Why did I lose the mark?

The answer that scoresInfiltration is the downward movement of water from the ground surface into the soil, and its rate depends on soil texture and structure, organic content, slope, vegetation cover and how much water the soil already holds. When rain falls faster than the soil's infiltration capacity, or when the soil is already saturated, the excess water cannot enter and flows over the surface instead - that is overland flow, either Hortonian (infiltration-excess) or saturation-excess. The distinction matters because infiltration feeds the soil and groundwater stores and delays water, while overland flow delivers it to the channel within minutes and produces the steep rising limb of a flood hydrograph.

The answer that scoresUrbanisation shortens the lag time and increases the peak discharge. Roofs, roads and pavements are impermeable, so infiltration falls to almost nothing and the rain that would have entered the soil store becomes overland flow. Gutters, drains, sewers and culverted channels then carry that water to the river in minutes rather than hours, and the vegetation that would have intercepted and transpired some of it has been removed. The result is a storm hydrograph with a steep rising limb, a short lag and a high peak, followed by a rapid recession once the drains have emptied - and because baseflow is lower, the river also falls further in dry weather.

The answer that scoresA dam reduces flood risk rather than removing it. It stores water during a flood and releases it at a controlled rate, which lowers the peak discharge downstream and lengthens the lag. But it is designed for a specific event, and a flood larger than the design capacity must be spilled, so the protection has a ceiling; sediment trapped behind the dam starves the downstream channel and delta, which can increase erosion and lower the bed; the reservoir drowns land and displaces communities; and a failure would be catastrophic. It also only controls the main river above it, so tributaries joining below the dam can still flood the town. That is why a dam is normally one part of a strategy that also includes floodplain zoning, warning and floodplain storage.

The answer that scoresIn a meander the flow is helicoidal: the water spirals, with the surface flow thrown outwards against the outer bank, then diving and returning along the bed towards the inner bank. The fastest flow, the thalweg, is therefore close to the outer bank, where hydraulic action and abrasion undercut it to form a steep river cliff. On the inside of the bend the water is slower and shallower, so it deposits its load and builds a gently sloping point bar or slip-off slope. The cross-section becomes asymmetric and the bend migrates outwards and downstream. When two bends meet, a flood may cut across the neck, leaving an oxbow lake sealed by deposition at both ends.

The answer that scoresThe recurrence interval of a flood is calculated by dividing the number of years of record by the number of times a flood of that magnitude has been recorded, so a flood recorded four times in a hundred years has a recurrence interval of 25 years. A 100-year flood has a 1 per cent probability of being equalled or exceeded in any single year, which is a statement about the odds, not about the calendar: the chance is the same next year whether or not one has just happened. Recurrence intervals are useful for hazard mapping and for setting the design standard of a defence, but they are only as reliable as the length and quality of the record, and a changing climate or a changing catchment can make the historical record an unreliable guide to the future.

The answer that scoresThe ozone layer is in the stratosphere and absorbs ultraviolet radiation from the Sun, so its thinning allows more ultraviolet radiation to reach the surface, with consequences for skin cancer, eyes and ecosystems. It does not affect the amount of shortwave radiation reaching the ground in the wavelengths that heat the surface. Global warming is caused by the enhanced greenhouse effect: emissions of carbon dioxide, methane, nitrous oxide and F-gases increase the atmosphere's absorption of outgoing longwave radiation, more of that energy is re-emitted downwards as back radiation, and the global energy budget is pushed out of balance. The two issues are linked only in that some of the gases that destroy ozone, such as CFCs, are also powerful greenhouse gases.

The answer that scoresSensible heat transfer is the heat you can feel: the surface warms the air in contact with it by conduction, that air expands and rises by convection, and the heat is carried upward where it can be measured as a temperature change. Latent heat transfer is hidden: evaporation at the surface uses energy to change liquid water into vapour without raising its temperature, the vapour is carried upward, and when it condenses it releases that energy into the surrounding air, warming it. Latent heat is the larger of the two transfers globally, and it is what powers a cumulonimbus, because the latent heat released as the rising air condenses keeps the air warmer than its surroundings and keeps it rising.

The answer that scoresThe Earth's axis is tilted at about 23.5° and keeps pointing in the same direction as the planet orbits, so the hemisphere tilted towards the Sun receives radiation at a higher angle and for more hours of the day, and the hemisphere tilted away receives it at a lower angle for fewer hours. A higher angle concentrates the same beam into a smaller area and shortens the path through the atmosphere, so more energy reaches the surface per square metre, and longer days give more hours of heating. That is why the northern hemisphere has summer in June and July, when it is tilted towards the Sun, even though the Earth is then near its furthest point from the Sun.

The answer that scoresA large eruption injects sulphur dioxide into the stratosphere, where it forms a haze of sulphuric acid droplets that reflects shortwave radiation back to space and raises the albedo of the stratosphere, so the surface cools. Mount Pinatubo in 1991 lowered global average temperature by about 0.5°C for roughly two years, until the aerosols settled out. The eruption also releases carbon dioxide, but the quantity is tiny compared with the emissions from burning fossil fuels, and it is not enough to offset the cooling. Volcanic eruptions are therefore a short-term physical control on the energy budget, not a cause of long-term warming.

The answer that scoresOrographic rainfall occurs when prevailing winds carry moist air towards a mountain range and the air is forced to rise over it. As the air rises it expands and cools at about 6.5°C per km, its relative humidity rises until it reaches the dew point, condensation begins on nuclei, and cloud forms with rain or snow on the windward slope. Once over the crest the air descends, compresses and warms, so its relative humidity falls and the cloud clears, leaving the leeward side in a rain shadow that is drier and often warmer. The windward slope may receive well over 2 000 mm a year while the leeward side receives only a few hundred, which is why the Atacama Desert lies east of the Andes.

The answer that scoresOceanic plates are 5-10 km thick, composed of basalt and gabbro, with a density of about 3.0 g/cm3, and they are young - nowhere more than about 200 million years old - because ocean crust is continually created at ridges and destroyed at trenches. Continental plates are 30-70 km thick, made largely of granite and older metamorphic rock, with a density of about 2.7 g/cm3, and contain rocks up to nearly 4 billion years old. At a destructive boundary the older, colder oceanic plate is the denser one, so it is subducted beneath the continental plate and sinks, while the lighter continental plate is crumpled upwards into fold mountains at the margin.

The answer that scoresAs basaltic lava erupts at a mid-ocean ridge and cools below about 580 °C, iron-rich minerals align with the Earth's magnetic field at that moment. Because the field reverses polarity every few hundred thousand years, successive bands of new crust record alternating normal and reversed polarity. These bands appear as magnetic stripes running parallel to the ridge, arranged in mirror image on either side of the axis, because new crust forms at the axis and is carried away in both directions. Dating shows the crust gets older and the sediment thicker with distance from the ridge, which confirms that the ocean floor is spreading and allows the rate - typically 1 to 10 cm a year - to be calculated.

The answer that scoresA slope fails when the shear stress on it exceeds the shear strength of the material. Geology sets that strength: weak clays and shales, joints and bedding planes dipping downslope, and a permeable layer resting on an impermeable one all reduce it. Topography sets the stress: a steep slope, a concave profile or a toe undercut by a river or a road. Human activity makes it worse by cutting into the toe, loading the crest, removing deep-rooted vegetation and leaking water into the ground. Intense rainfall then acts as the trigger: it adds weight, lubricates the failure surface and raises pore water pressure, which pushes the grains apart and reduces the effective stress holding the slope together, so a slope that was already close to failure gives way.

The answer that scoresA rotational slide, or slump, fails along a curved shear surface: the mass rotates backwards as it moves, producing a steep concave scar at the head, a bulging toe at the foot and blocks that are tilted back towards the slope. The material remains a coherent mass and the failure surface can be identified. A mudflow is a flow of fine sediment with a high water content that moves down a channel like a thick fluid, with no failure surface and no coherent blocks; a debris flow is the same behaviour but carries coarse rock and boulders. The practical difference is in management: slumps are treated by drainage and by reducing the slope angle, while flows have to be kept away from people by check dams, channel works and planning that keeps housing off the valley floor.

The answer that scoresA retaining wall at the toe can hold back a shallow failure and protect a road or a building immediately behind it, but it is expensive, needs maintenance and foundations, and does nothing about the water that is the usual cause of failure, so a larger slide can simply go over or around it. Drainage is usually the more effective measure because it lowers pore water pressure, and it is cheaper. Grading, pinning and netting each protect a specific site, while afforestation and planning restrictions are cheap, address the cause and prevent new risk, but act slowly and depend on enforcement. Monitoring and hazard mapping do not stabilise anything but allow people to be evacuated. The defensible judgement is that a combination is needed, with drainage and planning control doing most of the work, and that the choice depends on the value of what is being protected and on whether the authorities can enforce the rules.

The answer that scoresBangladesh has a high population density - over 1 000 people per square kilometre - because a very large population lives in a small area of flat, fertile, monsoon-fed delta land. The Ganges-Brahmaputra delta provides deep alluvial soils and reliable water for intensive rice cultivation, which has supported dense settlement for centuries, and the population has grown rapidly since death rates fell while birth rates stayed high. The density is not evenly spread either: it is highest on the most productive farmland and in and around Dhaka, and lower in the hill districts of the Chittagong Hill Tracts. Density is an average, and it must be explained by the factors that concentrate people, not by the size of the population alone.

The answer that scoresThe DTM is a descriptive model, not a predictive one. It was built from the experience of western Europe, where death rates fell first as food supply, medicine and sanitation improved and birth rates followed later as families adjusted to lower child mortality. Its strengths are that it summarises that sequence clearly, allows countries to be compared and placed in stages, and explains why growth is fastest when death rates have fallen but birth rates have not. Its limitations are that it is Eurocentric and assumes every country follows the same path; fertility has fallen in poor countries such as Bangladesh and Sri Lanka without the economic development the model implies; it makes no allowance for migration; stage 5 was added later; it cannot handle reversals such as the effect of HIV/AIDS on death rates in southern Africa; and national averages hide large regional differences. It is useful for comparison and for showing the pressures a population structure creates, but not for predicting when a particular country's population will stop growing.

The answer that scoresNatural increase is the excess of births over deaths in a population, usually expressed per 1 000 people per year; net migration is immigration minus emigration. Population change is the sum of the two. A country can have strong natural increase and a falling population if emigration is large enough, as several Pacific island states and parts of eastern Europe have experienced, and a country can have natural decrease and a growing population because of immigration, as Germany and Canada have at times. When a question asks why a population is growing or falling, both components have to be considered before choosing which one dominates.

The answer that scoresFor the destination, migrants fill labour shortages in health, care, construction, agriculture and technology, pay taxes and consume goods, which raises output. The costs are pressure on housing, school places and health services where arrival is rapid and concentrated, downward pressure on wages at the bottom of the labour market, and social tension where integration is poor; migrants themselves may be tied to one employer, paid below the minimum and housed in poor conditions. For the source area the flow brings remittances, relieves unemployment and can return skills and capital, but it also removes trained nurses, teachers and engineers - a brain drain - separates families and leaves the economy dependent on money earned abroad. The size of each impact depends on the number of migrants, the type of flow, and the population structure, income level and skill profile of both countries.

The answer that scoresThe pyramid shows far more men than women in the 25-44 age groups, and a base that is narrow relative to the working-age population. That shape is produced by large-scale in-migration of young male construction and service workers, most of whom are temporary and cannot bring their families, so the children who would normally fill the base are largely absent. The Qatari national population itself has a different structure, with relatively high fertility, but it is a small minority of the total. The lesson is to read the sex imbalance, the bulge and the base together, and to check whose population the diagram actually shows before explaining the shape by fertility or ageing.

The answer that scoresWater stress is the condition where demand is close to the available supply, so rivers are fully allocated, users compete and groundwater begins to be drawn down, but supply has not yet run out. Water scarcity is an actual shortage relative to demand, and it takes two forms. Physical water scarcity means the resource is genuinely too small or has been over-abstracted, as in much of North Africa and the Arabian peninsula, and needs new water or lower demand. Economic water scarcity means the water is there but people cannot reach it because there is no money for wells, pipes, treatment or maintenance, as across much of sub-Saharan Africa, and needs investment rather than rainfall.

The answer that scoresDesalination is valuable because it is the only supply that does not depend on rainfall, and it has made domestic supply secure in the Gulf states, Israel, Malta, Cyprus, Spain, Australia and Singapore. But it is expensive to build and to run, because reverse osmosis needs large amounts of energy, which also means carbon emissions unless the power is renewable. It produces brine that must be discharged without smothering the sea bed, and the cost per cubic metre makes the water affordable for taps and industry but rarely for irrigation, which is where most water in a dry country is actually used. It also leaves a city dependent on a small number of large plants. So desalination buys water security for cities, not for agriculture, and it works alongside reuse and demand management rather than replacing them.

The answer that scoresA dam stores wet-season flow for the dry season, and it usually brings hydropower, irrigation and flood control with it, which is why multipurpose schemes such as the Tennessee Valley Authority transformed whole regions. The costs are just as real. The reservoir floods land and forest and displaces the people living there. It traps the sediment that would have rebuilt the delta, so deltas erode and channels downstream scour. Evaporation from a large lake surface is high in hot climates. Capacity falls as the reservoir silts up, so the benefit declines over decades. Fish migrations may be blocked, and the dam is a single, very expensive structure that fails catastrophically when it fails. The judgement depends on how much storage is worth against those losses, and on who receives the benefits and who bears them.

The answer that scoresAgriculture accounts for about 70 per cent of global freshwater withdrawals, industry and energy about 20 per cent and domestic use about 10 per cent, though the shares shift as economies industrialise. Rain-fed farming does use soil moisture rather than abstracted water, which is why the figure refers to withdrawals, mostly for irrigation. Industry's share is smaller in volume but it takes water in concentrated, metered places, and much of it is non-consumptive: a power station returns most of its cooling water to the river, whereas an irrigated field loses most of its water to the crop and the air. That difference between withdrawal and consumption is why irrigated farming is the focus of demand management.

The answer that scoresDrought is a trigger, but the reason it becomes a crisis is human. In much of the Sahel the rainfall is low and variable, yet the deeper causes of insecurity are economic water scarcity: households cannot pay for connections, villages lack wells, pumps and spare parts, water quality is poor, and institutions cannot maintain what is built. Building a dam or pipeline without recurrent funding, trained mechanics and a tariff that covers costs produces a structure that fails within a few years. The effective response combines small-scale supply - boreholes, protected springs, rainwater harvesting, sand filters - with demand management and, above all, maintenance and local management, so that the system still works in the third dry year rather than only the first.

The answer that scoresUrbanisation is the increase in the proportion of a country's population that lives in urban areas, driven by rural push factors such as poverty and land pressure, urban pull factors such as work and education, natural increase among a young urban population and the reclassification of settlements as urban. Counter-urbanisation is the movement of people out of the city to smaller towns and rural areas, usually beyond the commuting belt, driven by congestion, pollution, house prices, crime and retirement, and made possible by motorway access and, more recently, remote working. It raises house prices in the receiving villages, turns them into commuter settlements and leaves the inner city with a falling population - but the built-up area does not shrink.

The answer that scoresManufacturing has decentralised from inner-city sites to edge-of-city industrial estates and motorway junctions for several connected reasons. Inner-city plots are small, hemmed in by housing and cannot be expanded, and they were built for canal and rail freight rather than for container lorries, so access for heavy goods vehicles is poor. Land values in the inner city are now far higher for housing, offices and retail than for industry, so competition for space pushes factories out, and congestion and clean-air controls make heavy traffic in the centre difficult. Edge-of-city sites offer large, flat, cheap plots with direct motorway access, room for parking and expansion, and lower land values. The old docks were abandoned at the same time because container ships could not use them, which created the derelict land later used for regeneration.

The answer that scoresA primate city is the largest city in a country and is disproportionately larger than the second city, commonly at least twice its population, and it dominates politics, finance, media, transport and higher education. Its causes are largely historical and political: a colonial port or administrative capital from which railways and roads radiated, continued concentration of government and private investment in the capital after independence, urban bias in subsidies and infrastructure, and rural-urban migration. For the country the consequences cut both ways. It provides agglomeration economies, a gateway for foreign investment and a national symbol, but it also produces regional inequality because the rest of the country is starved of investment, congestion and housing shortages that the city cannot manage, and vulnerability because a single disaster in one city damages a large share of national output.

The answer that scoresCuritiba's bus rapid transit, introduced from 1974, gave the city a high-capacity public transport system at a fraction of the cost of a metro, with dedicated lanes, tube stations and free transfers from feeder routes, and the city tied higher-density development to those transport axes so that land use and transport reinforced each other. Pedestrianisation kept retail in the centre, parks were designed with lakes that hold floodwater, and the Green Exchange scheme trades food and bus passes for sorted waste in low-income districts, which raises recycling rates and gives informal waste pickers a recognised role. Environmentally and economically the scheme has been a relative success: lower fuel use, shorter journeys and far lower car use than in comparable Brazilian cities, achieved cheaply and quickly. Socially the record is more mixed, because land values along the improved corridors have risen, the poorest households have been pushed to the periphery, the system is now overcrowded and its fleet ageing, and the metropolitan area beyond the city boundary has grown without the same planning. The fair judgement is that it was a cost-effective success in transport and waste management that did not solve inequality, and that its transferability depends on political continuity rather than on the technology.

The answer that scoresThe rainforest grows on oxisols (latosols), which are deep, strongly acidic and heavily leached. High temperatures and heavy rainfall drive rapid hydrolysis and leaching, which removes silica and the bases and leaves kaolinite clay and iron and aluminium oxides, so the soil holds few available nutrients and has little capacity to retain more. Almost all of the nutrient capital is in the biomass, and the cycle is tight: litter decomposes within days, and the nutrients released are taken up immediately by a dense mat of shallow roots and by mycorrhizal fungi. That is why the forest can be luxuriant on poor soil, and why clearing it destroys the nutrient store and leaves a thin, infertile, easily eroded soil behind.

The answer that scoresA bornhardt is a large dome of bare granite rising from a plain. Granite is cut by sets of joints, and in a hot, wet climate water moving down those joints hydrolyses the feldspar into kaolinite clay, rotting the rock from the surface downwards and from each joint inwards. Rock between widely spaced joints survives as sound core stones, and where a very large mass survives, it forms the core of a dome. When the climate becomes drier, or the vegetation is cleared, the soft regolith is stripped away by running water and the core stone is exposed as a bornhardt. Pressure release then opens sheeting joints parallel to the surface and slabs spall off, which rounds the dome. The smooth dome surface is called a dwala, and the lowest, barely raised form is a ruware.

The answer that scoresTower karst forms by carbonation. Rainwater absorbs carbon dioxide from the air and, especially, from the soil and vegetation, forming weak carbonic acid, which converts calcium carbonate in the limestone into soluble calcium bicarbonate that is carried away in solution. The limestone must be thick, pure and well jointed, and the climate hot and wet so that the reaction is fast. Water follows the vertical joints, widening them into clefts, and where the rock between them is removed the residue stands as towers. Lateral solution and river erosion at the base of the towers undercut them and remove the debris, so that eventually isolated towers stand on a planed, alluvial plain, as at Guilin and Yangshuo in southern China. Rivers transport the waste; solution does the sculpting.

The answer that scoresA climatic climax is the community in equilibrium with the climate, and in the humid tropics that is evergreen rainforest. In the seasonally humid tropics the climax is a dry forest or woodland, and the open savanna that covers much of that zone is usually a plagioclimax - a community held below the climax by human activity, principally repeated burning and grazing, and maintained only while those continue. A subclimax is different: it is held back by a physical factor such as a long dry season, waterlogging or a thin soil, not by people. The distinction matters because a plagioclimax will revert towards forest if burning and grazing stop, whereas a subclimax will not, and because derived savanna produced by clearing rainforest is entirely a human creation.

The answer that scoresIn a normal year the trade winds blow westwards across the Pacific, piling warm surface water up in the west, so the rising air and heavy convectional rain sit over Indonesia and northern Australia while the eastern Pacific is cooler and drier. In an El Nino year the trade winds weaken or reverse, the warm water spreads eastwards and the main area of rising air moves with it. Indonesia, Papua New Guinea, northern Australia and north-east Brazil then become unusually dry, with a raised risk of fire and a shorter wet season, while Peru and Ecuador receive torrential rain and floods. A La Nina year strengthens the normal pattern instead. For this topic, the El Nino drought is the important effect, because it dries the forest and the savanna and makes them burn.

The answer that scoresA constructive wave has a long period and a low height, and the beach it reaches is permeable, so much of the swash soaks into the sand and the backwash is weak. The swash is therefore stronger than the backwash: sediment is carried up the beach and left there, and the beach builds up and becomes wider and flatter. A destructive wave is steeper and higher, is generated by strong winds or a storm, and arrives at a beach that is already saturated or made of fine sediment, so the water cannot soak away. The backwash is then stronger than the swash, and it drags sediment down the beach face and offshore, leaving a narrower and steeper beach and an offshore bar. The label describes which flow dominates, so it is the state of the beach, not simply the size of the wave, that decides the name.

The answer that scoresLongshore drift is the zigzag movement of sediment along the coastline. Waves usually approach the beach at an angle, so the swash carries sand and shingle up the beach in the direction the wave is travelling; the backwash then returns straight down the slope under gravity, at right angles to the shoreline. Each wave therefore moves sediment a little further along the coast, and the direction of movement is set by the prevailing wave approach. The backwash on its own only moves sediment down the beach face and offshore, which is how a storm profile is cut. Longshore drift is what carries sediment into a bay, past a change in the coastline's direction, and out across the gap to build a spit.

The answer that scoresReef-building coral is a colony of small animals called polyps, each of which secretes a skeleton of calcium carbonate, and the reef is the accumulated skeleton of thousands of years of growth. The polyps contain single-celled algae called zooxanthellae, which photosynthesise and supply the coral with food and oxygen while the coral gives them shelter and nutrients. When the water stays a degree or two above the normal summer maximum for weeks, the coral expels the algae, loses its colour and most of its food supply, and turns white: that is bleaching. It can recover if the water cools quickly, but prolonged heat kills it. Pollution, sediment and physical damage weaken reefs and make bleaching more likely, but the trigger for mass bleaching events is marine heat, which is why local management alone cannot prevent it.

The answer that scoresSea walls are appropriate where high-value land, a town or essential infrastructure must be protected, because they prevent both erosion and flooding at that point. They are not appropriate everywhere. A wall reflects wave energy, which can scour away the beach in front of it, leaving the wall standing at the foot of a bare shore; it interrupts the natural exchange of sediment between the beach and the offshore zone; and it is expensive to build and to maintain for ever, so once it exists the coast behind it has to be defended whatever happens. The same is true of groynes, which hold sediment on one beach by taking it from the next. The defensible approach is to decide, for each sediment cell, where the line should be held and where it should be allowed to move - protecting towns with hard engineering, using beach nourishment and dune management where there is space, and accepting managed realignment where the land is of low value. The evaluation has to say who is protected and who is not.

The answer that scoresMagnitude is the energy released by the earthquake, measured on the moment magnitude scale, which has replaced the Richter scale; it is logarithmic, it does not saturate, and one earthquake has one magnitude wherever it is recorded. Intensity is the severity of shaking and damage at a particular place, measured on the Modified Mercalli scale from I to XII, and it is assessed from what people felt and what happened to buildings and the ground. That is why the same earthquake has a high Mercalli value close to the epicentre and a low one hundreds of kilometres away, and why Mercalli is used to map the pattern of damage rather than to compare the size of two earthquakes.

The answer that scoresA tsunami is generated when a shallow subduction-zone earthquake displaces the sea floor vertically, or when a large volume of sediment collapses into the sea. In the open ocean it travels at up to about 800 km/h with a very long wavelength and an amplitude of only about a metre, so ships pass over it without noticing. As it reaches shallow water it slows, the water piles up and it comes ashore as a series of surges that can run hundreds of metres inland, with the first surge often not the largest. A storm surge is different: it is a rise in sea level driven by low pressure and onshore winds during a tropical cyclone, it lasts for hours, and it floods rather than strikes.

The answer that scoresLava flows destroy buildings, roads and farmland and can cut off communities, but they usually move slowly enough for evacuation, so they rarely kill. The lethal hazards are pyroclastic flows, which sweep down the slopes at up to 700 km/h at several hundred degrees Celsius and give no time to escape; lahars, which run down river valleys like wet concrete and can occur years after the eruption; ash fall, which collapses roofs, kills crops and pasture and closes airspace; and toxic gases such as carbon dioxide, which is denser than air and collects in valleys and hollows. This is why management has to match the hazard - evacuation and exclusion zones for pyroclastic flows and lahars, strong roofs and ash clearance for ash fall - and why a volcano that has been quiet for a century is more dangerous than one that erupts every few years.

The answer that scoresA tropical cyclone needs sea surface temperatures of at least about 26-27 °C to a depth of roughly 50 m, but it also needs the Coriolis effect to set the converging air spinning, which is why the belt of formation lies between about 5 and 20 degrees of latitude and why there are no cyclones at the equator itself. It also needs low vertical wind shear, moist unstable air through a deep layer and a pre-existing disturbance to start it. The same phenomenon is called a hurricane in the North Atlantic and eastern North Pacific, a typhoon in the western North Pacific, and a cyclone in the Indian Ocean and around Australia.

The answer that scoresWildfire causes divide into physical and human. Physical causes are desiccation of vegetation by heat and drought, lightning strikes, and sparks from falling rocks. Human causes are accidental - unattended campfires, equipment use or malfunction such as power lines, and rubbish such as glass acting as a lens or a discarded cigarette - and deliberate, chiefly arson. Fire is also part of some ecosystems: eucalypt forest and Mediterranean scrub depend on it to release seed and return nutrients, so management aims to control fuel and reduce risk to people rather than to eliminate fire. That is why controlled burning, grazing and firebreaks are used to break up continuous fuel, and why total suppression eventually produces a larger, more intense fire.

The answer that scoresA rock pedestal forms where a resistant cap rock lies over softer rock. Sand is carried by saltation within roughly a metre of the ground, so abrasion is concentrated at that height and cuts into the softer rock beneath the cap, undercutting it. The cap protects the rock above and the pedestal is left with a narrow stem and a wide overhanging top, which is why it is also called a mushroom rock. Where several hard and soft beds alternate, a stack of waists forms, one at each soft layer, and the whole feature records how close to the ground the sand is carried.

The answer that scoresDesertification is the degradation of land in arid and semi-arid areas, meaning a lasting fall in biological productivity. It is caused by physical factors - low and highly variable rainfall, drought, high evapotranspiration, thin soils and wind erosion - acting on land already weakened by human factors such as overgrazing, overcultivation, clearance of trees and shrubs for fuel, and irrigation without drainage. The two interact: a drought that a well-managed landscape would survive destroys a degraded one. The result is falling yields, dust, erosion and the abandonment of land, and it can happen hundreds of kilometres from any desert. It is not the physical advance of sand across a boundary.

The answer that scoresChemical weathering is limited in hot deserts but it does occur. Dew, fog and occasional heavy rain keep rock damp long enough for reactions, and the water carries dissolved carbon dioxide from the air and soil, forming weak carbonic acid. Hydrolysis attacks feldspar in granite, converting it to clay minerals and releasing soluble ions, so the rock weakens and crumbles to a coarse residue called grus. Carbonation dissolves calcium carbonate in limestone and chalk, and reprecipitates it near the surface as calcrete when the water evaporates. The products matter: the clay and fine material are what deflation lifts, and the calcrete forms a hard cap that later protects mesas and buttes. Chemical weathering is subordinate to physical weathering, but its products are essential to the landscape.

The answer that scoresA barchan is a crescent-shaped dune formed where the sand supply is limited, the ground beneath is firm, and the wind blows consistently from one direction. Sand is carried up the long, gentle windward slope and avalanches down the steep lee slip face at the angle of repose of dry sand, about 32 to 34 degrees, so the whole dune migrates downwind. The horns point downwind: they are the thin edges of the crescent, which advance more slowly than the thick centre. Star dunes, by contrast, are pyramidal with three or more radiating arms and form where winds blow from several directions, so sand is piled up rather than carried away. Seif dunes are long ridges parallel to a bimodal wind.

The answer that scoresSalinisation is the accumulation of soluble salts in the soil. In irrigated dryland it usually happens where water is applied without adequate drainage: the excess percolates down, the water table rises towards the surface, and capillary action draws the salty groundwater back up. Evaporation at the surface leaves the salts behind as a crust, so the concentration in the root zone rises until crops fail and the land is abandoned. On coasts the same problem arrives from the sea when heavy abstraction allows salt water to intrude into the aquifer. The remedy is drainage plus controlled leaching, along with drip irrigation and, in some soils, gypsum; simply adding more water makes it worse.

The answer that scoresNet zero greenhouse gas emissions means that the greenhouse gases still being released are balanced by an equal amount removed from the atmosphere, so the net addition is zero. The removals can be natural - forests, peatland, soils and the ocean - or engineered, such as carbon capture and storage combined with bioenergy. Some emissions, from agriculture and from industries such as cement and aviation, are extremely hard to eliminate, so the strategy is to cut everything that can be cut and remove the rest. Net zero matters because global temperature stops rising once the net addition of carbon dioxide falls to zero: warming depends on the accumulated stock in the atmosphere, not on the emissions of any single year. That is why the date of net zero, rather than the date emissions peak, determines the final temperature.

The answer that scoresSea level rises for two reasons. First, thermal expansion: the ocean has absorbed about nine-tenths of the excess heat trapped by greenhouse gases, and water expands as it warms - this has been the larger contributor so far. Second, melting land ice: mountain glaciers, the Greenland ice sheet and the West Antarctic ice sheet add water that was previously stored on land, and these are the larger long-term risk because they hold enough water to raise sea level by many metres. Local relative sea level can rise faster than the global average where the land is sinking, as in many deltas where groundwater and sediment have been extracted. The impact is then concentrated in events: a higher baseline means storm surges reach further inland, and a one-in-a-century flood becomes a one-in-a-few-decades flood.

The answer that scoresThe five categories are conditional projections, not five possible forecasts. Each one is a consistent story about emissions - rising throughout, roughly maintained, or peaking and then falling - and each produces a different amount of warming, sea-level rise and extreme weather. The very high category follows if emissions keep increasing; the very low category follows if emissions peak almost immediately and fall steeply to net zero around mid-century. Which path the world takes depends on policy, technology and behaviour, so the categories describe choices, not certainties. Even the very low projection is not an absence of change: it still means roughly 1.4 °C of warming, more heatwaves, and sea level that continues to rise after 2100.

The answer that scoresThe choice of strategy depends on wealth, the length and type of coast, the value of the land being protected, the technical capacity of the state and the number of people affected. The Netherlands can afford protection because it is rich, its exposed coastline is relatively short, and the land behind it carries a global port and dense cities. Bangladesh has a far longer and more dynamic delta coast, a much lower income, and a population that depends on farming land that is renewed by sediment rather than fixed behind concrete. Hard defences there are built where the value justifies it, but the main effort goes into compromise - cyclone shelters, warning systems, floating gardens, salt-tolerant crops - because these are affordable, can be delivered community by community, and do not depend on a single structure surviving an event above its design standard. Building Dutch-style defences everywhere would also be maladaptation, since embankments that trap water and block silt can lower the land behind them.

The answer that scoresNature-based solutions are valuable and available now: protecting existing forest, peatland and mangroves avoids emissions that would otherwise occur, and restoring them removes carbon while improving biodiversity, soils and flood defence. But they cannot carry the whole task. Land is finite, storage saturates as ecosystems mature, fire, disease and future clearance can release the carbon again, and measurement is uncertain. Offsetting has further weaknesses: additionality (would the reduction have happened anyway?), permanence, leakage (protection in one place pushing damage to another), double counting and low prices that make it cheaper to buy a credit than to cut your own emissions. The credible route to net zero is therefore deep cuts in emissions from energy, industry, transport and land use first, with nature-based solutions and engineered removals used for the residual emissions that cannot be eliminated.

The answer that scoresRenewables emit little or no carbon dioxide, sulfur dioxide or particulates when generating, which is a genuine and large gain for air quality and climate. But they are not impact-free. A solar or wind farm needs far more land than a gas-fired station for the same output, which takes farmland and habitat; the panels, turbines, cables and batteries require copper, lithium, nickel, cobalt, silicon and rare earths, all of which have to be mined, with the acid drainage, dust and land damage that mining causes; wind turbines kill birds and bats; large dams flood valleys, displace people, trap the sediment that sustains deltas and emit methane as drowned vegetation rots; and biomass is renewable only if the crop is regrown, which competes with food for land and water. The honest conclusion is that renewables shift and reduce environmental damage rather than abolishing it, and that the size of the gain depends on where the materials come from, where the projects are sited and how the land is managed afterwards.

The answer that scoresEnergy security means energy that is available when it is needed, affordable, and not dependent on a single supplier or fuel. Nigeria has large oil and gas reserves but poor energy security: its gas-fired and hydro stations deliver only a fraction of their installed capacity, transmission is limited, tariffs do not cover costs, and more than a third of the population has no connection, so households and businesses depend on petrol and diesel generators, and cooking depends on wood and charcoal. Costa Rica has no fossil reserves but much better energy security in its power sector, because it generates more than 98 per cent of its electricity from domestic hydro, geothermal and wind. Its weakness is elsewhere: it imports all its transport fuel, so it is exposed to oil prices. The comparison shows that security depends on generation that works, a grid that carries the power, tariffs that pay for it and institutions that maintain it, and that a country can be resource-rich and energy-poor at the same time.

The answer that scoresEutrophication is the enrichment of water with nitrate and phosphate, mostly from fertiliser washed off farmland and from poorly treated sewage. The nutrients allow algae to grow explosively into a bloom on the surface; when the algae die they sink and are decomposed by bacteria, which use up the dissolved oxygen in the water, so fish and other animals suffocate and the water turns cloudy and green. The management therefore has to target nutrients: soil testing and precision application so that less fertiliser is used, buffer strips and wetlands between fields and watercourses to trap runoff, limits on nitrate in vulnerable zones, better sewage treatment, and in some cases a reduction in livestock numbers in the catchment. Because the nutrients travel through groundwater and rivers, the measures have to be applied across the whole catchment, not on one farm.

The answer that scoresRestoration is a sequence. The pit is backfilled and the slopes are regraded so that they are stable and drain; subsoil and topsoil, stripped and stored before quarrying began, are replaced, because soil cannot be manufactured; acidic spoil is neutralised with lime or organic matter and drainage is passed through constructed wetlands so that metals settle out; then native species suited to the soil and climate are planted and protected until they are established. Beyond that, a successful scheme decides the afteruse - a lake for wildlife and recreation, woodland, a solar farm, housing or industry - and is secured by planning conditions, an environmental impact assessment before licensing, discharge permits and a financial bond so that the public does not pay if the company disappears. Monitoring continues after closure, because acid drainage and settling carry on for years. Success is judged by whether the land is stable, uncontaminated and useful afterwards, which is why the Lusatian Lakeland in Germany and the Eden Project in a former china clay pit in Cornwall count as successes and a grassed-over spoil heap usually does not.

The answer that scoresThe sources of urban pollution in the syllabus are domestic, construction, manufacturing and power generation, transport, and waste, both solid and sewage - and their balance differs with income. In low-income cities the dominant problems are domestic and infrastructural: wood, charcoal, coal and kerosene burned indoors for cooking, no sewerage, waste that is not collected and is burned or dumped in drains, and workshops and industry with no separation from housing and no enforcement of standards, while population growth outruns the pipes. In middle-income cities the pressure shifts to motorisation, construction dust and industrial expansion. In high-income cities sewage and coal smoke have largely been dealt with, and the remaining problems are traffic, nitrogen dioxide, waste volumes driven by consumption and contaminated brownfield land - often with the worst pollution exported to the countries that make their goods. Per-person emissions are frequently higher in rich cities even though their air is cleaner, so 'fewer cars' is not the same as 'less pollution'.

The answer that scoresAfrican countries account for a small share of world trade by value, but many of them export very large volumes. Nigeria, Angola and Libya ship crude oil, Zambia and the Democratic Republic of the Congo ship copper and cobalt, and Cote d'Ivoire and Ghana ship cocoa, but these commodities are bulky and low-priced per tonne, and most are exported unprocessed, so the country captures a small share of the final price. Manufactured goods are light and expensive, and the design, processing, branding and retail stages that capture most of the value are concentrated in high-income countries and in the NEEs. The result is that a country can fill ships and still earn little, which is why the inequality in trade is an inequality of value added, not simply of the amount of goods moved.

The answer that scoresTourism is a form of invisible trade, because what the visitor is buying is a service - a night in a hotel, a flight, a guided walk, a meal - rather than a good that is shipped across a border. The spending counts as an export for the destination country and an import for the country the visitor came from, and it appears in the balance of trade as receipts rather than as merchandise. The value is measured in receipts instead of arrivals, and the two can diverge sharply: a country with huge numbers of cruise passengers or day-trippers may earn less than one with far fewer visitors who stay longer and pay more, and a large share of what visitors spend may leak out again through imported food, foreign-owned hotels and overseas tour operators.

The answer that scoresWhether aid benefits the receiving country depends on its type and on the context. Humanitarian aid delivered quickly through experienced agencies saves lives and is hard to criticise, and long-term development aid that builds schools, clinics, roads and tax systems can raise capacity permanently. But tied aid must be spent on the donor's goods and services, often at above-market prices, so part of its value returns to the donor; conditionality attached to loans and grants has forced policies that cut public spending and hurt the poorest; food aid and donated second-hand clothing can destroy local markets; and large inflows can push up prices and the exchange rate, or be lost to corruption where institutions are weak. Aid is therefore most effective when it is untied, predictable, channelled through systems the recipient government controls, and matched to what the country says it needs.

The answer that scoresTNCs bring real benefits to host countries: capital, formal employment, training, technology transfer, tax revenue, and access to export markets that a local firm could not reach on its own, which is how garment assembly and electronics production began in many NEEs. But the benefits are partial. The most valuable stages of a global production network - design, branding, marketing, finance and retail - usually stay in the home country, so the host country captures only the assembly wage; inputs are often imported, so the local value added is small; profits are repatriated rather than reinvested; wages may be low and conditions poor where labour organisation is weak; environmental standards may be lower than at home; and the investment is footloose, moving to whatever country offers cheaper labour or better incentives. The balance depends on the terms the host government negotiates: local content requirements, training commitments, tax rules and labour and environmental enforcement decide whether a factory becomes a permanent part of the economy or an enclave.

The answer that scoresEcotourism is intended to be low-impact, nature-based travel that funds conservation and benefits local people, and it can work: in parts of Costa Rica, in Kenya's conservancies and in the Galapagos it pays for rangers, research and community income, and it gives local people a reason to protect habitat rather than clear it. But 'eco' is a claim rather than a guarantee. The carbon cost of the flight to a remote destination is often the largest environmental impact of the trip and is rarely counted; lodges still need land, water, energy and waste disposal; the presence of visitors disturbs wildlife even at low densities; and certification schemes vary in strength, so an operator can label a conventional resort as eco without changing much. The honest test is not the label but the numbers: how many visitors, how much water and land, where the waste goes, who receives the money, and what the journey emitted.

The answer that scoresCholera is caused by the bacterium Vibrio cholerae and is spread by the waterborne route: the bacterium leaves an infected person in their faeces and is swallowed by the next person in contaminated drinking water or food. It is infectious rather than contagious, because it is caught from the environment and not from the breath or touch of a patient. After the Haiti earthquake of 2010 the water and sewerage network in Port-au-Prince was destroyed, people moved into crowded camps with shared water points and latrines, and the bacterium spread rapidly along the Artibonite river, producing hundreds of thousands of cases. That is why the control is chlorinated water, sewers and oral rehydration rather than isolation of patients.

The answer that scoresMalaria is caused by a parasite of the genus Plasmodium, most severely P. falciparum, and it is transmitted by the bite of an infected female Anopheles mosquito, which is the vector. The parasite has to develop inside the mosquito before it can be passed on, and that development needs temperatures above about 16-18 °C, so malaria is absent from highlands above that limit and from cool seasons. The mosquito also needs standing or slow-moving water to breed and humid, vegetated shade in which to rest, so transmission peaks in and after the rainy season and is highest where drainage is poor, irrigation provides breeding sites or people store water near their homes.

The answer that scoresAn outbreak is a rise in cases above the expected level in a small, defined area; an epidemic is a rise above the expected level across a country or region; and a pandemic is an epidemic that has spread across a world region or globally, so that several continents are affected at once. Endemic is different in kind: a disease is endemic when it is permanently present at a predictable, expected level, as malaria is across much of sub-Saharan Africa. Severity is a separate question, which is why the 2009 H1N1 influenza pandemic was caused by a strain that was milder than feared for most people but still reached more than 200 countries.

The answer that scoresLife expectancy in low-income countries is lower mainly because a much larger share of deaths occurs in infancy and childhood, from diarrhoeal disease, lower respiratory infections, malaria, measles and neonatal conditions, and because adults die earlier from communicable disease and from conditions that would be treatable with better healthcare. The infant mortality rate is the measure that shows this most clearly, and it responds quickly to clean water, nutrition, vaccination and access to care. Age structure works the other way on the crude death rate: a country with a large elderly population records more deaths per 1 000 people even when its healthcare is good, which is why age-specific rates and life expectancy are better comparisons than the crude death rate.

The answer that scoresInfluenza vaccines protect against the strains expected to circulate that season, but the virus changes in two ways. Antigenic drift is the gradual accumulation of small changes in the surface proteins, so immunity from previous infection or vaccination no longer matches and the vaccine has to be reformulated every year; that is why annual epidemics recur and why people catch influenza repeatedly. Antigenic shift is a major change, usually when a strain from an animal population - pigs, which can be infected by both bird and human viruses, or poultry - acquires the ability to infect people efficiently, producing a virus to which almost nobody has immunity. That is the pandemic risk, and it is why surveillance of both human and animal strains matters more than any single vaccine.