SACE Stage 2 Earth and Environmental Science cheat sheet

Four spheres: geosphere (rock, mantle, core), atmosphere (gas envelope), hydrosphere (all liquid water plus ice), biosphere (living organisms and organic matter); each sphere exchanges matter and energy with the others.

Open vs closed system: Earth as a whole is essentially a closed system for matter but an open system for energy; a single reservoir (e.g. a lake) is usually open for both.

Energy balance: incoming solar radiation equals reflected shortwave (albedo × incoming) plus emitted longwave; a sustained imbalance changes global mean temperature.

Albedo values: fresh snow ~0.8-0.9, ocean ~0.06-0.1, forest ~0.1-0.15, desert sand ~0.3-0.4; ice-albedo feedback is positive because melting lowers albedo and increases absorption.

Carbon cycle fluxes: photosynthesis removes CO2, respiration and decomposition return it, ocean uptake and carbonate burial are long-term sinks, and combustion of fossil carbon is a fast source.

Rock cycle pathways: melting produces magma, crystallisation produces igneous rock, weathering and erosion produce sediment, compaction and cementation produce sedimentary rock, heat and pressure produce metamorphic rock.

Plate boundaries: divergent (new crust, mid-ocean ridges), convergent (subduction or collision, volcanoes and earthquakes), transform (lateral sliding, earthquakes); earthquakes mark brittle failure where stress exceeds rock strength.

Feedback decision rule: positive feedback amplifies the initial change (permafrost thaw releasing methane), negative feedback dampens it (increased plant growth absorbing CO2); name the direction of change before labelling.

Residence time = reservoir size ÷ flux in or out (same units); long residence times (deep ocean carbon) indicate slow response, short ones (atmospheric water vapour, days) indicate fast response.

Resource classification: a reserve is the portion of a resource that is identified, economically extractable with current technology; a resource includes undiscovered and currently uneconomic material.

Renewable resources are replenished at a rate comparable to use (solar, wind, sustainably managed biomass); non-renewable resources (coal, oil, gas, most metal ores) form over geological timescales.

Fossil fuel formation: buried organic matter is heated and compressed over millions of years; coal forms from terrestrial plant material, oil and gas from marine plankton in source rocks, and hydrocarbons migrate into porous reservoir rocks sealed by impermeable cap rock.

Energy density comparison: uranium fuel far exceeds coal, oil and gas per unit mass; lower energy density means more fuel mass and more waste per unit of energy delivered.

Ore grade is the concentration of the target element in rock; cut-off grade is the minimum grade that is economic to mine, and it rises with energy, labour and transport costs.

Environmental impacts of mining: acid mine drainage from sulfide minerals, tailings storage failure, dust, habitat removal, and groundwater drawdown; rehabilitation aims to restore stable landforms and vegetation.

Groundwater: an aquifer is permeable rock storing and transmitting water; a confining layer restricts flow; recharge rate sets the sustainable extraction limit, and over-extraction causes water-table decline and land subsidence.

Soil as a resource: fertility depends on texture, organic matter, pH and nutrient content; salinisation from irrigation and erosion from clearing are the main degradation pathways.

Sustainability test for a resource use: extraction rate ≤ regeneration rate (renewables) or ≤ rate of substitution plus recycling (non-renewables), with impacts kept within assimilative capacity.

Greenhouse mechanism: atmospheric gases absorb and re-emit outgoing longwave radiation; the most important are water vapour, CO2, methane, nitrous oxide and ozone, in order of overall contribution.

Radiative forcing is the change in net energy flux at the top of the atmosphere in W/m², positive for warming; CO2 forcing scales approximately with the logarithm of concentration.

Keeling curve evidence: continuous CO2 measurements since 1958 show a steady rise plus an annual sawtooth from Northern Hemisphere photosynthesis; ice cores extend the record back about 800,000 years.

Distinguish weather from climate: weather is short-term atmospheric state, climate is the statistical distribution of conditions over decades (conventionally 30 years).

Proxy evidence for past climate: ice-core isotopes and trapped air bubbles, tree rings, corals, speleothems, pollen, ocean sediment isotopes, and historical records.

Attribution reasoning: natural forcings (solar variability, volcanic aerosols) cannot explain the simultaneous tropospheric warming and stratospheric cooling, whereas greenhouse forcing does.

Ocean response: the ocean absorbs over 90% of excess heat, causing thermal expansion and sea-level rise; ocean acidification follows from dissolved CO2 forming carbonic acid.

Projection versus prediction: projections are conditional on stated emission scenarios and are not forecasts of a single outcome; uncertainty ranges widen with time and scenario spread.

Mitigation reduces forcing (cut emissions, enhance sinks); adaptation reduces vulnerability (sea walls, drought-resistant crops, early-warning systems); a response can be both.

Variable control: identify independent (manipulated), dependent (measured) and controlled variables; a fair test changes only the independent variable.

Validity is whether the method measures the intended quantity; reliability is whether repeated measurements give consistent results; accuracy is closeness to the true value.

Precision versus accuracy: random errors scatter results about the mean and reduce precision; systematic errors shift all results the same way and reduce accuracy.

Uncertainty rules: for addition or subtraction add absolute uncertainties; for multiplication or division add percentage uncertainties; quote a result as value ± uncertainty with matching decimal places.

Repeat readings and calculate the mean; a mean reduces random error but not systematic error, and anomalous points are identified against the spread of repeats, not discarded silently.

Graphing: plot the independent variable on the x-axis and the dependent on the y-axis, label units, and use a line of best fit rather than joining every point; slope of a linear fit gives the rate.

Correlation does not establish causation; a causal claim needs a plausible mechanism, temporal order (cause before effect), and exclusion of confounding variables.

Evaluation of a conclusion: check whether the data range supports extrapolation, whether uncertainties overlap between groups, and whether sample size and sampling method permit generalisation.

Sampling bias: random sampling supports generalisation, while convenience or single-site sampling limits conclusions to that site and condition.