Study Concludes That an Aggressive Global Cellulosic Biofuels Program Could Have Unintended Consequences
|Carbon balance associated with all land use change and that directly associated with biofuels over the period 2000-2050 as simulated by the deforestation (a) and intensification (b) scenarios. Melillo et al. (2009) Click to enlarge.|
An aggressive global cellulosic biofuels program could contribute substantially to future global-scale energy needs, but could have significant unintended environmental consequences, according to a recent report by the MIT Joint Program on the Science and Policy of Global Change.
Using simulation modeling, the researchers explored two scenarios for cellulosic biofuels production: the clearing of large swathes of natural forest, or the intensification of agricultural operations worldwide. The greenhouse gas implications of land-use conversion differ substantially between the two scenarios, but in both, numerous biodiversity hotspots suffer from serious habitat loss, the study found.
Elements and parameters of the study included:
Cellulosic biofuels contribute 141 EJ yr-1 in the deforestation scenario and 128 EJ yr-1 in the intensification scenario—levels of energy supply large enough to meet at least 10% of the projected global energy requirement in 2050.
A computable general equilibrium (CGE) model of the world economy, the MIT Emissions Predictions and Policy Analysis model, was coupled with a process-based terrestrial biogeochemistry model, the Terrestrial Ecosystem Model to generate global land-use scenarios and to explore some of the environmental consequences of an aggressive global cellulosic biofuels program over the first half of the 21st century.
The biofuels scenarios are linked to a global climate policy to control greenhouse gas emissions from industrial and fossil fuel sources that would, absent feedbacks from land-use change, stabilize the atmosphere’s CO2 concentration at 550 ppmv.
The climate policy makes the use of fossil fuels more expensive and speeds up the introduction of biofuels, and ultimately increases the size of the biofuel industry, with additional effects on land use, land prices, and food and forestry production and prices.
At the beginning of the 21st century, about 31.5% of the total earth land area of 133 million km2 was in agriculture; 12.1% (16.1 million km2) in crops and 19.4% (25.8 million km2) in pasture, with no land devoted to cellulosic biofuels.
In the deforestation scenario, the researchers estimate that by 2050, the land area in cellulosic biofuels will grow to 14.8 million km2, which is 11.1% of the earth’s total land area. They also project that the area of croplands will grow to about 20.0 million km2 and the area in pasture will shrink slightly to 24.5 million km2. The growth of croplands by 3.9 million km2 in the deforestation scenario is in response to increased food demands globally.
In the intensification scenario, they estimate that the land area in cellulosic biofuels will grow to 13.9 million km2, and that by mid-century croplands area will grow by about 2 million km2 to almost 18 million km2 (Table 1), while pasture areas will shrink by almost 8 million km2 to just under 18 million km2. Despite the loss of pastures, the intensification of land use allows nearly as much production of food as in the deforestation scenario.
Carbon debt. The combination of energy production from biofuels together with agriculture expansion results in an initial carbon loss from land ecosystems, referred to as a carbon debt.
The deforestation scenario results in a direct carbon debt of 21 Pg C by 2050, with much of this carbon coming from areas once covered by tropical forests in Brazil and in Southeast Asia. The indirect carbon debt could be as large as 82 Pg C, giving a total carbon debt of 21-103 Pg C. This carbon debt is equal to 8-37% of the cumulative fossil fuel emissions for the period 2000-2050 in the climate policy imposed here to limit these emissions.
Even considering the best-case, where total carbon debt is 21 Pg C, we estimate that the carbon debt associated with biofuels establishment in the deforestation scenario will last until the middle of the 21st century; that is, no net greenhouse gas reductions will be realized from biofuels until about 2045. Moving towards the worst-case, where total carbon debt is 103 Pg C, it becomes clear that these large emissions from land-use change would substantially undermine the efforts to stabilize climate.—Melillo et al. (2009)
The results of the intensification scenario differ dramatically. Energy production from biofuels results in a direct carbon credit of 4 Pg C by 2050. The small carbon gains in many of the areas devoted to bioenergy production are mostly in soils in response to nitrogen fertilization, which stimulates plant growth and carbon inputs to soil. However, the indirect carbon debt is still potentially as large as 38 Pg C, giving a total carbon debt of -4 to 34 Pg C over the first half of the 21st century.
While the upper bound on the total carbon debt is still substantial, it is much less than the 103 Pg C in the deforestation scenario. This upperbound debt of 34 Pg C in the intensification scenario is nearly repaid by the middle of the 21st century.—Melillo et al. (2009)
Impacts on Biodiversity. Both the deforestation and intensification scenarios project that by the middle of the 21st century, many regions will substantially increase the fraction of land they devote to meeting the combined demands for food and biofuels at the expense of natural ecosystems including a number of biodiversity “hotspots” in the sub-tropics and tropics.
The researchers project that sub-Saharan Africa will be the region devoting the largest area to biofuels production during 2050, followed by Latin America.
In both scenarios in our analysis, we project the loss of large areas of forest and savanna habitats due to the direct and indirect effects of implementing a large-scale biofuels program, although the areas lost are smaller in the intensification scenario. These losses have the potential to put thousands of endemic plant and animal species at risk across the globe, especially in the sub-tropical and tropical regions.—Melillo et al. (2009)
The study did not account for all of the unintended consequences of intensification that are likely to occur—such as the greater use of agricultural chemicals, increased confined livestock production and overgrazing—and these additional effects are likely to differ between the two scenarios, the researchers wrote.
Existing and proposed emissions trading systems are, in principle, a superior approach for controlling greenhouse gases, but also fail to fully protect or provide incentive to increase carbon stocks in vegetation and soils. These poorly designed policies put carbon stocks in vegetation and soils at risk and in doing so potentially undermine the goal of stabilizing the atmospheric concentration of carbon dioxide at the desired target.
Even though we see the potential for considerable intensification of production on land, and therefore a less than one-for-one conversion of land to meet biofuels demands, the risks of converting land in biodiversity hotspots are substantial. With the loss of biodiversity comes a cascade of environmental consequences including the loss of critical ecosystems services. It is clear that we must think holistically and proceed cautiously as we develop policies to use plant-based biofuels to combat global warming.—Melillo et al. (2009)
Melillo, J.M., A.C. Gurgel, D.W. Kicklighter, J.M. Reilly, T.W. Cronin, B.S. Felzer, S. Paltsev, C.A. Schlosser, A.P. Sokolov and X. Wang (2009) Unintended Environmental Consequences of a Global Biofuels Program (Report 168, January 2009)