Biology › Ecology, populations and environmental change › The carbon cycle and climate change
The carbon cycle and climate change
The carbon cycle moves carbon between the atmosphere, oceans, living organisms, soils, rocks and fossil fuels. Before industrialisation, the large natural flows were roughly in balance over the long term, although the stores varied. Burning fossil fuels, making cement and changing land use now add more carbon to the atmosphere than is removed. Land and oceans absorb part of the extra, and the rest raises the concentration of carbon dioxide in the air.
Key terms Every definition in ecology, populations and environmental change, lesson by lesson
Before this Nutrient cycles: nitrogen, phosphorus and eutrophication · The Calvin cycle · Glycolysis, ATP and the link reaction
COMMON MISCONCEPTION
The carbon dioxide people add to the air is a large amount compared with the carbon dioxide living things move around.
Natural carbon fluxes are large but were close to long-term balance before industrialisation. Fossil-fuel use and land-use change add a smaller one-way net flux; land and ocean sinks absorb part of it and the remainder accumulates in the atmosphere. Attach a year and source to the figures used.
What you should be able to do
- Assemble the carbon cycle from the processes that move carbon between the air, living organisms, the soil and the sea.
- Explain how fossil fuels were formed, and why burning them transfers geological carbon to the atmosphere far faster than natural processes do.
- Describe the evidence that atmospheric carbon dioxide is rising, and explain the shape of the record.
- Explain how deforestation, consumption per person and land-use choices contribute to the rise, alongside population size.
- Describe the effects of a changing climate on the distribution and abundance of species, and hence on biodiversity.
- Describe the evidence for climate change from temperature records, carbon dioxide records, pollen in peat bogs and dendrochronology, and separate correlation from cause in it.Edexcel A
- Explain how climate models are built and tested, and why their projections have limitations and are given as ranges.Edexcel A
- Describe how changing rainfall patterns and seasonal cycles affect the distribution, development and life cycles of plants and animals.Edexcel A
- Describe the roles of scientific journals, peer review and conferences in validating evidence about climate change.Edexcel A · Edexcel B
- Investigate the effect of temperature on the development of an organism, and calculate a development rate and Q10 from the results.Edexcel A
Stores, transfers, and a cycle in approximate balance
Before industrialisation, the global carbon cycle was in approximate long-term dynamic balance, although stores and atmospheric carbon dioxide varied.
Carbon is the element every organic molecule is built on, and there is a fixed amount of it. It is not made or destroyed by anything biology does; it is moved between places where it is held. Those places are called stores, or reservoirs, and the processes that move carbon between them are transfers, or fluxes. Keep the two terms distinct: a store is a place where carbon is held, and a transfer is a process that moves it.
The stores are the atmosphere, where carbon is carbon dioxide gas, and living organisms, where it is carbohydrate, lipid and protein. Dead organic matter in soil is another. So are the oceans, where it is dissolved carbon dioxide, hydrogencarbonate ions and the tissues of marine organisms. Sedimentary rock, chiefly limestone, is much the largest store of all. The last store is fossil fuels. The transfers are the processes you already know, viewed from the outside.
Photosynthesis takes carbon dioxide out of the air and fixes it into organic molecules. It is the only sizeable route from the atmosphere into living things. Feeding moves that carbon along a food chain, which is a transfer between organisms, not into or out of the air. Respiration returns carbon dioxide to the air from every living thing, producers and consumers alike. Decomposition is respiration too, carried out by bacteria and fungi on dead organisms and on faeces. It is the route by which carbon locked into a body gets back into circulation instead of accumulating. The sea both dissolves carbon dioxide from the air and releases it back, at rates that depend on temperature: cold water holds more dissolved gas than warm water.
Describe the drawing
A list of nine carbon transfers, one to a line. Each line names the process on the left, then shows a labelled chip for the store the carbon leaves, an arrow, and a chip for the store it enters, with the yearly quantity in gigatonnes on the right. The six natural transfers are drawn in cyan and the three human ones in coral, and two summary lines at the foot show the natural exchanges balancing while the human ones leave a net gain in the air.
Two of the transfers are slow enough to be worth separating out. Marine organisms build shells and skeletons of calcium carbonate. When they die those settle, compact and become limestone, which takes carbon out of circulation for many millions of years. Weathering of that rock and volcanic activity slowly return it.
Some dead organic matter fell into conditions with little or no oxygen. There, decomposition was slowed and altered but not stopped, so organic matter accumulated faster than it was broken down. It was buried, compressed and heated into fossil fuels. Coal formed mostly from the trees of swamp forests some three hundred million years ago; oil and natural gas formed mostly from marine plankton. Every one of them is carbon that photosynthesis removed from the air and that was buried before decomposition returned it.
- Carbon store
- A place where carbon is held, such as the atmosphere, living organisms, soil, the oceans, sedimentary rock or fossil fuels.
- Carbon transfer
- A process that moves carbon from one store to another, such as photosynthesis, respiration, decomposition or combustion.
- Fossil fuel
- Coal, oil or natural gas, formed over millions of years from the remains of organisms that were buried in low-oxygen conditions before decomposers could break them down fully.
The net transfer added by human activity
The natural transfers above continue. In addition, human activity has greatly accelerated the transfer of geological carbon to the atmosphere. Combustion of fossil fuels returns, in a couple of centuries, carbon that took hundreds of millions of years to be buried. It is being returned at about ten thousand million tonnes of carbon a year. Deforestation adds to it twice over. Burning or rotting the felled trees releases the carbon they held, and the forest that would have gone on fixing carbon is no longer there to do it. Cement manufacture, which drives carbon dioxide out of limestone, adds a further few per cent.
The relative sizes matter. Photosynthesis and respiration move of the order of a hundred and twenty gigatonnes of carbon a year in each direction. The human addition is of the order of eleven, under a tenth of the natural gross flux. Those are recent published estimates and should be quoted with a year and a source, since both are revised. The natural fluxes approximately cancel over the long term. The human addition does not, so the difference accumulates year on year. Land plants and the oceans absorb roughly half of the addition, so about half remains in the atmosphere each year and the concentration rises.
Describe the drawing
A graph of carbon dioxide concentration in parts per million against year from nineteen sixty to twenty twenty five. A coral line climbs from a little above three hundred and sixteen to beyond four hundred and twenty, getting steeper as it goes, and carries a small regular saw-toothed wobble of a few parts per million within each year.
Population size is one of several contributing factors. The human population was about one thousand million in 1800 and two thousand million by 1930, and passed eight thousand million in 2022. Energy use per person rose over the same period as well. Population growth has slowed, since the global growth rate peaked in the 1960s and family sizes have fallen almost everywhere. But the population is still rising, and the total demand for energy, food and land rises with it.
Population is not the whole account, though. Environmental pressure depends on population, consumption per person, technology and land-use choices. All four bear on the pressures treated in this unit and in the biodiversity lesson: land cleared for farming, forests felled for timber, fisheries taken past what they can replace, and now the composition of the atmosphere.
The figures below are approximate annual averages for the 2010s. Published global carbon-budget estimates are revised every year, so quote the year and the source with any flux you use.
| Transfer | Direction | Roughly, in gigatonnes a year |
|---|---|---|
| Photosynthesis | Air into living organisms | 120 |
| Respiration by producers | Living organisms into air | 60 |
| Respiration and decay by consumers and decomposers | Living organisms and soil into air | 60 |
| Exchange with the oceans | Both ways, and roughly equal | 90 |
| Burning fossil fuels | Fossil fuels into air | About 10 |
| Deforestation and land clearance | Living organisms into air | About 1.5 |
| Taken up by oceans and land plants | Air into stores | About 6 |
Evidence that the climate is changing
The Edexcel A specification names four kinds of evidence for climate change: records of carbon dioxide levels, temperature records, pollen in peat bogs and dendrochronology. The first two are direct measurements. The other two are proxy records, which preserve something living that depended on the climate and reach back thousands of years.
Temperature records. Thermometer readings from weather stations and ships have been compiled into a global record from 1850, and the Central England Temperature series runs from 1659. Several groups compile the global record independently and their results agree closely. The Intergovernmental Panel on Climate Change (IPCC) assessed the global mean surface temperature for 2011 to 2020 as about 1.1 °C above the 1850 to 1900 average. The early decades have few stations, mostly on northern-hemisphere land, and a station's readings shift when its instruments or surroundings change, for example as a town grows round it; compilers correct for these and publish the corrections.
Carbon dioxide records. The direct record from Mauna Loa begins in 1958. Older air comes from ice cores: as snow is compacted into ice it seals in bubbles of the air of its time, and the layers can be dated. Antarctic cores reach back about 800 000 years, over which carbon dioxide stayed between about 180 and 300 parts per million. The present value, over 420, is outside that range.
Pollen in peat bogs. Peat forms where the ground is waterlogged, acidic and short of oxygen. Decomposition there is slow and dead plant material builds up in layers, the oldest at the bottom, burying the pollen that lands on it. The outer wall of a pollen grain is made of sporopollenin, which resists decay, and the shape of a grain identifies the plant.
A core is taken, the pollen in each layer is counted under a microscope, and the layers are dated by radiocarbon dating of the peat. Each plant grows only within a range of temperature and rainfall. So a change in the proportions of pollen types up the core records a change in the vegetation, from which the climate is inferred. British cores show tundra plants after the last ice age giving way to birch and pine, then to oak, elm and lime as the climate warmed.
The limitations: wind-pollinated plants release far more pollen than insect-pollinated ones and are over-represented, and some pollen blows in from far away. Vegetation also changes for reasons other than climate, as when early farmers cleared woodland and tree pollen fell.
Dendrochronology is the dating of wood by its annual rings. A tree in a seasonal climate adds one ring of xylem a year, wide early wood in spring and narrow, dense late wood in summer. Counting in from the bark dates every ring. Ring width depends on that year's growing conditions, chiefly temperature and rainfall, so a run of wide and narrow rings records good and poor years. Trees of one region share the pattern. It can be matched between living trees, old timbers and wood preserved in bogs, extending the dated record back thousands of years.
The limitations: a narrow ring could mean a cold summer, a dry one, an insect attack or shading, so width cannot be pinned on one cause. It records only the growing season. And many tropical trees lay down no annual rings.
Correlation and cause. In Antarctic ice, temperature and carbon dioxide rise and fall together for 800 000 years. The correlation does not say which drives which, and at the end of each ice age the causation ran both ways. Changes in the Earth's orbit started a warming, the warming oceans released carbon dioxide, and that carbon dioxide added to the warming.
The claim that the present warming is caused by the carbon dioxide people add rests on more than correlation. There is a mechanism measured in the laboratory: the absorption of infrared radiation by carbon dioxide. There is a known source, since the extra carbon has the isotope composition of fossil fuels. And the alternatives fail, since the Sun's output has not risen over the decades of fastest warming.
- Proxy record
- Evidence preserved from the past, such as pollen in peat or tree rings, from which a condition that was not measured directly, such as temperature, is inferred.
- Dendrochronology
- Dating wood by counting and matching its annual growth rings, whose widths also record the growing conditions of each year.
- Ice core
- A cylinder of ice drilled from an ice sheet, in which bubbles of trapped air preserve the composition of the atmosphere when each layer formed.
What a changing atmosphere does to living things
Carbon dioxide is a greenhouse gas: it is transparent to the short-wavelength radiation arriving from the Sun and absorbs some of the long-wavelength radiation the warmed Earth emits, so raising its concentration raises the mean surface temperature. Methane, released by cattle, leaks from oil, gas and coal extraction, rice paddies and landfill, and increasingly by thawing permafrost, does the same job far more strongly per molecule while being much less abundant. The ecological question is what a change does to organisms whose tolerances evolved under the old one.
Distribution. Every species has a range of conditions it can survive in, so a change in climate moves the places where those conditions are found. In practice ranges have been moving polewards and uphill. British butterflies and dragonflies have extended their northern limits by tens of kilometres over a few decades, tree lines on mountains have crept upwards, and fish stocks in the North Sea have shifted north. Species already at the top of a mountain or the edge of a continent have nowhere to move to, which is why arctic and alpine species are the ones most often listed as at risk.
Timing. Many life cycles are triggered by temperature, and warming brings events forward. Trees come into leaf earlier, insects emerge earlier and birds lay earlier. The problem is that not everything shifts by the same amount, because different species take their cue from different things. A bird that times laying by day length, which has not changed, can find its chicks hatching after the peak of the caterpillars they were meant to eat. That mismatch is one of the clearest measured effects on abundance.
Describe the drawing
Two graphs one above the other on the same spring date axis from the middle of April to the middle of June. Each carries a cyan curve for caterpillar abundance and an amber curve for chick demand, with the region where the two curves overlap shaded grey and a dotted vertical line at the peak of chick demand. In the upper graph the two peaks are close together and the shaded region is broad. In the lower graph, after one point two degrees of warming, the cyan curve has slid six days to the left while the amber curve is in exactly the same place, so the shaded region is narrower and the cyan curve has already fallen well down its slope by the time the dotted line is reached.
The oceans. About a quarter of the carbon dioxide released dissolves in seawater, where it forms carbonic acid and lowers the pH, a change usually called ocean acidification. It makes building calcium carbonate shells and skeletons harder, which affects corals, molluscs and much of the plankton at the base of marine food chains. Warming does more immediate damage to reefs: above a threshold temperature corals expel the photosynthetic protoctists living in their tissues and bleach, and repeated bleaching kills them. A reef is a habitat for a very large number of other species, so losing it is a loss of species diversity as well as of habitat diversity.
Describe the drawing
Three drawings of one pair of measurements. Along the top, carbon dioxide in the air passes into the water, becomes carbonic acid, and splits into a hydrogen ion, ringed in amber, and a hydrogencarbonate ion. Below on the left, three cyan bars give the pH of the surface ocean as eight point two before the industrial era, eight point one now and eight point zero after one further tenth, and the three bars are so nearly the same height that the change is invisible. On the right the identical three values are redrawn as hydrogen ion concentration in nanomol per cubic decimetre, six point three, seven point nine and ten, each bar twenty six per cent taller than the one before it. Along the foot, three amber bars show the carbonate ion left for building shells falling from one hundred per cent to seventy nine and then sixty three.
Biodiversity overall. The three effects work together. Species with narrow tolerances, small ranges or poor powers of dispersal lose out; generalists with wide tolerances spread; and communities that took a long time to assemble come apart at different rates. The result is not simply fewer species everywhere but a redistribution, with local diversity rising in some places and falling in others while the number of species on the planet falls. Habitat fragmentation increases the effect, since a species whose range must shift may be unable to cross a city or a motorway, which is one argument for wildlife corridors.
TRY IT: Reading a distribution shift
The northern limit of a British butterfly species was recorded in 1970 and again in 2020. Over that period the mean summer temperature in Britain rose by about 1.2 °C, and the northern limit moved 220 km north. The species overwinters as an egg on a single food plant, which has not moved north.
Explain the shift in the northern limit, and suggest why the species may still decline overall.
Check your answer
The butterfly can survive only within a range of temperatures. Warming has moved the places where those temperatures occur northwards, so the conditions once found at the old northern limit now occur further north, and adults dispersing into that area can now survive and breed there.
The shift is a change in distribution: the species has followed its tolerable conditions north. Fifty generations is enough time for some evolution, but none is needed to explain a shift of this kind.
It may still decline because the food plant has not moved. The butterfly can only complete its life cycle where its food plant grows, so the area it can actually occupy is the overlap of the two ranges, and that overlap can shrink even while the butterfly's own climatic range moves.
The southern limit is likely to be retreating at the same time, as conditions there become too warm, so the range moves rather than expands. Habitat fragmentation may also prevent the species from reaching newly suitable areas at all, since it has to cross farmland and towns to get there.
Distinguish the mechanism, a change in where the tolerable conditions occur, from the outcome, which depends on what else must shift with the species.
Changing rainfall and seasons
Warmer air holds more water vapour, about seven per cent more for each degree, so more water evaporates and more falls when it rains. The change is uneven: some regions are becoming wetter and others drier, over most land with good records the heaviest falls have become heavier and more frequent, and wet and dry seasons are starting and ending at different times. The UK Climate Projections (UKCP18) project wetter winters and drier, hotter summers for the UK on average.
Distribution. A plant's range ends where the soil is too dry for its seedlings to establish, or so waterlogged that its roots run short of oxygen. Where a region dries, drought-tolerant species with deep roots, thick waxy cuticles and few or sunken stomata spread, and plants of damp ground retreat; where it becomes wetter, the reverse. Animals follow their food plants and the water they need. Amphibians breed in standing water, and a pond that dries before the tadpoles metamorphose produces no young that year. Mosquitoes, whose larvae live in water, can spread where new breeding water appears, carrying malaria or dengue with them.
Development and life cycles. In much of the world water, not temperature, sets the season. Seeds need water to germinate, because the water they absorb activates the enzymes that mobilise their food store. So desert annuals germinate, flower and set seed in the weeks after heavy rain, and the large grazers of the African savanna migrate with the rains to the new grass. Rains can arrive late, fail, or fall as downpours that run off instead of soaking in. Any of these leaves germination, flowering and breeding mistimed, the same mismatch as the caterpillars and birds above with rainfall as the cue.
A plant short of water closes its stomata, which also cuts carbon dioxide uptake, so it grows and develops more slowly. In flooded soil the roots cannot respire aerobically, and a crop or tree waterlogged through the growing season may die.
Seasonal cycles. Warmer winters and earlier springs lengthen the temperate growing season, and insects with several generations a year can fit in an extra one. But seeds and buds that must be chilled before they break dormancy can be held back by a mild winter.
Models of future climate, and their limits
The simplest prediction is an extrapolation: extend a record's trend beyond its last point. That assumes whatever produced the trend carries on unchanged, which is reasonable a short way beyond the data and less so further out. The carbon dioxide curve depends on how much fuel people decide to burn, and that cannot be read from an extrapolation of its past.
A climate model is the physics of the atmosphere, oceans, land and ice written as equations and solved by computer. The Earth is divided into a three-dimensional grid of cells, commonly 100 kilometres or more across, and for each cell, at each time step, the model calculates temperature, pressure, wind and humidity and the energy and water passing to neighbouring cells. Its inputs drive the climate from outside: the Sun's output, volcanic dust, and greenhouse gases and other pollutants. For the future these come from emissions scenarios, alternative assumptions about population, energy use and policy.
A model is tested by running it over the past, sometimes called hindcasting, and comparing its output with the temperature record. Models reproduce the warming of the last century when the human increase in greenhouse gases is included. They fail to reproduce the warming since the mid-twentieth century when it is left out. That is one of the main lines of evidence that people are the cause.
| Limitation | Why it matters |
|---|---|
| Future emissions are unknown | Each scenario gives a different answer, and by 2100 the choice of scenario matters more than the choice of model |
| Clouds and other processes smaller than a grid cell are approximated | How clouds respond to warming is the main reason models give different warming for the same rise in carbon dioxide |
| Feedbacks are incompletely known | Melting ice exposes darker surfaces that absorb more sunlight, thawing permafrost releases carbon dioxide and methane, and warmer oceans dissolve less carbon dioxide, each by an uncertain amount |
| Thresholds the past record never crossed | A model checked against past climate cannot be checked against conditions that have not yet happened |
| Regional and rainfall projections are less reliable | Rainfall depends on small-scale processes and the grid is coarse |
| Natural variability cannot be forecast | A large volcanic eruption or a strong El Niño moves a particular year away from the trend |
So a projection is given as a range, from many models run on the same scenarios, and stated for each scenario separately.
Describe the drawing
A chart with five horizontal bars, one for each emissions scenario from very low to very high, on a warming axis from nought to six degrees. Each cyan bar spans the very likely range of projected warming for 2081 to 2100 and a coral dot marks the best estimate: very low, 1.0 to 1.8 with 1.4; low, 1.3 to 2.4 with 1.8; intermediate, 2.1 to 3.5 with 2.7; high, 2.8 to 4.6 with 3.6; very high, 3.3 to 5.7 with 4.4. The bars move right and grow longer as emissions rise. A dashed amber vertical line at 1.1 degrees marks the warming already observed in 2011 to 2020.
Reading a projection
Use the figure. A newspaper reports that 'scientists predict the world will warm by 4.4 °C this century'. Explain what is wrong with the report.
Show the working
4.4 °C is the best estimate for the very high scenario only, in which emissions keep rising steeply. It is a projection conditional on that scenario, not a prediction.
Even for that scenario the models give a range, 3.3 to 5.7 °C, mainly because of clouds and other feedbacks, and a single number hides it.
The projected warming depends chiefly on the scenario: 1.0 to 1.8 °C in the very low scenario against 3.3 to 5.7 °C in the very high one, ranges that do not overlap. The values are for 2081 to 2100 relative to 1850 to 1900, and that baseline has to be stated with them.
- Climate model
- A computer simulation of the atmosphere, oceans, land and ice built from physical laws, used to project future climate under a given set of inputs.
- Emissions scenario
- A stated assumption about future emissions of greenhouse gases, based on population, energy use and policy, which a climate model is run on.
How climate evidence is checked: journals, peer review and conferences
A measurement or a model result becomes accepted evidence about the climate in the same way as any other scientific claim, and the Edexcel B specification names three parts of the process.
Scientific journals. Researchers publish their work as papers in journals, giving the methods in enough detail for someone else to repeat them, the data, the analysis, the conclusions with their uncertainty, and who funded the work. Publication puts the claim where anyone in the field can check and test it.
Peer review. Before a journal publishes a paper, the editor sends it to other scientists in the same field, usually anonymously. They judge whether the methods are valid, the analysis sound and the conclusions supported by the data, and recommend publication, revision or rejection. Peer review does not prove a paper right: reviewers do not repeat the work and can miss errors or fraud. It filters out work with obvious flaws.
Scientific conferences. Researchers present results, often before publication, and are questioned on them in public by others in the field, so groups using different methods or data hear each other's findings and argue out disagreements.
A result is accepted when independent groups get the same answer. The groups that compile the global temperature record use different methods and corrections and find the same warming, and Berkeley Earth, set up in 2010 partly to test published criticisms of the existing records, matched them. The IPCC does no research of its own: its reports assess the published, peer-reviewed literature, and each draft is reviewed by experts and governments with every comment and response recorded.
The process is meant to make a conclusion independent of who reached it, and does so imperfectly. Research funded by an interested party, a fossil-fuel company or a campaign group, may be framed to suit the funder, so journals require funding to be declared. A claim in a press release, a blog or an unreviewed report has been through none of this checking.
- Peer review
- The evaluation of a scientific paper by independent experts in the same field before it is published, to check that its methods, analysis and conclusions are valid.
- Scientific journal
- A publication in which researchers report their methods, data and conclusions, usually after peer review, so that others can check and repeat the work.
- Scientific conference
- A meeting at which researchers present and discuss their work, often before publication, and have it questioned by others in the field.
Core practical 13: temperature and the development of organisms
Edexcel A CP13
Pearson Edexcel Biology A (9BN0) sets Core Practical 13: Investigate the effects of temperature on the development of organisms (such as seedling growth rate, brine shrimp hatch rates). It draws on statements 5.15 and 5.16: the effect of temperature on the rate of enzyme activity, and what that means for the development of plants and animals as the climate warms.
Development is a long series of enzyme-controlled reactions. In a seedling, or in an animal that does not hold its body temperature steady, those reactions run at the temperature of the surroundings. The prediction is that development speeds up as temperature rises, up to an optimum, and slows or fails above it as enzymes and other proteins begin to denature.
Brine shrimps (Artemia) suit the investigation. Their dried eggs, sold for aquarium use, stay dormant until they are put into salt water. Then they hatch within a few days into swimming larvae that are easy to see with a hand lens.
The apparatus: brine shrimp eggs, and salt water made up as the supplier directs, in one batch. Identical containers hold the same volume. Water baths or incubators are set to a range of temperatures, for example 15, 20, 25, 30 and 35 °C, each with a thermometer. One light source is placed the same distance from every container. You also need a hand lens or a binocular microscope, a fine paintbrush and a pipette.
| Step | What to do | Why |
|---|---|---|
| 1 | Put the same volume of salt water from the one batch into each container, and let each reach its temperature before any eggs go in | Salinity and volume are then the same everywhere, and the eggs start at the temperature being tested |
| 2 | Count 50 eggs into each container with a moist paintbrush under the hand lens | A known number of eggs lets the hatch be given as a percentage |
| 3 | Keep every container in the same light, and read its thermometer each time results are taken | Light affects hatching, and a reading shows whether the temperature held |
| 4 | Every 12 hours for four days, count the larvae that have hatched, removing each with a pipette as it is counted | Removing counted larvae stops any being counted twice |
| 5 | Set up three containers at each temperature | Replicates show how far results vary between batches |
The same design works with seedlings. Germinate seeds of one species at each temperature and measure the shoot length each day; the growth rate is the gradient of the straight part of a graph of length against time, in mm per day.
The independent variable is temperature. The dependent variable is the percentage of eggs hatched by each time, and from it the development rate, 1 ÷ the time for half the eggs to hatch. Control the salinity, the volume of water, the number of eggs and the batch they come from, the light, and the surface area of water open to the air, since oxygen dissolves in through it. Plot the percentage hatched against time for each temperature, read off the time to 50% hatch, and plot the rate against temperature. A temperature coefficient can be found for any 10 °C interval.
Q10 = rate at (T + 10) °C ÷ rate at T °CThe same temperature coefficient as for an enzyme-controlled reaction; a value near 2 means the rate roughly doubles for a 10 °C rise
Hatch times and a temperature coefficient
These results are invented to show the processing. The mean time for half the eggs to hatch was 72 h at 15 °C, 44 h at 20 °C, 30 h at 25 °C and 22 h at 30 °C. At 35 °C only 20% of the eggs had hatched after four days. Calculate the development rate at each temperature, and Q10 between 20 °C and 30 °C, and explain the result at 35 °C.
Show the working
Rate = 1 ÷ time: 1 ÷ 72 = 0.0139 h⁻¹ at 15 °C; 1 ÷ 44 = 0.0227 h⁻¹ at 20 °C; 1 ÷ 30 = 0.0333 h⁻¹ at 25 °C; 1 ÷ 22 = 0.0455 h⁻¹ at 30 °C.
Q10 = 0.0455 ÷ 0.0227 = 2.0, so development roughly doubled in speed for a 10 °C rise, as expected for enzyme-controlled reactions.
At 35 °C fewer than half the eggs hatched in four days, so no rate can be calculated. Above the optimum, hydrogen bonds and other bonds holding the enzymes' tertiary structure break, active sites change shape, and development slows or stops, so many embryos die.
For a warming climate the result cuts both ways: a few degrees can speed the development of an organism in the lower part of its range, and push one already near its optimum beyond it.
| Source of error | Effect | Improvement |
|---|---|---|
| Larvae swim, so they are missed or counted twice | Counts are inaccurate | Remove each larva as it is counted; count under a lamp, which draws them to one side |
| Not every egg is viable | The percentage hatched is low at every temperature | Compare temperatures on the same batch of eggs, and give results relative to the best temperature |
| The temperature drifts, or the room warms a water bath | The temperature tested is not the one stated | Use thermostatically controlled baths and record the temperature at every reading |
| Water evaporates faster at higher temperatures | Salinity rises in the warmer containers | Cover each container loosely and check the volume |
| Counts are 12 hours apart | The time to 50% hatch is imprecise | Count more often near the expected time and read the time from a graph |
Safe and ethical use. Brine shrimps are animals: use no more eggs than the question needs, and at the end dispose of the larvae as your school's risk assessment directs, never by releasing them into ponds or drains. Water baths and incubators bring mains electricity close to water, so keep plugs and cables dry. Salt water stings the eyes and cuts, so wear eye protection when making it up and wash your hands after handling the eggs and cultures. Check the CLEAPSS guidance and your school's risk assessment before starting.
- Development rate
- How fast an organism passes through a stage of its development, measured as 1 ÷ the time the stage takes, for example 1 ÷ the time for half a batch of eggs to hatch.
In the exam
- Name the process rather than the organism: photosynthesis, respiration, decomposition or combustion, with the store it moves carbon from and the store it moves it to.
- State that decomposition is respiration by decomposers, and that they release carbon dioxide by respiring, rather than that they break things down.
- Include the low-oxygen condition in an account of fossil fuel formation: decomposition was slowed and altered, not stopped, so organic matter accumulated faster than decomposers returned it.
- State both the size and the direction of the human transfer: it is small beside the natural gross fluxes, and it is a net one-way addition that accumulates.
- Answer a distribution question through tolerance: the species survives within a range of conditions, and warming moves where those conditions occur.
- For pollen in peat, give the reasons the record survives: waterlogged, acidic, anaerobic peat slows decomposition, and the sporopollenin wall of the grain resists decay.
- Call a climate model's output a projection for a stated scenario, and give a named limitation, such as clouds, feedbacks or unknown future emissions, rather than 'models are not accurate'.
- Give a development rate as 1 ÷ time, so a shorter time to hatch is a faster rate, and use the rates, not the times, to calculate Q10.
Check yourself
A student writes: 'Burning fossil fuels is a problem because it creates carbon dioxide, and the amount of carbon on Earth is therefore increasing.' Explain what is wrong with this statement, and give a correct account of why burning fossil fuels raises the concentration of carbon dioxide in the atmosphere.
Answer
Combustion does not create carbon. The carbon in a fossil fuel was already on Earth, and burning it transfers that carbon from one store, the fuel, to another store, the atmosphere. The total amount of carbon is unchanged.
The carbon in fossil fuels was originally removed from the atmosphere by photosynthesis, hundreds of millions of years ago. The organisms that fixed it were buried in conditions with little or no oxygen, so decomposition was slowed and much of the carbon was locked away instead of being returned to the air.
Before industrialisation the cycle was in approximate long-term dynamic balance. Photosynthesis removes roughly a hundred and twenty gigatonnes of carbon a year from the atmosphere and respiration and decomposition return about the same, while the oceans dissolve and release roughly equal amounts. Those fluxes approximately cancel.
Burning fossil fuels adds a transfer into the atmosphere with no matching transfer out. About ten gigatonnes of carbon a year are released, with a further one to two from deforestation, which also removes the trees that would have fixed carbon.
Oceans and land plants absorb roughly half of that addition, so about five and a half gigatonnes a year remain in the atmosphere. The annual addition is small compared with the natural flows, but because it is a net addition it accumulates, and the concentration has risen from about 280 parts per million before industrialisation to over 420 parts per million in the 2020s. Quote current concentrations and fluxes with their year and source.
The demand behind it depends on population, consumption per person, technology and land-use choices. The human population has risen from about one thousand million in 1800 to over eight thousand million, and energy use per person has risen as well.
Questions
Question 15 marksOCR A · Edexcel A · Edexcel B
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.
| 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 |
Mark scheme
- 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 24 marksOCR A · Edexcel A · Edexcel B
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.
Mark scheme
- 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 34 marksOCR A · Edexcel A · Edexcel B
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.
Mark scheme
- 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
All 26 questions with mark schemes, the other 23 set out in full on the question page.
Worth remembering
- Stores hold carbon; transfers move it. Photosynthesis, respiration, decomposition and combustion are transfers.
- Decomposition is respiration carried out by bacteria and fungi on dead material, and it stops carbon accumulating in dead bodies.
- Fossil fuels formed where dead organisms were buried in low-oxygen conditions that slowed decomposition, so most of their carbon was not returned to the air.
- Before industrialisation the cycle was in approximate long-term dynamic balance. Combustion greatly accelerated the transfer of geological carbon to the air, deforestation added more, and about half of what is added stays there.
- A changing climate moves the places where a species can survive, so ranges shift polewards and uphill, and species with nowhere to go are the ones at risk.
- Temperature and carbon dioxide records are measured directly; pollen in peat and tree rings are proxies from which past climate is inferred.
- Climate models give ranges for each emissions scenario, because clouds, feedbacks and future emissions are uncertain.
- Rainfall sets where many species can live and when they germinate, breed and migrate.
- Peer review filters a claim before publication; independent groups repeating the work validate it afterwards.
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- Assemble the carbon cycle from the processes that move carbon between the air, living organisms, the soil and the sea.
- Explain how fossil fuels were formed, and why burning them transfers geological carbon to the atmosphere far faster than natural processes do.
- Describe the evidence that atmospheric carbon dioxide is rising, and explain the shape of the record.
- Explain how deforestation, consumption per person and land-use choices contribute to the rise, alongside population size.
- Describe the effects of a changing climate on the distribution and abundance of species, and hence on biodiversity.
- Describe the evidence for climate change from temperature records, carbon dioxide records, pollen in peat bogs and dendrochronology, and separate correlation from cause in it.Edexcel A
- Explain how climate models are built and tested, and why their projections have limitations and are given as ranges.Edexcel A
- Describe how changing rainfall patterns and seasonal cycles affect the distribution, development and life cycles of plants and animals.Edexcel A
- Describe the roles of scientific journals, peer review and conferences in validating evidence about climate change.Edexcel A · Edexcel B
- Investigate the effect of temperature on the development of an organism, and calculate a development rate and Q10 from the results.Edexcel A
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WORKBOOK
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