Lucas Reijnders’s research while affiliated with University of Amsterdam and other places

In the scientific literature, the terms sustainable, green, ecofriendly and environment(ally) friendly are used regarding magnesium alloys applied in cars. When sustainability is defined as remaining within safe planetary boundaries for mankind or as conserving natural capital for transfer to future generations, current alloys based on primary magnesium applied in cars are not sustainable. Current alloys based on primary magnesium are not green, ecofriendly or environmentally friendly when these terms mean that there is no burden to the environment or a minimal burden to the environment. Available environmental data do not support claims that current alloys based on magnesium originating from the Pidgeon process, which replace primary mild conventional steel in automotive applications, can be characterized as green, ecofriendly or environmentally friendly. There are options for substantially reducing contributions to the life cycle environmental burden of magnesium alloys. Minimizing the life cycle environmental burden of magnesium alloys may enable them to be characterized as environmentally friendly, ecofriendly or green in the sense of a minimal burden to the environment.

Recycling of non-product outputs containing substantial amounts of rare elements originating in nanomaterial syntheses is relatively attractive as rare elements tend to be more valuable than abundant elements.

Regarding the achievement of worldwide agricultural climate neutrality, the focus is on a worldwide net-zero emission of cradle-to-farmgate greenhouse gases (GHGs), while, when appropriate, including the biogeophysical impacts of practices on the longwave radiation balance. Increasing soil carbon stocks and afforestation have been suggested as practices that could be currently (roughly) sufficient to achieve agricultural climate neutrality. It appears that in both cases the quantitative contributions to climate neutrality that can actually be delivered are very uncertain. There is also much uncertainty about the quantitative climate benefits with regard to forest conservation, changing feed composition to reduce enteric methane emission by ruminants, agroforestry and the use of nitrification and urease inhibitors to decrease the emission of N2O. There is a case for much future work aimed at reducing the present uncertainties. The replacing of animal husbandry-based protein production by plant-based protein production that can reduce agricultural GHG emissions by about 50%, is technically feasible but at variance with trends in worldwide food consumption. There is a case for a major effort to reverse these trends. Phasing out fossil fuel inputs, improving nitrogen-use efficiency, net-zero GHG-emission fertilizer inputs and reducing methane emissions by rice paddies can cut the current worldwide agricultural GHG emissions by about 22%.

The terms sustainable and sustainability are currently often used in scientific journals, including Energies. There are cases where these terms are defined or operationalized, but more often they are not. This is problematic, as there are reportedly hundreds of (different) definitions and operationalizations (in terms of standards or goals) of sustainability. This large number has its roots in history. Many current definitions and operationalizations of sustainability are social constructs. As these constructs vary, there can be variation in the characterization of specific ways to provide energy as sustainable or not sustainable. There are also definitions of sustainability that have emerged from the sciences. These definitions can also lead to differences in the characterization of specific ways to provide energy as sustainable or not sustainable. In view thereof, there is a case to define and/or operationalize sustainable and sustainability when these terms are used in the context of energy.

Life-cycle assessment (LCA) leads to indicators of cradle-to-grave environmental burdens or impacts of products such as microalgae-based fuels. Such indicators can regard pollution, natural resources and ecosystems. Comparing LCAs of product life cycles may provide answers to the question whether specific burdens or impacts are smaller or larger. Such answers have limited certainty. Uncertainty is relatively large for products that are, as yet, not widely commercially produced, such as microalgae-based fuels. Whether environmental burdens or impacts are sustainable requires a definition of sustainability. This definition can serve to provide targets that may be considered sustainable. LCA may be used to establish the distance(s) to such targets, with a zero distance corresponding to sustainability. In practice, there are hundreds of specific definitions or operationalizations of sustainability. Many of these start from the point of view that sustainability is a subjective concept or a social construct. Many of such operationalizations comprise social, economic and environmental goals. Conflicts between achieving these goals have led to the advice to treat environmental sustainability separately. There are also operationalizations of environmental sustainability that have emerged in the environmental sciences. Such operationalizations of sustainability include conserving the carrying capacity of ecosystems, the conservation of natural capital, and staying within planetary boundaries. The development of targets for LCA based on the latter operationalizations is, as yet, a work in progress. However, in view of these operationalizations, weak points of biodiesel produced from autotropHic microalgae are indicated.

There are reportedly hundreds of definitions for sustainability. Many of such definitions comprise social, economic and environmental sustainability. It has been argued that combining environmental life cycle assessment, social life cycle assessment and life cycle costing is suitable to assess the sustainability of product life cycles. (Environmental) life cycle assessment is a widely applied tool to assess the environmental burdens and impacts of product life cycles. Life cycle costing focuses on costs of product life cycles, whereas definitions or operationalizations of economic sustainability can also focus other economic matters. Social life cycle assessment addressing social sustainability is a work in progress. Four relatively important operationalisations of sustainability are considered as to their suitability for the derivation of sustainability targets. The development of targets for life cycle sustainability assessment based on these operationalisations is found to be largely a work in progress. Applications of distance-to-target methodology to assess biofuel sustainability are discussed.

Substitutability of natural capital by human-made capital would seem to be limited. When human-made capital substitutes natural capital, there are currently commonly long-lasting negative impacts of such substitutions on constituents of natural capital. Long-lasting negative impacts on natural capital can be considered at variance with justice between the generations. In view thereof, there is a case to define (environmental) sustainability as keeping natural capital intact for transferral to future generations. A major problem for such conservation regards natural resources generated by geological processes (virtually non-renewable resources), especially regarding geochemically scarce elements. Substitution of virtually non-renewable resources by generating equal amounts of renewables has been proposed as a way to conserve natural capital. However, renewables substituting for fossil carbon compounds are currently associated with negative impacts on constituents of natural capital to be transferred to future generations. The same holds for the substitution of widely used geochemically scarce virtually non-renewable copper by abundant resources generated by geological processes. Though current negative impacts of substitutions on natural capital can be substantially reduced, their elimination seems beyond the scope of what can be achieved in the near future. The less strict “safe operating space for humanity”, which has been used in “absolute sustainability assessments” is, however, not a proper alternative to keeping natural capital intact for transferral to future generations.

Life cycle assessment (LCA) assesses the environmental impacts of products from cradle to grave. It is also possible to assess parts of the life cycle. Available peer-reviewed LCAs dealing with microalgae-based products and processes are reviewed. In processes that require extraction of dry algae, the combined harvesting, drying, and extraction steps are associated with relatively large environmental impacts. In addition, the cultivation stage has been identified as linked to relatively large environmental impacts. Proteins derived from cyanobacteria tend to have larger estimated life cycle environmental impacts than proteins derived from cultivated terrestrial plants such as maize and soybean. Microalgal biomass-based feed for carnivorous fish might have a lower life cycle environmental burden than conventional fish meal. Whether commercial microalgal fuels will have a climate benefit compared with fossil fuels appears to be uncertain. The energetic return on energy invested (EROI) of all microalgal fuels for which peer-reviewed LCAs are available does not meet the criterion of 5–8, which would allow for widespread application.

Biofuels that are applied using advanced conversion technologies are promoted as green alternative to fossil fuels. The water footprint, the freshwater consumption, linked to current life cycles of such biofuels is much larger than the corresponding consumption associated with fossil fuels. Wind‐ and photovoltaic energy are also characterized by water footprints that are much lower than the corresponding consumption linked to biofuels. Food production and biofuel production increasingly compete for scarce freshwater resources and freshwater stress may increase when biofuel production expands. Availability of freshwater may be a limiting factor in biofuel production. Biofuel life cycles can also be associated with substantial water pollution burdens of nitrate, phosphate, pesticides, and organic substances. On the other hand, it has been proposed to use wastewater for the production of microalgal biofuels. So far, however, no full‐scale production based on this proposal that meets standards achieved by state‐of‐the‐art wastewater treatment facilities has been reported. In many respects relevant to green perspectives, except the consumption of geochemically scarce mineral resources, solar and wind‐based technologies tend to outperform biofuel‐based energy supply technologies. There would seem to be a case for modest expectations as to the share of biofuels that are applied using advanced conversion technologies in future energy supply.