Sustainability and the Future

In 2009, scientist Johan Rockström and colleagues published the planetary boundaries framework identifying 9 Earth-system processes that together define a "safe operating space" for humanity. Each boundary represents a threshold; exceeding it risks triggering non-linear, possibly irreversible changes in Earth systems.

The 9 planetary boundaries are:

1. Climate change (CO₂ concentration, radiative forcing)
2. Biosphere integrity (biodiversity loss rate)
3. Land-system change (forest cover)
4. Freshwater use
5. Biogeochemical flows (nitrogen and phosphorus cycles)
6. Ocean acidification
7. Atmospheric aerosol loading
8. Stratospheric ozone depletion
9. Novel entities (synthetic chemicals, plastics, nuclear waste)

A 2023 update found that 6 of the 9 boundaries have already been exceeded: climate change, biosphere integrity, land-system change, freshwater use, biogeochemical flows, and novel entities. Ocean acidification is approaching its boundary. Only stratospheric ozone (recovering since the Montreal Protocol) and atmospheric aerosol loading remain within safe limits.

The dominant economic model has historically been linear: take raw materials, make products, dispose of waste. This model works well in the short term but is physically impossible to maintain indefinitely on a finite planet: it depletes resources and generates mounting waste.

The circular economy model redesigns this flow. Rather than ending at waste, materials are kept in use as long as possible through a hierarchy of strategies:

• Refuse/Reduce: avoid creating the product or waste in the first place
• Reuse: use the product again for its original purpose (refillable bottles, second-hand clothing)
• Repair: fix broken products rather than replace them (right-to-repair legislation)
• Remanufacture: rebuild products to original specification using reclaimed components (automotive, electronics)
• Recycle: recover raw materials from end-of-life products (last resort, loses product value)

Australia has introduced several circular economy initiatives including a ban on single-use plastics, the National Recycling Framework, and producer responsibility schemes for e-waste and batteries. The European Union's Circular Economy Action Plan is the most comprehensive global example.

Can economic growth and environmental sustainability coexist? This is one of the most contested questions in environmental policy.

Green growth proponents argue yes: economic growth can continue if it is decoupled from material use and emissions. Renewable energy replaces fossil fuels; clean technology replaces dirty technology; GDP can keep rising while environmental impact falls. Evidence: many wealthy nations have reduced territorial CO₂ emissions while growing economically.

Degrowth proponents argue no: while efficiency improvements help, absolute decoupling of GDP from material and energy throughput has not been achieved at sufficient scale. The rebound effect (efficiency gains lead to more consumption, not less) is empirically well-documented. They argue GDP is a flawed measure of wellbeing, and that rich countries should reduce material consumption so global resource budgets can be shared more equitably.

The Stern Review (2006) provided a key economic argument for action: doing nothing about climate change would cost 5–20% of GDP permanently, while acting now costs approximately 1% of GDP per year. This framing, climate action as investment, not sacrifice became influential in policy debates. However, critics note that the actual policy costs depend heavily on the transition speed and technology availability.

The concept of the personal carbon footprint was popularised by BP in a 2004 advertising campaign. By shifting focus to individual choices, BP successfully redirected public attention from corporate responsibility (extracting and selling fossil fuels) to consumer behaviour (driving and flying). This rhetorical move, blaming individuals for systemic problems caused by corporate decisions, is a well-documented public relations strategy.

Does this mean individual action is pointless? The answer is more nuanced:

• Direct impact is limited individually but significant at scale: if 10% of Australians shifted to a plant-rich diet, the emissions impact would be comparable to removing millions of cars.
• Individual choices signal markets: consumer demand shifts investment. The rapid fall in electric vehicle prices was driven partly by growing consumer demand, which triggered investment in manufacturing scale.
• Social norms matter: visible individual action can shift what is considered normal, creating conditions for political and corporate change.
• However, systemic change is irreplaceable: no amount of individual recycling can offset the emissions from a power grid running on coal. Policy (carbon pricing, renewable standards, building codes) can change behaviour at scale in ways that individual choice cannot.

Intergenerational equity is the underlying ethical principle: decisions made today about resource use and emissions constrain the options available to future generations. A fossil-fuel-dependent civilisation that depletes the atmospheric carbon budget leaves future generations a more difficult world to inhabit.

Card 5 ranked recycling as the last resort in the circular hierarchy. To judge that ranking you need to know what actually happens after the kerbside truck leaves, because every stage of recycling is a physical or chemical separation that costs energy and loses a little material.

Stage 1, sorting at a Materials Recovery Facility. A mixed load is separated using differences in physical properties, not by hand:

• Screens and trommels sort by size and shape, so flat cardboard rides over the top while round containers roll and fall through.
• Overhead magnets lift out steel cans.
• Eddy-current separators remove aluminium, which is not magnetic. A rapidly spinning magnetic field induces a current in the conductive aluminium; that current generates its own opposing magnetic field, and the can is flung off the end of the belt into its own bin.
• Near-infrared optical sorters shine infrared light on each item. Each polymer reflects a different pattern, so a computer can tell PET from HDPE from PVC in milliseconds and fires a jet of air to kick it out of the stream.
• Glass is crushed into cullet and sorted by colour, because mixed-colour cullet can only be made into green or brown glass.

Stage 2, reprocessing, and this is where the materials stop being equal:

• Aluminium is simply remelted. Melting a can uses roughly 95% less energy than smelting new aluminium from bauxite ore, and the metal is chemically unchanged, so it can go round indefinitely with no loss of quality.
• Glass is remelted with a little raw sand, soda ash and limestone. Cullet melts at a lower temperature than the raw ingredients, so each cycle saves energy, and glass too can be recycled endlessly.
• Paper is pulped in water. Every cycle shortens the cellulose fibres, so paper survives only about 5 to 7 cycles before the fibre is too short to bind, and it is made into progressively lower grades: office paper, then cardboard, then tissue.
• Plastic is washed, shredded to flake, then melted and re-pelletised. Heat and residual contamination break some of the long polymer chains, so the recycled pellet is weaker and often discoloured. Most of it becomes a lower-grade product such as fibre or pipe rather than a new bottle.

Paper and plastic are therefore downcycled: the material is recovered but the quality is not. Aluminium and glass are genuinely recycled. That difference, not enthusiasm, is what decides where a material sits in the hierarchy.

Mechanical recycling has a ceiling: it recovers the material but degrades it. Scientists working on that ceiling have taken three different routes, and each one attacks a specific failure of the mechanical process.

Enzymatic recycling attacks the quality loss. In 2016, Japanese researchers sampling sediment outside a bottle-recycling plant in Sakai found a bacterium, Ideonella sakaiensis, that was living on PET. It makes an enzyme, PETase, that cuts the polymer back into its two building blocks, terephthalic acid and ethylene glycol. Those monomers can be purified and re-polymerised into PET that is indistinguishable from new. A French team later engineered a plant-compost enzyme, LCC, to do the same job far faster and at industrial temperatures, depolymerising about 90% of a batch of PET in around 10 hours. This is a depolymerisation reaction, chemically the reverse of the reaction that built the plastic, which is why the product does not lose quality.

Chemical recycling attacks contamination. Pyrolysis heats mixed plastic waste to several hundred degrees with no oxygen present, so it cannot burn. Instead the long polymer chains crack into short ones, producing a synthetic oil that a refinery can use as feedstock. Its advantage is that it accepts the mixed, dirty, multi-layer plastic that a Materials Recovery Facility rejects. Its limitation is energy: heating waste that hard consumes much of the benefit, and yields of usable product are still modest.

Urban mining attacks scarcity. A tonne of gold ore typically holds a few grams of gold. A tonne of discarded mobile-phone circuit boards holds hundreds of grams, along with copper, silver, palladium and rare earths. Recovery uses hydrometallurgy, dissolving the metals into solution and then precipitating each one back out, or bio-leaching, in which bacteria oxidise metal compounds into soluble forms at ordinary temperatures. Electronic waste is one of Australia's fastest-growing waste streams, so the ore is arriving whether or not we mine it.

None of the three yet runs at the scale of mechanical recycling, and each is limited by energy and cost rather than by chemistry. That is the honest evaluation the syllabus asks for: they are promising, they are not a reason to keep making single-use products.