Transcript Document
Role of land surface models in GCMs • • • • • • Provides the boundary conditions at the land-atmosphere interface – e.g. albedo, surface temperature, surface fluxes Partitions available energy at the surface into sensible and latent heat flux components Partitions rainfall into runoff and evaporation – Evaporation provides surface-atmosphere moisture flux – River runoff provides freshwater input to the oceans Provides the carbon fluxes at the surface (photosynthesis, respiration) Updates state variables which affect surface fluxes – e.g. snow cover, soil moisture, soil temperature, vegetation cover, leaf area index LSM cost is actually not that high ( ~10% of full coupled model) Role of land surface models in GCMs The land-surface model solves (at each timestep) – Surface energy balance (and other energy balances, e.g. in canopy, snow, soil) • S + L = S + L + E + H + G – S, S are down(up)welling solar radiation – L , L are down(up)welling longwave radiation – is latent heat of vaporization, E is evaporation – H is sensible heat flux – G is ground heat flux – Surface water balance (and other water balances such as snow and soil water) • P = ES + ET + EC + RSurf + RSub-Surf + ∆SM / ∆t – P is rainfall – ES is soil evaporation, ET is transpiration, EC is canopy evaporation – RSurf is surface runoff, RSub-Surf is sub-surface runoff – ∆SM / ∆t is the change in soil moisture over a timestep – Carbon balance (and plant and soil carbon pools) • NPP = GPP – Ra = (∆Cf + ∆Cs + ∆Cr) / ∆t • NEP = NPP – Rh • NBP = NEP - Combustion – NPP is net primary production, GPP is gross primary production – Ra is autotrophic (plant) respiration, Rh is heterotrophic (soil) respiration – ∆Cf, ∆Cs, ∆Cr are foliage, stem, and root carbon pools – NEP is net ecosystem production, NBP is net biome production – Combustion is carbon loss during fire Surface energy balance and surface temperature Surface energy balance (S-S) + L = L[Ts] + H[Ts] + E[Ts] + G[Ts] L (Ts 273.15)4 (1 ) L H C p E Gk (Ta Ts ) raH C p ea e*[Ts ] raW Ts Tsoil z Atmospheric forcing S - incoming solar radiation L - incoming longwave radiation Ta – air temperature ea – vapor pressure Surface properties S - reflected solar radiation (albedo) - emissivity raH – aerodynamic resistance (roughness length) raW – aerodynamic resistance (roughness length) Tsoil – soil temperature k – thermal conductivity z – soil depth With atmospheric forcing and surface properties specified, solve for temperature Ts that balances the energy budget Turbulent fluxes Logarithmic wind profile in atmosphere near surface 16 Height above ground (m) 14 12 10 8 6 4 Day 14 2 Day 12 Day 35 0 1 2 3 4 5 6 7 8 9 10 -1 Wind speed ( m s ) z / u* u / z 1/ k u2 u1 u / k ln z2 / z1 * u ( z ) u / k ln z / z0M * u ( z ) u / k ln z d / z0M * Similar logarithmic profiles for temperature and vapor pressure Turbulent fluxes (u us ) / raM u / raM H C p raM (a Ts ) raH 1 2 ln z d m ( ) k u z0 M 2 raH 1 z d z d ( ) ln m ( ) ln h 2 z0 H k u z0 M raW 1 z d z d ( ) ln m ( ) ln w 2 z0W k u z0 M Height z raM raM raH = raM + r*b z0M r *b z0H us = 0 Wind speed uz z Temperature s Community Land Model Hydrometeorology Reflected Solar Radiation Absorbed Solar Radiation Photosynthesis Sensible Heat Flux Latent Heat Flux Longwave Radiation Hydrology Momentum Flux Wind Speed 0 ua Precipitation Evaporation Interception Canopy Water Transpiration Emitted Longwave Radiation Diffuse Solar Radiation Biogeophysics Throughfall Stemflow Sublimation Melt Soil Heat Flux Evaporation Infiltration Surface Runoff Snow Soil Water Heat Transfer Redistribution Drainage Community Land Model • Land model for Community Climate System Model • Developed by the CCSM Land Model Working Group in partnership with university and government laboratory collaborators Bonan et al. (2002) J Climate 15:3123-3149 Oleson et al. (2004) NCAR/TN-461+STR Dickinson et al. (2006) J Climate 19:2302-2324 Energy fluxes: radiative transfer; turbulent fluxes (sensible, latent heat); heat storage in soil; snow melt Hydrologic cycle: interception of water by leaves; infiltration and runoff; snow accumulation and melt; multi-layer soil water; partitioning of latent heat into evaporation of intercepted water, soil evaporation, and transpiration Community Land Model Dynamic vegetation 0 15003000 g CO2g-1s-1 g CO2g-1s-1 0 -10 25 60 Temperature (C) 0 15 30 Autotrophic Respiration Temperature (C) Litterfall 0 500 1000 Heterotrophic Respiration Ambient CO2 (ppm) 0 1 2 Vapor Pressure Foliage Deficit (Pa) Nitrogen (%) Nutrient Uptake 8 1 0 15 30 Temperature (C) 1 5 4 3 2 1 0 Nov Dec g CO2g-1s-1 6 4 2 0 0 -10 25 60 Temperature (C) Jul Aug Sep Oct 0 -1 -2 Foliage Water Potential (MPa) 0 -10 25 60 Temperature (C) 6 0.3 Jan Feb Mar Apr May Jun 0 500 1000 PPFD (molm-2s-1) 0.01 Root Leaf Area Index 6 4 2 0 Growth Respiration 0.5 Leaf phenology Sapwood Relative Rate 6 4 2 0 Foliage Relative Rate g CO2g-1s-1 g CO2g-1s-1 Photosynthesis g CO2g-1s-1 Ecosystem carbon balance 0 0 100 Soil Water (% saturation) Needleleaf evergreen tree Broadleaf deciduous tree Vegetation dynamics Bonan et al. (2003) Global Change Biology 9:1543-1566 Levis et al. (2004) NCAR/TN-459+IA Bonan & Levis (2006) J Climate 19:2290-2301 First-generation models Evaporation E = Ep =1 for ww0 = w/w0 for w<w0 Precipitation Sensible heat H C p E Ta Ts ra C p ea e*[Ts ] ra / Latent heat Ta ea ra ra/ Ts e*(Ts) Water depth, w Critical depth, w0 Runoff Simple energy balance model: (1-r)S + L = L[Ts] + H[Ts] + E[Ts] Prescribed surface albedo Bulk parameterizations of sensible and latent heat flux No influence of vegetation on surface fluxes Prescribed soil wetness factor or calculated wetness from bucket model No soil heat storage Manabe (1969) Monthly Weather Review 97:739-774 Williamson et al. (1987) NCAR/TN-285+STR Green world vs desert world Two climate model experiments Wet – evapotranspiration not limited by soil water; vegetated planet Dry – no evapotranspiration; desert planet July surface temperature (C) July precipitation (mm/day) Wet soil Wet soil Dry soil Dry soil Dry soil warmer than wet soil Shukla & Mintz (1982) Science 215:1498-1501 Dry soil has less precipitation Second-generation models Vegetation and hydrologic cycle Biosphere-Atmosphere Transfer Scheme (BATS) Dickinson et al. (1986) NCAR/TN-275+STR Simple Biosphere Model (SiB) Sellers et al. (1986) J Atmos Sci 43:505-531 Snow-Covered Ground 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Visible Canopy Albedo Fractional Canopy Absorption Radiative transfer Near-infrared 0 1 2 3 4 5 6 Leaf Area Index (m2 m-2) Sunlit Leaf Area Index random horizontal vertical 2.0 1.5 1.0 0.5 Zenith Angle = 30 0.0 0 1 2 3 4 5 Leaf Area Index 6 Random leaf orientation Zenith angle = 45 Near-infrared Visible 0 1 2 3 4 Leaf Area Index (m2m-2) 3.0 2.5 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 7 8 5 6 Plant canopy Sensible Heat Ta Ta Ta Ta raH raH raH rs raH rc Tac Tacu racu Tv Ts rac Ts Bulk Surface No Vegetation Bulk Surface With Vegetation Tacl racl Tg One Vegetation Layer rcu Tvu rcl Tvl Tg Two Vegetation Layers Latent Heat ea ea raW ea ea raW raW rs raW rc es es rs Bare Surface No Vegetation Bulk Surface With Vegetation rcu evu rcl evl eacu eac rac ev eg One Vegetation Layer racu eacl racl eg Two Vegetation Layers Leaf stomatal resistance Stomatal Gas Exchange Leaf Cuticle High CO2 Dry Air CO2 H2O Guard Cell Guard Cell Photosynthetically Active Radiation Chloroplast Low CO2 light CO2 + 2 H2O CH2O + O2 + H2O Moist Air Stomata Open: • High Light Levels • Moist Leaf • Warm Temperature • Moist Air • Moderate CO2 • High Leaf Nitrogen Stomata Close (Smaller Pore Opening): • Low Light Levels • Dry Leaf • Cold Temperature • Dry Air • High CO2 • Low Leaf Nitrogen Leaf boundary layer Dry Air High CO2 H2O Boundary Layer Thickness: 1 to 10 mm rb Leaf Cuticle rb Height CO2 Sensible Heat Guard Cell Guard Cell High 0 Wind Speed Photosynthetically Active Radiation Chloroplast rs Low CO2 light CO2 + 2 H2O CH2O + O2 + H2O Moist Air Low Temperature High Plant canopy Leaf Boundary Layer rb/2 Stomata Ts Total Resistance Ta ei=e*(Ts) Canopy rb/(2L) Sensible Heat rb/2 es Tac 1.65rs Latent Heat (rs+rb) ei=e*(Ts) es 1.65rs/L 1.37rb ci cs rb/L rs/L ea ca Photosynthesis (1.65rs+1.37rb) Total Resistance Ta Sensible Heat rb/(2L)+ra ea Latent Heat (rs+rb)/L+ra Ts rb rs Atmosphere Surface Layer ra ra eac 1.37rb/L Photosynthesis ci cs ca (1.65rs+1.37rb)/L Soil temperature Vertical Heat Transfer Fin F k FT I k FT I Hz K Hz K y x 5 mm A 41C F 15 . change in storage = flux in - flux out F I G HJ K T c xyz ( Fin Fout )xy t z T I F F I F G Ht J KG Hz J K T I F T I F 2T I cF G k kG J G J J Ht Kz H z K Hz 2 K c Fout 15 mm B 37C F G H W 41o C 37 o C 0.01 m mo C F 600 W / m 2 I J K Hydrologic cycle Reflected Solar Radiation Absorbed Solar Radiation Photosynthesis Sensible Heat Flux Latent Heat Flux Longwave Radiation Hydrology Momentum Flux Wind Speed 0 ua Precipitation Evaporation Interception Canopy Water Transpiration Emitted Longwave Radiation Diffuse Solar Radiation Biogeophysics Throughfall Stemflow Sublimation Melt Soil Heat Flux Evaporation Infiltration Surface Runoff Snow Soil Water Heat Transfer Redistribution Drainage Soil water – Richards equation Vertical Water Flow ( z ) O F I L M N z P Q k Hz 1K L( z) O F 1I k F 1I F k M k P N z Q Hz K Hz K F k Fin y x change in storage = flux in - flux out z I F G Ht J Kxyz ( Fin Fout )xy / t F / z L F M H NG IJ O KP Q k 1 t z z 50 mm A Fout =-478 mm z=500 mm F 2 550 mm B =-843 mm z=0 mm L M N mm ( 478 mm 500 mm) ( 843 mm 0 mm) hr 500 mm F 3.46 mm / hr O P Q Land degradation Climate feedback Overgrazing Drought Reduced Vegetation Cover Dead vegetation in drought-stricken area, Sol-Dior area, Senegal (FAO, Ch. Errath) Less Rainfall Increased Albedo Decreased Clouds And Convection Goat seeks food in the sparsely vegetated Sahel of Africa (US AID) Decreased Net Radiation Subsidence Surface Cooling Charney (1975) QJRMS 101:193-202 Charney et al. (1975) Science 187:434-435 Land degradation Climate model experiments Degradation scenario - the vegetation type within the shaded area was changed to type 9 to represent degradation: less vegetation, lower LAI, smaller surface roughness length, higher albedo, sandy soil Broadleaf evergreen tree Broadleaf shrub/ground cover Broadleaf tree/ground cover Clark et al. (2001) J Climate 14:1809-1822 Broadleaf shrub/bare soil Land degradation Climate impacts July-August-September precipitation differences (mm/day) due to degradation. Differences that are significant at the 95% confidence level are shaded and the degraded area is enclosed by a solid line. July-August-September mean differences due to degradation. Values are means over the degraded area. D–C is the difference between degraded and control values. Clark et al. (2001) J Climate 14:1809-1822 Tropical deforestation Settlement and deforestation surrounding Rio Branco, Brazil (10S, 68W) in the Brazilian state of Acre, near the border with Bolivia. The large image covers an area of 333 km x 333 km. (NASA/GSFC/LaRC/JPL) (National Geographic Society) Tropical deforestation Warmer, drier tropical climate Annual response to Amazonian deforestation in various climate model studies. albedo and z0 indicate the change in surface albedo and roughness due to deforestation (+, increase; -, decrease). T, P, and ET are the simulated changes in temperature, precipitation, and evapotranspiration. Shading denotes warmer, drier climate. Surface Change Study albedo z0 Climate Change T P ET (C) (mm) (mm) Dickinson and Henderson-Sellers (1988) + - +3.0 0 -200 Lean and Warrilow (1989) + - +2.4 -490 -310 Nobre et al. (1991) + - +2.5 -643 -496 Dickinson and Kennedy (1992) + - +0.6 -511 -256 Mylne and Rowntree (1992) + unchanged -0.1 -335 -176 Henderson-Sellers et al. (1993) + - +0.6 -588 -232 Lean and Rowntree (1993) + - +2.1 -296 -201 Pitman et al. (1993) + - +0.7 -603 -207 Polcher and Laval (1994a) + unchanged +3.8 +394 -985 Polcher and Laval (1994b) + - -0.1 -186 -128 Sud et al. (1996) + - +2.0 -540 -445 McGuffie et al. (1995) + - +0.3 -437 -231 Lean and Rowntree (1997) + - +2.3 -157 -296 Hahmann and Dickinson (1997) + - +1.0 -363 -149 Costa and Foley (2000) + - +1.4 -266 -223 Third-generation models Stomatal Gas Exchange CO2 Net Photosynthesis (molm-2s-1) Guard Cell Guard Cell Leaf Cuticle 8 7 6 5 4 3 2 1 0 -1 H2O Photosynthetically Active Radiation Low Moist CO2 Air Chloroplast 0 200 400 600 800 1000 1200 1400 light CO2 + 2 H2O CH2O + O2 + H2O Stomata Open: • High Light Levels • Moist Leaf • Warm Temperature • Moist Air • Moderate CO2 • High Leaf Nitrogen 0 200 400 600 800 1000 1200 1400 Photosynthetic Photon Flux Density (molm-2s-1) Bonan (1995) JGR 100:2817-2831 Denning et al. (1995) Nature 376:240-242 Denning et al. (1996) Tellus 48B:521-542, 543-567 Photosynthetic Photon Flux Density 8 7 6 5 4 3 2 1 0 -1 Photosynthesis Transpiration 0 5 10 15 20 25 30 35 40 45 50 Stomatal Conductance (mmol CO2m-2s-1) 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Transpiration (mmol H2Om-2s-1) 50 45 40 35 30 25 20 15 10 5 0 Net Photosynthesis (mol CO2m-2s-1) Stomatal Conductance (mmol CO2m-2s-1) Photosynthetic Photon Flux Density (molm-2s-1) Leaf stomatal resistance A (h /100) P 1 gs m n s b rs cs wc is the rubisco-limited rate of photosynthesis, wj is light-limited rate allowed by RuBP regeneration rubisco-limited rate is wc Vmax (ci * ) ci Kc (1Oi / Ko ) RuBP regeneration-limited rate is wj J (ci * ) 4(ci 2* ) Photosynthesis (mol CO2m-2s-1) An min(wc , w j ) Rd Light Response Curve 20 18 16 14 12 10 8 6 Wj Wc Wj 4 2 0 0 100 200 300 400 500 600 700 800 900 1000 Photosynthetic Photon Flux Density (mol photonsm-2s-1) Canopy resistance Percent Of Full Solar Radiation 0 7 7 6 6 Height (meters) Height (meters) Oak Forest 5 4 3 2 40 60 80 100 4 5 5 4 3 2 1 1 0 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 0 Leaf Area Index 20 LAI Radiation 0 1 2 3 Cumulative Leaf Area Index CO2 fertilization and stomatal conductance Leaf photosynthesis and conductance response to atmospheric CO2 concentration, light-saturated (a) (b) (c) Dependence of leaf-scale photosynthesis for C3 and C4 vegetation on external CO2 concentration The C3 photosynthesis curves for unadjusted (C and P) and down-regulated (PV) physiology Dependence of stomatal conductance on CO2 concentration for the unadjusted and downregulated cases. Photosynthesis increases and stomatal conductance decreases with higher atmospheric CO2 Bounoua et al. (1999) J Climate 12:309-324 CO2 fertilization and stomatal conductance CO2 fertilization (RP, RPV) reduces canopy conductance and increases temperature compared with radiative CO2 (R) Amazonian evergreen forest, diurnal cycle January Bounoua et al. (1999) J Climate 12:309-324 Canadian evergreen forest, diurnal cycle July CO2 fertilization and stomatal conductance Global climate: Reduced conductance Reduced evaporation Reduced precipitation Warmer temperature Bounoua et al. (1999) J Climate 12:309-324 Fourth-generation of models Dynamic vegetation 0 15003000 g CO2g-1s-1 g CO2g-1s-1 0 -10 25 60 Temperature (C) 0 15 30 Autotrophic Respiration Temperature (C) Litterfall 0 500 1000 Heterotrophic Respiration Ambient CO2 (ppm) 0 1 2 Vapor Pressure Foliage Deficit (Pa) Nitrogen (%) Nutrient Uptake 8 1 0 15 30 Temperature (C) 1 5 4 3 2 1 0 Nov Dec g CO2g-1s-1 6 4 2 0 0 -10 25 60 Temperature (C) Jul Aug Sep Oct 0 -1 -2 Foliage Water Potential (MPa) 0 -10 25 60 Temperature (C) 6 0.3 Jan Feb Mar Apr May Jun 0 500 1000 PPFD (molm-2s-1) 0.01 Root Leaf Area Index 6 4 2 0 Growth Respiration 0.5 Leaf phenology Sapwood Relative Rate 6 4 2 0 Foliage Relative Rate g CO2g-1s-1 g CO2g-1s-1 Photosynthesis g CO2g-1s-1 Ecosystem carbon balance 0 0 100 Soil Water (% saturation) Needleleaf evergreen tree Broadleaf deciduous tree Vegetation dynamics Foley et al. (1996) GBC 10:603-628 Levis et al. (1999) JGR 104D:31191-31198 Levis et al. (2000) J Climate 13:1313-1325 Cox et al. (2000) Nature 408:184-187 ATMOSPHERE E, H, x,y, S, L, (CO2) T, u,v, q, P S, L, (CO2) BIOGEOPHYSICS canopy physics water radiative energy balance aerotransfer temperature dynamics balance Greening of a land surface model VEGETATION DYNAMICS plant functional type (presence, extent) height plant carbon litter and soil carbon canopy physiology photosynthesis stomatal conductance (GPP) litter GPP soil water leaf temperature soil temperature daily leaf area index litter carbon soil carbon rain green maximum leaf area index DAILY STATISTICS phenology •10-day mean temperature •10-day mean photosynthesis •growing degree-day accumulation BIOGEOCHEMISTRY heterotrophic respiration (RH) summer green fire probability net primary production •growth efficiency bioclimatology •frost tolerance •heat stress soil organic matter fire occurrence •moisture •fuel load mortality fire resistance combustion plant, litter competition aboveground space establishment •potential rate •canopy gap bioclimatology •frost tolerance •heat stress •winter chilling •growing season warmth •low precipitation ANNUAL STATISTICS bioclimatology •minimum monthly temperature (20-year mean) •growing degree-days above 5C (20-year mean) •precipitation •growing degree-days above heat stress Net CO2 GPP-RA-RH Bonan et al. (2003) Global Change Biology 9:1543-1566 •leaf litter •sapwood to heartwood •root litter •fire season length •net primary production •GPP and potential GPP GPP-RA 20-minutes ecophysiology turnover mortality soil PHENOLOGY soil/snow/ice physics energy temperature water balance balance autotrophic respiration (RA) maintenance growth •foliage,stem,root allocation •leaves •stems •roots •seeds Daily Yearly Model validation – tower fluxes Boreal Ecosystem Atmosphere Study (BOREAS) Model Tower Observations Bonan et al. (1997) J Geophys Res 102D:29065-29075 Simulated Leaf Area Index Three types of phenology • Evergreen • Raingreen • Summergreen Bonan et al. (2003) Global Change Biology 9:1543-1566 Model validation – global net primary production Annual net primary production (g C m-2 yr-1) Vegetation Type Simulated Observed Tropical broadleaf evergreen forest 1278 1250900 Tropical broadleaf deciduous forest 886 825475 Temperate broadleaf deciduous forest 579 600325 Boreal deciduous forest 346 425200 Boreal needleleaf evergreen forest 385 325200 Temperate/boreal mixed forest 576 525275 Grassland 175 575475 Tundra 159 150200 Bonan et al. (2003) Global Change Biology 9:1543-1566 Vegetation dynamics Boreal forest succession Bonan et al. (2003) Global Change Biology 9:1543-1566 Global biogeography Greening of North Africa Climate 6000 years BP Increased Northern Hemisphere summer solar radiation Strengthened African monsoon Wetter North African climate allowed vegetation to expand Two climate model experiments Desert North Africa Green North Africa Kutzbach et al. (1996) Nature 384:623-626 Climate model experiments show: • Strengthened monsoon due to radiative forcing • Vegetation forcing similar in magnitude to radiative forcing Greening of North Africa Present Day Biogeography (percent of grid cell) 6kaBP DynVeg Soil Texture – 0 kaBP Precipitation Change From Present Day Dominant forcing Increase in evaporation Decrease in soil albedo Albedo Vegetation and soil Orbital geometry Levis et al. (2004) Climate Dynamics 23:791-802 Effect of boreal forests on climate Vegetation masking of snow albedo Maximum satellite-derived surface albedo during winter Robinson & Kukla (1985) J Climate Appl Meteor 24:402-411 Tree-covered land has a lower albedo during winter than snow-covered land Colorado Rocky Mountains Effect of boreal forests on climate Climate model simulations show boreal forest warms climate Climate Model Simulations: Forested - Deforested Temperature Difference (°C) 14 January 12 April July 10 October 8 6 4 2 0 -2 70 60 50 40 30 20 10 0 Latitude (degrees N) Forest warms climate by decreasing surface albedo Warming is greatest in spring but is year-round Warming extends south of boreal forest (about 45°N) Bonan et al. (1992) Nature 359:716-718 Effect of boreal forests on climate Boreal forest expansion with 2CO2 warms climate Mean annual temperature (2CO2) Dominant forcing Decrease in albedo [Carbon storage could mitigate warming] Additional temperature change with vegetation Bonan & Levis, unpublished Land cover change as a climate forcing Land cover change as a climate forcing Future IPCC SRES Land Cover Scenarios for NCAR LSM/PCM Forcing arises from changes in Community composition Leaf area Height [surface roughness] Surface albedo Turbulent fluxes Hydrologic cycle Also alters carbon pools and fluxes, but most studies of land cover change have considered only biogeophysical processes Feddema et al. (2005) Science 310:1674-1678 Land use climate forcing SRES B1 SRES A2 2050 Dominant forcing Brazil – albedo, ET U.S. – albedo Asia - albedo 2100 PCM/NCAR LSM transient climate simulations with changing land cover. Figures show the effect of land cover on temperature (SRES land cover + SRES atmospheric forcing) - SRES atmospheric forcing Feddema et al. (2005) Science 310:1674-1678 Carbon cycle feedback Three climate model simulations to isolate the climate/carbon-cycle feedbacks • Prescribed CO2 and fixed vegetation (a 'standard' GCM climate change simulation) • Interactive CO2 and dynamic vegetation but no effect of CO2 on climate (no climate/carbon cycle feedback) • Fully coupled climate/carbon-cycle simulation (climate/carbon cycle feedback) Effect of climate/carbon-cycle feedbacks on CO2 increase and global warming Prescribed CO2 and fixed vegetation Interactive CO2 and vegetation, no climate change Fully coupled Carbon budgets for the fully coupled simulation Cox et al. (2000) Nature 408:184-187