The global scientific community first began to consider, in earnest, the Earth’s capacity to sustain humanity in the classic 1972 study Limits to Growth.1 The primitive model put forward by Meadows and colleagues used the best projected population and economic growth trajectories of the time to produce a model showing that, by the early 21st century, humanity would have begun bumping up against serious global limits, including energy and food. More recently, planetary boundary thinking has refreshed the original conceptual basis of Limits to Growth,2 but the essential message is the same: we must learn to live more sustainably within the bounds of the global biosphere.
In the last few decades, as climate change has become a more urgent problem, renewable energy generated from biological materials, or bioenergy, has been touted as a low- or zero-carbon alternative to fossil fuels. However, this solution has set up a clash between the natural resources of an expanding bioenergy sector and those of global food security.
Energy economists looking for pathways to a low-carbon world have assumed a massive acceleration in bioenergy production. Meanwhile, global food security experts search desperately for ways to feed more than nine billion people by mid-century.3-6 Can the biosphere accommodate both? Or, do we now face limits to continued growth in bioenergy? Will some sort of global land prioritization become necessary?
Understanding the global potential for bioenergy means putting a figure on net primary production, that is, the overall carbon drawn by green plants from the air in a given period of time, stored as plant biomass. Global net primary production is estimated at about 54 petagrams (Pg) of carbon per year—a figure that has not changed much in the 35 years since satellites first began gathering the necessary global data. Annual global terrestrial plant growth is an important planetary boundary; a process sufficiently constrained by global geochemistry that it cannot be increased significantly.7 In other words, this is a potential limit to resource consumptive economic growth.
Land Available for Bioenergy is Limited
The next question is how much of this plant biomass are humans using now? This is known as Human Appropriation of Net Primary Production and includes the total amount of food, paper, and wood products we consume, as well as feed given to livestock.8 Our best current estimate is that humanity consumes 38 percent of global plant biomass per year, which implies that our species still has 62 percent available for future use. However, it would be simplistic to presume this means there is still more than ample land available for bioenergy production. A detailed analysis showed that what appears to be readily accessible plant biomass is actually a mix of unharvestable root growth, wilderness and protected areas, and marginal land on which the energy required for harvesting would exceed the energy gained. A more realistic estimate of potentially available terrestrial plant biomass, then, is about 10 percent, or 5 Pg.9
How far would this 5 Pg go in satisfying global energy demand? A rigorous study of global bioenergy potential found that, depending on the rate of use, bioenergy can satisfy at best between 12 and 35 percent of current global energy demand.10
Many believe that biofuels such as ethanol from cellulosic waste (e.g. sawmill waste, forest residues, and waste paper) will provide much of this energy source. But, there is a struggle with optimizing chemical conversion efficiencies and identification of sufficiently dependable feedstock sources. Using food (such as corn) to generate biofuels has also proven to be politically and ethically risky as food prices inevitably increase, a burden borne disproportionately by poor people. Single-crop approaches to turning biomass into energy have been shown to be hard to bring to market.
With the global population expected to increase 30 percent by 2050, food demand rising by 70–100 percent, and energy demand roughly doubling, if global biospheric plant growth cannot be significantly enhanced, then it seems that humanity is headed for a future where food and bioenergy compete for land and the annual plant production.
New Biosystems: Combining the Best of Engineering and Ecology
Returning to some ecological systems thinking may be helpful. It might just be possible to devise bioenergy systems that, integrated with food production and other co-products, are overall more efficient, and this can become economically viable. Recent experiments in modeling more efficient systems are showing impressive results. These take various biomass inputs, generate electric power and useable heat energy, and then route the waste and carbon dioxide through complex processes to generate various additional products.
One example is the Green Powerhouse Prototype in Montana,11 which takes in a variety of feedstocks, primarily residue from forestry and agriculture, and generates electricity, syngas, and biochar via pyrolysis. The system directs both waste heat and CO2 to a greenhouse where vegetables are grown and algae are harvested for high protein feed for fish, among other things.. Four different marketable products are generated: electricity, organic vegetables, soil conditioners, and bio-oils.
Other recent advances in integrated biosystems like this include:
- Hydroponic and aquaponic systems (or polycultural fisheries) that produce all of the nutrients required for crop fertilizers and irrigation.
- Vertical growing systems that maximize production per square meter of growing space using LED lighting systems and climate control technologies.
- A variety of renewable energy systems, such as biomass cogeneration, solar photovoltaics, passive solar design, wind power, etc.
- Permaculture or agro-ecological technologies, such as biochar, nutrient and water recycling, biodigestion, and microbial soil conditioning.
These new, integrated food-energy greenhouses can potentially produce commercial volumes of high-quality products, very efficiently, in virtually any location. Their use of otherwise wasted plant residues, advanced growing techniques, biomass gasification, and thermal heating and cooling systems also serves to keep operating costs and carbon footprints low, though initial infrastructure costs are high.
Crucially, these systems can cut the land area for cultivation and the water used in irrigation by 90 percent compared to open-field crops. These biosystems are not meant to replace all open-field agriculture, but to aid in growing high-value crops in areas where the local climate would not otherwise support production, and to efficiently use plant residues now going into landfills or being burned. Further, because these biosystems can be located anywhere (although preferably near population centers), storage and transport costs can be reduced compared to current transcontinental food distribution.
Critical to the success of these biosystems is, first and foremost, proximity to ongoing biomass sources—be it forest residues, agricultural residues, food waste, or garbage. Ideally, this use of biomass would also serve to solve other resource problems. For example, the mountain pine beetle has killed millions of hectares of forest in the western United States and Canada. Conversely, forests are severely overgrown on many sites in semi-arid western North America, producing heightened future fire risk.12 Salvaging wood residue from forest fires and thinning activities simply for electric power production does not stack up economically as a stand-alone industry. Only an integrated system with multiple saleable products can be economically viable. Also, most intensively-managed bioenergy crops consume so much energy in production that final gains in carbon footprints are lost.
It seems logical that by mimicking the biogeochemical cycling of natural ecosystems—where output from one process becomes input to the next, driven only by solar energy—we will produce the most efficient bioenergy systems with the least waste. Indeed, energy will be but one of an assortment of products. The key challenge is to make the economics of such systems match their ecological efficiency. There is a long history of policy, regulations, protected monopolies, tax holidays, and the like that have built the current commodity economics that incentivize waste. If ecological efficiencies were valued correctly, and ecosystem degradation not externalized economically, the functional beauty of these integrated biosystems would be evident. Maybe then, the concept of garbage, whether tossed into the landfill or the atmosphere, will fade to become a quaint memory of the past.
This contribution is based on deliberations in the session ‘Biomass as a natural resource’ at the IARU Sustainability Science Congress 2014.
- Meadows, D.H. et al. The Limits to Growth (Potomac Associates—Universe Books, New York, 1972).
- Rockstrom, J. et al. A safe operating space for humanity. Nature 461 (2009): 472-5.
- Foley, J.A. et al. Solutions for a cultivated planet. Nature 478 (2011): 337-42.
- Ray, D.K., N.D. Mueller, P.C. West, and J.A. Foley. Yield Trends Are Insufficient to Double Global Crop Production by 2050. PLoS ONE 8 (2013): 66428.
- Tilman, D. et al. Global food demand and the sustainable intensification of agriculture. PNAS 108 (2011): 20260-4.
- West, P.C. et al. Leverage points for improving global food security and the environment. Science 345 (2014): 325-8.
- Running, S.W. A measurable planetary boundary for the biosphere. Science 337 (2012): 1458-9.
- Krausmann, F. et al. Global human appropriation of net primary production doubled in the 20th century. PNAS 110 (2013): 10324-9.
- Haberl, H. et al. Bioenergy: how much can we expect for 2050? Environmental Research Letters 8 (2013): 031004.
- Smith, W.K., M. Zhao, and S.W. Running. Global Bioenergy Capacity as Constrained by Observed Biospheric Productivity Rates. BioScience 62 (2012): 911-22.
- Algae Aquaculture Technologies [online] (2016) http://www.algaeaqua.com/full/index.html.
- Rasker, R. Resolving the increasing risk from wildfires in the American West. Solutions 6 (2015): 48-55.