On the amount of mitigation required to solve the carbon problem: new constraints from recent carbon cycle science. Stephen W. Pacala,
Outline 1. Effort required to solve the carbon and climate problem depends on the stability of the terrestrial carbon sink, whose cause is unknown. 2. A new global forest inventory matches the size and spatial distribution of the sink and identifies its likely causes. 3. Some good news here - evidence that the sink in primary tropical forests is caused by CO 2 fertilization. All else equal, this portion of the sink should increase over time. 4. Recently published evidence that the sink is declining in efficiency.
Key risks that increase with warming (IPCC 2007) fast warming EU target in-the-bank Lock in ice sheet melt 30% global extinction Reduced crop yields, low-latitudes Negative impacts some regions Based on Kerr, Science 23 Nov 2007
CO 2 emissions and eventual temperature increase for a range of policy outcomes (IPCC 2007)
Atmospheric CO 2 needs to be kept below 450 ppm for 2 degrees, but we are out of time. However, this answer depends on the airborne fraction.
Key Diagnostic of the Carbon Cycle Evolution of the fraction of total emissions that remain in the atmosphere CO 2 Partitioning (PgC y -1 ) 10 8 6 4 2 Total CO 2 emissions Atmosphere 1960 1970 1980 1990 2000 2010 Data: NOAA, CDIAC; Le Quéré et al. 2009, Nature Geoscience
Fate of Anthropogenic CO 2 Emissions (2000-2008) 1.4 PgC y -1 4.1 PgC y -1 45% 3.0 PgC + y-1 7.7 PgC y 29% -1 26% 2.3 PgC y -1 Le Quéré et al. 2009, Nature Geoscience; Canadell et al. 2007, PNAS, updated
What causes the land sink? CO 2 fertilization Climate change Land use change
Norby et al. (2006)
Annual tree basal area increment (standardized by pre-treatment mean) Korner et al. 2005
Catastrophe from a global failure of CO 2 fertilization at double pre-industrial CO 2. Shevliakova et al. PNAS (2010) +218GtC - 444GtC With CO2 fertilization Without CO2 fertilization
Maximum Atmospheric Co2 Concentration (ppm) 1000 900 800 700 600 500 400 Land sink fails Both sinks fail BAU airborne fraction 2010 2020 2030 2040 2050 2060 Solving the carbon Stable Atmospheric Fraction problem becomes Land SInk Fails considerably Atmospheric Fraction Equals 1 more difficult if the sink declines. Year That 50-Year Decarbonization Begins
What can we learn by measuring from recent field measurements? Global forest inventory: Birdsey et al. (in prep.) from 2010 meetings in China and Princeton.
Global Forest Inventory (2000-2007) Quality Carbon Sink GtC/y (SD) Cause Net forest carbon flux (atmos. land) 1.5 (0.5)* Missing forest carbon sink 2.9 (0.8)* Secondary forest regrowth + tropical. deforestation + tropical primary growth enhancement Net flux minus tropical deforestation. LeQuere et al. (2009) missing forest carbon sink from atmospheric and oceanic measurements for 2000-2008. 3.0 (0.9) Net flux minus tropical deforestation. * Birdsey et al. (2010) with Shevliakova et al. (2009) satellite correction for tropical secondary forest.
Comparison of Regional Land Sink Estimates (GtC/y (SD)) from Inverse Modeling without Priors (Sarmiento et al. 2009), and Forest Inventories Estimation Net sink from Net sink from Method NH Land Tropical Land Atmos. & Ocean Data Inversion 1.0 (0.5) 0.1 (1.1) Forest Inventory 1.2 (0.4) 0.3 (0.5)
Global Forest Inventory (2000-2007) Region Sub-region Carbon Sink Cause Globe 1.5 (0.5) Global secondary regrowth + tropical deforestation + tropical growth enhancement Boreal Russia Canada and Alaska 0.5 (0.2) 0.5 (0.2) 0.0 (0.1) Economic collapse Fire and insects Temperate US Europe China 0.7 (0.2) 0.24 (0.05) 0.21 (0.05) 0.18 (0.05) Secondary regrowth Secondary regrowth Forest planting Tropical 1.1 + 0.6 1.4 = 0.3 (0.5) South America 0.4 + 0.4 0.6 = 0.2 (0.3) Primary growth enhancement + secondary regrowth - primary deforestation Africa 0.5 + 0.1 0.2 = 0.4 (0.3) Primary growth enhancement + secondary regrowth - primary deforestation Asia 0.2 + 0.1 0.6 = -0.3 (0.4) Primary growth enhancement + secondary regrowth - primary deforestation
The mean age of forest stands has increased in the temperate and boreal countries because of decreased harvesting. Land use change causes the northern sink.
Global Forest Inventory (2000-2007) Region Sub-region Carbon Sink Cause Boreal Russia Canada and Alaska 0.5 (0.2) 0.5 (0.2) 0.0 (0.1) Economic collapse Fire and insects Temperate US Europe China 0.7 (0.2) 0.24 (0.05) 0.21 (0.05) 0.18 (0.05) Secondary regrowth Secondary regrowth Forest planting Tropical 1.1 + 0.6 1.4 = 0.3 (0.5) South America 0.4 + 0.4 0.6 = 0.2 (0.3) Primary growth enhancement + secondary regrowth - primary deforestation Africa 0.5 + 0.1 0.2 = 0.4 (0.3) Primary growth enhancement + secondary regrowth - primary deforestation Asia 0.2 + 0.1 0.6 = -0.3 (0.4) Primary growth enhancement + secondary regrowth - primary deforestation
Biomass trend, 123 RAINFOR plots 1980-2005 Phillips, Aragao et al. 2009. Science
Biomass change, 79 AfriTRON plots 1968-2007 Fraction of forest cover 100% < -4.0-4.0-2.0-2.0-1.0-1.0-0.1-0.1 0.0 0.0 0.1 0.1 1.0 0% 1.0 2.0 2.0 4.0 Changes in Biomass stocks, Mg ha -1 a -1 > 4.0 Lewis et al. 2009, Nature.
Following Caspersen et al. (2000),many analyses now show that growth rates of trees are not increasing across the temperate and boreal zones. In contrast the tropical sink appears to be caused by CO 2 fertilization. This is good news.
Western Amazonian Forests Eastern & Central Amazonian Forests 3.0 2.5 2.0 3.0 2.5 2.0 1980s 1990s Interval 1 Interval 2 Annual rate, % 1.5 1.0 Annual rate, % 1.5 1.0 0.5 0.5 0.0 Stand BA growth Stand BA mortality Stemrecruitment Stemmortality 0.0 Lewis et al. 2004. Phil Trans Royal Society B. Stand BA growth Stand BA mortality Stemrecruitment Stemmortality Oliver Phillips pers. comm.
Good News Although 50% of the current sink should decline as secondary forests mature, the other 50% should increase as CO 2 rises. Mystery There appears to be little or no CO 2 fertilization in temperate and boreal forests (no room for it given the size of the land use sinks, no record of ubiquitous growth enhancement).
The CO 2 fertilization sink is mostly cancelled by N- limitation in the north, but not in most of the tropics because of nitrogen fixing trees. Residual terrestrial sink 1800 to 2000 (C-only) Effects of N cycle on residual sink (C-only minus C-N) Gerber et al. (2010)
Airborne Fraction Why has the airborne fraction been constant at 45% for fifty 1.0 years?. 0.8 0.6 0.4 0.2 1960 1970 1980 1990 2000 2010 Le Quéré et al. 2009, Nature Geoscience; Canadell et al. 2007, PNAS; Raupach et al. 2008, Biogeosciences
All else equal, the efficiency of the land sink should have declined. CO 2 fertilization. Should decrease in efficiency because of enzymatic saturation. Land use change. Should decrease in efficiency because of recovery from disturbance.
Gloor et al. 2010, Atmos. Chem. Phys. 10, 1-13. Let Δ c be the change in atmospheric CO 2 since the pre-industrial. Then: ) (, ) ( 1 1 ) ( 1 :, 1 1 1, ) ( t f dt d A where A dt df t f dt df t f dt da and so where t f dt d c F F s F l o s s c c Bacastow and Keeling (1979): Constant A F with constant sink strength (τ s ) and exponential fossil fuel growth: f(t)=ae bt.
But CO 2 emissions did not grow exponentially. CO 2 emissions (PgC y -1 ) 10 8 6 4 2 Fossil fuel Land use change 1960 1970 1980 1990 2000 2010 Le Quéré et al. 2009, Nature Geoscience; Data: CDIAC, FAO, Woods Hole Research Center 2009
Residuals exhibit a trend the efficiency of the sink is decreasing (>10% over 40 yrs.).
Summary Our fate depends on a sustainable carbon sink. The terrestrial carbon sink is caused by land use change, primarily in the north, and by CO 2 fertilization of tropical forests. The portion of the sink caused by successional recovery will decline, but the portion caused by CO 2 fertilization should increase. The CO 2 fertilization of northern forests may be N- limited.