Questions on the carbon cycle and climate change

Question 1

The table gives the concentration of carbon dioxide in the air and two measurements made on surface seawater over the same period, constructed for this question rather than measured. Describe what happens to the seawater as the concentration of carbon dioxide in the air rises, and describe the effect of that change on organisms that build shells and skeletons of calcium carbonate.

Accepted Answer

B1 as the carbon dioxide in the air rises from 354 to 414 parts per million, the pH of the surface seawater falls from 8.11 to 8.05 and the concentration of carbonate ions falls with it, from 240 to 216 µmol kg⁻¹ B1 about a quarter of the carbon dioxide released dissolves in the sea, where it reacts with water to form carbonic acid B1 the carbonic acid releases hydrogen ions, which lower the pH; the change is called ocean acidification, although the water stays alkaline throughout the period in the table B1 those hydrogen ions combine with carbonate ions to form hydrogencarbonate ions, which is the fall in the carbonate column, so there is less carbonate available for building calcium carbonate B1 corals, molluscs and much of the plankton at the base of marine food chains therefore deposit their shells and skeletons more slowly and existing structures dissolve more readily, so the food chains built on those plankton are affected as well

Question 2

The northern limits of several British insect species have moved north over the past fifty years. Suggest why, and suggest why this may still lead to a fall in biodiversity overall.

Accepted Answer

B1 each species survives only within a range of temperatures, and rising mean temperature has moved the area in which those conditions occur northwards B1 individuals dispersing into the newly suitable area can now survive and reproduce there, so the recorded limit moves, while the southern limit retreats as conditions there become too warm B1 species already at the northern edge of a land mass, or at the top of a mountain, have nowhere suitable to move to and are lost B1 the species a population depends on, such as a larval food plant or the insects a bird feeds its chicks, may not shift by the same amount or at the same time, so ranges and timings no longer overlap

Question 3

Compare carbon dioxide with methane as greenhouse gases, referring to how each warms the surface, to how much of each is in the air, and to where the added gas comes from.

Accepted Answer

B1 both are transparent to the short-wavelength radiation arriving from the Sun and both absorb part of the long-wavelength radiation the warmed Earth emits, so both raise the mean surface temperature B1 methane absorbs far more strongly per molecule than carbon dioxide, but there is far less of it in the air, so carbon dioxide accounts for more of the warming overall B1 the carbon dioxide added comes chiefly from burning fossil fuels, from deforestation and from cement manufacture, whereas the methane added comes chiefly from cattle, rice paddies, landfill and leaks from oil, gas and coal extraction, with thawing permafrost a small but growing source B1 the methane from thawing permafrost is released by the warming itself, so that transfer grows as the temperature rises, whereas the size of the carbon dioxide transfer depends on how much fuel people burn

Question 4

A student says that because the carbon people add to the air each year is less than a tenth of the carbon photosynthesis removes, the human contribution cannot matter. Discuss this claim.

Accepted Answer

B1 the arithmetic is right: photosynthesis removes about 120 gigatonnes of carbon a year, while combustion and deforestation together add about 11.5, which is under a tenth of it B1 against the claim: the natural transfers are a balanced pair, because respiration and decomposition return each year about as much as photosynthesis takes, so the natural cycle leaves the amount held in the air unchanged B1 against the claim: the human transfer has no matching return, and although the oceans and land plants take up roughly half of it, about 5.5 gigatonnes stays in the air every year and the surplus accumulates year on year B1 a judgement: comparing one transfer with another is the wrong test, and the right one is whether a transfer is balanced; the accumulation has taken the concentration from about 280 parts per million before industrialisation to over 420 today

Question 5

A woodland bird times its egg-laying by day length, but its chicks hatch just after the peak abundance of the caterpillars it feeds them. Suggest why this mismatch has developed as the climate has warmed, and suggest one consequence for the bird population.

Accepted Answer

B1 caterpillar emergence is triggered by temperature, so warming has brought the caterpillar peak forward in the year B1 the bird's laying date is cued by day length, which does not change as the climate warms, so when its chicks hatch, and need most food, has not shifted by the same amount B1 the two events, once closely matched, have drifted apart, so chicks now hatch after rather than at the peak of the food supply B1 chicks receive less food during their critical growth period, so fewer survive to fledge, which can reduce the size of the bird population over time

Question 6

Climate change requires many species to shift the area they occupy, but habitat fragmentation can prevent a population from reaching a newly suitable area. Suggest how connecting fragments of habitat with a wildlife corridor could reduce the resulting loss of biodiversity, and suggest one situation in which a corridor would not solve the problem.

Accepted Answer

B1 a corridor of suitable habitat linking fragments gives individuals a route by which they can disperse from an area becoming unsuitable into one that has newly become suitable, rather than being blocked by farmland, a city or a motorway B1 a species able to reach the newly suitable area can survive and breed there, so its population, and the local biodiversity, is maintained as its old range becomes unsuitable B1 a corridor does not help a species already at the top of a mountain or the edge of a landmass, since there is no further suitable area beyond it for a corridor to lead to B1 nor does it help a species that must shift faster than it can disperse even along a clear route, since a corridor removes the barrier to movement without increasing the rate at which the species itself can move

Question 7

Explain why the rise in atmospheric carbon dioxide cannot be explained by population growth on its own, referring to what else the environmental pressure a population creates depends on.

Accepted Answer

B1 population size is one factor behind the demand for energy, food and land, but it is not the whole account of that demand B1 consumption per person also matters, since energy use per person has risen over the same period that population has risen, so total demand has grown faster than population alone would predict B1 the technology used changes how much carbon dioxide a given amount of consumption releases, for example whether energy is generated by burning fossil fuels or by a low-carbon method B1 land-use choices, such as clearing forest for farming, add a further transfer of carbon to the atmosphere that is independent of how many people there are or how much each one consumes

Question 8

Describe how pollen preserved in a peat bog is used as evidence of past climate.

Accepted Answer

B1 peat is waterlogged, acidic and anaerobic, so decomposition is slow and plant material and pollen build up in layers, the deepest oldest B1 the sporopollenin wall of the pollen grain resists decay, and each plant's pollen has a distinctive shape, so the grains in each layer can be identified and counted B1 the layers of a core are dated, for example by radiocarbon dating of the peat B1 each plant species grows only within a range of temperature and rainfall, so a change in the proportions of pollen types up the core shows a change in vegetation, from which the climate at that time is inferred

Question 9

Explain how dendrochronology provides evidence about past climate, and explain one limitation of the evidence.

Accepted Answer

B1 a tree in a seasonal climate adds one ring of xylem a year, so counting rings in from the bark dates each ring to its year B1 the width of a ring depends on the growing conditions that year, such as temperature and rainfall, so wide rings indicate good years and narrow rings poor ones B1 matching the pattern of wide and narrow rings between living trees, old timbers and preserved wood extends the dated record back thousands of years B1 limitation: ring width responds to several factors together, such as temperature, rainfall, pests and shading, so a narrow ring cannot be attributed to one cause; or it records only the growing season; or tropical trees often lack annual rings

Question 10

Antarctic ice cores show that atmospheric carbon dioxide and temperature rose and fell together over the last 800 000 years. A student concludes that this proves rising carbon dioxide causes rising temperature. Evaluate this conclusion.

Accepted Answer

B1 the ice-core data show a strong correlation between carbon dioxide and temperature, which is consistent with carbon dioxide causing warming B1 a correlation alone cannot show which variable is the cause, or rule out a third factor affecting both B1 at the end of ice ages warming started by changes in the Earth's orbit released carbon dioxide from warming oceans, so temperature also drove carbon dioxide and the causation ran both ways B1 the causal claim is supported by other evidence: a mechanism, since carbon dioxide absorbs infrared radiation; models that reproduce recent warming only when the extra carbon dioxide is included; so the data support but do not on their own prove the conclusion

Question 11

Explain why projections of future global temperature made by climate models are given as a range of values for each of several emissions scenarios, rather than as a single figure.

Accepted Answer

B1 future emissions depend on decisions about energy use, population and policy that have not been made, so the models are run on several scenarios, each giving a different projection B1 processes smaller than a grid cell, especially clouds, are approximated, and different models approximate them differently, so they give different warming for the same scenario B1 feedbacks such as melting ice reducing reflection of sunlight, or thawing permafrost releasing methane, are not known precisely B1 natural variability such as volcanic eruptions cannot be forecast, so many models are run and the spread of their results is reported as a range

Question 12

The table gives the IPCC's projected warming for 2081 to 2100, relative to 1850 to 1900, for two emissions scenarios. Calculate the width of the very likely range for each scenario, and suggest why the range is wider for the very high scenario.

Accepted Answer

M1 width = upper value − lower value A1 very low: 1.8 − 1.0 = 0.8 °C; very high: 5.7 − 3.3 = 2.4 °C, three times as wide B1 more warming gives the uncertain feedbacks, such as clouds, melting ice and permafrost, more to act on, so the models diverge further B1 a high scenario takes the climate further beyond conditions the models have been tested against, so the extrapolation is less certain

Question 13

Explain how scientific journals, peer review and scientific conferences help to validate new evidence about climate change.

Accepted Answer

B1 publishing in a journal gives the methods, data and conclusions in enough detail for other scientists to check and repeat the work B1 peer review means experts in the field check the methods, analysis and conclusions before publication, so work with obvious flaws is revised or rejected B1 at conferences results are presented, often before publication, and questioned by other scientists, and groups using different data or methods share findings B1 evidence is accepted when independent groups repeat the work and get the same result, as when separate groups compiling the global temperature record find the same warming

Question 14

Name the process that transfers carbon from the atmosphere into living organisms, the process that returns it from living organisms to the atmosphere, and the process that returns carbon locked in dead organisms to the atmosphere.

Accepted Answer

B1 photosynthesis, carried out by producers B1 respiration, carried out by all living organisms B1 decomposition, which is respiration carried out by saprobiotic bacteria and fungi on dead material and faeces

Question 15

Explain how coal was formed, and explain why the carbon in it had not been returned to the atmosphere before people began to mine it.

Accepted Answer

B1 it formed from the remains of plants, chiefly the trees of swamp forests, that were buried under sediment B1 the conditions were anaerobic, so decomposition was slowed and the organic material accumulated faster than it was broken down B1 over millions of years heat and pressure converted it to coal, so the carbon that photosynthesis had removed from the air stayed out of the cycle

Question 16

Photosynthesis removes about 120 gigatonnes of carbon from the atmosphere each year and respiration and decomposition return about 120 gigatonnes. Burning fossil fuels adds 10 gigatonnes and deforestation adds 1.5 gigatonnes, while oceans and land plants absorb 6.0 gigatonnes of the addition. Calculate the net increase in the mass of carbon in the atmosphere each year, and calculate this as a percentage of the carbon removed by photosynthesis.

Accepted Answer

M1 total human addition = 10 + 1.5 = 11.5 gigatonnes A1 net gain = 11.5 − 6.0 = 5.5 gigatonnes of carbon a year A1 5.5 ÷ 120 × 100 = 4.6 per cent, to two significant figures

Question 17

The concentration of carbon dioxide in the atmosphere falls slightly during the northern hemisphere summer and rises again during the northern winter. Explain this pattern.

Accepted Answer

B1 most of the world's land, and therefore most of its vegetation, lies in the northern hemisphere B1 in the northern summer the rate of photosynthesis across that vegetation exceeds the rate of respiration and decomposition, so more carbon dioxide is removed from the air than is returned to it B1 in the northern winter many plants have lost their leaves or are dormant, so photosynthesis falls below respiration and decomposition and the concentration rises again

Question 18

Explain why the ocean both takes up carbon dioxide from the air and releases it back, and explain why a warmer ocean is expected to hold less dissolved carbon dioxide.

Accepted Answer

B1 carbon dioxide dissolves into the ocean from the air and comes back out of the ocean into the air, so the ocean is both a carbon store and the site of a two-way transfer with the atmosphere B1 the rate of each direction depends on temperature, and cold water is able to hold more dissolved gas than warm water B1 as the ocean warms, its capacity to hold dissolved carbon dioxide falls, so it is expected to absorb a smaller proportion of what is emitted over time

Question 19

Explain what happens to a coral reef when sea temperature rises above a threshold, and explain why the loss of a reef reduces biodiversity beyond the corals themselves.

Accepted Answer

B1 above a threshold temperature, corals expel the photosynthetic protoctists living in their tissues, which is called bleaching, and repeated bleaching kills the coral B1 a coral reef is the habitat for a very large number of other species, so losing the reef removes their habitat as well as removing the coral that built it B1 the loss of the species that depended on the reef habitat is therefore a loss of both habitat diversity and species diversity, beyond the direct effect on the corals

Question 20

A region is projected to receive less rain in spring and summer. Suggest how this could change the distribution of plant species in the region.

Accepted Answer

B1 species whose seedlings need moist soil to establish would fail to regenerate, so their range would contract to the wetter parts of the region B1 drought-tolerant species with xerophytic features, such as deep roots, thick waxy cuticles or few sunken stomata, would survive and could spread into the space B1 competition between species would change, and animals that depend on the plants that are lost would also decline or move, so the community changes as well as single ranges

Question 21

In a semi-arid grassland, the seasonal rains have started later and become less reliable. Explain how this could affect the life cycles of the plants and of a grazing animal that migrates with the rains.

Accepted Answer

B1 seeds need water to germinate, because absorbed water activates the enzymes that mobilise the food store, so germination and growth are delayed or fail B1 plants short of water close their stomata, taking up less carbon dioxide, so photosynthesis, growth, flowering and seed production are reduced B1 the grazers' migration and breeding are timed to the new growth that follows the rains, so if the rains are late or fail, young are born when food is scarce and survival falls

Question 22

Describe the peer review process for a scientific paper, and describe one of its limitations.

Accepted Answer

B1 before publication the journal's editor sends the paper to independent scientists working in the same field, usually anonymously B1 they check that the methods are valid, the analysis sound and the conclusions supported by the data, and recommend publication, revision or rejection B1 limitation: reviewers do not repeat the work, so errors or fraud can be missed; or reviewers may be biased towards accepted ideas, so peer review does not prove a paper correct

Question 23

In an investigation of temperature and the development of brine shrimps, the mean time for half the eggs to hatch was 50 hours at 18 °C and 24 hours at 28 °C; the results are invented for this question. Calculate the temperature coefficient, Q10, for hatching between 18 °C and 28 °C.

Accepted Answer

M1 rate = 1 ÷ time: 1 ÷ 50 = 0.020 h⁻¹ at 18 °C and 1 ÷ 24 = 0.042 h⁻¹ at 28 °C M1 Q10 = rate at 28 °C ÷ rate at 18 °C, or 50 ÷ 24 A1 Q10 = 2.1

Question 24

In the same investigation, fewer than half of the brine shrimp eggs kept at 36 °C had hatched after four days, although hatching had been fastest at 30 °C. Explain this result.

Accepted Answer

B1 development depends on enzyme-controlled reactions, and 36 °C is above the optimum temperature for them B1 hydrogen bonds and other bonds holding the tertiary structure break, so the active sites change shape and the enzymes are denatured B1 fewer enzyme-substrate complexes form, so development slows or stops and many embryos die before they hatch

Question 25

State the difference between a carbon store and a carbon transfer, giving an example of each.

Accepted Answer

B1 a store is a place where carbon is held, such as the atmosphere, living organisms, soil, the ocean, sedimentary rock or fossil fuels B1 a transfer is a process that moves carbon from one store to another, such as photosynthesis, respiration, decomposition or combustion

Question 26

The following are all part of the carbon cycle: the atmosphere, photosynthesis, sedimentary rock, respiration, fossil fuels, decomposition. Identify which three of these are stores and which three are transfers.

Accepted Answer

B1 the atmosphere, sedimentary rock and fossil fuels are stores, each a place where carbon is held B1 photosynthesis, respiration and decomposition are transfers, each a process that moves carbon from one store to another

Year	Carbon dioxide in the air / parts per million	pH of surface seawater	Carbonate ions / µmol kg⁻¹
1990	354	8.11	240
2000	369	8.09	232
2010	389	8.07	224
2020	414	8.05	216

Scenario	Best estimate / °C	Very likely range / °C
Very low emissions (SSP1-1.9)	1.4	1.0 to 1.8
Very high emissions (SSP5-8.5)	4.4	3.3 to 5.7