Environmental Sustainability: Unit Synthesis and Review

Answer Key (Teacher)

1. Warm-up (a): CO₂ was roughly flat at about 280 ppm before 1900, then rose steeply (an exponential or accelerating curve) to about 414 to 420 ppm by 2020. Before 1900 it changed only a few ppm over a century; after 1950 it rose from about 311 ppm to over 410 ppm, around 100 ppm in roughly 70 years, far faster than any earlier period on the graph. Award marks for: correct overall trend; quoting figures; comparing the two rates.
2. (b): Any two independent lines, e.g.: the 800,000-year ice-core record shows today's CO₂ is higher than at any point in that record; the falling ¹³C/¹²C ratio fingerprints the extra CO₂ as fossil-fuel carbon; attribution climate models only reproduce the observed warming when human emissions are included; measured ocean heat uptake, sea-level rise and ocean acidification all match a warming driven by greenhouse gases. 1.5 marks per valid, correctly explained line.
3. (c): A carbon atom locked in coal/oil/gas underground is burned and released as CO₂ into the atmosphere, where it traps heat; some dissolves into the ocean forming carbonic acid (driving acidification); it can be taken up by plants such as mangroves and stored as blue carbon, or locked back into soil/rock by good land management. Sustainability means managing these carbon, water and nutrient cycles so the systems we depend on keep working. Marks for a correct multi-stage pathway and the link to cycle management.
4. Evaluate (a): Accurate part: Earth's climate genuinely has changed naturally before (ice ages, past high-CO₂ periods) without humans (1 mark). Reasoning error: it confuses the fact that change can happen with the cause and rate of today's change, a non-sequitur; past natural change does not mean the current rapid, human-fingerprinted change is also natural or harmless (1 mark).
5. Evaluate (b): The Keeling Curve shows about 2.5 ppm per year today versus roughly 10 ppm per 1,000 years in natural glacial cycles, around 60 to 70 times faster. Such rapid change pushes Earth systems past planetary boundaries (several already crossed, including climate and biodiversity), and ecosystems and species cannot migrate or adapt fast enough, so the speed, not just the amount, of change is what threatens reefs, food systems and communities. Marks for: rate comparison; planetary boundaries; why speed harms ecosystems/people.
6. Evaluate (c): Intergenerational equity means meeting present needs without compromising future generations, so we have a duty not to pass on a degraded climate. The Stern Review estimated strong early action costs about 1% of global GDP, while inaction could cost 5 to 20%, so acting now is also far cheaper and avoids locking in irreversible impacts (e.g. coral reef loss, sea-level rise) that future generations cannot undo. A full answer states the principle, applies it to future generations, and uses the 1% vs 5 to 20% figures.