The BAU scenario in this analysis forecasts increasing carbonization and warming of the atmosphere and oceans. This results in climate change, which will magnify demographic pressures on food and water security as well as forests and other ecosystems and functional biodiversity. In many ways, however, this baseline assumes the possibility of carrying on with business-as-usual through the end of the present century. Most importantly, it is possible that depletion of cheap fossil fuel reserves or growing appreciation of the threat posed by climate change will lead to greater-than-forecast demand for biomass. In the closed land system, failure to prepare for this transition will exacerbate competition for arable land, degrade food security, and accelerate deforestation and biodiversity loss.

In terms of emissions and temperature change, the BioEnergy and Alg-Fuel scenarios do not differ significantly from BAU. Ultimately, land scarcity precludes the possibility of establishing plantations and energy crops to produce enough biomass to substitute for a significant fraction of fossil fuel consumption [30, 32]. This leaves CCS (on scales that have not yet been proven feasible) or other so far unidentified technologies as alternatives for rapid, urgent emissions mitigation. Further, although the agricultural and plantation yields assumed in this analysis are conservative, harvesting of tree crowns and agricultural residues can deplete topsoils of vital nutrients, increasing dependence on fertilizers. In addition, an increase in demand for land-based biomass such as is foreseen in BioEnergy scenarios carries several hidden costs. In addition to land use change consequences, risks include disruption of ecosystems services including soil carbon sinks, biodiversity, and water cycles [7, 33, 34]. The direct and indirect costs of bioenergy production must be weighed against the benefits of this approach.

On the other hand, we have demonstrated the theoretical and technological potential of microalgal feedstock to relieve land scarcity, allowing arable land to be leveraged to produce cleaner energy while addressing the threats of climate change, deforestation, eutrophication, and food and water scarcity. In recognition of the fact that these systems can be engineered to produce biomass without generating a commensurate burden on critical ecosystems cycles and services, algacultural systems have already been established at some experimental farms to overcome biomass shortages in dry seasons.

The AEF currently produces algal biomass for feed at a cost of $1,840 per dry ton (cf. Additional file 1: Table S2). Prices as low as $500 per dry ton are generally seen as feasible, but only if CO\(_2\) (57 % of AEF costs) can be sourced at no expense. At these prices, algae could have supplied 40 % of global feedstock in 2013 at a cost of 250–920 billion US$(2013). As a comparison, the global gross production value of livestock in the same year was 1,262 billion US$(2013), and fossil fuel subsidies totaled 550 billion US$(2013) [27, 35]. This estimate does not account for the value of algal co-products, nor does it include the land value created by the transformation of low-productivity pastures into plantations.

Algal biomass is already a viable alternative to fishmeal ($1,880 ton\(^{-1}\)) and should eventually compete with soymeal (average $375 ton\(^{-1}\) over the last year) [36]. Critically, growth in demand for conventional protein sources has driven up the prices of both commodities (soy: 88 %, fishmeal: 165 % over the last decade), and this long-term trend can be expected to continue. On the other hand, research and investment in algacultural pilot programs will lead to higher productivities and lower costs for the cultivation, harvesting, and processing of algae at industrial scales. Looking forward, microalgae production systems represent an ideal transition technology from fossil fuels to bioenergy. For governments looking for shortcuts to sustainable development, algal feedstock manages to satisfy the competing imperatives of food security and climate mitigation by reducing resource burdens while commodifying CO\(_2\). On large scales, this establishes the conditions for cascading greenhouse emissions savings and a return to preindustrial atmospheric carbon concentrations.

To be sure, there exist a number of challenges to engineering and operating algacultural systems on the scale envisioned in this analysis. Systems must be engineered which are robust against contaminant or mutant strains of algae, zooplankton, and viruses and other pathogens, which represent threats to stable and highly-productive monocultural systems [20]. As potential sites for colocation with carbon sources are exhausted, carbon capture and transportation infrastructures will need to be expanded at the same rate as algacultural production systems in order to maintain high productivities and low costs. Large scale carbon capture for algae production and permanent sequestration will likely increase energy consumption and costs, as will energy-intensive methods of algal biomass processing. For freshwater systems as well as those that fertilize with wastewater, the feasibility of recycling water through successive harvests in open and closed systems must be studied. Finally, the supply of nutrients to large scale algae production would necessitate an international market for manure and other forms or recycled nutrients from animal and human sources. Additional research should also be done to match leakage points in global nitrogen and phosphorous cycles with algacultural production systems, thereby minimizing the nutrient loads of these systems as well as the deleterious effects of agricultural runoff.

Apart from obstacles to large scale algae production, biodiversity loss–already a problem in the BAU and BioEnergy scenarios—would likely be exacerbated by even greater conversion of pasture and rangeland to energy crop plantations [37]. The elimination of pasture will require higher livestock densities, though rotational grazing in silvipastoral systems can mitigate animal crowding while fertilizing plantations, and the replacement of low quality feedstocks with algae may reduce reliance on antibiotics. Finally, afforestation and reforestation on the scale envisioned here would lower terrestrial albedo in these areas, potentially blunting the cooling impact of greater carbon sequestration [38]. Despite these unresolved questions, this analysis demonstrates that algacultural feedstock at any scale represents a promising and simultaneous solution to food security and climate change, and that these systems merit greater attention and closer scrutiny than they have thus far received.