Forty years after “The Limits to Growth”: A focus on global food production

Satellite images by Earth Observatory (2006); composition by WK Smith (2012).

By Bill Smith

“Population, when unchecked, increases in a geometrical ratio.  Subsistence increases only in an arithmetical ratio.  A slight acquaintance with numbers will show the immensity of the first power in comparison of the second.” 

Thomas Robert Malthus, An Essay on the Principle of Population, (1798)

This year marks the 40th anniversary of the seminal book The Limits to Growth, which details the first, global-scale computer model (developed by researchers at MIT) to analyze the potential future outcomes of unchecked economic and population growth in a world of limited resources.  The main factors considered by the analysis included world population, industrialization, pollution, food production, and resource depletion.  The take-home conclusion of the analysis was simple: if we control growth and resource consumption, a “stabilized world” is achievable (Figure 1a); if growth and consumption continues unchecked, “overshoot and collapse” is the only system response (Figure 1b).  So how has humanity responded since the publication of this thought-provoking, anxiety-inducing analysis …

… business as usual.  In fact, a recent study that compared 30 years of actual data with the original Limits to Growth analysis showed that we continue to march directly along one of the “overshoot and collapse” scenarios (Turner and colleagues 2008).  While Limits to Growth clearly didn’t spark a paradigm shift for humanity, the concept of defining global-scale limits to growth has spread throughout the scientific community.

Figure 1. Possible theoretical scenarios describing the intersection of exponential growth and finite boundaries. (a) Scenario describing “stabilization” in which growth behaves as a logistic curve that approaches, but never exceeds, the theoretical system carrying capacity. This figure also illustrates the potential trade-off between conservation and system carrying capacity. For instance, conservation in the form of ‘protected lands’ prevents resource consumption on those lands which lowers the carrying capacity of the system. (b) Scenario describing “overshoot and collapse” where growth exceeds the system carrying capacity, resulting in cataclysmic population collapse and a potential long-term reduction of the system carrying capacity. Image credit: WK Smith (2012).

The idea of Planetary Boundaries – introduced by Rockstrom and colleagues (2009) – is the most notable extension of the original Limits to Growth concept.  The analysis identifies nine boundaries of critical importance to the maintenance of a “safe planetary operating space” for humanity, including climate change, ocean acidification, ozone depletion, nutrient cycles, freshwater use, land use change, biodiversity loss, aerosol loading, and chemical pollution.  The analysis goes on to conclude that humanity has already crossed the boundaries for climate change, biodiversity loss, and perturbation of the nitrogen cycle – while we quickly approach the boundaries for freshwater use, land use change, ocean acidification, and perturbation of the phosphorus cycle.  Upon further inspection, it becomes clear that food production – a factor considered by the original Limits to Growth analysis – is a major driver of a number of these defined boundaries.  For instance, food production directly drives land use change, biodiversity loss, nitrogen and phosphorus cycle perturbations (i.e. fertilization application), and freshwater use (i.e. irrigation application).

Figure 2. The original Planetary Boundaries and their relationship with net primary production (NPP) as proposed by Running (2012). Original image by Rockstrom and colleagues (2009); modifications by SW Running and WK Smith (2012).

Most recently, Running (2012) introduced Net Primary Production (NPP) – total terrestrial plant growth – as a new planetary boundary that integrates the previously defined planetary boundaries related to food production (Figure 2).  A main advantage of this perspective is that we can directly measure global NPP using satellites, a method that has been in practice for the past few decades.  Thus, we have the ability to monitor any changes due to the conversion of natural ecosystems to urban, agriculture, or bioenergy systems.  This perspective builds on previous work by Vitousek and colleagues (1986), where it was estimated that humans appropriate 40% of total global vegetation growth for food production or other ecosystem services.  We recently extended this line of thinking by evaluating how much of the 60% of remaining NPP was available for human consumption (Smith and colleagues 2012).  We concluded that less than 10% of biospheric productivity remains on the table for humanity if we aim to maintain our current protected and wilderness areas (Figure 3).

Figure 3. Current partitioning of global primary production (NPP) according to Smith and colleagues (2012). Increases in food and bioenergy production beyond the ‘available’ fraction will either require intensified production on currently ‘human appropriated’ land or conversion of ‘unavailable’ land (i.e., currently protected and wilderness land). Intensification will likely require additional fertilization and irrigation inputs, which could push humanity past nitrogen, phosphorus, and freshwater planetary boundaries. Conversely, the conversion of protected and wilderness land could push humanity past land use, biodiversity, and NPP planetary boundaries. Given agricultural output must double by 2050 to feed an additional 2-3 billion people, these detrimental trade-offs are likely unavoidable under current agricultural practice. Abbreviations: petagrams carbon (PgC); million square kilometers (Mkm2); global primary energy consumption (GPEC).Image credit: SW Running and WK Smith (2012).

Looking forward, all research suggests an impending planetary boundary for food production which may be reached in the next few decades.  Current projections suggest global food demand will more than double by 2050, mainly due to a 40% increase in population coupled with an increased global appetite for meat.  This looming threat to humanity has been termed “The Other Inconvenient Truth” by Dr. Jonathan Foley, one of the original authors of the Planetary Boundary analysis (TEDx talk).  A full understanding of the above information may result in the building of anxiety in the pit of your stomach.  You have two ways to alleviate this anxiety: 1) accept current trends and conditions and hope for the best, or 2) accept the responsibility associated with changing them.  One of these choices has the potential to better the world for our children and future generations, while the other continues humanity’s march off the cliff (with our children skipping behind us).

For more info: see a previous post on current U.S. biofuel policy and the implications for food production, follow me on twitter, or check out my ECE introductory blog and profile.