Environmental issues and management

Energy resources: the types, their uses and their issues

Electricity is not a fuel. It is a way of moving energy from whatever was burned, split or turned to the wires that carry it, so when a question asks about energy resources it is really asking which primary source a country uses to generate its electricity. The syllabus divides them into renewable sources, which natural processes replenish as fast as they are used, non-renewable fossil fuels, which are a finite store laid down millions of years ago, and nuclear power, which is neither, because it depends on a finite mineral but releases no carbon dioxide as it generates.

Wind is turned into electricity by turbines, onshore where land is cheaper and offshore where the wind is stronger and steadier but the foundations and cables cost far more. Its fuel is free and it emits nothing in use, but output depends on the weather, so a wind farm typically delivers only about a third of its rated capacity over a year, and some people object to the look and the noise of turbines on their skyline. Solar arrives as light, which photovoltaic (PV) panels convert directly into electricity: the panels are cheap, modular and can sit on a roof or in a desert-scale farm, but output falls with cloud, latitude and season, is zero at night, and a solar farm needs far more land than a power station to produce the same annual output.

Tidal power uses the rise and fall of the tide, either by holding water behind a barrage and releasing it through turbines or by placing turbines in the tidal stream. Its great advantage is that tides are predictable decades ahead, unlike wind and sun, but there are very few sites with a large enough tidal range, capital costs are enormous and a barrage changes the estuary behind it; the La Rance barrage in Brittany and the Sihwa Lake barrage in South Korea are the two best-known. Hydroelectric power (HEP) uses falling water, either stored behind a dam and released through turbines or, in a run-of-river scheme, taken straight from the flow. It is the cheapest large-scale renewable once built and can be ramped up in seconds, but large dams flood valleys, displace people, trap the sediment that used to renew the floodplain, and produce methane as drowned vegetation rots under the reservoir.

Geothermal power uses heat from the Earth, drilled as steam or hot water in volcanic and rift areas such as Iceland, Kenya's Rift Valley and the Philippines. It runs day and night whatever the weather, which makes it unusually valuable, but it can only be developed where the heat is near the surface and the drilling is expensive. Biomass is plant material or organic waste burned or digested to make electricity and heat, and the syllabus expects all three forms: solid (wood, wood pellets, straw), liquid (biodiesel and ethanol from crops) and gaseous (biogas from anaerobic digestion of manure, food waste or landfill). It is renewable only if what is burned is regrown, and growing fuel crops competes with food for land and water.

Coal is the cheapest fuel per tonne and the dirtiest per unit of electricity, producing more carbon dioxide than any other source and releasing sulfur dioxide, nitrogen oxides and ash; mining it also scars land and pollutes water. Gas burned in a combined-cycle turbine produces roughly half the carbon dioxide of coal for the same electricity and can be started and stopped quickly, which is why it is used to back up wind and solar, though methane leaking from wells and pipes is a powerful greenhouse gas in its own right. Oil is mainly a transport fuel, but it is burned in diesel generators where there is no grid and in peaking plants at times of high demand, and it is expensive. Nuclear power splits uranium atoms to make heat, produces no carbon dioxide at the point of generation and runs for years on one fuel load, but stations cost billions and take a decade or more to build, the waste stays radioactive for thousands of years, and accidents, however rare, are feared.

• Renewables in the syllabus: wind, solar, tidal, hydroelectric power (HEP), geothermal, and biomass in solid, liquid and gaseous forms.
• Non-renewables: coal, gas and oil. Nuclear is a fourth category - low carbon, but finite and very expensive to build.
• The two limits that matter for every renewable are intermittency (wind and solar cannot be switched on) and land take (they are dilute, so they need large areas).
• Tidal is predictable decades ahead, which no other renewable is; geothermal runs continuously, which no other weather-dependent renewable does.
• HEP is the cheapest renewable per unit once built and the easiest to ramp up, but dams flood valleys, trap sediment and emit methane.
• Biomass is only renewable if the crop is regrown, and it competes with food for land and water.
• Gas produces about half the carbon dioxide of coal per unit of electricity and is flexible, which is why it is called a transition fuel.

Domestic fuels, and the global pattern of energy production and consumption

Not all energy is electricity. In many low-income countries most household energy is a combustible fuel burned for cooking and heating: wood, charcoal (wood heated without air so that it is lighter and burns hotter), animal dung and crop residues. These fuels are often gathered free, they need no grid and no meter, and for hundreds of millions of households they are the only option. They are also the most damaging: burned indoors on an open fire or a traditional stove they produce household air pollution, one of the largest environmental health risks in the world, and the burden of collecting them falls mostly on women and children. Around fast-growing cities, charcoal production strips woodland in a widening ring, and dung and crop residues that should go back into the soil are burned instead, so the soil loses the nutrients it needs.

Globally, production and consumption are rising, because population is rising and because each person uses more energy as incomes grow. Electricity's share is rising fastest, because electricity can be generated from any source and then used for heat, transport and industry, which is the whole basis of the energy transition. The pattern is very uneven. China is the world's largest producer and consumer of electricity, generating more in recent years than the United States and the European Union combined, and it is also the largest producer and user of coal, the largest builder of wind and solar capacity and the dominant manufacturer of both. The United States is second, with a mix of gas, coal, nuclear and a fast-growing share of wind and solar. India is third and rising quickly, with coal still supplying most of its electricity.

Consumption per person is even more unequal than total consumption. A person in the United States, Canada, Australia or the Gulf states uses many times the electricity of a person in sub-Saharan Africa, where in some countries a whole household uses less in a year than a single appliance in a rich country uses in a month, and where hundreds of millions of people have no connection at all. This matters twice over. Almost all of the growth in global demand in the next few decades will come from Asia and Africa rather than Europe or North America, and any agreement to cut emissions has to leave room for poor countries to grow.

The trends are the ones to quote. Fossil fuels still supply the great majority of the world's primary energy, and coal use has plateaued or fallen in Europe and North America while still growing in parts of Asia. Gas has grown as a transition fuel, cleaner than coal and flexible enough to back up renewables. Renewables are the fastest-growing source: solar PV has become the cheapest way to add new generating capacity in most parts of the world, and wind and solar together now supply a large and rising share of electricity in countries as different as Denmark, Uruguay, Chile and China. Nuclear output has been roughly flat, with new stations in China and closures in Europe and North America. None of this has yet reduced total fossil fuel use, because demand has grown faster than the clean capacity added to the system.

• Domestic combustible fuels: wood, charcoal and animal dung, burned for cooking and heating where there is no gas or electricity supply.
• They cause indoor air pollution, deforestation around cities and the loss of nutrients that dung and residues would have returned to the soil.
• China is the largest producer and consumer of electricity and of coal; the United States is second; India is third and growing fastest.
• Per-person consumption in North America, Australia and the Gulf is many times that of sub-Saharan Africa, where many people have no connection at all.
• Trends: total demand rising; electricity's share rising; coal flat or falling in HICs but still growing in parts of Asia; gas as a transition fuel; renewables the fastest-growing source; nuclear roughly flat.
• Fossil fuels still supply most primary energy because demand has grown faster than the renewable capacity added.

The energy transition: environmental, social, economic and political factors, scale, storage and grids

The energy transition is the shift from fossil fuels to sustainable sources across the whole energy system, not only electricity: heat for homes and industry, fuel for transport, and the energy used to make steel, cement and fertiliser. Electricity is the easiest part, and it is where most progress has been made; heat and heavy industry are the hardest, and they are where most emissions now sit.

The environmental factors are not all in favour of renewables. A solar or wind farm needs far more land than a gas-fired station for the same output, which puts pressure on farmland, moorland and valued landscapes, and the materials - copper for cables, lithium, nickel and cobalt for batteries, rare earths for magnets, silicon and silver for panels - have to be mined somewhere, with the damage that mining causes. Wind turbines kill birds and bats, large dams change rivers and block fish migration, and some solar thermal plants use a lot of water for cooling. Set against that, renewables cut carbon dioxide, sulfur dioxide and particulate emissions, and the air-quality gain alone is measured in millions of lives.

The social factors are about who gains and who pays. Closing a coal mine or a steelworks destroys the jobs and the identity of a whole town, and retraining a miner in his fifties for work in a turbine factory is not a simple swap. New wind farms and power lines attract local opposition from people who accept the need for them but do not want them on their own horizon. Biofuel crops compete with food, and large solar parks in poor countries have displaced smallholders. The transition also raises the price of energy at the moment it is carried out, which falls hardest on the poorest households, who already spend the largest share of their income on fuel.

The economic factors decide the pace. Renewable projects are capital-heavy and fuel-free: almost all the cost is paid up front and the electricity is then nearly free, so the cost of borrowing matters enormously, and a poor country with a high interest rate pays far more for the same wind farm than a rich one. Fossil plants are cheap to build and expensive to run, which makes them attractive to a government short of capital. Existing power stations, mines and pipelines become stranded assets if they cannot be used, and writing them off is politically painful. Grids, storage and back-up capacity all have to be paid for, and consumers notice the bill long before they notice the benefit.

The political factors are about security and popularity. A country that imports oil and gas sends money abroad and is exposed to price shocks and to the foreign policy of its suppliers, so producing its own wind, sun or hydro is a matter of national security as well as climate policy. Against that, fossil fuel subsidies are often the largest single item of energy policy in a low-income country, because they buy quiet streets, and removing them has caused riots. Targets such as net zero pledges and nationally determined contributions set a direction, but they only bite if planning rules, taxes and grid investment follow.

Scale and efficiency are where most marks are lost. A plant's capacity is its maximum output; what it actually delivers over a year is that capacity multiplied by its capacity factor, and capacity factors differ hugely. A nuclear station runs at more than 80 per cent of its capacity, a coal or gas station at 40 to 60 per cent, a good wind farm at 25 to 45 per cent and solar PV at 10 to 20 per cent in a temperate country. So 1 000 MW of solar nameplate capacity is not a replacement for 1 000 MW of coal. Conversion efficiency matters too: a coal station turns only about 35 to 40 per cent of the energy in the coal into electricity, a combined-cycle gas plant about 55 to 60 per cent, and a hydro station more than 90 per cent of the energy of the falling water, which is why HEP is so cheap once it is built. A country can add gigawatts of renewables and still see fossil generation rise if demand is growing faster.

Storage and grids are the physical limit on how much variable renewable electricity a system can absorb. Wind and solar cannot be switched on when demand peaks, so a system with a lot of them needs something else to fill the gap. That means pumped storage, where water is pumped uphill when electricity is cheap and released when it is needed; batteries, which respond in milliseconds but store only a few hours; hydrogen, made with surplus electricity and used later in industry or fuel cells; interconnection to a neighbour whose weather is different; and demand flexibility, paying industry to use power when it is plentiful. The grid itself has to be strengthened, because the best wind and sun are often far from the cities that need the power, and a weak grid cannot carry it, which is why curtailment - switching off working turbines because the network cannot take the output - happens even in rich countries. Small islands and remote regions face the hardest version of the problem, because they have no neighbour to trade with.

Decarbonising manufacturing is the frontier. Electricity is only part of industrial energy use; the rest is heat, and much of it comes from burning coal or gas on site. Steel is made by burning coke with iron ore in a blast furnace, which releases carbon dioxide twice over, from the fuel and from the ore itself, so the cleaner route is to reduce the ore with hydrogen made from renewable electricity, or to melt scrap in an electric arc furnace. Cement releases carbon dioxide from the limestone itself as well as from the fuel, so it needs carbon capture or the substitution of part of the clinker. Aluminium is already made by passing electricity through alumina, so its emissions are only as clean as the grid that supplies it. Fertiliser is made from gas, and green ammonia from hydrogen is the replacement. All of these need cheap clean electricity, new plant and a customer willing to pay a green premium, which is why progress depends on policy and on the price of carbon as much as on technology.

• Environmental: land take, mined materials, habitat and bird strikes, methane from reservoirs, water use for cooling - against large cuts in carbon dioxide, sulfur dioxide and particulates.
• Social: jobs in coal and heavy industry, retraining, local opposition to turbines and pylons, biofuel crops competing with food, and higher bills for the poorest.
• Economic: capital-heavy projects, the cost of borrowing, subsidies, stranded assets, grid and storage investment, and competition from cheap gas.
• Political: import dependence and energy security, fossil fuel subsidies, net zero targets and planning rules.
• Scale and efficiency: capacity is not output - multiply by the capacity factor (nuclear over 80 per cent, wind 25-45 per cent, solar 10-20 per cent).
• Storage and grid: pumped storage, batteries, hydrogen, interconnection and demand flexibility, plus transmission to carry power from windy and sunny places to cities.
• Decarbonising manufacturing: hydrogen steel, carbon capture in cement, green electricity for aluminium and green ammonia for fertiliser.

National factors: why countries choose the energy mix they do

No country picks its energy mix from a list of the best options. A mix is the outcome of nine factors acting together, and the syllabus names all nine: resource endowment, climate, pollution, income characteristics, investment in energy technology, balance of types of economic activity, sustainability, energy policy and energy security. Some push supply, some pull demand, and some decide what is allowed.

Resource endowment is the starting point. A country with coal seams, oil fields, big rivers, volcanoes, deserts or a long windy coast will usually use what it has, because importing energy costs foreign currency and exposes it to someone else's decisions. Climate shapes both supply and demand: a wet, mountainous country has HEP, a hot sunny one has solar potential and a large cooling load, a cold one has a winter heating peak, and a country with a strong monsoon has hydro output that swings between flood and drought.

Pollution has become a factor on the supply side as well as an outcome. Cities that cannot meet air-quality limits have to move away from coal, and a carbon price or an emissions trading system makes fossil generation more expensive. Income characteristics decide what is affordable: a high-income country can fund offshore wind and nuclear, subsidise insulation and electric cars and borrow cheaply, while a low-income country struggles to raise capital for a hydro dam even when the resource is excellent, and its households cannot pay a tariff that covers the cost of supply.

Investment in energy technology and the research base decide how fast a country can build. Denmark built a wind industry and India built a solar industry, and both now have cheap domestic supply chains as well as export earnings. The balance of types of economic activity sets the shape of demand: an economy dominated by heavy industry and mining needs large, steady supplies for smelters and machinery, a service economy needs less, and an economy that is electrifying heating and transport needs its grid to grow faster than its economy.

Sustainability commitments - a net zero target, a renewable portfolio standard, a pledge to end coal - change what is permitted and what is financed. Energy policy covers tariffs, subsidies, taxes, auctions, planning rules and ownership, and it is often the factor that decides whether a project happens at all. Energy security is the requirement that energy is available, affordable and not dependent on a single supplier or a single fuel, and it explains choices that look irrational on climate grounds: Japan restarted nuclear and built gas capacity after Fukushima, Germany kept coal burning longer after losing Russian gas, and many countries hold strategic reserves of oil.

Put the nine factors together and you can explain almost any mix. Norway has hydro because of its mountains and rain. France went nuclear in the 1970s because it had no oil and wanted independence. Saudi Arabia uses oil and gas because it sits on them. Denmark has wind because it has a windy coast, no fossil reserves and a policy choice made after the oil shocks of the 1970s. Kenya has geothermal because the Rift Valley runs through it. Brazil has hydro and biofuels because of its rivers and its sugar cane.

Two contrasting countries: Costa Rica and Nigeria

The syllabus asks for two contrasting countries, and the contrast that matters is not only income but the relationship between resources, institutions and outcomes. Costa Rica and Nigeria make a sharp pair: one has almost no fossil fuel and nearly all its electricity from renewables, the other is one of the world's largest oil and gas producers and cannot keep its grid running.

Costa Rica is an upper-middle-income country of about five million people with no oil, no coal and no gas of its own. Its electricity comes almost entirely from renewables - around 70 per cent from hydropower in the wet volcanic highlands, with geothermal from its volcanoes, wind from the breezy highlands and a little biomass - and it has generated more than 98 per cent of its electricity from renewables in most years since the mid-2010s. The factors behind the choice are endowment (heavy tropical rainfall, steep terrain and a volcanic chain), the absence of fossil fuels, a state-owned utility able to plan over decades, and a national commitment to environmental protection that also underpins its ecotourism industry. The challenges begin with the fact that this is renewable electricity, not renewable energy: transport still runs almost entirely on imported oil, so the country's total energy use is far less clean than its power sector. Hydro dominance also makes the system vulnerable to drought in El Nino years, when reservoirs fall and diesel plants are started, and a small system has limited neighbours to trade with when its own rivers fail.

Nigeria is a lower-middle-income country of more than 200 million people and one of the largest oil and gas producers in the world. Its grid is supplied mainly by gas-fired stations and by large hydro stations on the Niger and its tributaries such as Kainji, Jebba and Shiroro, with solar contributing very little. Installed capacity is more than 10 000 MW on paper, but only a fraction is available on a typical day because of gas supply problems, ageing plant, transmission bottlenecks and vandalism, so the grid delivers a few thousand megawatts to a country that needs many times that. More than a third of the population has no connection at all, and those who do rely on petrol and diesel generators, which are expensive, noisy and polluting, or on wood and charcoal for cooking. The factors behind the mix are a huge gas endowment, tariffs that do not cover the cost of supply, chronic underinvestment and a long history of subsidies that left the sector unprofitable. The challenge of energy security here is not a shortage of resources but a failure of delivery: the fuel is there and the electricity is not.

Evaluating the two. Costa Rica shows that a middle-income country can run a power system on renewables, and that endowment plus consistent policy can achieve in a few decades what much richer countries have not. It also shows the limits of that achievement: a clean grid with a dirty transport system is only part of a transition, and a hydro-heavy system needs storage, interconnection and demand management to survive a dry year. Nigeria shows the opposite lesson, that owning fossil fuels is not the same as having energy security, because security depends on plant that works, a grid that carries the power, tariffs that pay for it and institutions that maintain it. Judged as strategies, Costa Rica's has succeeded within its electricity sector and stalled outside it, while Nigeria's has failed on delivery despite abundant resources - which is why its most effective recent programmes have been off-grid, such as solar mini-grids and solar home systems that bypass the grid altogether.

Rural degradation 1: extraction, agriculture and large-scale projects

Pollution is the release of a harmful substance into land, air or water; degradation is the loss of the quality of an environment - its soil, vegetation, water or biodiversity - whether or not a pollutant caused it. The syllabus groups the human activities that do this on a large scale in rural areas into three: rock and mineral extraction, agricultural practices and large-scale infrastructure projects.

Mining and quarrying removes rock, soil and vegetation over areas measured in square kilometres, and it damages all three media. On the land, open-cast pits and quarries leave holes, overburden and spoil heaps of waste rock, and underground working can cause subsidence; the tailings - the fine waste left after the ore is concentrated - are stored as slurry behind dams, and when those dams fail the result is catastrophic, as at Brumadinho in Brazil in 2019. In the air, blasting, crushing, hauling and wind-blown spoil produce dust and fine particulates, and smelting releases sulfur dioxide and heavy metals. In water, rain falling on spoil and tailings produces acid mine drainage, where sulfide minerals oxidise to form sulfuric acid that dissolves metals and kills life in the streams it reaches, while processing plants use and discharge water carrying cyanide, mercury or sediment. Artisanal gold mining adds mercury, which washes into rivers and accumulates in fish and in the people who eat them.

Agricultural practices degrade land through what they take out and what they put on. Taking out: continuous cropping without fallow or organic matter depletes nutrients and structure, ploughing up and down slopes and leaving soil bare between crops causes soil erosion, heavy machinery compacts it, and overgrazing strips the vegetation so that wind and rain take the topsoil. Putting on: nitrate and phosphate fertilisers leach into groundwater and run off into rivers and lakes, where they cause eutrophication - an algal bloom that dies, decomposes and strips the water of oxygen, killing fish. The same process creates the seasonal dead zone in the Gulf of Mexico at the mouth of the Mississippi. Pesticides kill non-target insects and accumulate up the food chain. Irrigation in dry climates raises the water table and brings salts to the surface, so salinisation turns productive land white and sterile, as it has across the irrigated plains of the Indus basin and around the Aral Sea. Livestock add methane from digestion, ammonia and nitrates from slurry, and a demand for animal feed that drives the clearance of forest and savanna.

Large-scale infrastructure changes the movement of water, sediment and animals. A dam floods the valley behind it, drowning farmland, forest and settlements and displacing the people who lived there; it traps the sediment that used to reach the delta, so the coast downstream erodes and the delta sinks; it blocks migrating fish; and the drowned vegetation decomposes without oxygen and releases methane. Roads, railways, pipelines and power lines cut habitats into fragments, open previously remote land to logging and settlement, and cause erosion and landslides where they cut across slopes. The clearest single example is the Aral Sea, where Soviet-era canals took the water of two rivers to grow cotton and the sea shrank to a fraction of its former size, leaving fishing ports stranded in desert, a salt plain whose dust blows across the region, and a local climate made harsher.

Rural degradation 2: managing extraction and agriculture

Management in rural areas works on the same principle as a doctor's: stop the damage at source if you can, treat what has already happened if you cannot, and monitor to check that the treatment worked. For extraction that means restoring the landscape and controlling the company through monitoring and legislation; for agriculture it means farming in ways that conserve resources, improve soil and protect biodiversity.

Restoring an extractive landscape is a sequence, and examiners reward the sequence. First recontour the site: backfill the pit with waste rock, grade the slopes so that they are stable and drain freely, and reshape the spoil heaps to fit the surrounding landform. Second replace the soil: subsoil and topsoil are stripped and stored before mining begins and put back afterwards, because soil takes centuries to form and cannot be manufactured. Third treat the chemistry: lime or organic matter is mixed into acidic spoil to neutralise it, and drainage is passed through constructed wetlands so that metals settle out before the water leaves the site. Fourth revegetate with native species suited to the soil and climate, and protect them until they are established. Fifth decide the afteruse, and this is where the best schemes earn credit: a pit can become a lake for recreation and wildlife, a spoil heap can be planted as woodland, a level site can carry solar panels, housing or industry, and a deep mine can be used for research or for pumped storage. The Eden Project in Cornwall was built in a worked-out china clay pit, and the Lusatian Lakeland in eastern Germany has turned a chain of exhausted open-cast lignite mines into a flooded landscape of lakes.

Monitoring and legislation are what make restoration happen and keep it honest. Before a site is licensed, an environmental impact assessment identifies the likely damage and the measures that will be needed. The licence or planning permission can then impose conditions: limits on dust, noise and discharges; a water abstraction and discharge permit; a duty to restore the land to an agreed standard; and a financial guarantee or bond held by the government so that if the company walks away the taxpayer does not pay for the clean-up. During operation, regulators inspect, sample water and air, and require the operator to report, and satellites and drones now make it possible to watch vegetation cover, subsidence and tailings dams from outside. After closure the site has to be monitored for years, because acid drainage and settling continue long after the last truck leaves. The weakness is always enforcement: rules that exist but are not enforced, and fines smaller than the cost of compliance, change nothing.

Sustainable agriculture follows the same logic of use less, lose less, put back more. To conserve resources: soil testing so that only the nutrients actually needed are applied, precision farming guided by GPS so that fertiliser and pesticide go only where they are needed, drip irrigation and watering scheduled by soil moisture rather than by habit, rainwater harvesting and storage, and renewable energy generated on the farm. To improve soil health: crop rotation including a legume that fixes nitrogen, cover crops and reduced or zero tillage so that the soil is never left bare, returning manure and crop residues, and controlling machinery traffic to avoid compaction. To encourage and protect biodiversity, keep hedgerows, field margins, beetle banks and ponds, and put buffer strips of grass beside watercourses to trap soil and nutrients before they reach the stream. Add agroforestry, where trees grow in the same field as crops or livestock; integrated pest management, which uses predators, resistant varieties and targeted spraying instead of routine prophylactics; organic systems that avoid synthetic inputs; and, in the right places, taking marginal land out of production altogether and rewilding it. Rotational grazing, covered slurry stores and anaerobic digestion of manure deal with the livestock side.

The honest evaluation is that these practices work but are adopted unevenly. They save money in the long run and often raise yields on degraded land, but they need knowledge, capital and advice, and they can reduce output in the short term, which matters to a farmer with no savings to fall back on. They depend on payment schemes where the market does not reward them, and on advisory services that many countries have cut. And they work at field scale, while eutrophication and erosion are catchment-scale problems, so one farmer doing everything right changes little unless his neighbours do the same. That is why catchment partnerships, nitrate limits in vulnerable zones, buffer-strip requirements and agri-environment payments matter as much as individual choice.

• Restoration sequence: recontour, replace topsoil and subsoil, treat acidity and drainage, revegetate with native species, then establish an afteruse.
• Afteruses that count as success: lakes and wetlands for wildlife and recreation, woodland, solar farms, housing or industry, research sites, pumped storage.
• Legislation: environmental impact assessment before licensing, permit conditions on dust, noise and discharges, restoration duties, and financial bonds for closure.
• Monitoring: inspection and water and air sampling during operation, satellite and drone checks on vegetation and tailings dams, and long-term monitoring after closure.
• Conserving resources: soil testing, precision farming, drip irrigation, moisture scheduling, rainwater harvesting, on-farm renewables.
• Soil health: crop rotation with legumes, cover crops, reduced tillage, manure and residues, controlled traffic.
• Biodiversity: hedgerows, field margins, ponds, buffer strips, agroforestry, integrated pest management, organic systems, rewilding of marginal land.

Urban degradation 1: why cities concentrate pollution

A city concentrates people, buildings, vehicles and industry into a small area, and that concentration is exactly what makes it polluting. Building density and height trap pollutants between the walls of the street and cut the wind that would disperse them. The same surfaces of concrete, brick and asphalt absorb heat, so the city is several degrees warmer than the countryside around it - the urban heat island - which raises the demand for cooling and speeds the reactions that form smog. High population means a great many people cooking, heating, travelling and producing waste in the same place. Transport networks funnel vehicles into shared corridors where emissions are highest. The concentration of economic activity puts factories, power stations, workshops and warehouses next to homes. A lack of open space means less interception of rainfall and less filtration of air, so water runs off paved surfaces into drains and rivers instead of soaking away.

Domestic sources are the least visible and often the largest in low-income cities: coal, wood, charcoal and kerosene burned for cooking and heating in unventilated rooms, untreated sewage, and rubbish burned in the street or the yard. Construction produces dust and fine particulates from demolition, excavation and cutting stone, plus noise, and spoil and packaging that are dumped rather than reused. Manufacturing and power generation release sulfur dioxide, nitrogen oxides, particulates and heavy metals from stacks, solvents and fumes from workshops, and heat and chemicals into rivers. Transport is the dominant source in most large cities: nitrogen oxides and fine particulates from diesel, carbon monoxide and hydrocarbons from petrol, and the photochemical smog that forms when sunlight acts on them, with tyres, brakes and road surfaces adding particulates that have nothing to do with the engine. Waste, both solid and sewage, pollutes land and water: landfill produces methane and leachate, the liquid that drains from rotting waste and carries metals and organic matter into groundwater; open dumps are burned and scavenged; and untreated sewage discharges pathogens, nutrients and plastics into rivers and the sea.

Similarities and differences between LICs, MICs and HICs. In all three the list of sources is the same and in all three the poorest residents live in the worst of it. The differences are in the balance. In low-income cities the dominant problems are domestic: solid fuel burned indoors, no sewerage, waste that is not collected and is burned or dumped in drains, and industries mixed in among houses with no separation and no enforcement of standards. Rapid population growth outruns the pipes, so a network designed for a fraction of the present population has to serve all of it. In middle-income cities the pressure shifts to the consequences of growth: motorisation and congestion, construction dust from a permanent building boom, industrial expansion, and the air pollution that comes with burning more coal and oil, often with regulation that exists on paper and is weakly enforced. In high-income cities the classic killers of the nineteenth century, cholera from sewage and choking coal smoke, have been dealt with, and the remaining problems are the ones that come with affluence: traffic and nitrogen dioxide, waste volumes driven by consumption, brownfield land contaminated by former industry, and the export of the worst pollution to the places that make their goods.

Urban degradation 2: sustainable practices in six areas, and why cities plan

In the domestic environment the priorities are clean fuel, clean water and efficient use. Switching cooking from wood, coal or kerosene to LPG, electricity or biogas removes household air pollution at a stroke, which is why subsidised LPG schemes have done more for health in some cities than any measure aimed at traffic. Sewerage and treatment - a sewer network, treatment works and safe disposal of sludge - is the single most effective public health intervention a city can make, and where a sewer is impossible in the short term, improved latrines that are regularly emptied are the interim. In the home, insulation, double glazing, draught-proofing, efficient appliances, LED lighting, low-flow fittings and water meters all cut the energy and water a household needs, and rainwater harvesting reduces both flooding and demand.

In construction, sustainable practice means designing for less: using recycled and low-carbon materials, keeping existing buildings instead of demolishing them, cutting waste through prefabrication and site waste management plans, suppressing dust with water and screens, bunding and covering spoil, protecting drains from runoff, and reusing brownfield land rather than building on green fields. Standards such as BREEAM and LEED, and building codes that set insulation and energy limits, turn good intentions into requirements. Energy conservation at city scale means district heating and cooling, combined heat and power, smart meters and tariffs that reward shifting demand, retrofitting older housing and requiring new buildings to be efficient, because the cheapest unit of energy is always the one that is not used.

Small-scale, on-site generation changes the shape of the system. Rooftop solar PV on homes, schools and warehouses generates where the power is used, so less is lost in transmission and the grid is under less strain at peak. Solar water heaters and heat pumps cut the gas bill; micro-wind suits exposed sites; biogas digesters turn food waste and sewage sludge into gas and fertiliser; and combined heat and power units in factories, hospitals and housing estates supply electricity and useful heat together. In low-income cities the same idea appears as solar home systems and mini-grids that bypass a grid which will not arrive for years, which is how Bangladesh installed millions of solar home systems and how Kenyan companies sell pay-as-you-go solar.

In transport the hierarchy is: avoid the trip, shift it to a better mode, then improve the vehicle. Avoid by planning compact, mixed-use neighbourhoods so that people do not need to drive, and by charging for road space through congestion charging, road pricing and parking controls. Shift by investing in public transport - metros, light rail, bus rapid transit - and in walking and cycling infrastructure, which is the cheapest and cleanest option and has to be safe before people will use it. Improve by tightening emission standards, electrifying buses and fleets, and banning the oldest and dirtiest vehicles through low emission zones. Singapore combines quotas on new cars with electronic road pricing; London charges cars to enter the centre and has an ultra-low emission zone; Bogota built a bus rapid transit system rather than a metro because it was affordable.

Waste management follows the same hierarchy of reduce, reuse, recycle, recover and, last, dispose. Segregation at source is the precondition for everything else. Composting and anaerobic digestion turn food and garden waste into soil and gas, and recycling recovers materials, though it needs a market as well as a collection system, which is why informal waste pickers and their cooperatives are part of the real system in many cities and work better when they are organised and paid. Landfill gas can be captured and burned for electricity, and a sanitary landfill with a clay or membrane liner, leachate collection and treatment replaces the open dump. Sewage needs at least secondary treatment, and combined sewers that overflow in storms have to be separated or given storage; London's deep interceptor tunnel under the Thames was built for exactly this reason.

Why cities need planning and controls. A city is a set of shared systems - water, drains, roads, air, waste - and none of them works if each owner does as they like. Almost every reason for planning is about a cost that one person imposes on everybody else.

The instruments are zoning and land-use maps, building codes, density and height limits, emission and discharge permits, protected areas, development charges and planning conditions, and enforcement. The weakness is always the same: rules are only as good as the authority that enforces them, and enforcement is weakest exactly where the pressure is greatest and the residents have least voice.

Evaluating urban management. The measures that stop pollution at source - clean cooking fuel, sewerage, public transport, efficient buildings, planning - are the cheapest per unit of damage avoided, the most permanent, and they protect everyone regardless of income. The measures that treat pollution after it is produced - treatment works, landfill liners, scrubbers on chimneys - are necessary but expensive and do nothing about the volume. The main obstacles are money, institutional capacity and politics: a city that cannot collect its waste cannot enforce a segregation rule, and a mayor who bans old vehicles loses the votes of the people who drive them. The distributional point is the one that lifts an answer: the poorest residents usually contribute least to urban pollution and suffer most from it, because they live beside the road, the dump and the factory and cannot afford the cleaner alternative.

• Separate incompatible uses: a chemical works is not built beside a school, and a noisy road is not run past a hospital.
• Provide infrastructure at the density it will be used: pipes and buses are only affordable when buildings are close together and laid out in advance.
• Protect the environment everyone shares: a river, a catchment or an airshed, where one polluter damages everybody.
• Keep land open: green belts, parks and urban growth boundaries limit sprawl and give people somewhere to breathe.
• Reduce risk: keep building off floodplains and unstable slopes, and enforce standards that resist hazards.
• Make the city work economically: congestion and ill health are costs on every business in it.
• Enforce as well as write the rules: a plan that is not policed changes nothing, which is why enforcement capacity matters as much as the regulation itself.