Why did I lose the mark?

The answer that scoresLimestone is permeable. Water passes through the pore spaces between the grains and along the joints and bedding planes, and because the weak acid in rainwater slowly dissolves the calcium carbonate, those cracks are widened into channels and caves over time. That is why rivers disappear into sinkholes and springs emerge further down the valley in limestone country, and why a village well in limestone can be polluted very quickly.

The answer that scoresWeathering is the breakdown of rock in place, by freezing and thawing, by plant roots or by chemical reactions such as the solution of calcium carbonate in weak acid. Erosion is the wearing away and removal of that material by a moving agent such as a river, a glacier, the wind or the waves. Deposition is when the agent loses energy and drops the load, and the sediment then settles and accumulates as sedimentation.

The answer that scoresLimestone is changed by heat and pressure into marble, as the calcium carbonate recrystallises into interlocking calcite crystals; this is why marble contains no fossils. Shale or mudstone is changed at lower heat and pressure into slate, as the plate-like clay minerals recrystallise in alignment, which is why slate splits into thin sheets that are good for roofing.

The answer that scoresBasalt is extrusive: the lava erupted at the surface, where it lost heat quickly, so there was little time for crystals to grow and the rock is fine-grained. Granite is intrusive: the magma was insulated by the rock around it and cooled slowly, so its crystals grew large enough to be seen with the naked eye. Slow cooling gives large crystals, fast cooling gives small crystals.

The answer that scoresAn ore is rock containing minerals and metals in a concentration high enough for the metal to be extracted at a profit. Rock carrying only a trace of metal is not an ore, and rock that is economic today becomes waste if the metal price falls.

The answer that scoresA surface mine disturbs the pit itself, the spoil heaps, the tailings dam, the processing plant, the haul roads and the land under the rail or conveyor link, and it lowers the water table and spreads dust and noise for kilometres around. That is why an environmental impact assessment covers a much wider area than the orebody.

The answer that scoresRestoration makes the land stable, safe and usable again by replacing the overburden, re-grading the slopes, improving the soil with lime, fertiliser and organic matter, planting trees and preventing acid drainage. It rarely recreates the original habitat, and because it takes decades and must be paid for after the mine has closed, permission is usually tied to a restoration bond or to restoring the site in stages.

The answer that scoresThese methods are chosen where conventional mining would not pay - spoil heaps, tailings, low-grade ore and contaminated ground - because the metal is too thinly spread to dig, crush and process at a profit. Bioleaching uses bacteria that dissolve the metal into a solution over months, and phytomining uses hyperaccumulator plants that are harvested and burned so that the metal can be recovered from the ash. Both are slow, both produce acid that must be contained, and neither can replace a large open pit for a rich deposit.

The answer that scoresA finite resource is a natural resource that is used up at a faster rate than it is replaced. Rocks, ores and minerals are finite because new ore bodies are formed by geological processes over millions of years, while a single mine can extract a deposit in a few decades. The rate of use is therefore far higher than the rate of replacement, so the total supply is fixed.

The answer that scoresRecycling extends the life of a reserve, because it supplies metal without mining new ore, and metals keep their properties through repeated recycling. It cannot supply everything, however: some metal is lost at every stage, alloys are hard to separate, and collection only works where there is infrastructure and a market. Demand for metals such as copper and lithium is rising, so reduce, reuse and substitution are needed as well, since those strategies avoid extraction altogether.

The answer that scoresLegislation sets the rules and enforcement makes them work. In Ghana, illegal small-scale gold mining polluted the Pra and Ankobra rivers despite the law; a ban and a task force reduced it but did not stop it, because thousands of people depended on the income, the sites were remote and equipment could be moved. Effective enforcement needs inspectors, monitoring, fines that are larger than the profit from breaking the law, and alternative livelihoods for the people affected.

The answer that scoresRestoration replaces the overburden, improves the soil, uses bioremediation and plants trees, and the site can be repurposed as a lake, landfill, recreation ground or nature reserve, so future generations inherit usable land instead of a hazard. The ore itself is not replaced: it was formed over millions of years and has been extracted and dispersed. That is why restoration must be combined with reduce, reuse, recycling, efficiency and substitution, which limit how much ore is taken in the first place.

The answer that scoresNuclear power is non-renewable (finite) because it uses uranium, which is mined from a limited ore body and cannot be replaced. Its advantage is that splitting uranium releases no carbon dioxide at the power station, but it leaves radioactive waste that must be stored safely for thousands of years, and the fuel is still finite, so nuclear power cannot replace fossil fuels permanently.

The answer that scoresWind and solar are intermittent: output varies with wind speed and with daylight, so they cannot be relied on to meet demand on their own. A country needs backup generating capacity such as gas turbines, or storage such as batteries or pumped hydro, to cover the gaps, and it also needs enough capacity installed to cover the times when the wind and the Sun produce less than their average.

The answer that scoresPhotovoltaic (PV) cells convert light energy directly into electrical energy, with no steam and no turbine, so they work even on a roof. Concentrating solar power (CSP), as at Noor Ouarzazate in Morocco, uses mirrors to focus sunlight onto a fluid, and the hot fluid raises steam that drives a turbine - the same steam cycle as a fossil-fuel station, but with the Sun as the heat source.

The answer that scoresDemand for energy is affected by transport, personal and national wealth, climate, human population size and industry, and also by disruption to supply, unreliable supply and scarcity of resources. Wealth raises the amount each person can afford to use, climate decides how much heating or air conditioning is needed, industry adds very heavy loads, and a sudden loss of supply raises prices so that users consume less.

The answer that scoresEnergy conservation means using less energy by changing behaviour, such as lowering a thermostat, switching off standby loads or taking the bus. Energy efficiency means getting the same useful output from a smaller energy input, such as an LED lamp or a condensing boiler. Both reduce demand, but conservation depends on people and efficiency depends on equipment, and greater efficiency can be partly cancelled by the rebound effect when the money saved is spent on using more of the same service.

The answer that scoresBlue hydrogen is produced from natural gas by steam methane reforming, with the carbon dioxide by-product captured and stored, so it still begins with a fossil fuel and depends on the capture working and on methane not leaking. Green hydrogen is produced by the electrolysis of water using electricity from renewable resources, so it releases no carbon dioxide if the electricity is genuinely renewable. Blue is cheaper and uses existing gas infrastructure today; green is the near-zero-carbon route but is more expensive and loses a large share of the energy used to make it.

The answer that scoresA heat pump works like a refrigerator in reverse: a refrigerant cycle driven by a compressor absorbs heat from the ground or the outside air and releases it inside the building. A coefficient of performance of 4 means that 1 unit of electrical energy moves 4 units of heat into the building, so it is far less carbon intensive than a gas boiler or an electric heater. The limits are that performance falls if the building is poorly insulated or the radiators are too small to run at a low temperature, and that the emissions saved depend on how the electricity is generated.

The answer that scoresBatteries store energy for hours. They charge when wind and solar output is high and prices are low, discharge into the peak, and steady the grid frequency in milliseconds, which is why they are so useful with a high renewable share. They cannot carry energy from summer to winter, because the capacity required would be impossibly large and expensive and the batteries would self-discharge and degrade. Seasonal storage needs other methods such as pumped hydro-electric storage, hydrogen or large thermal stores, used together with a mix of generating sources so that the wind is always blowing or the sun shining somewhere.

The answer that scoresFracking is the extraction of natural gas or petroleum from shale rock by hydraulic fracturing. Water, proppant and a small amount of chemicals are pumped into the impermeable shale at very high pressure, which cracks the rock and creates pathways so that the gas can flow to the well, and the proppant left in the fractures holds them open once the pressure is released.

The answer that scoresA ban removes the risk at the site completely, which is a real benefit for local water supplies and for people living nearby. But the demand for gas continues, so the country imports it, and the environmental costs of producing it move abroad. If the gas is not available, electricity may be generated from coal, which produces about twice the carbon dioxide of gas for the same output. This is why many governments regulate fracking with impact assessments, cementing standards, water recycling and seismic monitoring instead of banning it outright.

The answer that scoresFracturing itself produces micro-earthquakes that nobody notices. The felt and damaging earthquakes are caused when flowback and produced water are pumped into a deep disposal well, because the injected water raises the pressure in the pore spaces of the rock and reduces the friction holding a fault closed. The fault then slips. This is managed by monitoring the site with seismometers and applying a traffic light system that pauses or stops work, and by recycling water so that less has to be disposed of - and in Oklahoma, by limiting the volumes that may be injected.

The answer that scoresMethane is a much more powerful greenhouse gas than carbon dioxide, particularly over the short term, so a small percentage leak can remove the climate advantage that gas has over coal and make the climate case for fracking collapse. Leakage is reduced by capturing gas through a green completion instead of flaring it, by using low-bleed valves and seals, and by running leak detection and repair programmes on wells, compressors and pipelines. The limitation is cost: small leaks are difficult to find and expensive to fix, so operators have to be required to do it.

The answer that scoresA clay soil holds water and nutrients well but drains badly and is poorly aerated and hard to cultivate, so it is not the best medium for most crops. A loam is better because it combines the good points of sand, silt and clay: enough clay and silt to hold water and nutrient ions, and enough sand and large pore spaces to drain excess water and let air back in to the roots, while still being easy to cultivate.

The answer that scoresFertiliser raises yield only where the nutrient was the limiting factor. A balanced approach supplies the ions the crop needs, in the amounts a soil test shows are missing, while correcting the other limiting factors - soil water, drainage, pH, structure - so that the crop can actually use them. Applying more than the crop can take up wastes money and pollutes water.

The answer that scoresWeather is the day-to-day conditions of the atmosphere in a location - temperature, rainfall, wind, cloud and sunshine - which change from hour to hour and day to day. Climate is the average pattern of those conditions at a place over many years. A single storm is weather; the fact that a region has a wet season and a dry season every year is climate.

The answer that scoresA pH of 4.5 is acidic, so the soil is too acid for most crops and phosphate may be locked up while aluminium can become toxic to the roots. The farmer should apply lime, which is calcium carbonate, to raise the pH towards 6 to 7, choosing the amount from a soil test because too much lime makes other nutrients unavailable. Raising the pH also has to be repeated, because the soil re-acidifies with continued cropping.

The answer that scoresIntensive farming uses very high inputs of labour, capital, water, fertiliser or machinery on each hectare to get a high yield from that hectare, and it can be done on a small plot, for example a glasshouse of tomatoes or a two-hectare market garden. Extensive farming is the opposite: a very large area with few inputs per hectare and a low yield per hectare, such as cattle ranching on dry grassland. The two words describe inputs and yields per hectare, not the size of the farm or the size of the business.

The answer that scoresThe two have different strengths. Inorganic NPK fertiliser supplies nitrogen as nitrate, phosphorus as phosphate and potassium as potassium ions in a known concentration and ratio, so a deficiency can be corrected quickly, but it adds no organic matter and its surplus nitrate is easily leached. Organic fertiliser such as manure, crop residue and mulch adds those nutrients more slowly and in smaller amounts, but it also adds the organic matter that gives the soil its structure, its water-holding capacity and its soil organisms. The best practice is to use both: return organic matter to keep the soil in condition and top up with inorganic nutrients when a soil test shows a shortage.

The answer that scoresOver-irrigating causes three separate problems. Water logging, because the pore spaces are full of water and the roots cannot get the oxygen they need; salinisation, because evaporation leaves the dissolved salts behind and they accumulate in the topsoil until the crop will not grow; and soil erosion, where the surplus water runs off bare or sloping ground and carries the nutrient-rich topsoil with it. Trickle irrigation avoids all three by putting a small amount of water at the base of each plant, and drainage or a fallow period may be needed to recover a field that is already affected.

The answer that scoresResistance develops because the insects that survive a spray pass on the genes that let them survive it, so the population becomes more resistant each generation. A stronger dose speeds this up and also causes pest resurgence, because the broad-spectrum spray kills the predators and parasites that were keeping the pest in check. The farmer should instead rotate between chemicals with different active ingredients, use biological control alongside them, rotate and intercrop so the pest cannot build up, monitor the crop and spray only when the pest passes the level at which the damage costs more than the treatment.

The answer that scoresSoil erosion happens wherever bare soil is exposed to moving water or wind. In humid regions the main agent is water: raindrops detach the particles, run-off carries them down the slope and cuts rills and then gullies. In semi-arid regions during drought, wind is the main agent and takes the fine silt, clay and organic matter. Desertification is the longer-term result of erosion and vegetation loss in dry regions, not the only place where erosion occurs.

The answer that scoresContour ploughing means ploughing across the slope, along the contour lines. Each furrow then runs along the hillside like a small dam, so it slows the run-off and gives the water time to soak in, and soil washed out of one furrow is trapped in the next one down. The water that does reach the bottom has to be led away in a safe outlet, such as a grassed waterway, or it will break through the furrows and cut a gully.

The answer that scoresTerracing cuts the slope into flat steps, so rain soaks in where the crop is growing and the risers hold the soil and slow the run-off, which reduces erosion a great deal. The limit is maintenance: the risers, drains and outlets must be repaired after storms, an exceptional storm can overtop the steps, and abandoned terraces collapse and release a large volume of soil at once. Building them is also very labour-intensive, so on a gentle slope contour ploughing, bunds and a cover crop may give most of the benefit for far less work.

The answer that scoresApplying NPK fertiliser can replace the nitrogen, phosphorus and potassium that were carried away, but the field is still shallower, so it holds less water and dries out faster, and its structure is poorer, so the next storm detaches more soil. The organic matter that held the crumbs together, stored the nutrients and fed the soil organisms is lost with the topsoil and takes years to rebuild. On steep, thin soils the lost ground cannot be replaced at all, so preventing the erosion is far cheaper than treating the symptoms.

The answer that scoresAn aquifer is a permeable rock, such as sandstone, chalk or gravel, that holds water in its pores and allows it to flow. The water table is the upper surface of the saturated zone; it rises in the wet season and falls in the dry season and when the aquifer is pumped. Water is abstracted through a well or borehole, and the level falls if pumping is faster than the recharge seeping down from the surface.

The answer that scoresPotable water is water that is safe to drink. Screening, sedimentation and filtration remove solids, but only chlorination disinfects the water, killing bacteria and viruses and leaving a residual that protects it in the pipes. Water can be perfectly clear and still carry pathogens, so clarity is not proof of safety.

The answer that scoresDesalination is valuable because it gives a supply that does not depend on rainfall, but it is one of the most expensive sources of fresh water per cubic metre. It should be compared with the alternatives: repairing leaks, metering and charging by volume, reusing treated water for irrigation and flushing, and drip irrigation are all usually cheaper per cubic metre. Desalination is justified where those savings have already been made and there is still a shortfall, and where cheap energy is available.

The answer that scoresA dam floods land upstream, so villages, farmland, forest and sometimes archaeological sites are lost and the residents have to be resettled. It traps sediment, so the river downstream erodes its bed and banks and floodplain soils lose their fertility; it blocks migratory fish such as salmon; water is lost by evaporation from the reservoir; and on an international river the flow and fisheries of the next country are changed. The benefits - flood control, hydro-electric power, irrigation, storage, transport, recreation, tourism and fish farming - must be weighed against these costs, with a judgement about who gains and who loses.

The answer that scoresThe nitrate and phosphate enrich the water, so algae grow explosively and form a bloom on the surface. The bloom blocks the light, so the submerged plants die, and the algae die as the nutrients are used up and sink. Bacteria decompose the dead material and use oxygen, so the biochemical oxygen demand rises and the dissolved oxygen falls. The fish suffocate from lack of oxygen, and the final anaerobic decomposition gives the water its black colour and foul smell.

The answer that scoresBioaccumulation is the build-up of a toxic substance in a single organism over time, because it takes the substance in from the water, the sediment or its food faster than it can excrete it, so older individuals carry more. Biomagnification is the increase in concentration of that substance as it is passed up a food chain, because each predator eats many prey and so collects the toxin from all of them. Mercury released into Minamata Bay in Japan was bioaccumulated by the fish and biomagnified up to the people who ate them, who suffered severe damage to the nervous system.

The answer that scoresChlorination (or ultraviolet light or ozone) is a disinfection step that kills bacteria and viruses. Nitrate and phosphate have to be removed by tertiary treatment, by stripping phosphate out with iron or aluminium salts, removing nitrate by denitrification or in a constructed wetland, before the effluent is discharged. Without that stage the effluent still enriches the river, and it will still bloom and lose its oxygen.

The answer that scoresThe acid water damages the gills and the eggs of the fish and kills the invertebrates they feed on, and as the water becomes more acid it releases aluminium from the soil, which coats the gills and suffocates the fish. Lakes on granite and other hard, insoluble rocks cannot neutralise the acid, so their pH falls furthest, which is why the lakes of southern Norway and south-west Sweden lost their fish. Lakes on limestone or chalk are buffered by calcium carbonate, which neutralises the acid, and their fish survive. Liming a lake replaces some of that missing neutralising capacity, but it must be repeated and it does not stop the acid falling.

The answer that scoresMalaria is spread by mosquitoes, not by water. The vector is the female Anopheles mosquito and the pathogen is the Plasmodium parasite, which passes from an infected person to the mosquito in a blood meal and from the mosquito to the next person in its bite. Control is by personal protection - insecticide-treated nets, repellent, antimalarial drugs and vaccination - and by vector control: draining and covering breeding areas, spraying with insecticide, sterilising male mosquitoes and biological control such as larvivorous fish. Cleaning the drinking water supply controls cholera, typhoid and diarrhoeal disease, not malaria.

The answer that scoresNets work against Anopheles because it bites at night, and that is also why they protect a whole household and not just the sleeper. Dengue must be controlled differently, because Aedes aegypti bites during the day: water containers, tyres, gutters and other small containers of clean water are emptied, scrubbed and covered so that the larvae have nowhere to develop, drains are cleared, repellent and window screens are used in the daytime, and insecticide is applied or the mosquito population is reduced by releasing sterile or Wolbachia-carrying males.

The answer that scoresVaccination is valuable during an outbreak because it protects people quickly while the cause is being dealt with, but the definitive measures are the ones that separate sewage from drinking water: handwashing with soap, adequate sanitation and sewage treatment, and a potable supply by chlorination or boiling, with water protected between the source and the home. In a city with no sewers and a contaminated supply, vaccination without those measures simply leaves the source in place and cases will continue.

The answer that scoresControl works on the parasite's life cycle. Patients are treated with praziquantel to clear the worms; the snail intermediate host is killed with molluscicides or removed by clearing the aquatic plants that shelter it; piped water, washing areas and latrines are provided so that people do not have to enter the water to wash or defecate into it; and health education warns children and workers away from shallow water where the larvae are. Treating people alone leads to quick reinfection while the snails and the contaminated water remain.

The answer that scoresMarine aquaculture is the farming of marine species in captivity, limited to fish, crustaceans and seaweeds. The stock is controlled by people from the hatchery to the harvest, so it is the opposite of a wild fishery, in which nobody controls how many animals exist or where they go.

The answer that scoresFarmed and wild fish are linked through the net. Sea lice that multiply on the crowded farmed stock are carried by the tide onto wild salmon smolts leaving the rivers; infections can spread from the pens to passing wild fish; nutrients from uneaten feed and faeces leave the pens and enrich the water; and when a net tears, farmed fish escape, interbreed with wild relatives and compete with them for food and spawning sites. Only a closed or recirculating system removes the connection.

The answer that scoresA larger mesh lets juveniles escape before they are landed, so they survive to breed and the spawning stock is maintained. It does reduce the catch of small fish in the short term, and some fish are still killed in the net, but the long-term yield of the fishery is higher than if the juveniles were removed.

The answer that scoresUneaten feed, faeces and ammonia leave the pens as nitrogen, phosphorus and organic carbon. The dissolved nutrients encourage phytoplankton and algal growth in a plume down-current, while the organic waste settles under the cages, where bacteria break it down and use up the oxygen in the sediment. The seabed under the farm becomes black and oxygen-poor and the bottom community dies, which is nutrient enrichment turning into pollution, not food.

The answer that scoresBooms are used to encircle a slick around a leaking ship, to deflect a slick away from a sensitive coast, or to seal off the mouth of a creek or lagoon. They contain the oil, and skimmers or sorbents then remove it from inside the boom. Booms only work in calm water, because a current carries the oil under the boom and waves wash it over the top, and they need anchors, boats and trained crews to deploy and tend them.

The answer that scoresA dispersant is useful when a large slick in deep, open water would otherwise reach the coast before it could be recovered, because it stops the slick forming and speeds up dilution and bacterial breakdown. It must not be used in shallow water or near coral reefs, shellfish beds, mangroves or seagrass, where the dispersed oil does more damage than floating oil would. Skimmers and sorbents remove oil, dispersants only relocate it.

The answer that scoresSome of the oil always escapes recovery: a slick that has spread thinner than about a millimetre is too thin for skimmers and too thin to burn, emulsified oil cannot be pumped or dispersed easily, and oil in the sediment or buried in a beach is released again by storms for years. On a reef or in a mangrove swamp the remaining oil cannot be removed at all, which is why preventing the spill and containing it at sea matter so much.

The answer that scoresA big tanker accident releases a huge volume in one place in a few days, and it deserves the attention it gets. But chronic pollution from pipelines, ageing wells and routine discharges releases a steady supply of oil, close to coasts and over decades, rather than once. The Niger Delta, where thousands of small leaks have polluted creeks, mangroves and farmland for years, shows how much damage the slow, chronic source can do.

The answer that scoresA bioplastic is a plastic made from biological raw materials instead of fossil fuels, and it can be biodegradable or non-biodegradable. A biodegradable plastic is one designed to decompose in water or soil, broken down by bacteria and fungi into water, biomass and gases, and its rate of decomposition depends on biotic factors such as the decomposers present and abiotic factors such as temperature, moisture and oxygen. Only a plastic that is both bio-based and designed to be biodegradable, and then placed where decomposition can happen, will break down quickly.

The answer that scoresRecycling is worth doing for clean single-polymer streams such as PET bottles and HDPE containers, because it saves energy and oil and keeps material in use. But it has real limits: mixed and contaminated plastic cannot be recycled economically, separate collection is essential, quality falls with each cycle, and demand for recycled material is uncertain. So recycling is one part of a package that has to be led by reduction and reuse, better collection and safe disposal, and legislation that makes producers responsible for their packaging.

The answer that scoresMicroplastics are plastics less than 5 mm in length. Primary microplastics are manufactured at that size and used in commercial products, including microbeads in facial scrubs and toothpaste and the nurdles that are melted down to make plastic products. Secondary microplastics are formed when larger plastics break down, for example fibres shed from synthetic clothing in a washing machine and particles worn off car tyres, which are washed into drains and rivers and reach the sea.

The answer that scoresClean-up projects protect wildlife locally, restore beaches and raise awareness, but they remove only a small fraction of the plastic already in the sea and are expensive. The cause is plastic continuing to enter the environment, so the effective response works upstream: avoiding single-use plastics, alternative packaging, safe disposal and better collection, recycling where it genuinely works, extended producer responsibility and legislation such as the Basel Convention plastics amendment. A judgement worth making is that clean-ups and source reduction are complementary, but only reduction changes the trend.

The answer that scoresClean dry air is about 78% nitrogen and 21% oxygen by volume. Together these two gases are 99% of the air, which is why argon, at about 0.9%, is only the third most abundant gas, and carbon dioxide is about 0.04%.

The answer that scoresThe ozone layer is in the lower stratosphere, roughly 15 to 35 km above the surface. It absorbs most of the harmful ultraviolet radiation from the Sun before it reaches the surface, which reduces skin cancer and cataracts and limits damage to crops and to phytoplankton in the oceans. Ozone formed at ground level in the troposphere is a component of photochemical smog and damages lungs and crops.

The answer that scoresTemperature falls with height in the troposphere, rises with height in the stratosphere, falls again in the mesosphere and rises again in the thermosphere - down, up, down, up. The troposphere is heated from below by the surface, the stratosphere is heated from within by ozone absorbing ultraviolet radiation, the mesosphere has almost no source of heat, and the thermosphere absorbs very short-wavelength ultraviolet radiation and X-rays.

The answer that scoresEven in a heavily polluted city the main composition is unchanged: about 78% nitrogen and 21% oxygen. The pollution is a small addition to that mixture, concentrated in the troposphere because that is where it is released and where weather can disperse or wash it out. Carbon dioxide is about 0.04% of clean air and measured in parts per million, because such a small number needs a sensitive unit for change to be visible.

The answer that scoresSolar radiation passes through the atmosphere and is absorbed by the land and oceans and partly reflected back into space. The warmed surface re-emits radiation as infrared, and greenhouse gases absorb this infrared radiation and emit the energy again as thermal (heat) energy, some of it back towards the surface, which keeps the surface warmer than it would otherwise be. Adding more greenhouse gases makes more of this infrared radiation return to the surface, so the Earth warms.

The answer that scoresThe enhanced greenhouse effect is caused by the increased concentration of greenhouse gases such as carbon dioxide, methane and nitrous oxide from human activity, which absorb and re-emit infrared radiation from the warmed surface. Ozone depletion is a separate issue: CFCs in aerosols and refrigerants break down ozone in the stratosphere, and the resulting thinning lets more ultraviolet radiation reach the Earth's surface, increasing rates of cancer and cataracts and damaging vegetation. Some CFCs contribute to warming as well, but the ozone hole is not the cause of global warming.

The answer that scoresClimate change adaptation is altering behaviour, practices and infrastructure to adapt to the impacts of climate change. Reducing emissions is mitigation, which limits how much the climate changes in the future. Adaptation includes growing crops that resist warmer, drier or wetter conditions, improving flood defences, making laws about building near coasts and on floodplains, and changing building design and materials such as raising houses on stilts or using cool roofs. Both are needed: mitigation alone cannot prevent the impacts already set in motion by the greenhouse gases already released, and adaptation alone cannot keep up with the impacts if emissions continue to rise.

The answer that scoresThe impacts are uneven. Crops in tropical and subtropical regions are likely to suffer because they are already close to their maximum temperature, and drought, flooding, salinity and more frequent extreme weather destroy harvests and soil. Warm winter temperatures allow pests and invasive species to survive and spread. Some cooler regions may gain a longer growing season, but the carbon dioxide fertilisation effect is limited by shortages of water and nutrients, and heavier rain can wash nutrients and topsoil away. Overall, food production becomes less reliable and prices rise, so the poorest households are hit hardest.

The answer that scoresUnpolluted rain is already slightly acidic at about pH 5.6 because it dissolves carbon dioxide. Acid rain is more acidic than that because it also absorbs sulfur dioxide, from the combustion of fossil fuels that contain sulfur compounds and from volcanic gases, and oxides of nitrogen, formed when nitrogen from the atmosphere reacts with oxygen at the high temperature of a vehicle engine. Sulfur dioxide is oxidised to sulfur trioxide and then to sulfuric acid, and nitrogen dioxide forms nitric acid, so the rain can fall below pH 4 and the acid mist on high ground is stronger still.

The answer that scoresSulfur dioxide and oxides of nitrogen leave through tall chimneys, which spread them high into the air, and the wind carries them hundreds or thousands of kilometres in a few days. The gases are oxidised on the way, so by the time they reach the ground they may have crossed several borders. That is why the lakes of Norway and Sweden were acidified by emissions from the United Kingdom and central Europe, and why the Adirondack Mountains in New York State were acidified by power stations in the Midwest. A tall chimney lowers the concentration in the town beneath it but does not remove the gas, so it exports the problem rather than solving it, and the only real answer is an international agreement on emissions.

The answer that scoresSpreading calcium carbonate or calcium oxide on the lake or its catchment neutralises the acid and raises the pH, and it does allow fish to breed again, which is why thousands of lakes in Norway and Sweden have been limed. But the acid continues to fall, so the lime has to be applied again every few years, the cost never ends, large doses can harm some species, and it is impossible to lime every stream in a catchment. Liming is a holding measure that buys time until emission controls reduce the deposition, and the permanent solution is to cut the sulfur dioxide and oxides of nitrogen at their sources.

The answer that scoresA three-way catalytic converter uses platinum, palladium and rhodium on a ceramic honeycomb to turn oxides of nitrogen into nitrogen and oxygen, and carbon monoxide and unburned hydrocarbons into carbon dioxide and water. It does not remove sulfur dioxide, because that gas comes from the sulfur compounds in the fuel rather than from the combustion chemistry, so the sulfur must be dealt with by using low-sulfur fuel or by flue-gas desulfurisation at a power station. The converter also needs unleaded fuel, because lead poisons the catalyst, and it only works once the exhaust is hot, so the first few minutes of a journey are uncontrolled.

The answer that scoresOzone depletion is the thinning of the ozone layer in the lower stratosphere, mainly by chlorine released from CFCs, so that more ultraviolet radiation reaches the surface. It is not a hole through which heat escapes and it does not cause global warming. The two problems are separate: ozone depletion is about ultraviolet radiation and is caused by CFCs; global warming is about infrared radiation and is caused by greenhouse gases. They overlap only slightly - CFCs and HFCs are also greenhouse gases, and a thinner ozone layer over the Antarctic changes the winds of the Southern Hemisphere.

The answer that scoresUV radiation breaks the carbon-chlorine bond in a CFC and releases a chlorine atom. That atom takes an oxygen atom from an ozone molecule, forming ClO and O2, and the ClO then takes a spare oxygen atom and becomes a chlorine atom again. Because the chlorine is regenerated it acts as a catalyst, and it goes on destroying ozone until it reacts to form a reservoir compound such as HCl or ClONO2. That is why a small mass of CFC causes damage out of all proportion to its size.

The answer that scoresThe Montreal Protocol has reduced emissions sharply, and stratospheric chlorine has been falling since it peaked in the late 1990s, but recovery is slow. It is expected to be complete around the middle of the century, with the Antarctic hole last, and it can be delayed by natural variation such as a large volcanic eruption. Recovery also depends on finishing the job: phasing out HCFCs and HFCs, and recovering and destroying the CFCs still held in old fridges, air conditioners and foams instead of letting them leak.

The answer that scoresAlternatives must be judged on both ozone and climate. HCFCs contain chlorine and still deplete ozone, although far less than CFCs, so they are only transitional. HFCs contain no chlorine and do not deplete ozone, but they are strong greenhouse gases, so the Kigali Amendment of 2016 added a timetable to phase them down. Hydrocarbons such as butane and propane, ammonia, carbon dioxide and the newer hydrofluoroolefins avoid both problems but introduce others: hydrocarbons are flammable, ammonia is toxic, and high-pressure carbon dioxide systems need redesigned equipment and retrained technicians. The Montreal Protocol itself uses different timetables and finance for developing countries, which is part of why it works.

The answer that scoresAbout 90% of the energy is lost during the life processes of the organisms: movement, respiration, digestion and excretion. Respiration releases most of it as heat to the surroundings, so it is no longer available to the next trophic level, and further energy is locked in parts that are not eaten, such as roots, bones, hair and shells.

The answer that scoresThe habitat is the place where the organism lives, such as the mud at the bottom of a pond. The niche is its role in the ecosystem: what it feeds on, where and when it feeds, how it reproduces and what preys on it. Two species with identical niches in the same habitat will compete, and one will out-compete the other.

The answer that scoresA pyramid of numbers can be inverted: a single oak tree supports thousands of insects, so the bar for the producers is narrower than the bar above it. A pyramid of biomass can also be inverted in a sea, because the phytoplankton are eaten as fast as they reproduce, so the mass of the consumers present at that instant is greater than the mass of the microscopic producers. Only a pyramid of energy is always a true pyramid, because energy is lost at every step and can never increase up the levels.

The answer that scoresProducer 30 000 kJ -> primary consumer 10% of 30 000 = 3 000 kJ -> secondary consumer 10% of 3 000 = 300 kJ. Applying the 10% at each step is why food chains are usually short: after four or five trophic levels almost none of the original energy is left.

The answer that scoresRemoving the forest removes the canopy that intercepts rainfall and the litter and roots that keep the soil open. Less water infiltrates and more runs over the surface, so much more of the same rainfall reaches the river, and it reaches it faster, which makes the flood peak higher and sooner. The soil washed off the slopes settles in the channel as silt, raising the bed and reducing the capacity of the river, so it overflows more often. Flow in the dry season falls, because less water was stored in the soil during the wet season.

The answer that scoresLight decreases sharply from the top of the forest downwards. The emergents stand in full sunlight, the canopy below them catches most of what is left, the understorey receives only dim, dappled light, and the forest floor receives very little. Plants in the lower layers are shade-tolerant ferns, mosses and seedlings, and they grow quickly only when a gap opens in the canopy above them.

The answer that scoresSustainable management of forests means balancing the needs of the environment, wildlife and humans while conserving forests for future generations. Trees may be harvested, but only at a rate that the forest can replace, so that the carbon store, the water cycle, the soil, the biodiversity and the products people need are all still there for the next generation. Total protection is one strategy within sustainable management, but it is not the same thing, and it can fail if local people lose their livelihoods.

The answer that scoresSelective logging can be sustainable only if it is planned and enforced: the roads and extraction routes are kept to a minimum, the damage to the remaining trees and soil is small enough for the canopy to close again, and enough seed trees are left for the forest to regenerate. Its limitations are that machinery and haul roads still damage the forest that is left, that the species that need shade are harmed by the new gaps in the canopy, and that illegal felling is very difficult to control in remote areas where there is no supervision.

The answer that scoresBiodiversity is the variety of living organisms. It has three levels: genetic diversity, which is the variation between individuals of a species; species diversity, which is the number of species and how abundant each one is; and ecosystem diversity, which is the range of habitats and the processes going on in them. A single-species pine plantation is green, but it holds far less biodiversity than the mixed forest it replaced, because both the species diversity and the genetic diversity have collapsed.

The answer that scoresThe remaining forest is broken into small patches surrounded by farmland the species cannot cross. Each patch holds a smaller population, which loses genetic variation through inbreeding and is more easily wiped out by disease, fire or a storm. The edges of the patches are drier, brighter and windier and are open to predators and weeds from the fields, so species that need the sheltered interior disappear. Because individuals can no longer move between patches, a patch that loses a species cannot be recolonised, and species cannot shift their range as the climate warms.

The answer that scoresCITES regulates international trade. Appendix I species cannot be traded commercially; Appendix II species can be traded only with an export permit from the country of origin showing that the export will not harm the species; Appendix III species are controlled at the request of one country. Poaching continues because much of the demand is met through domestic markets and smuggling across poorly controlled borders, permits and paperwork can be forged, and enforcement depends on customs officers and patrol budgets. A ban can also raise the price of illegal horn and ivory, which makes poaching more profitable.

The answer that scoresA seed bank is ex-situ conservation: it stores the seeds of a species, and the genetic variation they carry, in a cold dry store where they can survive for decades. It does not conserve the ecosystem, so a species restored from a bank still needs somewhere to grow, with its pollinators, its soil and the right climate. Species with recalcitrant seeds, such as oaks and many rainforest trees, cannot be stored this way. The seeds must also be germinated, grown and re-harvested at intervals to stay viable, which costs money for ever, and a bank is useful only if its material is actually used for reintroduction.

The answer that scoresEvery part of the field must have an equal chance of being sampled. Divide the field into a grid, use a random number generator or random number table to give the coordinates of each quadrat, and place the frame there even if the spot looks bare. Then sample enough quadrats to be representative and report the mean and the range, so that the estimate can be trusted and the survey can be repeated.

The answer that scoresFrequency = (number of quadrats containing the species / total number of quadrats) x 100, so a frequency of 60% means the daisy was found in 6 of 10 quadrats. Percentage cover records how much of each quadrat is occupied by the species, estimated with a frame divided into 100 small squares. Report both figures, and say which one the investigation set out to measure.

The answer that scoresReport the catch as a measure of activity, for example the mean number of beetles per trap per night, and use it to compare places or conditions sampled in exactly the same way. If the total population must be estimated, use a method designed for it, such as mark-release-recapture, and accept that even then the figure is an estimate, not a count.

The answer that scoresSample several quadrats in each field, placed randomly or at fixed intervals, and report the mean number of species and the range for each. Only when the ranges do not overlap, or the difference between the means is much larger than the variation within each field, can a statement about the two fields be supported. The number of samples should be decided after a pilot study of how variable each field is.

The answer that scoresAt a conservative plate boundary the two plates slide past one another, either in opposite directions or in the same direction at different speeds. The rough edges catch and grip, so the rock on either side bends and stores strain for years, and when the rock finally breaks or the two sides jerk past each other the stored energy is released as an earthquake. No crust is created or destroyed and no magma is formed, so the only hazard is the earthquake itself. The San Andreas Fault in California and the Enriquillo Fault in Haiti are both examples.

The answer that scoresThe focus is the point inside the Earth where the rock breaks and the stored energy is released as seismic waves. The epicentre is the point on the Earth's surface directly above the focus, and it usually experiences the strongest shaking and the worst damage. The depth of the focus matters: a shallow-focus earthquake, less than about 70 km down, releases its energy close to the surface and causes far more damage than a deep-focus earthquake of the same magnitude.

The answer that scoresLava flows destroy property, farmland and roads and can set fire to buildings, but they move slowly enough for people to escape. The most lethal volcanic hazards are pyroclastic flows, the fast-moving clouds of hot gas, ash and rock that sweep down the slopes and destroy everything in their path; lahars, the mudflows of ash and water that fill valleys like wet concrete; ash falls, which collapse roofs and can suffocate people and animals; and gases, because carbon dioxide is denser than air and collects in valleys and hollows where it can kill, as it did at Lake Nyos in Cameroon in 1986.

The answer that scoresMagnitude, measured on the Moment Magnitude Scale, is a measure of the energy released and is the same wherever it is recorded; it is logarithmic, so each whole number is about 32 times more energy than the one below it. What kills people is how hard the ground shakes where they live, which is the intensity, and that depends on the depth of the focus, the distance from the epicentre, whether the ground is solid rock or waterlogged sand that liquefies, whether buildings were designed and built to resist shaking, and whether there was a warning, a plan and a trained rescue service. A shallow magnitude 7 earthquake in a poor city of unreinforced concrete kills far more people than a deeper one beneath a city of engineered buildings.

The answer that scoresTropical cyclones form between 5 and 30 degrees of latitude north and south of the Equator, not at the Equator itself. They need a sea surface temperature of at least 27 °C, warm water at least 60 m deep, weak wind shear, and a Coriolis effect strong enough to spin the inflowing air into a rotating storm. Latitude supplies the spin, the warm deep water supplies the energy.

The answer that scoresThe eyewall has the strongest winds and the heaviest rain, because the pressure gradient is steepest there, and this is where most damage is done. The eye is calm with the lowest pressure, and the passage of the eye gives a short false lull: people should stay in shelter until the whole storm has passed, because the winds quickly return from the other side.

The answer that scoresA storm surge is a dome of water driven ashore by the wind and raised further by the low pressure, so sea level rises across a long stretch of coast and stays high for hours around high tide. It floods low-lying deltas and coastal cities, drowns people, salts farmland and freshwater wells, and does far more damage than the waves alone.

The answer that scoresAccurate forecasting is only the first link. Effective management needs the warning to be broadcast in a form everyone receives, agreed evacuation routes with transport, shelters that are reachable and stocked, drills so people know what to do, building regulations so houses survive, and rescue and relief teams ready to act. The comparison to use is Odisha in 2019, where the same forecast became a mass evacuation and very few deaths, and New Orleans in 2005, where a good forecast did not make up for people having no means of leaving.

The answer that scoresThe causes are the physical processes - heavy or prolonged rainfall, snowmelt, saturated and compacted soil, impermeable rock, steep slopes and small drainage basins - together with the human changes to the basin, such as urbanisation, deforestation, poor farming practice and the drainage of wetlands. Damage to roads, bridges and crops is an impact that follows the flood.

The answer that scoresUrbanisation increases flood risk. Impermeable surfaces reduce infiltration and increase surface run-off; gutters, drains and sewers deliver that run-off to the river quickly; vegetation that intercepted rain and used it is removed; and building on the floodplain takes away the ground that used to store the floodwater. The lag time shortens and the peak discharge rises, so flash flooding becomes more likely downstream.

The answer that scoresA short lag time gives a flashy hydrograph: the rising limb is steep, the peak discharge is high and it arrives soon after the rainfall, so there is little time to warn or evacuate people. Long lag times, produced by gentle slopes, permeable rock, deep soil, dense vegetation and wetland storage, give a flatter hydrograph with a lower peak.

The answer that scoresLevees reduce the frequency of flooding for the land immediately behind them, but they do not remove the risk. They need constant maintenance, they can be overtopped or breached, they carry the flood peak downstream to the next community, and they stop silt being deposited on the fields. They work best as one part of a strategy that also stores water upstream, keeps new building off the floodplain and warns people in time.

The answer that scoresA drought is a period of dry weather that is longer or more severe than normal compared with the long-term records for that place. It is defined by the shortage relative to the local average, not by a complete absence of rain, and it can be described as meteorological, hydrological or agricultural depending on whether the rainfall, the water stores or the soil moisture are below normal.

The answer that scoresThe rainfall deficit is natural, but its severity is not. Pumping groundwater faster than it is recharged lowers the water table, so there is less in store when the rain fails; deforestation and overgrazing reduce infiltration and send rain running off the surface; growing demand from cities, farming and industry means the same dry year causes a worse shortage; and climate change is increasing both evaporation and the frequency of extreme weather. People can also manage a drought by monitoring and warning, by conserving water and by increasing supply.

The answer that scoresDrought has a chain of impacts. Water sources dry up, so cities ration supply and rivers become too low for navigation. Hydropower output falls when reservoirs are low, and thermal and nuclear stations may lack cooling water, so power cuts follow. Factories that need water for processing must cut production and shed jobs, so wages and export earnings fall. Dry vegetation increases the risk of wildfires and drought increases dust storms, both of which reduce air quality and harm health. Where the loss of a harvest destroys the only livelihood, families migrate to towns or across borders, and competition for water and grazing can turn into conflict.

The answer that scoresDrought-resistant and early-maturing crops are one useful measure among several, not a complete answer. They raise yields when the rains are late or short, but a crop still needs water to grow, so they must be combined with water storage, irrigation efficiency such as drip irrigation, mulching and conservation agriculture to keep moisture in the soil, land management to reduce desertification, and early warning so that seed is planted when the forecast is favourable. Improved seed also has to be bought again each season, which many smallholders cannot afford, and the choice of crop has to match what the local market will buy.

The answer that scoresWork out both: density = population / area in km2. If Country A has 80 million people in 2 million km2 the density is 40 people per km2, and if Country B has 20 million in 250,000 km2 the density is 80 people per km2 - so the smaller country is the denser one. Always divide before you compare, and quote the unit.

The answer that scoresA wide base that narrows quickly to a pointed top shows a youthful population: a high birth rate, a large young dependent group and a shorter life expectancy, as in Malawi. An ageing population has a narrow base, showing a low birth rate, and long bars in the older age groups, as in Sweden or Japan, with more women than men at the top of the pyramid.

The answer that scoresSubtract the dependents first: 60 - 24 = 36 million people of working age. Then divide the dependents by the working-age population and multiply by 100: (18 + 6) / 36 x 100 = about 67 - that is, about 67 dependents for every 100 people of working age. State who the dependents are, because a ratio of 67 built from children and one built from elderly people mean different things for the government.

The answer that scoresIf 250,000 people immigrated and 150,000 emigrated, net migration is 250,000 - 150,000 = +100,000, so the country gained 100,000 people through migration. If 150,000 immigrated and 250,000 emigrated, net migration would be -100,000 and the population would fall by that amount, other things being equal. Total population change = natural increase + net migration.

The answer that scoresNatural increase is the birth rate minus the death rate, so if the death rate falls while the birth rate stays high, natural increase becomes larger and the population grows more quickly. In stage 2 clean water, sanitation, vaccination and medicine cut the death rate sharply while the birth rate barely moves, so the gap between the lines is at its widest and the population rises faster than at any other stage. Growth slows in stage 3, when the birth rate finally starts to fall, and the population levels off only in stage 4, when the two rates come together.

The answer that scoresUse the rule of 70: doubling time is about 70 divided by the annual percentage growth rate, so 70 / 4 = 17.5, about 18 years. The compounding also works the other way, which is why the world population is not growing as fast as many candidates assume: the growth rate peaked at a little over 2 per cent a year in the late 1960s, so the doubling time then was about 35 years, and with a growth rate now below 1 per cent a year it is much longer. Still, an 18-year doubling time is fast enough that a country growing at 4 per cent a year would increase its population eightfold in about 54 years.

The answer that scoresIn stage 2 the death rate falls steeply and the birth rate stays high, so natural increase reaches its maximum and the population curve is at its steepest. In stage 3 the birth rate falls as girls stay longer in school and contraception spreads, so the gap narrows and the rate of growth slows - but the population still rises, often by a great deal, because the birth rate remains well above the death rate. The population stops growing only in stage 4, when the two rates meet at a low level.

The answer that scoresRural to urban migration is the main cause of urban growth, driven by push factors such as low farm income and land shortage and by pull factors such as jobs, schools and clinics. Natural increase does add to the total, and it becomes more important over time because the people who migrate are young and are starting their families, and cities also grow when surrounding land is reclassified as urban. The three causes should be given in that order, with migration first, and urbanisation should be defined as the rising proportion of a population that lives in towns and cities.

The answer that scoresAn anti-natalist policy tries to reduce the birth rate, using incentives for small families, disincentives such as fines, or legal limits on the number of children, as in China's one-child policy. A pro-natalist policy tries to increase the birth rate, using baby bonuses, family allowances, tax allowances, subsidised childcare and parental leave, as in France, Japan and Singapore.

The answer that scoresThe one-child policy reduced the birth rate and slowed the growth of the population, but the population still grew, because the large number of people already born went on having children. The policy's real effects were on the structure of the population: a badly unbalanced sex ratio because families preferred sons, and rapid ageing, so that one adult may now support two parents and four grandparents. When the policy was relaxed to two children in 2015 and three in 2021, the birth rate did not recover.

The answer that scoresVaccination, maternal care, clean water and treatment of disease lower the death rate, particularly among young children, so more people survive and the population grows faster while the birth rate remains high. In the long run fewer deaths lead to fewer births, because parents who expect their children to live no longer need to have extra children as insurance for their old age, but this takes a generation and depends on family planning and education being available at the same time.

The answer that scoresFree contraception raises use and lowers the birth rate, but only if the supply actually reaches the people who need it and they are free to use it. Family planning works best when it is combined with girls' education, which raises the age of marriage, with healthcare, which makes parents confident that their children will survive, and with advice given by someone the community trusts. Iran and Thailand both lowered fertility quickly with a combination of cheap supplies, local health workers and education, and Iran's programme later lost ground when the government withdrew its support.