Climate change impacts and governance

The IPCC and the five categories of projected climate change

The Intergovernmental Panel on Climate Change (IPCC) produces the projections this topic is built on. It was set up in 1988 by the World Meteorological Organization and the United Nations Environment Programme, and it is an intergovernmental research group: governments nominate the scientists and approve the Summary for Policymakers line by line. It does not run its own experiments. Its role is to bring together experts from many fields, gather the published evidence, and produce assessment reports that say what is known, what is uncertain and what is likely. The reports are thoroughly reviewed and updated in a regular cycle - the sixth assessment report appeared between 2021 and 2023 - and they aim to be objective and comprehensive rather than to campaign.

The IPCC does not issue a single forecast, because the future depends on what the world chooses to do. It produces a set of projections, and this syllabus groups them into five categories: very high, high, intermediate, low and very low. Very high means the largest amount of global warming; very low means the smallest. The categories are not five guesses about the same thing - each is a complete, internally consistent story about emissions, and they differ in three ways you must be able to describe: whether emissions keep increasing, are roughly maintained, or rise to a peak and then reduce; the year emissions peak, if they peak at all; and the year of net zero, if net zero is reached at all.

The gap between the categories is the point. Best estimates in the sixth assessment report put warming by 2100, compared with the pre-industrial average, at about 1.4 °C under very low and about 4.4 °C under very high. That is the difference between a world that is difficult but manageable and one in which parts of the tropics become too hot for outdoor work and low-lying coasts have to be abandoned. Learn the shape of the ladder and one figure at each end; you are not required to know the details of the pathways behind the scenarios.

Net zero holds the ladder together. Net zero greenhouse gas emissions means that the gases still being released are balanced by an equal amount removed from the atmosphere, by forests, soils, the ocean or engineered storage, so the net addition is zero. It does not mean emissions stop. It matters because global temperature stops rising once the net addition of carbon dioxide reaches zero: warming depends on the total accumulated in the atmosphere, not on the rate in any single year. The year of net zero, not the year of peak emissions, is what decides how hot the world finally gets.

What each category changes: sea level, rainfall and extreme weather

Sea-level rise has two causes and you need both. First, thermal expansion: water expands as it warms, and the ocean has absorbed the great majority of the extra heat trapped by greenhouse gases. Second, melting land ice - mountain glaciers, the Greenland ice sheet and the West Antarctic ice sheet - which adds water that was previously stored on land. Thermal expansion has been the larger contributor so far, but the ice sheets are the larger long-term risk because they hold enough water to raise sea level by many metres. Projected rise by 2100 runs from about 0.3 to 0.6 m under very low to about 0.6 to 1.0 m under very high, and it does not stop in 2100.

The coastal impact is greater than the global average figure suggests. Many low-lying deltas and coastal cities are also subsiding as groundwater and sediment are compacted, so relative sea level rises faster than the global mean. A higher baseline means the same storm surge reaches further inland, and a flood that was a one-in-a-century event becomes a one-in-a-few-decades event. Add saltwater intrusion into aquifers, beach erosion and the loss of coastal wetlands, and the result is not only flooding but the slow loss of land, soil and drinking water.

Precipitation changes in both amount and pattern, and the syllabus expects both. The broad rule is that wet places get wetter and dry places get drier: high latitudes and the deep tropics receive more, while the subtropical dry belts - the Mediterranean, southern Africa, the south-west of North America, much of Australia - receive less. Within that pattern, rain becomes more concentrated: heavier bursts separated by longer dry spells, so the total may barely change while both floods and droughts become more common. Snow falls as rain in more places, and the timing of the monsoon becomes less reliable.

Extreme weather responds to warming faster than average conditions do. Heatwaves become hotter, longer and more frequent; heavy rainfall becomes more intense because warm air holds more water vapour, roughly seven per cent more for each degree; drought deepens where rainfall falls; tropical cyclones carry heavier rainfall and a larger share of the most intense storms; and marine heatwaves become more common. Under the very low projections every one of these changes is smallest, and under the very high projections every one is greatest - the categories do not only change the average, they change the tails of the distribution, which is where the damage is done.

Cold environments: the fastest and most visible change

Cold environments in this syllabus are the high-latitude areas - the Arctic and the Antarctic - and high-altitude areas such as the Himalayas, the Andes and the Alps. They warm faster than the global average for two reasons: when bright snow and ice melt they are replaced by dark ocean or dark rock, which absorb far more sunlight (the ice-albedo feedback), and the cold, stable air over ice traps heat near the surface. Arctic warming has been roughly two to four times the global rate, which is why the changes there are the clearest evidence that the projections are already arriving.

Snow and ice cover is shrinking. Arctic sea ice has declined by roughly 13 per cent per decade in September since satellite records began in 1979, and the ice that survives the summer is thinner and younger. Mountain glaciers are retreating almost everywhere, the Greenland ice sheet is losing mass, and the snow season starts later and ends earlier. The consequences run through the whole physical system: less white surface means more absorbed heat, which means more melting, and the fresh water running off the ice raises global sea level.

Permafrost is ground that stays below 0 °C for at least two years, and it underlies much of the Arctic and the high mountains. As it thaws, three things happen. The ground slumps and collapses, cracking roads, pipelines, airport runways and the foundations of buildings. Frozen plant and animal remains begin to decompose, releasing carbon dioxide and methane that had been locked away for thousands of years. And the surface becomes more prone to fire and to the release of further carbon, which is a positive feedback that pushes warming further. These three effects make permafrost one of the most important tipping points in the topic.

People and ecosystems at high latitude are on the front line. Ice-dependent species lose their hunting and breeding platforms; Indigenous communities lose travel routes, hunting grounds and the predictability of the seasons; buildings and infrastructure built on frozen ground become unsafe. There are also new opportunities - longer growing seasons, easier access to mineral and oil reserves, and shorter Arctic shipping routes - which is why the Arctic is a useful example of the way climate change redistributes advantage as well as damage. In the Antarctic the pattern is more complex: the sea ice has fluctuated, while the ice shelves and the West Antarctic ice sheet are thinning from below where warmer ocean water reaches them.

• Temperature: Arctic warming at roughly two to four times the global rate, driven partly by the loss of reflective ice.
• Snow and ice: shrinking September sea ice, retreating glaciers, a shrinking Greenland ice sheet, a shorter snow season.
• Permafrost: ground slumping, damaged infrastructure, and the release of carbon dioxide and methane as frozen organic matter thaws.
• Ecosystems and people: loss of hunting and breeding habitat, disrupted Indigenous livelihoods, new shipping routes and resource access.
• Antarctic: ice shelves thinning from below by warm ocean water, with glacier acceleration and a growing contribution to sea level.

Marine environments: sea level, sea temperature and circulation

The sea-level rise and coastal flooding described earlier belong here too, but the coastal consequences are worth spelling out. Rising water combines with tides, storm surges and waves, so the damage is done by events rather than by the average. Low-lying deltas, coral atolls and coastal cities face erosion, saltwater intrusion into soils and aquifers, the loss of mangroves and wetlands that buffer the coast, and eventually the abandonment of land. Because a large share of the world's population and most of its largest cities are on coasts, a change measured in centimetres has consequences measured in billions of dollars and in millions of displaced people.

Sea surface temperature is the second change. The ocean has absorbed about nine-tenths of the excess heat trapped by greenhouse gases, so it has warmed substantially, and short periods of extreme heat - marine heatwaves - have become more frequent and longer. Warm water holds less dissolved oxygen and drives coral bleaching, the poleward migration of fish stocks and the loss of the kelp forests and seagrass meadows that nurse juvenile fish. For people, the effect arrives through fisheries: catches fall or shift beyond the reach of the boats and harbours built for them, and tropical cyclones draw extra energy from warmer surface water.

Ocean circulation is the third. The global conveyor belt, of which the Atlantic Meridional Overturning Circulation (AMOC) is the Atlantic limb, moves warm surface water northwards, where it cools, becomes saltier and denser, sinks near Greenland and Iceland, and returns southwards at depth. Fresh water from melting Greenland ice and heavier rainfall dilutes the surface, making it less dense and less able to sink, so the circulation is projected to weaken. A serious weakening would cool the North Atlantic region even as the world warms, shift tropical rain belts southwards, change monsoon rainfall, and raise sea level along the north-east coast of the United States as water piles up.

The three marine changes interact with the cold environments rather than sitting apart from them. A warmer ocean melts ice shelves from below, which lets glaciers behind them flow faster and raises sea level; melting ice and extra rainfall freshen the surface and slow the circulation; and a slower circulation changes where heat is stored, which feeds back into the atmosphere. This is why the syllabus asks for the interactions between cold, marine and atmospheric environments - the system is one connected set of loops, not three separate topics.

Non-linear change: feedback loops and tipping points

The projections are often drawn as smooth lines, but the real system does not respond smoothly. A feedback loop is a mechanism in which a change produces a further change that either amplifies the original (a positive feedback) or damps it down (a negative feedback). Positive feedbacks accelerate change and make the higher categories worse than a simple linear reading suggests; negative feedbacks slow change but are weaker, slower and, in several cases, weakening as the planet warms.

Three positive feedbacks do most of the work in this topic. The ice-albedo feedback: bright ice reflects most of the sunlight that hits it, while dark ocean and dark land absorb it, so melting ice warms the surface, which melts more ice. The permafrost carbon feedback: thawing ground releases carbon dioxide and methane from frozen organic matter, and both are greenhouse gases, so the thaw amplifies the warming that caused it. The water vapour feedback: warm air holds more water vapour, and water vapour is itself a greenhouse gas, so warming increases the greenhouse effect - this is the strongest feedback of all, and it is why the climate is more sensitive than carbon dioxide alone would make it.

The main negative feedbacks are the uptake of carbon by the land and the ocean, and the possibility of more cloud reflecting more sunlight. Plants grow faster in a carbon-rich atmosphere and the ocean dissolves carbon dioxide, and between them they currently remove about half of what humans emit. But both sinks are weakening as the ocean warms and acidifies and as forests are cleared and burned, and neither can keep pace with rising emissions. Clouds are the largest uncertainty in the projections: some cloud changes cool, others warm, and the net effect is still being pinned down.

A climate tipping point is a threshold beyond which a small additional push sets off a change that continues on its own and is very difficult or impossible to reverse. The syllabus names four families: sea ice loss (an ice-free Arctic summer), ice sheet loss (Greenland and West Antarctica), permafrost melt, and ocean circulation collapse (the AMOC). Each is a positive feedback with a threshold, and each affects the others - a thawing Greenland raises sea level, freshens the Atlantic and slows the circulation, which shifts rainfall, which affects permafrost and forests.

Tipping points matter for the magnitude and speed of change. They make the difference between categories larger than the emissions numbers imply, because passing a threshold adds warming that no emissions cut can quickly undo. They make change faster, because a self-sustaining process runs ahead of the forcing that started it. And they make some changes effectively permanent on human timescales, which is the argument for treating the very low category as a limit to be defended rather than a target to be approached at leisure.

Socio-economic impacts and the pattern of vulnerability

The syllabus groups socio-economic impacts under areas of human habitation, food production and property, and then asks how those impacts are distributed. For habitation, the risks are coastal and river flooding, heat that makes work dangerous, water shortage, and the slow loss of land under settlements - all of which push people to move, usually within a country and usually towards cities that are themselves exposed. Informal settlements on floodplains and estuaries concentrate the risk, because the people with least money live on the land nobody else wanted.

For food production, the effects run through yields, growing seasons and pests. Heat and drought cut yields of staple cereals in the tropics and subtropics; changed seasons disrupt planting and harvesting; warmer winters let pests and diseases survive; livestock suffer heat stress; and fisheries shift as stocks move polewards. Some high-latitude regions gain from longer seasons and new crops, but the gains are smaller than the losses and are concentrated in countries that are already food-secure. The net effect is a redistribution of agricultural advantage away from the poorest and most densely populated regions.

For property, the losses are direct damage from floods, storms and wildfire, the cost of insurance and its withdrawal from exposed areas, the value lost when land is eroded or salinised, and the damage to infrastructure built on thawing permafrost or in the path of a shifting hazard. Property losses fall on governments and insurers as well as households, and in poor countries the state often has to rebuild the same road or embankment after every event. Globally, the money at risk is concentrated in rich coastal cities; the lives and livelihoods at risk are concentrated in poor ones.

Vulnerability is not the same as exposure. Exposure is being in the way of the hazard; vulnerability is the likelihood of being harmed by it, which depends on locational factors (low-lying land, storm tracks, drought-prone or flood-prone climates), on historical, political and economic factors (colonial legacies, conflict, debt, weak institutions, dependence on a single export crop), and on reliance on sensitive ecosystems (subsistence farming, fishing, pastoralism, glacier-fed water, coral reefs). A combination of these produces vulnerability far greater than the sum of the parts: a low-lying delta farmed by tenants with no insurance, no storage and no alternative income is exposed to the same storm as a wealthy coastal city but loses far more.

The IPCC identifies five groups as most vulnerable: least-developed countries, small-scale food producers, low-income households, tropical and subtropical small-island communities, and indigenous communities. In each case the reason is the same combination - high exposure, livelihoods that depend directly on climate, and limited money, technology or political power to adapt. Remember that this section explains why the impacts fall where they do; the strategies available to these groups are the subject of 11.3, and every answer about vulnerability should end by pointing forward to them.

Governance: the UNFCCC, the COP meetings and the Paris Agreement

Climate change is a global problem with no global government, so it is managed through a treaty process. The United Nations Framework Convention on Climate Change (UNFCCC) was agreed at the Rio Earth Summit in 1992 and came into force in 1994; almost every country is a party. Its objective is to stabilise greenhouse gas concentrations at a level that prevents dangerous interference with the climate system, within a timescale that lets ecosystems adapt and does not threaten food production. It also sets the principle of common but differentiated responsibilities, meaning that countries have contributed differently to the problem and have different capacities to act.

The Conference of the Parties (COP) is the annual meeting of the countries that have signed the convention, and it is where the decisions are actually taken. The IPCC and the UNFCCC are separate bodies with different jobs: the IPCC assesses the science, the UNFCCC provides the legal framework, and the COP is the negotiating forum. The link is deliberate - IPCC reports are timed to feed into the COP negotiations and into the periodic review of whether the world is on track, so the science arrives at the table just as governments have to respond to it.

The first binding attempt was the Kyoto Protocol of 1997, which set emission targets for developed countries only. It was limited: the United States never ratified it, Canada withdrew, and the largest and fastest-growing emitters had no targets at all. The Paris Agreement, agreed at COP21 in 2015, changed the architecture. Every country submits a nationally determined contribution (NDC) setting out what it will do, and revisions are expected every five years to raise ambition. The aims are to hold the rise in global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit it to 1.5 °C, to increase the ability to adapt, and to make finance flows consistent with both.

Paris also created the machinery that later COPs have filled in: a global stocktake every five years to assess collective progress, a transparency framework for reporting emissions, an adaptation goal, and commitments on finance and technology transfer. Its weakness is that it has no enforcement mechanism - a country that misses its NDC faces reporting requirements, peer pressure and reputational cost, not penalties. That is the central trade-off of the agreement: near-universal participation and national flexibility, in exchange for targets that are voluntary and, so far, insufficient.

Mitigation: the strategies aimed at net zero

Mitigation means reducing the amount of climate change by cutting emissions or removing greenhouse gases from the atmosphere. Adaptation means adjusting to the change that is already happening. The distinction is the single most examined idea in 11.3, and it is the one students blur most often: mitigation attacks the cause, adaptation deals with the effect, and neither is a substitute for the other.

The first family of mitigation strategies is sustainable energy: shifting electricity and heat from coal, oil and gas to wind, solar, hydro, geothermal and nuclear, using energy more efficiently, electrifying transport and heating, and building the grids and storage that a renewable system needs. The COP28 target of tripling renewable capacity by 2030 is the clearest example of a target in this family. The limitation is not the technology but the speed and the politics: fossil fuel infrastructure is long-lived, grids and storage are expensive, and cutting coal costs jobs and tax revenue in the regions that produce it.

Carbon offsetting pays for reductions made somewhere else - protecting a forest, planting trees, distributing efficient cookstoves or building a wind farm - and counts them against the buyer's emissions. The units are carbon credits, traded under the Paris Agreement's Article 6 rules or in voluntary markets. The criticisms are well established: additionality (would the reduction have happened anyway?), permanence (a forest can burn or be cleared later), leakage (protection in one place can push deforestation to another), double counting (two countries claiming the same cut) and measurement. Offsetting can finance genuinely useful projects, but it cannot be the main route to net zero, because the removals available are far smaller than current emissions.

Climate engineering covers two very different ideas. Carbon capture and storage (CCS) catches carbon dioxide at a power station or factory, compresses it, pipes it away and stores it in depleted oil and gas fields or deep saline rocks; it can also be combined with bioenergy or used to capture carbon directly from the air. It works, but it is expensive, it uses energy, and global capacity is tiny compared with emissions. Solar radiation management (SRM) is different in kind: it would reflect a small part of incoming sunlight, for example by releasing sulphate particles into the stratosphere, mimicking a large volcanic eruption. It could cool the planet quickly and cheaply, but it does not reduce emissions, it does not stop carbon dioxide building up in the oceans, it would alter rainfall patterns, and stopping it abruptly would cause a rapid rebound in temperature - the termination shock. It also raises a governance problem no institution currently solves: who decides the global thermostat?

Nature-based solutions use ecosystems to store carbon and to buffer the impacts. They come in three forms in this syllabus: protection of what already exists (guarding old-growth forest, peatland, mangroves and seagrass), restoration of what has been lost (rewetting drained peat, replanting mangroves, reforesting degraded land), and improvements to management (soil carbon in farmland, agroforestry, less destructive logging). They are attractive because they are relatively cheap, available now, and bring co-benefits for biodiversity, flood defence and livelihoods. Their limits are equally real: land is finite, storage saturates as ecosystems mature, fire and disease can release the carbon again, and measuring the gain is difficult.

• Sustainable energy: renewables, efficiency, electrification, grids and storage, and a phasedown of unabated coal - limited by cost, infrastructure lifetime and political resistance.
• Offsetting and credits: cheap and flexible, but weakened by doubts over additionality, permanence, leakage and double counting, and unable to substitute for cutting emissions at source.
• Carbon capture and storage: cuts emissions from heavy industry and can be combined with bioenergy, but is expensive, energy-hungry and small in scale.
• Solar radiation management: fast and cheap cooling in theory, but treats the symptom, disturbs rainfall, carries a termination shock and has no agreed system of governance.
• Nature-based solutions: protection, restoration and improved management of forests, peat, mangroves and soils - cheap with co-benefits, but limited by land, permanence and measurement.

Adaptation: protection, compromise and withdrawal

Adaptation accepts that some change is now unavoidable and adjusts to it. The syllabus divides adaptation into three approaches, and the division is worth memorising because it turns a vague answer into a structured one. Protection keeps the hazard out. Compromise (accommodation) lives with the hazard by changing how land and buildings are used. Withdrawal (retreat) moves people and assets out of the way.

Protection means hard defences and the ecosystems that act like them: dikes, sea walls, storm surge barriers, embankments, floodgates, beach nourishment, offshore breakwaters, and the conservation of dunes, mangroves and saltmarsh. It protects existing buildings and farmland, allows dense cities to keep working, and is the default in wealthy countries with valuable land to defend. It is also expensive to build and to maintain, it can encourage more development behind the defence, it can worsen erosion further along the coast, and it fails once an event exceeds its design standard - which is exactly what happened on the Japanese coast in 2011 and at New Orleans in 2005.

Compromise accepts that water and heat will sometimes win. It includes raising buildings on stilts, flood-proofing ground floors, amphibious housing that rises with the flood, floating agriculture in flooded fields, early warning systems that move people and livestock in time, insurance and micro-insurance, changed planting dates and crop varieties, water storage and rainwater harvesting, urban shade and cooling centres, and heat action plans. It is usually cheaper and more flexible than protection, and it can be adopted by poor communities, but it does not save fixed assets, it depends on people acting on warnings, and insurance becomes unaffordable precisely where risk is highest.

Withdrawal moves out of the way: managed retreat from a coastline, resettlement of a village, set-back lines that forbid new building near the shore, and buy-outs of repeatedly flooded homes. It is the only option that removes exposure permanently, and it can create space for floodplain or mangrove restoration that reduces risk elsewhere. Its costs are social and cultural as much as financial - people lose their homes, land, graves and community networks, compensation is contested, and resettlement schemes often fail when the new site has no work. It is politically the hardest of the three, which is why governments usually choose it last.

Whichever is chosen, there are limits and challenges. Adaptation has physical limits (an atoll cannot be raised, a glacier cannot be regrown), economic limits (the poorest cannot pay), and technical limits (no defence survives an event above its design standard). It is also vulnerable to maladaptation - a sea wall that shifts erosion downdrift, or a defence that attracts new building onto a dangerous coast. And because adaptation cannot prevent all loss, the residue becomes loss and damage, the issue that produced the fund agreed at COP27. Every adaptation choice is therefore also a question of justice: who is protected, who pays, and who is left to move.