecosystem Sciences Division/ESPM
University of California, Berkeley
2008 NCEAS WorkGroup on ‘Linking carbon storage in terrestrial ecosystems with other climate forcing agents’
If Papal Indulgences can save us from burning in Hell:Can Carbon Indulgences Save us from Global Warming?
Working Hypotheses
• H1: Forests have a negative feedback on Global Warming– Forests are effective and long-term Carbon Sinks– Landuse change (more forests) can help offset greenhouse gas
emissions and mitigate global warming
• H2: Forests have a positive feedback on Global Warming– Forests are optically dark and Absorb more Energy– Forests have a relatively large Bowen ratio (H/LE) and convect
more sensible heat into the atmosphere– Landuse change (more forests) can help promote global
warming
Issues of Concern and Take-Home Message
• Much vegetation operates less than ½ of the year and is a solar collector with less than 2% efficiency
– Solar panels work 365 days per year and have an efficiency of 20%+• Ecological Scaling Laws are associated with Planting Trees
– Self-Thinning Occurs with Time– Mass scales with the -4/3 power of tree density
• Available Land and Water– Best Land is Vegetated and New Land needs to take up More Carbon than
current land– You need more than 500 mm of rain per year to grow Trees
• The Ability of Forests to sequester Carbon declines with stand age• Energetic and Environmental Costs to soil, water, air by land use change
– Forests are Darker than Grasslands, so they Absorb More Energy– Changes in Surface Roughness and Conductance and PBL Feedbacks on
Energy Exchange and Evaporative cooling may Dampen Albedo Effects– Forest Albedo changes with stand age– Forests Emit volatile organic carbon compounds, ozone precursors– Forests reduce Watershed Runoff and Soil Erosion
• Societal/Ethical Costs and Issues– Land for Food vs for Carbon and Energy– Energy is needed to produce, transport and transform biomass into energy
Myhre et al 1998 GRL
Energetics of Greenhouse Gas Forcing:Doubling CO2 provides a 4 Wm-2 energy increase, Worldwide
Global Albedo
Albedo: Conifer Forests < Deciduous Forests < Grass<Crops
Changing Land from Forests to Grass can Increase Reflectance by 10 - 20 W m-2
Should we cut down dark forests to Mitigate Global Warming?:UpScaling Albedo Differences Globally, part 1
• Average Solar Radiation varies with Latitude: ~95 to 190 W m-2
• Land area: ~30% of Earth’s Surface• Tropical, Temperate and Boreal Forests: 40% of land• Forest albedo (10 to 15%) to Grassland Albedo (20%)• Area-weighted change in incoming Solar Radiation: 0.8 W m-2
– Smaller than the 4 W m-2 forcing by 2x CO2
– Ignores role of forests on planetary albedo, as conduits of water vapor that form clouds and reflect light
We must Consider Magnitude of Energy Forcing x Spatial Scale
Evaporative Cooling , normalized by Available Energy, is Greater over Green, Irrigated and
Fertilized Crops than over Temperate, Boreal and and Mediterranean Forests, which are limited by a
combination of N and H2O
Baldocchi and Xu, 2007 Adv Water Res; Baldocchi et al 1997 JGR
Rcanopy (s m-1)
10 100 1000 10000
LE
/LE
eq
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
Wheat
Corn
Boreal jackpine forest
Temperate deciduous forest
Mediterranean oak-grass savanna
Case Study:
Energetics of a Grassland and Oak Savanna
Measurements and Model
2006
Day
0 50 100 150 200 250 300 350
Ene
rgy
Flu
x D
en
sity
(M
J m
-2 d
-1)
0
5
10
15
20
25
30
35
Solar RadiationNet Radiation, GrasslandNet Radiation, Savanna
Case Study:Savanna Woodland adjacent to Grassland
1. Savanna absorbs much more Radiation (3.18 GJ m-2 y-1) than the Grassland (2.28 GJ m-2 y-1) ; Rn: 28.4 W m-2
2006
Day
0 50 100 150 200 250 300 350
PA
R A
lbed
o
0.00
0.05
0.10
0.15
0.20
0.25
0.30
GrasslandSavanna
2. Grassland has much great albedo than savanna;
Gs (mm s-1)
0 2 4 6 8 10 12 14 16
LE/L
Eeq
0.0
0.2
0.4
0.6
0.8
1.0
Savanna WoodlandAnnual Grassland
Monthly Averages
Landscape DifferencesOn Short Time Scales, Grass ET > Forest ET
Ryu, Baldocchi, Ma and Hehn, JGR-Atmos, in press
California Savanna
Hydrological Year
02_03 03_04 04_05 05_06 06_07
Eva
pora
tion
(mm
y-1
)
240
260
280
300
320
340
360
380
400
420
440
Oak WoodlandAnnual Grassland
Role of Land Use on ET:On Annual Time Scale, Forest ET > Grass ET
Ryu, Baldocchi, Ma and Hehn, JGR-Atmos, in press
2006
Day
0 50 100 150 200 250 300 350
La
ten
t H
ea
t F
lux
De
nsi
ty (
MJ
m-2
d-1
)
0
2
4
6
8
10
12
GrasslandSavanna
2006
Day
0 50 100 150 200 250 300 350
Sen
sibl
e H
eat
Flu
x D
ensi
ty (
MJ
m-2
d-1)
0
2
4
6
8
10
12
14
GrasslandSavanna
3. On Annual Time scales, Savanna Evaporates more than the grassland, so it Loses Less Longwave Energy through Evaporative Cooling
u*, oak woodland, daily average
0.0 0.2 0.4 0.6 0.8 1.0
u*,
gras
slan
d,
daily
ave
rage
0.0
0.1
0.2
0.3
0.4
0.5
2002
4a. U* of tall, rough Savanna > short, smooth Grassland
4b. Savanna injects more Sensible Heat into the atmosphere because it has more Available Energy and it is Aerodynamically Rougher
2006
Day
0 50 100 150 200 250 300 350
Sen
sibl
e H
eat
Flu
x D
ensi
ty (
MJ
m-2
d-1)
0
2
4
6
8
10
12
14
GrasslandSavanna
2006
Day
0 50 100 150 200 250 300 350
Air
Te
mp
era
ture
(C
)
0
10
20
30
40
GrasslandSavanna
5. Mean Potential Temperature differences are relatively small (0.84 C; grass: 290.72 vs savanna: 291.56 K); despite large differences in Energy Fluxes--albeit the Darker vegetation is Warmer
Compare to Greenhouse Sensitivity ~2-4 K/(4 W m-2)
2006, Ione, CA
Potential Temperature, Grassland
275 280 285 290 295 300 305 310 315
Po
ten
tial
Tem
per
atu
re, O
ak S
avan
na
275
280
285
290
295
300
305
310
315
b[0] -2.67b[1] 1.012r ² 0.953
Landscape Modification of Energy Exchange in Semi-Arid Regions:Theoretical Analysis with a couple Surface Energy Balance-PBL Model
Vapor Pressure
LongwaveEnergy
ShortwaveEnergy
Sensible HeatLatent Heat
PBL Height
Time 1
Time 2
Time 3
Temperature
Conceptual Diagram of PBL Interactions
Time (hrs)
6 8 10 12 14 16 18
pbl h
t (m
)
0
500
1000
1500
2000
2500
3000
Time (hrs)
6 8 10 12 14 16 18
e a (P
a)
0
500
1000
1500
2000
2500
3000
H and LE: Analytical/Quadratic version of Penman-Monteith Equation
•The Energetics of afforestation/deforestation is complicated
•Forests have a low albedo, are darker and absorb more energy
•But, Ironically the darker forest maybe cooler (Tsfc) than a bright grassland due to evaporative cooling
•Forests Transpire effectively, causing evaporative cooling, which in humid regions may form clouds and increase planetary albedo•Due to differences in Available energy, differences in H are smaller than LE
Axel Kleidon
ET-PBL Model
Time
4 6 8 10 12 14 16 18 20
Air
Tem
pera
ture
, K
290
292
294
296
298
300
302
304
albedo = 0.15; Rc=320 s/malbedo = 0.30; Rc = 2560 s/m
Theoretical Difference in Air Temperature: Grass vs Savanna
Summer Conditions
ET-PBL Model
Time
4 6 8 10 12 14 16 18 20
Air
Tem
pera
ture
, K
286
288
290
292
294
296
albedo = 0.25; Rc = 160 s/malbedo = 0.15; Rc = 160 s/m
Temperature Difference Only Considering Albedo
Spring Conditions
And Smaller Temperature Difference considering PBL, Ra and albedo….!!
Summer Conditions
Time (hours)
4 6 8 10 12 14 16 18 20
Ta
ir (K
)
286
288
290
292
294
296
298
grass, albedo = 0.30; Rc = 2560 s/m; Ra = 40 s/msavanna, albedo=0.15; Rc = 320 s/m; Ra= 10 s/m
u* savanna = 2 u* grassland
Juang et al. GLR 2007
Positive and Negative Feedbacks on dT
Excellent Contribution, but did not consider PBL Feedbacks
Conclusions
• Albedo, alone, should not be considered when assessing the effects of Land Use Change on the Climate System– Aerodynamic and Surface Resistance and PBL dynamics are
important, too
• Darker Vegetation Absorbs more Energy, but experiences greater Latent Heat Exchange– Evaporative Cooling offsets the Albedo Effect
– Tsfc: savanna < Tsfc: grassland
– Tair: savanna > Tair: grassland
• PBL Entrainment and Roughness differences Dampens Temperature Differences between two Near-by and Contrasting Land Surfaces
Tonzi Ranch
Vaira Ranch
km2 MJ m-2 y-1 albedo albedoarea rad change wt value
tropical 1.75E+07 6.00E+09 0.05 0.15 5.25E+15temperate 1.00E+07 5.00E+09 0.05 0.15 2.50E+15boreal 1.30E+07 4.00E+09 0.1 0.1 5.20E+15
Earth 5.10E+08 sum 1.30E+16ave time/land 0.805
W m-2
Should we cut down dark forests to Mitigate Global Warming?:UpScaling Albedo Differences Globally, part 2
FLUXNET database
Latitude
0 10 20 30 40 50 60 70 80 90
Rg
(MJ
m-2
y-1
)
0
2000
4000
6000
8000
Time
6 8 10 12 14 16 18
Tsf
c (o K
)
280
285
290
295
300
305
310
PBL Feedbackno PBL feedbackair temperature with feedback
PBL feedbacks affect Tsfc, Tair and LE
Time
6 8 10 12 14 16 18LE
(W
m-2
)
100
200
300
400
500
600
700
800
PBL feedbackno pbl feedback
Rc (s/m)
10 100 1000
LE
(Rc)
/LE
eq
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Coupled surface and PBL evaporation model(after McNaughton and Spriggs, 1989)
u: 1.5 m s-1
u: 3.0 m s-1
u: 6.0 m s-1
ESPM 228 Adv Topics Micromet & Biomet
Time
8 10 12 14 16 18
PB
L H
t (m
)
0
500
1000
1500
2000
2500
3000
Rc = 10 s m-1
Rc = 20 s m-1
4080160320640measured, Oak Ridge, TN
Test of PBL Scheme
ESPM 129 Biometeorology
u
z10
10
u
z10
15
u kzu
z* ~
Changes in roughness and displacement with Canopy Height
Assume Common Regional Wind Speed at Blending Height, aloft
Annual budget of energy fluxes
Tonzi site Vaira site
6.6 GJ/m2/yr 6.6 GJ/m2/yr
-0.01 GJ/m2/yr 0.05 GJ/m2/yr
G G
0.97 GJ/m2/yr
0.75 GJ/m2/yr
LE
LERnet3.18 GJ/m2/yr
Rnet2.28 GJ/m2/yr
1.45 GJ/m2/yr
H
1.93 GJ/m2/yr
H
EF: 0.23Ω: 0.16Gs: 3.42 mm/secGa: 50.64 mm/secSWC at surface: 0.19
EF: 0.29Ω: 0.27Gs: 3.95 mm/secGa: 25.14 mm/secSWC at surface: 0.12
Ryu et al JGR in press
California Savanna
Hydrological Year
02_03 03_04 04_05 05_06 06_07
Eva
pora
tion
(mm
y-1
)
240
260
280
300
320
340
360
380
400
420
440
Oak WoodlandAnnual Grassland
Role of Land Use on ET:On Annual Time Scale, Forest ET > Grass ET
Ryu, Baldocchi, Ma and Hehn, JGR-Atmos, in press
You Need Water to Grow Trees!
[N]ppt/Eeq
0.1 1 10 100
LAI
0.1
1
10
various functional types:Baldocchi and Meyers (1998)savanna:Eamus et al. 2001
b[0] -0.785b[1] 0.925r ² 0.722
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