BIOENERGY: THE NEED FOR ADDITIONAL CARBON

Transcription

1 BIOENERGY: THE NEED FOR ADDITIONAL CARBON TIM SEARCHINGER, PRINCETON UNIVERSITY JUNE, 2011

2 Gross terrestrial carbon sink (2.8) & ~1 Gt logging regrowth (slightly exceeds logging source) 2

3 BOTH BIOMASS AND FOSSIL FUEL COMBUSTION EMIT CARBON DIOXIDE, POTENTIAL SAVINGS COME FROM PLANT UPTAKE 3

4 BIOENERGY IS A FORM OF LAND-BASED CARBON OFFSET Land grows plants whether for bioenergy or not: * forest * food Only ADDITIONAL plant growth helps 4

5 Put another way, biofuel GHG benefits come not from changing what goes up, but adding to what comes down or stays down 5

6 90 Scientist Letter to Congress Bioenergy can reduce atmospheric carbon dioxide if land and plants are managed to take up additional carbon dioxide beyond what they would absorb without bioenergy. Alternatively, bioenergy can use some vegetative residues that would otherwise decompose and release carbon to the atmosphere rapidly. Whether land and plants sequester additional carbon to offset emissions from burning the biomass depends on changes both in the rates of plant growth and in the carbon storage in plants and soils. W. Schlesinger, Donald Kennedy, Sallie Chilsolm, Norm Christensen, Gretchen Daily, Gene Likens, Dan Kammen, Tom Lovejoy, Michael Oppenheimer, Stuart Pimm, Phil Robertson, Stephen Schneider, Robert Socolow,Dan Sperling, John Terborgh et al. 6

7 Large Bioenergy Potential Studies Most potential arable land IPCC 2007 chapter 8-1.3billion hectares and/or All forest growth in excess of harvest (Smeets 2008)and/or All abandoned cropland (Hoodwijk (2004) and/or Hundreds of millions of hectares of grazing or other land savannah (Fischer 2001; Smith 2007) Recounts existing forest, forest re-growth, net terrestrial carbon sink, land counted for grazing

8 To produce 20% of world energy demand by 2050 (IEA target), roughly 170 EJ, world would have to double harvest of plant material (assuming biomass is used as efficiently as fossil fuels!). World Plant Harvest Today from Haberl, European Environmental Agency Presentation (October, 2010) (below) (energy content of biomass converted from biomass use quantities in Haberl et al., PNAS (2007)) Above-ground biomass uptake (net primary production) Exajoules/year Natural vegetation 1,309 Human-induced reduction 68 Current total 1,241 Human harvest Primary crops 64 Harvested crop residues 54 Biomass grazed 71 Wood removals (FAO) 36 Possible additional fuel wood not counted by FAO 12 8

9 IPCC Guidelines IPCC 2000 Land Use Report (p. 355): Because fossil fuel substitution is already rewarded by excluding emissions from the combustion of bioenergy, to avoid underreporting... any changes in biomass stocks on lands... resulting from the production of biofuels would need to be included in the accounts. EPA Call for Information: IPCC guidelines exclude bioenergy emissions to avoid double-counting 9

10 Credit for Plant Growth Explains Findings of Greenhouse Gas Benefits in LCAs EU JRC Source of fuel* Producing Feedstock (crude oil or crop) Refining Tailpipe Emissions Fermentation emissions Total GHGs & % Increase for Biofuel Without Plant Credit Credit for Plant Growth Total GHGs & % Savings for Biofuel Gasoline EU wheat ethanol (+96%) (-29%) Diesel EU Biodiesel from Rape (+41%) (-47%) Greenhouse gas emissions and sinks (CO 2 Eqv.) per mega joule of fuel

11 CO 2 uptake Figure 1 Effect of switching from gasoline to biofuels grown on otherwise unproductive land Reduced atmospheric CO 2 through increased plant growth Unproductive land New crop growth Car, gasoline Car, ethanol Gasoline Use Ethanol Use 11

12 Using otherwise burned or decomposed crop residues for biofuels - Reduced emissions through reduced land sources X Burning or decomposing crop residues Car, gasoline Reduced emissions from Residues Car, ethanol 12

13 CO 2 uptake CO 2 uptake Figure 2 - Direct effect of switching from gasoline to biofuels that use existing crops No change in emissions Crop growth Crop growth Car, gasoline Gasoline Use Ethanol Use Car, ethanol 13

14 CO 2 uptake CO 2 uptake CO 2 em. Figure 3 - Indirect effect 1 of adopting ethanol Ethanol leads to less crop consumption for feed and food, which reduces CO 2 Crop growth Crop growth Car, gasoline Gasoline Use Livestock & human respiration, methane and wastes Car, ethanol Ethanol Use Reduced livestock & human respiration, methane and wastes (vertical arrows indicate carbon uptake and emissions)

15 Importance of Food Consumption Reduction in LCAs for Biofuels Model and Type of Ethanol GTAP US Maize Impact US Maize IMPACT EU Wheat FAPRI CARD EU Wheat GTAP EU Wheat Weight reductions from JRC 2010 Food Consumption Reduction by weight(exclusive of byproducts) and CO 2 savings from reduced respiration 52% = 56 g/mj 36% = 39 g/mj 47% = 50 g/mj 34% = 36 g/mj 46% = 49 g/mj

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17 CO 2 uptake CO 2 uptake CO 2, N 2 0 Figure 4 - Indirect effect 2 of adopting ethanol Ethanol leads to yield growth on existing farmland to replace diverted crops, absorbing more carbon and probably reducing CO 2 Crop growth Gasoline Use Car, gasoline Increased yields absorb more carbon on the same land but may increase emissions from fertilizer use Ethanol Use Car, ethanol

18 Compound Annual Growth Rate in Yield 6.0% Crop Yields Needed to Provide Food and 10.3% of World Transport Fuel (E4Tech Scenario) With and Without Biofuels Without Land use Change Compared To Trend and FAPRI Projections 5.0% 4.0% 2.6% 3.0% 0.9% 3.8% 2.0% 0.8% 3.1% 1.0% 0.0% 1.2% 1.6% 1.2% 1.4% 2.0% 0.6% 0.8% Cereals Cereals Cereals Oilseeds Oilseeds Oilseeds Sugar Crops 0.5% Sugar Crops -0.1% Sugar Crops 1.8% Palm Palm Palm -1.0% Trend Non Biofuel food demand Biofuel, adjusted for by products FAPRI Projection

19 CO 2 uptake CO 2 uptake CO 2 uptake Figure 5 - Indirect Effect 3 of adopting ethanol Ethanol leads to land use change, which increases crop growth, but sacrifices forest or grassland and probably causes net increase in CO 2 Crop growth Crop growth Land conversion Car, gasoline Gasoline Use Car, ethanol Ethanol Use

20 Land Carbon Cost Benefit of Using Land for Biofuel 3 t/ha/yr maize ethanol GREET (2009) 8.6 t/ha/yr cellulosic ethanol GREET (switchgrass 18 t/ha/yr, 359 l/t) Cost of Using Land for Biofuel Fallow land - forest regeneration, t/ha/yr Existing forest = t/ha/yr (over 30 years) plus lost forest growth Existing grassland/savannah (lose tons), t/ha/yr (over 30 years) plus lost forage for grazing animals

21 To earn 107 g/mj credit (or no ILUC): Reduced emissions from reduced food consumption + Increased carbon uptake from positive land use change + Increased carbon uptake from yield gains due to biofuels - Increased emissions from adverse land use change = 107 g/mj 21

22 Key Points There can be no GHG benefits from using existing crops for biofuels except through indirect effects If indirect effects are too uncertain to calculate, then one cannot assume any GHG reductions Cellulosic ethanol not necessarily better, depends on land use implications 22

23 Natural Forest (Melillo, Gurgel, et al. 2008) Natural Forest 23

24 Natural Forest ( Deforestation Scenario) Natural Forest 24