Humanity relies on global plant production for food, fibre and fuel. But how much of this plant production is available for bioenergy?
Let’s take what scientists call a top-down look at the active biospheric global carbon cycle. That is, we will analyze the largest components of the system and their interactions without getting bogged down in technical or local details. Why bother? Well, because for at least the last 40 years scientists globally have been speculating about “The Limits to Growth” and other possible limits to human consumption of global resources. Recent reframings of this theme, such as “Planetary Boundaries”, have rekindled discussion of whether we can measure global limits, and predict crisis points, far enough in advance to hopefully avoid them.
We start with global net primary production (NPP), the formal term for the total plant growth of the world, both on land and in the ocean. NPP is the foundation of the food chain of every ecosystem, and from a human centric point of view provides the basis for food, fibre and fuel for human consumption. We think the ocean and land NPP are roughly equal at 53-56Pg C yr (a Petagram is 1015 g, or 1012 kg), and there is evidence that year to year variability of the global totals is small. Ocean NPP is primarily consumed by zooplankton, not by humans, so our discussion now will be only about land NPP.
In a changing world, can we tell if land NPP is changing too? Is NPP, total plant growth, being enhanced by modern agriculture, or retarded by pollution or land clearing? We estimate that NPP has been at least constant, and has maybe increased slightly since satellite-based global calculations were started in 1981. A logical follow-on question that has been asked repeatedly over the last few decades is how much of this NPP are humans now using, in what is often referred to as Human Appropriation of NPP or HANPP for short. The most recent estimates are that humans consume about 40% of global land NPP for food (including feeding animals), wood and paper fibre, and fuel. An easy but erroneous interpretation of that figure would imply that 60% remains for potential future human use, i.e. that we appear to be far from any limit or boundary. First, 15Pg, or about 28% of NPP goes to the plant root growth, which with the exception of a few root crops like potatoes, can’t be harvested. Second, a geographically detailed analysis of this remaining “unused” NPP shows that most resides in national parks, wilderness areas, or remote locations that are not really available. These areas also provide essential ecosystem services such as biodiversity, recreation and aesthetic values that suggest we really do not want to exploit all of them. The best estimate of the remaining “available NPP” for new human use is 5Pg or about 10% of the total land NPP (see figure). There is a consensus among people working in global food security that this final 10% of available land NPP may well be needed to feed a growing global population.
But now a new competitor is emerging for a slice of the global carbon pie, bioenergy. Energy research works in different units, exajoules instead of petagrams, and the world now consumes around 550 Exajoules of total energy per year, 90% being from fossil fuel sources. Biomass currently contributes about 50 EJ of that total energy. If energy production prices increase, due to scarcity or policy, big energy companies may turn their attention to large scale biomass farming. The energy markets can out-compete the food markets financially, so bioenergy production could replace food production on a large enough scale to disrupt the current economics of agriculture and even generate food shortages. The ethanol fuel goals of the U.S, government doubled the commodity price of corn from around $2 to $4 per bushel in the last half dozen years, disrupting the global corn market. It is easy to imagine that any major development of bioenergy could be on a collision course with global food security. Most agricultural land is owned privately, and the landowner typically chooses management that provides maximum financial return.
Returning to our global NPP budget, translating the 5Pg of remaining “available” annual NPP into bioenergy would only amount to 100EJ, less than a fifth of current annual consumption, and that assumes all biomass was harvested and processed! While bioenergy may be a useful way to get utility from agricultural and forest residues, municipal trash and the like, only about 60EJ is available from residue sources. One can play with many of these assumptions and numbers, but overall it seems that bioenergy can at best contribute about 1/4 of global energy demand. However, this contribution is still meaningful, and could bring important income to farm and forest landowners. How do we build a policy environment for bioenergy and food production to coexist optimally? Should biomass be burned directly for electricity production, or processed to a liquid fuel like ethanol? When are crop residues more important as a nutrient source for the next crops than as bioenergy? Will conventional economic price signals be effective to insure adequate and equitable food production? Building a policy or incentive structure that protects agricultural production from bioenergy competition may be a necessary but difficult requirement that will challenge international diplomacy and treaty mechanisms.
Haberl H, Erb KH, Krausmann F, Gaube V, Bondeau A, Plutzar C, Gingrich S, Lucht W, Fisher-Kowalski M. (2007). Quantifying and mapping the human appropriation of net primary production in earth’s terrestrial ecosystems. Proceedings of the National Academy of Science USA 104: 12942-12947.