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The Physics and Ecology of Carbon Offsets: Case Study of Energy Exchange over Contrasting Landscapes, a grassland and oak woodland Dennis Baldocchi Ecosystem.

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Presentation on theme: "The Physics and Ecology of Carbon Offsets: Case Study of Energy Exchange over Contrasting Landscapes, a grassland and oak woodland Dennis Baldocchi Ecosystem."— Presentation transcript:

1 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’

2 If Papal Indulgences can save us from burning in Hell: Can Carbon Indulgences Save us from Global Warming?

3 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

4 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

5 Myhre et al 1998 GRL Energetics of Greenhouse Gas Forcing: Doubling CO 2 provides a 4 Wm -2 energy increase, Worldwide

6 Global Albedo Albedo: Conifer Forests < Deciduous Forests < Grass { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/12/3532521/slides/slide_6.jpg", "name": "Global Albedo Albedo: Conifer Forests < Deciduous Forests < Grass

7 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 CO 2 –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

8 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 H 2 O Baldocchi and Xu, 2007 Adv Water Res; Baldocchi et al 1997 JGR

9 Case Study: Energetics of a Grassland and Oak Savanna Measurements and Model

10 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

11 2. Grassland has much great albedo than savanna;

12 Landscape Differences On Short Time Scales, Grass ET > Forest ET Ryu, Baldocchi, Ma and Hehn, JGR-Atmos, in press

13 Role of Land Use on ET: On Annual Time Scale, Forest ET > Grass ET Ryu, Baldocchi, Ma and Hehn, JGR-Atmos, in press

14 3. On Annual Time scales, Savanna Evaporates more than the grassland, so it Loses Less Longwave Energy through Evaporative Cooling

15 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

16 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 )

17 Landscape Modification of Energy Exchange in Semi-Arid Regions: Theoretical Analysis with a couple Surface Energy Balance-PBL Model

18 Conceptual Diagram of PBL Interactions H and LE: Analytical/Quadratic version of Penman-Monteith Equation

19 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 (T sfc ) than a bright grassland due to evaporative cooling

20 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

21 Theoretical Difference in Air Temperature: Grass vs Savanna Summer Conditions

22 Temperature Difference Only Considering Albedo Spring Conditions

23 And Smaller Temperature Difference considering PBL, R a and albedo….!! Summer Conditions

24 Juang et al. GLR 2007 Positive and Negative Feedbacks on dT Excellent Contribution, but did not consider PBL Feedbacks

25 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 –T sfc : savanna < T sfc : grassland –T air : savanna > T air : grassland PBL Entrainment and Roughness differences Dampens Temperature Differences between two Near-by and Contrasting Land Surfaces

26

27 Tonzi Ranch Vaira Ranch

28 Should we cut down dark forests to Mitigate Global Warming?: UpScaling Albedo Differences Globally, part 2

29 PBL feedbacks affect T sfc, T air and LE

30 ESPM 228 Adv Topics Micromet & Biomet

31 Test of PBL Scheme

32 ESPM 129 Biometeorology Changes in roughness and displacement with Canopy Height Assume Common Regional Wind Speed at Blending Height, aloft

33 Annual budget of energy fluxes Tonzi siteVaira site 6.6 GJ/m2/yr -0.01 GJ/m2/yr0.05 GJ/m2/yr GG 0.97 GJ/m2/yr 0.75 GJ/m2/yr LE Rnet 3.18 GJ/m2/yr Rnet 2.28 GJ/m2/yr 1.45 GJ/m2/yr H 1.93 GJ/m2/yr H EF: 0.23 Ω: 0.16 Gs: 3.42 mm/sec Ga: 50.64 mm/sec SWC at surface: 0.19 EF: 0.29 Ω: 0.27 Gs: 3.95 mm/sec Ga: 25.14 mm/sec SWC at surface: 0.12 Ryu et al JGR in press

34 Role of Land Use on ET: On Annual Time Scale, Forest ET > Grass ET Ryu, Baldocchi, Ma and Hehn, JGR-Atmos, in press

35 You Need Water to Grow Trees!


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