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