The carbon cycle and climate change questions

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 is what lowers the pH; the change is called ocean acidification, although the water stays alkaline throughout the period in the table B1 those hydrogen ions react with carbonate ions and take them out of solution, which is the fall in the third 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 thawing permafrost 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 is what 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 rather than lost 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

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 9

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 decomposers could not respire the remains and the organic material was not 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 10

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 mass of carbon added to 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 11

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 12

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 13

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 14

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 15

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