D4.3 Climate change. Practice questions with markscheme.

• B: in I and II warming speeds decomposition, releasing greenhouse gases that cause further warming, so the change reinforces itself; III removes carbon dioxide from the air and so opposes the warming that caused it: a negative feedback;

• D: carbon fixed before the previous fire is legacy carbon: organic matter in the soil that escaped earlier fires; burning it releases more carbon than the forest has fixed since the last fire, so across the fire cycle the forest changes from net accumulation to net loss of carbon, a tipping point made more likely as warmer, drier conditions make fires more frequent and intense; A the old carbon came from the soil, not only from living trees; B old carbon surviving in the soil shows it had been accumulating, not being lost; C regrowth within one fire interval cannot recover carbon that accumulated over several;

• benefit: growing trees remove CO₂ from the air by photosynthesis and store carbon in biomass, and can also restore habitat/biodiversity;
• limitation: the carbon is stored only temporarily. It is released again if the forest burns, is felled, or dies from drought or pests;
• limitation: the land area and time needed to offset current emissions are very large, and new forest competes with land for food;
• judgement: sequestration can help but cannot substitute for cutting emissions at source, so it is best used alongside emission reductions, not instead of them, OWTTE;

• record the lowest and highest altitudes at which the plant grows, using transects / quadrats at set intervals of altitude from below the band to the summit;
• repeat the survey at the same (permanently marked) sites after several years / decades, or compare with historical surveys / herbarium records;
• an upward shift of the lower limit / of the mean altitude of the plant is evidence of an upslope range shift;
• record temperature at the sites (or use weather-station data) over the same period, to show that the shift is correlated with warming;
• repeat on several mountains / for several species, so that the shift is not just a local effect;
• record or rule out other factors that could cause the change, e.g. grazing / land use / trampling, since a correlation alone does not show causation, OWTTE;

Accept 'elevation' for altitude. Full marks require repeated surveys over time.

• I, tentacle;
• II, zooxanthella / (photosynthetic / mutualistic) alga living inside a coral cell;
• III, (calcium carbonate / limestone) skeleton;

Do not accept chloroplast or plant cell for II. Do not accept rock or shell alone for III.

• (a) [1]
  • as atmospheric CO₂ rises, the global temperature anomaly rises (positive correlation);
• (b) [3]
  • a correlation does not by itself establish causation: the two could both be driven by a third factor, or the association could be coincidental;
  • however, there is a well-understood physical mechanism (greenhouse gases absorb and re-radiate long-wave radiation from Earth), which provides a causal explanation and strengthens the case;
  • confidence comes from combining the correlation with the mechanism and with other evidence (e.g. laboratory measurements, models), rather than from the correlation alone, OWTTE;
• (c) [3]
  • strengths: they are based on established physics, can reproduce past/observed climate, and independent models broadly agree;
  • limitations: feedbacks and tipping points are uncertain, future emissions are unknown, and natural variability plus simplifications add uncertainty;
  • (so) projections are given as a range rather than a single value, and are reliable for broad trends while individual figures carry uncertainty, OWTTE;

• A: warming advances the temperature-cued event only; the photoperiod cue is unchanged by climate, so the two events fall out of synchrony and the reindeer miss the peak of plant growth;

• A: the plantation stores more carbon at 10 and 30 years (40 and 110 t ha⁻¹ against 15 and 70 t ha⁻¹), but it has almost stopped gaining carbon by 60 years (115 t ha⁻¹), whereas the native forest keeps accumulating and stores more (150 t ha⁻¹); so a judgement between plantations and rewilding for carbon sequestration depends on the time scale, which is part of the scientific debate; A is false at 60 years; B the plantation gained 110 t ha⁻¹ in 30 years against 70 t ha⁻¹; D both still gained carbon between 30 and 60 years;

• C: short-wave solar radiation passes through; the surface re-emits long-wave radiation, which greenhouse gases absorb and re-emit, warming the lower atmosphere;

• B: without upwelling, nutrients stay in deep water, so primary production by phytoplankton stays low through May and June (about 1 to 2 mg m⁻³ against 7.5 to 8 mg m⁻³); less energy is fixed at the base of the food chain, so less reaches zooplankton and fish while the chicks are being fed, and the later bloom comes after chick-rearing; A a later peak does not feed chicks in May and June; C the fish depend on energy fixed by phytoplankton; D the effect is on food supply, and the upwelling was late, not colder;

• D: carbon fixed before the previous fire is legacy carbon: organic matter in the soil that escaped earlier fires; burning it releases more carbon than the forest has fixed since the last fire, so across the fire cycle the forest changes from net accumulation to net loss of carbon, a tipping point made more likely as warmer, drier conditions make fires more frequent and intense; A the old carbon came from the soil, not only from living trees; B old carbon surviving in the soil shows it had been accumulating, not being lost; C regrowth within one fire interval cannot recover carbon that accumulated over several;

• the oceans absorb (a large fraction of) the extra atmospheric CO₂;
• dissolved CO₂ reacts with water to form carbonic acid (H₂CO₃ → H⁺ + HCO₃⁻);
• (so) ocean pH falls / H⁺ rises, and carbonate availability for shell-builders decreases, OWTTE;

• (a) [1]
  • 54 m (accept 53.75 m);
• (b) [1]
  • all four species shifted upslope / to higher altitude, by 60–75 m for three species but only 10 m for moss campion;
• (c) [2 max]
  • temperature falls with altitude (by roughly 0.6 °C per 100 m), so each species lives in a band of altitude with suitable temperatures;
  • warming shifts the band of suitable temperature to higher altitude;
  • seeds germinate and plants survive at higher sites than before, while plants at the lower edge of the range die out / are outcompeted, OWTTE;
• (d) [2 max]
  • it already grows close to the summit (2400 m of 2450 m), so there is little or no higher ground to move into;
  • as lower-altitude species move up they will compete with it / its suitable habitat shrinks;
  • (so) the species is at risk of local extinction on this mountain, OWTTE;
• (e) [1]
  • changes in grazing / land use / precipitation / snow cover (accept any reasonable factor);

• burning of fossil fuels (and cement production) releases CO₂;
• deforestation reduces CO₂ uptake (and releases carbon when burned);
• agriculture/livestock/waste release methane (and nitrous oxide);
• (so) atmospheric greenhouse-gas concentrations have risen (far) above pre-industrial levels;
• greenhouse gases absorb long-wave radiation emitted by Earth and re-radiate it, retaining heat;
• the enhanced greenhouse effect raises global mean temperature;
• positive feedback: melting ice lowers albedo, so more radiation is absorbed → more warming;
• positive feedback: thawing permafrost releases CH₄/CO₂ → more warming;
• warmer oceans absorb less CO₂ (and hold more water vapour, itself a greenhouse gas), OWTTE;