Role of Forest Ecosystems and Carbon Sequestion

The Physics and Ecology of Mining Carbon Dioxide from the Atmosphere by PlantsDennis BaldocchiProfessor of BiometeorologyEcosystem Sciences Division/ESPMUniversity of California, BerkeleyUniversity of Illinois, Feb, 2011

1Contemporary CO2 Record

What causes the wiggles, ups and downsand the trends2What We are ToldGlobal Mean Temperature will Increase by about 2 C (3.6 F) if CO2 Increases to 550 ppm by 2100Current [CO2] is over 380 ppm, a 100 ppm increase over pre-industrial levelsWe are releasing more than 8 PgC/y (1Pg = 1015g) by Fossil Fuel Combustion and Cement Production

Raupach, IBPG NewsletterMuch Confusion about:

How Much CO2 We can Emit to Prevent Certain Temperature Increase?How Fast Must We Reduce C Emissions and to What Extent?How Do We Convert Information on Emissions from PgC/y to Atmospheric Pool Size in terms of ppm CO2?

Information is Needed to Guide What We should Do?ESPM 111 Ecosystem EcologyHow much is C in the Air?:Resolving Differences between ppm and PgC?Mass of AtmosphereF=Pressure x Area=Mass x Acceleration=Mass x gSurface Area of the Globe = 4p R2Matmos = 101325 Pa 4p (6378 103 m)2/9.8 m2 s-1= 5.3 1021 g airCompute C in Atmosphere @ 380 ppm (380 10-6)

PgC/ppmP: atmospheric pressurepc: partial pressure CO2mc: molecular wt of C, 12 g/molema: molecular wt of air, 28.96 g/mole5CO2 in 50 years, at Steady-State8 GtC/yr, Anthropogenic Emissions45% retention; air-borne fraction8 * 50 * 0.45 = 180 GtC, Net C Fossil Fuel BurdenEach 2.19 GtC emitted causes a 1 ppm increase in Atmospheric CO2833 (@380 ppm) + 180 = 1013 GtC, atmospheric burden450 ppm is thought to be Threshold to Keep Global Warming Below +2.0 C (3.6 F).462 ppm with BAU in 50 years1.65 times pre-industrial level of 280 ppmBAU C emissions will be ~ 16 to 20 GtC/yr in 2050To stay under 462 ppm the world can only emit < 400 GtC of carbon, gross, into the atmosphere!Well Reach this Threshold in > short, smooth Grassland4b. Savanna injects more Sensible Heat into the atmosphere because it has more Available Energy and it is Aerodynamically Rougher

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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 WarmerCompare to Greenhouse Sensitivity ~2-4 K/(4 W m-2)

51Landscape Modification of Energy Exchange in Semi-Arid Regions:Theoretical Analysis with a couple Surface Energy Balance-PBL Model

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Conceptual Diagram of PBL Interactions

H and LE: Analytical/Quadratic version of Penman-Monteith Equation53

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 cooling54

Forests Transpire effectively, causing evaporative cooling, which in humid regions may form clouds and increase planetary albedoDue to differences in Available energy, differences in H are smaller than LE

Axel Kleidon55

Temperature Difference Only Considering AlbedoSpring Conditions, Both Systems Green, Low Rc56

Theoretical Difference in Air Temperature:Considering differences in Albedo and Surface Resistances

Savanna is Warmer than GrasslandSummer Conditions, Grass Dead, Trees TranspiringDifferent Albedo and Rc57And Smaller Temperature Difference considering PBL, Surface Roughness (Ra ) and albedo.!!Summer ConditionsGrass Dead, Trees Transpiring, Different Rc and Ra

And temperatures are about equal when albedo of the grass is 0.2558

Rectifier Effect Prevents Strong Drawdowns in CO22X CO2 is only 4 W m-259

Tsfc can vary by 10 C by changing albedo and Rs

Tair can vary by 3 C by changing albedo and Rs

Tair can vary by 3 C bychanging Ra and Rs

Tsfc can vary by 10 C by changing Ra and RsAre Ecological Solutions to Mitigating Global Warming a Band-Aid?

Are they Necessary because they Buy us Time?

Or, do they give Us, as a society, a False Sense of Security to Continue doing Nothing, or Little?

Will They, the Carbon Sinks, be Permanent?

Will Engineering the Environment for Carbon Capture Alone Engender Unexpected Consequences on the Climate System?

Closing CommentsKnobs We Can TurnFuture Carbon Emissions: Kaya IdentityPopulationPopulation expected to grow to ~9-10 billion by 2050Per capita GDP, a measure of the standard of livingRapid economic growth in India and ChinaEnergy intensity, the amount of energy consumed per unit of GDP.Can decrease with efficient technologyCarbon intensity, the mass of carbon emitted per unit of energy consumed. Can decrease with alternative energyC Emissions = Population * (GDP/Population) * (Energy/GDP) * (C Emissions/Energy)63Quo Vadis?Sure we'll live, we'll survive, it just might not be a very nice world., DDB, KTVU Interview, April 18, 2010

We must Reduce Carbon Emissions Immediately and Dramatically, rather than looking for Ecological Band Aids

To Remove our Dependence on Fossil Fuel, We Will Need to Re-Design Society which Depends on Energy for All its Work

This will involve Redesigning Energy Production and Infrastructure, Housing, Transportation Soon!

At the most radical stage, it may even require major Economic (e.g. Internalizing Externalities) and Government Changes and Major Reductions in Population GrowthUnintended Consequences include Poverty, Famine, and Conflict whether we make Changes, or NotVested Interests are Reluctant to ChangeEnergy/Cost Savings and Efficiencies Could be PositiveConcluding Issues to ConsiderVegetation operates less than of the year and is a solar collector with less than 2% efficiencySolar panels work 365 days per year and have an efficiency of 20%+Ecological Scaling Laws are associated with Planting TreesMass scales with the -4/3 power of tree densityAvailable Land and WaterBest Land is Vegetated and New Land needs to take up More Carbon than current landYou need more than 500 mm of rain per year to grow TreesThe ability of Forests to sequester Carbon declines with stand ageThere are Energetics and Environmental Costs to soil, water, air and land use changeChanges in Albedo and surface energy fluxes Emission of volatile organic carbon compounds, ozone precursorsChanges in Watershed Runoff and Soil ErosionSocietal/Ethical Costs and IssuesFood for Carbon and EnergyEnergy is needed to produce, transport and transform biomass into energyForests play positive roles for habitat, biodiversity, carbon storehouses and resources

66How Does Energy Availability Compare with Energy Use?US Energy Use: 105 EJ/year1018J per EJUS Population: 300 1063.5 1011 J/capita/yearUS Land Area: 9.8 106 km2=9.8 1012 m2 = 9.8 108 haEnergy Use per unit area: 1.07 107 J m-2Potential, Incident Solar Energy: 6.47 109 J m-2Ione, CAA solar system (solar panels, biomass) must be at least 0.1% efficient, working year round, over the entire surface area of the US to capture the energy we use to offset fossil fuel consumptionAssuming 20% efficient solar system8.11 1010 m2 of Land Area Needed (8.11 105 km2, the size of South Carolina)

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NASA GISSCO2, ppm+ 2C CaseWorking HypothesesH1: Forests have a Negative Feedback on Global WarmingForests are effective and long-term Carbon SinksLanduse change (more forests) can help offset greenhouse gas emissions and mitigate global warmingH2: Forests have a Positive Feedback on Global WarmingForests are optically dark and Absorb more EnergyForests have a relatively large Bowen ratio (H/LE) and convect more sensible heat into the atmosphereLanduse change (more forests) can help promote global warming

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Should we cut down dark forests to Mitigate Global Warming?:UpScaling Albedo Differences Globally, part 270Global GPP = 1033 * 110 1012 m2 = 113.6 PgC/y Annually-Integrated Measured GPP ~ 1000 gC m-2 y-1; Peak GPP < 3500 gC m-2 y-1

Stress Mean and Upper Limit71Its a matter of scaleA lot of trees need to be planted to offset our profligate carbon useUS accounts for about 25% of Global C emissions0.25*8.0 1015 gC = 2.0 1015 gC Per Capita Emissions, US2.0 1015 gC/300 106 = 6.66 106 gC/personEcosystem Service, net C uptake, above current rates~200 gC m-2Land Area Needed to uptake C emissions, per Person3.33 104 m2/person = 3.33 ha/personUS Land Area 9.8 108 ha10.0 108 ha needed by US population to offset its C emissions Naturally!

72Sheet1km2MJ m-2 y-1albedoalbedoarearadchangewt valuetropical1.75E+076.00E+090.050.155.25E+15temperate1.00E+075.00E+090.050.152.50E+15boreal1.30E+074.00E+090.10.15.20E+15Earth5.10E+08sum1.30E+16avetime/land0.805W m-2

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