Mission 1: In 1991 one volcano in the Philippines cooled the whole planet. How?

The atmosphere as an energy system

• Energy enters as shortwave radiation from the Sun and leaves as longwave (infrared) radiation from Earth.
• IPCC global averages: 340 W/m² of sunlight arrives at the top of the atmosphere; 100 W/m² is reflected straight back to space.
• Of the rest, 80 W/m² is absorbed by the atmosphere and 160 W/m² by the surface.

The outgoing side of the balance

• The warm surface emits 398 W/m² of longwave radiation.
• Greenhouse gases and clouds absorb much of it and send 342 W/m² back down (back radiation).
• 239 W/m² of longwave radiation leaves the top of the atmosphere.
• Incoming slightly exceeds outgoing: an energy imbalance of 0.7 W/m², so the climate system is gaining heat.

The natural greenhouse effect

• Greenhouse gases let most incoming sunlight through but absorb outgoing longwave radiation and re-emit it, some of it back towards the surface.
• Water vapour is the most important greenhouse gas, and carbon dioxide (CO₂) is the second.
• IPCC (2007): to emit about 240 W/m², a surface would have to be about −19 °C; the actual global mean surface temperature is about 14 °C.
• The natural greenhouse effect makes the difference of about 33 °C (14 − (−19), computed), and makes Earth habitable.

Forcings, and natural forcing 1: the Sun

• Anything that changes the energy balance is a forcing.
• Over the roughly 11-year sunspot cycle, total solar irradiance varies by about 0.1%.
• Over tens of thousands of years, changes in Earth's orbit (Milankovitch cycles) shift how sunlight is spread across seasons and latitudes: eccentricity about 100,000 years, obliquity about 41,000 years, and climatic precession (axial and orbital precession combined) about 23,000 years on average.
• NASA: Earth's current orbital positions "predict our planet should be cooling, not warming".

Natural forcing 2: volcanic eruptions and global dimming

• In June 1991 Mount Pinatubo, in the Philippines, injected about 15 million tons of sulfur dioxide into the stratosphere.
• The gas formed a haze of tiny droplets (aerosols) that "scatter and absorb incoming sunlight".
• Less sunlight reached the surface: global dimming. Stratospheric winds carried the haze around the world.

How big, and how long?

• Over the next 15 months, average global temperature dropped by about 0.6 °C.
• There was measurable cooling for almost two years. The droplets slowly fall out of the stratosphere, so the effect fades.
• Human pollution makes aerosols too. IPCC: other human drivers, "principally aerosols", cooled the planet by 0.0–0.8 °C (2010–2019), partly masking greenhouse warming.

Natural or human? Weighing the drivers (AO3)

• IPCC: observed warming from 1850–1900 to 2010–2019 was about 1.06 °C.
• Best estimate of the human-caused part over the same period: 1.07 °C (likely range 0.8–1.3 °C). Well-mixed greenhouse gases contributed 1.0 to 2.0 °C; other human drivers, principally aerosols, a cooling of 0.0 to 0.8 °C.
• Natural drivers (the Sun and volcanoes) changed temperature by only −0.1 to +0.1 °C.
• Exam move: natural forcings are real, but short-lived (volcanoes) or small (the solar cycle). Quote the size of each driver, not just its name.

Mission 2: Since 1979 the Arctic has warmed nearly four times as fast as the planet as a whole. Why there?

Feedback loops

• A feedback loop is a chain in which a change triggers effects that alter the original change.
• Positive feedback amplifies the change; negative feedback dampens it. Example of negative: a warmer surface emits more longwave radiation to space.
• Water vapour feedback: warmer air holds more water vapour, itself a greenhouse gas. IPCC: "This is a positive feedback". Water vapour responds to temperature rather than driving the warming.

Albedo: how reflective a surface is

• Albedo is the fraction of incoming sunlight a surface reflects (0 = none, 1 = all).
• Thick sea ice covered with snow reflects as much as 90% of incoming sunlight.
• Bare sea ice: about 0.5 to 0.7.
• The open ocean reflects only 6%.

The ice–albedo feedback

• Warming melts snow and sea ice, exposing darker ocean and land.
• Darker surfaces absorb more sunlight, which warms the surface and melts more ice.
• NSIDC: as melt ponds on the ice grow and deepen, "the albedo continues to decrease", leading to more absorbed sunlight and faster melting.
• It is a positive feedback.

Evidence: Arctic sea ice

• On 12 September 2026, Arctic sea ice reached its annual minimum: 4.60 million km² (preliminary), tied for the 10th-lowest in the satellite record.
• The record low was 3.39 million km², in 2012.
• NSIDC: the minimum is shrinking by 11.8% per decade relative to the 1981–2010 average, and the 20 lowest minimums have all occurred in the last 20 years.

Albedo on land

• Clean snow "reflects most of the visible sunlight". Arctic June snow cover on land shrank by 13.4 ± 5.4% per decade from 1967 to 2018, a loss of about 2.5 million km². Darker ground absorbs more sunlight: another positive feedback.
• Woody shrubs and trees are projected to cover 24–52% of Arctic tundra by 2050 (projection). Shrubs poking through snow darken the surface.
• Albedo change can push either way: changes in surface reflectance from land-use change, such as clearing forest for farmland, have had a small net cooling effect (−0.20 W/m² since pre-industrial times).

Arctic amplification

• Using several observational datasets, Rantanen and colleagues (2022) found the Arctic warmed "nearly four times faster than the globe" over 1979–2021.
• Earlier studies had reported two to three times; climate models rarely reproduce a four-fold ratio.
• IPCC: the causes of polar amplification "are well understood". Several feedbacks contribute, including surface albedo (ice and snow), the vertical temperature profile and clouds; this mission focuses on albedo.

Methane today

• Methane (CH₄) is the second most important human-influenced greenhouse gas after CO₂.
• In 2024 it reached 1,942 parts per billion, 166% above pre-industrial (before 1750) levels.
• About 65% of emissions (2010–2019) come from direct human sources.

Methane sources

• Total emissions, 2010–2019 (top-down estimate): 575 million tonnes a year.
• Human sources: agriculture and waste 228; fossil fuels 115; biomass and biofuel burning 27.
• Wetlands and inland waters: 248 million tonnes a year in the bottom-up estimate. Natural sources, especially wetlands, are the most uncertain part of the budget.

Methane feedback loops

• IPCC: the response of natural methane sources to future warming will be increased emissions, including "increased CH4 emissions from wetlands and permafrost thaw".
• Permafrost (permanently frozen ground): Arctic and boreal permafrost holds 1,460–1,600 billion tonnes of organic carbon, almost twice the carbon in the atmosphere (medium confidence).
• Permafrost temperatures have risen to record highs since the 1980s. Thawing lets microbes break down the carbon, releasing CO₂ and methane: a positive feedback.

How big, how soon? (AO3: uncertainty)

• IPCC: thawing permafrost "will lead to carbon release" (high confidence), but there is low confidence in the timing, the size, and the split between CO₂ and methane. Models project 3–41 billion tonnes of carbon released as CO₂ per 1 °C of warming by 2100.
• IPCC: it is very unlikely that gas clathrates in permafrost and under the seabed "will lead to a detectable departure from the emissions trajectory" this century.
• Methane growth swings from year to year: 17.69 ppb in 2021, 4.77 ppb in 2025 (NOAA). Its causes "are still debated".

A complex, interacting system (synthesis)

• Feedbacks link places: sea ice loss in the Arctic, permafrost thaw in Siberia and Canada, and tropical wetland methane all add to the same global energy balance.
• Some processes dampen change: land and ocean currently absorb roughly half of the CO₂ emitted each year (Mission 4).
• Exam move: draw a feedback loop with labelled arrows, name the place, and say whether it is positive or negative. Then say how certain the evidence is.

Mission 3: The UK's emissions fell by 54% between 1990 and 2025. Has the UK really halved its part in climate change?

The enhanced greenhouse effect

• Burning fossil fuels, deforestation, farming and industry add greenhouse gases faster than natural processes remove them.
• Higher concentrations absorb more outgoing longwave radiation: the enhanced greenhouse effect.
• IPCC: in 2019, CO₂ concentrations were higher than at any time in at least 2 million years, and methane and nitrous oxide higher than at any time in at least 800,000 years.

Concentrations today

• 2024 global averages: CO₂ 423.9 ppm; methane 1,942 ppb; nitrous oxide 338.0 ppb.
• These are 52%, 166% and 25% above pre-industrial (before 1750) levels.
• CO₂ rose by 3.5 ppm in 2024, the largest increase since modern measurements began in 1957.

Who emits most? Total emissions

• EDGAR estimates global greenhouse gas emissions in 2025 at 54.1 billion tonnes CO₂-equivalent, not counting land use and forestry.
• Shares: China 29.5%, United States 11.1%, India 8.3%, EU27 5.8%, Russia 4.8%, Indonesia 2.5%.
• Together these six emit 62% of the world total.

Per person, the ranking changes

• EDGAR estimates of emissions per person in 2025 (tonnes CO₂-equivalent): United States 17.53; China 11.11; EU27 7.06; world 6.62; India 3.11; Bangladesh 1.35.
• China's per-person figure rose from 3.15 in 1990 as it industrialised; the UK's fell from 13.27 to 5.45.

Emissions and development

• IPCC (2019 data, excluding land-use CO₂): emissions per person ranged from 2.6 to 19 t CO₂-eq across world regions; the least developed countries averaged 1.7 t and small island developing states 4.6 t.
• Across the world's households, the 10% with the highest emissions per person produce 34–45% of global consumption-based household emissions; the bottom 50% produce 13–15%.
• Emissions tend to rise with industrialisation and income; some high-income countries have since cut theirs.

History matters: cumulative emissions

• Shares of all fossil and industrial CO₂ emitted from 1750 to 2024: United States 23.5%, EU27 16.3%, China 15.4%, Russia 6.6%, UK 4.3%, India 3.6%.
• IPCC: the least developed countries contributed less than 0.4% of historical CO₂ from fossil fuels and industry (1850–2019); small island developing states 0.5%.
• Because CO₂ stays in the air for centuries, cumulative emissions drive today's warming.

Globalization and trade: whose emissions?

• Territorial emissions are released inside a country's borders.
• Consumption-based emissions are "released to the atmosphere in order to generate the goods and services consumed", wherever they were made.
• Trade moves emissions between countries' accounts: goods made in one country are consumed in another.

The UK's carbon footprint

• The UK's consumption-based carbon footprint was about 699 million tonnes CO₂-eq in 2023.
• Emissions embedded in imports rose from 31% of that footprint in 1996 to 53% in 2023.
• Imports from China account for 25% of import emissions.

The UK paradox (source evaluation)

• Since 1996, the UK's territorial emissions have fallen by 50%, but its carbon footprint by only 15%.
• The footprint peaked at 984 Mt in 2007 and has since fallen by about 29%.
• Defra links this to "a shift from manufacturing towards a service-based economy, alongside an increased reliance on imported goods and services". Real change at home matters too: the last coal power station closed in September 2024.

Exam move: four ways to measure responsibility (AO3)

• Total emissions put China first. Per person, the United States is far above China.
• Cumulative emissions put the United States first and the EU27 second (screen 3.8).
• Consumption-based accounts shift some of China's emissions to the countries that buy its goods.
• Each measure suits a different argument. Name the measure, the year and the source, and say whose perspective it supports.

Mission 4: Global sea level has risen about 10 cm since 1993, and the yearly rise has more than doubled. Where is the extra water coming from?

Water stored in ice

• NSIDC: ice sheets hold over 68% of Earth's fresh water.
• If all of it melted, Antarctica's ice sheet would raise sea level by about 58 m and Greenland's by about 7.4 m.
• Both are losing ice. IPCC ocean and cryosphere report, 2006–2015 average: Greenland about 278 billion tonnes a year and Antarctica about 155 billion tonnes a year.

Sea level is rising faster

• IPCC: global mean sea level rose 0.20 m between 1901 and 2018.
• The rate increased from 1.3 mm a year (1901–1971) to 1.9 mm (1971–2006) and 3.7 mm (2006–2018).
• NASA satellites: about 10 cm since 1993, with the yearly rate more than doubling.

Where the water comes from

• Two main causes: thermal expansion (seawater expands as it warms) and melting land ice (glaciers and ice sheets).
• IPCC, 1971–2018: thermal expansion explained 50% of the rise, glaciers 22%, ice sheets 20% and changes in land-water storage 8%.
• NASA: in recent years about two-thirds of the rise came from water added from land; in 2024, unusually, two-thirds came from thermal expansion, and the rise was 0.59 cm in the year.
• Melting sea ice adds almost nothing, because floating ice already displaces its own weight of water.

Projections and uncertainty

• IPCC projections for 2100, relative to 1995–2014: a likely rise of 0.28–0.55 m under very low emissions (SSP1-1.9) and 0.63–1.01 m under very high emissions (SSP5-8.5).
• Many low-lying megacities and small islands are projected to see extreme sea levels that historically occurred once a century at least once a year by 2050.
• Projections depend on the emissions path chosen. They are scenarios, not forecasts.

Carbon stores and flows

• Of the CO₂ emitted by humans in 2010–2019, 46% stayed in the atmosphere, the ocean took up 23% and land vegetation stored 31%.
• Carbon in frozen ground (permafrost, the guide's carbon "in ice"): 1,460–1,600 billion tonnes of organic carbon (Mission 2).
• Biosphere: forests and soils store carbon; deforestation and fires release it. Land-use change emitted about 4.1 billion tonnes of CO₂ in 2025 (projection).

Ocean acidification (source evaluation)

• Dissolved CO₂ makes seawater more acidic (lower pH; seawater remains alkaline). Surface ocean pH has fallen by 0.1 units since the industrial revolution, about a 30% increase in acidity.
• This makes it harder for organisms such as oysters and corals to build shells and skeletons.
• NOAA says the ocean absorbs "about 30%" of released CO₂; the IPCC gives 23% for 2010–2019. Different periods and methods give different shares: quote one, with its source.

Are the sinks weakening? (synthesis)

• IPCC: with higher emissions, the land and ocean sinks become less effective: they take up a smaller proportion of emissions.
• Global Carbon Budget 2025: 8% of the rise in atmospheric CO₂ since 1960 is due to climate change weakening the sinks.
• In 2024, wildfires and reduced uptake by land and ocean helped make CO₂'s record rise. Weaker sinks are another positive feedback.

Mission 5: In June 2021 a Canadian village reached 49.6 °C, a national record. Could that have happened without climate change?

Extremes are changing

• IPCC: it is "virtually certain" that hot extremes, including heatwaves, have become more frequent and more intense across most land regions since the 1950s.
• The frequency and intensity of heavy rainfall have increased since the 1950s over most land areas.
• It is "likely" that the share of tropical cyclones that are major (Category 3–5) has increased over the last four decades.

Every increment of warming matters

• A hot extreme over land that happened once in 10 years in a climate without human influence:
  • now (about 1 °C of warming) happens 2.8 times per decade and is 1.2 °C hotter
  • projected at 1.5 °C: 4.1 times, 1.9 °C hotter
  • at 2 °C: 5.6 times, 2.6 °C hotter
  • at 4 °C: 9.4 times, 5.1 °C hotter.

Attribution: the 2021 heatwave

• Attribution science compares how likely an event is today with how likely it would be in a modelled world without human-caused warming.
• In the last days of June 2021, Lytton, British Columbia, set Canada's all-time record of 49.6 °C.
• World Weather Attribution analysed the heatwave over the region 45–52°N, 119–123°W. It found the heatwave "virtually impossible without human-caused climate change".
• An event as rare as 1 in 1,000 years would have been at least 150 times rarer without it; the heatwave was about 2 °C hotter than it would have been at the start of the industrial revolution.

Drought: three types

• Meteorological drought: much less rainfall than normal.
• Agricultural and ecological drought: soil too dry for crops and plants. Higher temperatures raise evaporative demand, drying soils even when rainfall is normal.
• Hydrological drought: low rivers, lakes, reservoirs and groundwater.
• IPCC: a 10-year agricultural drought in drying regions now happens 1.7 times per decade (range 0.7–4.1).

Case study: the Horn of Africa drought, 2020–2023

• From October 2020, southern Ethiopia, southern Somalia and eastern Kenya had five failed rainy seasons in a row.
• World Weather Attribution: human-caused climate change made such agricultural droughts "about 100 times more likely" (a conservative estimate).
• In a world 1.2 °C cooler, the same low rainfall and evaporation "would not have led to drought at all".

Heat, not rainfall, made the difference

• Rainfall trends differ by season: the long rains are getting drier; the short rains are getting wetter.
• The main climate change effect was "the strong increase in evaporative demand caused by higher temperatures".

Who was hit

• Most vulnerable: households depending on rain-fed farming, agropastoralism and pastoralism.
• Somalia: a study commissioned by UNICEF and WHO, carried out by the London School of Hygiene and Tropical Medicine and Imperial College London, estimated that 43,000 "excess deaths" might have occurred in 2022; half may have been children under five, with the highest death rates around Bay, Bakool and Banadir.
• Movement: at least 180,000 refugees from Somalia and South Sudan crossed into drought-hit areas of Kenya and Ethiopia.

How certain? (AO3: uncertainty)

• Confidence differs by type of extreme. For heat, the IPCC's range is narrow (2.8 times, range 1.8–3.2); for drought in drying regions it is wide (1.7 times, range 0.7–4.1).
• For some events attribution finds no clear climate signal: World Weather Attribution could not conclude that the 2023 Panama Canal drying was due to human-caused climate change (Mission 7).
• Exam move: "climate change made it X times more likely" is stronger evidence than "climate change caused it". Quote the study, the place and the date.

Mission 6: Since the 1950s, many upper-ocean species have moved towards the poles by about 52 km every decade. Why move, and what happens to living things and farms on land?

Climate zones are moving

• IPCC: global warming has shifted climate zones in many regions, with arid zones expanding and polar zones contracting.
• As a result, many species have changed their ranges, numbers and the timing of seasonal activities.
• About half of the species assessed worldwide have shifted towards the poles or, on land, to higher elevations.

In the ocean

• In the upper ocean (top 200 m), marine species have shifted poleward by 52 ± 33 km per decade since the 1950s; seafloor species by 29 ± 16 km (very likely ranges).
• Species track the temperatures they are adapted to: their habitat moves, so they move.
• Seasonal activities shift too: migration, breeding or flowering can happen earlier or later. When predators and prey shift at different rates, food chains can fall out of step.

Biomes on the move

• Tundra: satellites show overall greening across the tundra, with some browning areas (IPCC).
• Woody shrubs and trees are projected to cover 24–52% of Arctic tundra by 2050 (projection).
• Boreal forest: projected to expand at its northern edge and shrink at its southern edge, where it gives way to lower-biomass woodland and shrubland.
• In high mountains, lower-elevation species are migrating upslope, and some snow-dependent species are declining.

A changing migration: North-east Atlantic mackerel (AO3)

• Since 2006 the stock has expanded its summer feeding migration west and north, into waters south of Iceland and towards the Irminger Sea.
• One common explanation is warming opening new waters in the north.
• Pacariz and colleagues (2016) propose another: falling nutrient (silicate) levels and a large stock reducing food in the east, pushing fish west.
• Lesson: a shifting migration is evidence of change, but its cause must be tested, not assumed.

Coral reefs: a habitat that cannot move fast

• The ocean has taken up more than 90% of the excess heat since 1970, driving marine heatwaves.
• From early 2023 to mid-2025, bleaching-level heat stress hit 84% of the world's coral reef area, in the Pacific, Atlantic and Indian Oceans; mass bleaching was documented in at least 83 countries and territories.
• In bleaching, heat-stressed corals expel the algae that feed them; repeated bleaching can kill them.

Limits to adapting

• IPCC: in terrestrial ecosystems, 3 to 14% of species assessed will likely face a very high risk of extinction at 1.5 °C of warming, and 3 to 18% at 2 °C.
• "Hard limits to adaptation have been reached in some ecosystems".
• Species that cannot move (corals) or have nowhere left to go (mountain tops, polar edges) are most at risk.

Agriculture: crop yields

• IPCC: farm productivity has grown, but climate change "has slowed this growth over the past 50 years globally".
• Yields of some crops (for example maize and wheat) have been hit in many lower-latitude regions, while some crops (maize, wheat, sugar beet) have gained in many higher-latitude regions.
• Africa's pastoral systems show lower animal growth rates and productivity.

Limits of cultivation: winners and losers

• Satellites show greening in parts of Asia, Europe, South America, central North America and south-east Australia, from longer growing seasons, CO₂ fertilisation, nitrogen deposition and land management.
• Longer growing seasons and the higher-latitude yield gains above suggest the limits of cultivation for some crops can shift poleward and uphill; at lower latitudes, heat and drought push the other way.
• But higher CO₂ can lower the nutritional quality of crops, and pests and diseases have both increased and decreased.

Soil erosion

• Mainly because of how land is farmed: soil is eroding from farmland 10–20 times faster than it forms under no-tillage farming, and more than 100 times faster under conventional tillage.
• Climate change can worsen land degradation through heavier rainfall, flooding, drought, heat, wind and sea-level rise. At about 1 °C of warming, the once-a-decade one-day heavy rainfall is already 6.7% wetter.
• Exam move: agriculture has winners and losers. Name the latitude, the crop and the scale, and weigh short-term gains (longer seasons) against long-term risks (heat, drought, erosion).

Mission 7: Rotterdam to Yokohama is about 11,200 nautical miles via Suez, but about 6,500 over the top of the world. Will melting ice make the Arctic a new shipping highway?

Health hazards: heat and disease

• Lancet Countdown 2025: the rate of heat-related deaths has risen 23% since the 1990s, to an average of 546,000 deaths a year.
• Heat exposure cost 640 billion potential working hours in 2024.
• IPCC: "The incidence of vector-borne diseases has increased from range expansion and/or increased reproduction of disease vectors".

Health: a projection

• WHO's 2014 assessment estimated that between 2030 and 2050 climate change would cause about 250,000 additional deaths a year, from undernutrition, malaria, diarrhoea and heat stress. It covers only "a subset of the possible health impacts".
• WHO: low-income countries and small island developing states "endure the harshest health impacts".

Migration and displacement

• IPCC: weather and climate extremes are "increasingly driving displacement in all regions".
• IDMC counted 29.9 million disaster displacements in 2025, across 140 countries and territories.
• Storms caused 17.9 million movements. Flood displacements fell to fewer than 7.9 million; wildfires caused more than 694,000. Geophysical hazards such as earthquakes caused about 2.5 million.

Reading displacement figures (AO3)

• These are flows: movements during the year, including repeat moves by the same people (see Unit 1).
• Weather-related hazards caused about 27.4 million of the 29.9 million (29.9 − 2.5, computed from). A weather disaster is not automatically caused by climate change: attribution needs evidence (Mission 5).
• IPCC (projection): at higher warming, "involuntary migration from regions with high exposure and low adaptive capacity would occur". "Climate refugee" is not a legal status (Unit 1).

Ocean transport routes

• The Arctic Council's shipping assessment compared Rotterdam to Yokohama: a nominal 11,200 nautical miles via the Suez Canal, against about 6,500 "across the top of the world".
• A shorter route could save sailing time and fuel, if ice allows.

Arctic shipping is growing

• The number of individual ships entering the Arctic Polar Code area rose 37% from 2013 to 2023, about 500 more ships.
• The distance they sailed rose 111%, from 6.1 million to 12.9 million nautical miles.
• The Arctic Council notes that less sea ice has "significant implications for longer navigation seasons". The IPCC also reports more summertime Arctic shipping, including tourism.

Limits and a counter-example (AO3)

• In 2009 the Arctic Council warned that sea ice "will be present for a majority of the year during the century". In 2021 the IPCC projected that the Arctic is likely to be practically sea-ice-free in September at least once before 2050. Both can be true: summer opens, winter stays icy.
• Panama Canal, 2023: a drought restricted the number and size of ships. The canal uses about 7 billion litres of fresh water a day from rain-fed Lake Gatún.
• World Weather Attribution could not conclude that the drying was due to human-caused climate change; it pointed to El Niño and rising water demand from urban and population growth.

Uneven effects (synthesis)

• IPCC: about 3.3 to 3.6 billion people live in contexts highly vulnerable to climate change.
• In 2010–2020, human mortality from floods, droughts and storms was 15 times as high in highly vulnerable regions as in regions with very low vulnerability.
• Exam move: effects are uneven in space (who is exposed), in time (projections differ by scenario) and between people (Mission 8).

Mission 8: About a quarter of the Netherlands lies below sea level. Tuvalu's land averages about 2 m above it. Which is more vulnerable, and why?

Risk

• IPCC: climate risks result "from the interaction of climate related hazards with vulnerabilities of societies and systems exposed". Textbooks often write this as Risk = Hazard × Exposure × Vulnerability.
• Hazard: the potential occurrence of a physical event or trend that may cause loss or damage (a storm surge, a heatwave, sea-level rise).
• Exposure: the presence of people, livelihoods, ecosystems and assets in places that could be adversely affected.
• Two places facing the same hazard can face very different risk.

Vulnerability and resilience

• Vulnerability: "the propensity or predisposition to be adversely affected", including sensitivity to harm and lack of capacity to cope and adapt.
• Resilience: the capacity to cope with a hazardous event or trend, "responding or reorganising in ways that maintain their essential function, identity and structure".
• Adaptation: adjusting to actual or expected climate to moderate harm or take advantage of opportunities.
• Adaptation can lower exposure (moving people and assets) or vulnerability (building capacity); it cannot change the hazard.

What makes people vulnerable?

• IPCC: vulnerability is higher where there is poverty, governance challenges, limited access to basic services and resources, violent conflict and climate-sensitive livelihoods such as smallholder farming, pastoralism and fishing.
• It is worsened by "inequity and marginalization linked to gender, ethnicity, low income or combinations thereof".
• Vulnerability differs between countries, and between people in the same place.

The Netherlands: high exposure

• PBL: 26% of the land is below sea level. With a further 29% susceptible to river flooding, 55% of the Netherlands is at risk of flooding; including unprotected river foreland, 59% is susceptible.
• The IPCC's 2007 report wrongly said 55% was below sea level. The error came from text supplied by PBL, which published the correction.
• Exam move: check the source behind a statistic, even an authoritative one.

The Netherlands: high capacity

• New flood-protection standards have been in the Water Act since 2017. By 2050, everyone behind a primary flood defence is to have a basic protection level: "1 in 100,000 annually (0.001%)", meaning the yearly chance of dying in a flood should be no more than 1 in 100,000.
• In the first national assessment, completed in 2021, about 67% of the national dike sections met the standard.
• The Delta Programme budget is about €29 billion for 2015–2050, against estimated costs of about €38 billion (± €4 billion): a funding gap.

Tuvalu: high exposure

• Tuvalu, a Pacific atoll nation, has a mean elevation of 2 m above sea level.
• Population: 9,492 (World Bank, 2025). NASA, using a figure of about 12,000, says more than half live on the largest islet, in Funafuti atoll.
• A 2023 assessment found sea level nearly 15 cm higher than 30 years earlier; NASA projects that by 2050 much of the land and critical infrastructure will sit below the average high-tide level.

Tuvalu: adaptation

• The Tuvalu Coastal Adaptation Project, funded by the Green Climate Fund, created 8 hectares of raised land on Fogafale, Funafuti's main islet (2025).
• Raising land is expensive and covers only a small area: it protects a core of homes and services, not the whole nation.

Tuvalu: mobility and its trade-off

• The Falepili Union treaty with Australia (signed 9 November 2023, in force 28 August 2024) gives Tuvaluans the choice to live, work and study in Australia. The first ballot, in 2025, offered up to 280 visa places.
• Australia recognises that Tuvalu's statehood continues "notwithstanding the impact of climate-change related sea-level rise".
• Trade-off: Tuvalu must agree with Australia any security or defence arrangement with another state.

Within a society: age and isolation

• In British Columbia's June–July 2021 heatwave, 619 deaths were identified as heat-related.
• 67% of those who died were 70 or older, 56% lived alone, and 98% died indoors, mostly in homes without adequate cooling.
• The coroners' panel recommended a provincial heat alert and response system.

Education, gender and risk perception

• Education: IIASA researcher Raya Muttarak: education is "more important than GDP in reducing mortality from natural disasters".
• Gender: the IPCC finds vulnerability worsened by inequity linked to gender; in the Somali drought, half of the estimated excess deaths may have been young children (Mission 5).
• Risk perception: in a 2024 UNDP survey, on average 71% of people in the nine small island developing states surveyed said they were more worried about climate change than the year before, against 53% globally. Risk perception can shape whether people prepare.

Mission 9: The least developed countries emitted less than 0.4% of all historical fossil CO₂, yet face some of the worst impacts. Who should cut emissions, and who should pay?

Mitigation and adaptation

• Mitigation: reducing the causes of climate change, by cutting emissions or removing greenhouse gases (renewable energy, carbon pricing, forests).
• Adaptation: adjusting to actual or expected climate and its effects, to moderate harm or take advantage of opportunities (sea walls, heat alerts, drought-tolerant crops).
• Governments act at both global and national scales.

Global agreements

• UN Framework Convention on Climate Change (UNFCCC): adopted 9 May 1992, in force 1994.
• Kyoto Protocol: adopted 11 December 1997, in force 2005. It set binding emission reduction targets for 37 industrialized countries and economies in transition. Limits: the United States signed but never ratified it, and Canada withdrew with effect from 15 December 2012.
• Paris Agreement: adopted 12 December 2015, in force 4 November 2016. The United States' withdrawal took effect on 27 January 2026; the UNFCCC now lists 194 Parties.

How the Paris Agreement works

• Goal: hold warming "well below 2°C above pre-industrial levels" and pursue efforts "to limit the temperature increase to 1.5°C".
• Each country sets its own target in a nationally determined contribution (NDC), on a five-year cycle; progress feeds a global stocktake.
• The UNFCCC's principle: "common but differentiated responsibilities and respective capabilities", with developed countries to "take the lead" (Article 3.1).
• Strength: almost universal participation. Limit: each country chooses its own target.

The gap between pledges and goals

• 2024 was likely the first calendar year above 1.5 °C (1.55 ± 0.13 °C); 2025 was 1.44 °C (± 0.13 °C). The Paris goal is judged over decades, not single years.
• UNEP (2025): with all NDCs fully implemented, warming projections for this century are 2.3–2.5 °C; current policies alone would lead to up to 2.8 °C. The multi-decade average will "very likely" exceed 1.5 °C within the next decade.
• The IPCC (2022) had found that policies in place at the end of 2020 implied a median 3.2 °C by 2100.

The carbon budget

• A carbon budget is the total CO₂ that can still be emitted for a given chance of staying below a temperature limit.
• Global Carbon Budget 2025: the remaining budget for 1.5 °C is 170 billion tonnes of CO₂.
• Projected 2025 emissions: 38.1 billion tonnes from fossil fuels plus 4.1 billion tonnes from land-use change.

Sources far from the impacts

• The least developed countries contributed less than 0.4% of historical CO₂ from fossil fuels and industry (1850–2019); small island developing states 0.5%.
• The largest cumulative emitters are the United States and the EU27 (screen 3.8).
• Yet IPCC: deaths from floods, droughts and storms were 15 times as high in highly vulnerable regions as in regions with very low vulnerability (2010–2020). The sources of emissions are spatially distant from the places most impacted.

Finance responses, and their limits (AO3)

• The 2009 goal of US$100 billion a year was first met in 2022, when developed countries provided and mobilised US$115.9 billion (OECD).
• COP29 (2024) set a new goal: US$300 billion a year by 2035 for developing countries, and called for efforts by all actors, public and private, to scale finance up to US$1.3 trillion a year by 2035.
• A Fund for responding to Loss and Damage, agreed under decision 2/CP.27 and made operational at the following session, assists developing countries "particularly vulnerable to the adverse effects of climate change".
• Evaluate: these are goals and funds, not delivered outcomes; the earlier US$100 billion goal was first met only in 2022.

A national example: the UK

• The UK's Climate Change Act 2008 set a legal duty to cut the net UK carbon account for 2050 by at least 80% from 1990; on 27 June 2019 this became 100% (net zero).
• The Act sets five-year carbon budgets and created the independent Committee on Climate Change.
• 2025 emissions: 367 million tonnes CO₂-eq (provisional), 54% below 1990. The last coal power station closed in September 2024.

Evaluating the UK's approach (AO3)

• Strengths: a legal target, five-year budgets and independent advice; emissions have more than halved since 1990.
• Limits: domestic transport is now the largest source (31% of net emissions); the consumption-based footprint has fallen much less than territorial emissions (Mission 3).
• Exam move: judge a national strategy against its own target, against other measures of emissions, and against what remains hardest to cut.

Mission 10: In May 2024 a company opened what it called the world's largest plant for pulling CO₂ out of the air, in Iceland, designed to capture up to 36,000 tonnes a year. Could machines like it clean up our emissions?

Putting a price on carbon: cap and trade

• Carbon pricing makes polluters pay, through a carbon tax or an emissions trading system (ETS). IPCC: by 2020 these covered over 20% of global emissions, "although coverage and prices have been insufficient to achieve deep reductions".
• In cap and trade, the government sets a cap on total emissions from covered sectors and issues allowances up to it. Firms that cut emissions cheaply sell spare allowances; others buy them. The cap falls over time.
• The EU Emissions Trading System was launched in 2005.

The EU ETS: results (source evaluation)

• The European Commission: by 2023, emissions from the power and industry plants covered had fallen by about 47% compared with 2005.
• Target: a 62% cut by 2030, compared with 2005. Since 2024 it has also covered maritime transport. Its cap is absolute.
• Coverage: the Commission says about 40% of EU emissions; ICAP's 2023-data figure is about 35%. Different years and definitions: name the source.

The EU ETS: limits (AO3)

• Surplus: over-allocation of free allowances, the 2008 financial crisis and overlapping renewables policies built up a surplus of about 140% of annual ETS emissions by 2018. A Market Stability Reserve has removed surplus allowances since 2019.
• Attribution: not all of the 47% fall is due to the ETS. Bayer and Aklin (2020) estimate it saved about 1.2 billion tonnes of CO₂ in 2008–2016, 3.8% compared with no carbon market.
• Carbon leakage: companies may "relocate their production to countries with weaker emission constraints". Sectors at risk receive some free allowances.

China's national ETS

• Began operating in 2021; now covers more than 3,300 companies in power, steel, cement and aluminium smelting.
• It covers around 8 billion tonnes of CO₂, more than 60% of China's CO₂ emissions.
• It is intensity-based: it limits emissions per unit of output, not total emissions. A move to an absolute cap is planned.
• 2025 average prices: China US$9.85 a tonne; EU US$84.01 a tonne, about 8.5 times China's (computed).

Carbon offsetting

• Offsetting: a company, person or country pays for an emissions cut or removal elsewhere (for example protecting a forest) and counts it against its own emissions.
• At COP29, countries agreed "the final building blocks" for how carbon markets will operate under the Paris Agreement.
• Tests of a real offset: additionality (the cut would not have happened anyway), permanence (forest carbon can burn) and leakage (logging may simply move elsewhere).

Do forest offsets work? (AO3)

• West and colleagues (2023, Science) examined 26 avoided-deforestation offset project sites in six countries on three continents.
• They found "most projects have not significantly reduced deforestation", and where they did, reductions were "substantially lower than claimed".
• The problem: the projects' baselines (predicted deforestation without the project) overstated what would have happened. One study of 26 sites is strong evidence, but not the last word.

Technology for mitigation

• IPCC: between 2010 and 2019, unit costs fell by 85% for solar energy, 55% for wind energy and 85% for lithium-ion batteries.
• IPCC: deploying carbon dioxide removal (CDR) "to counterbalance hard-to-abate residual emissions is unavoidable if net zero CO2 or GHG emissions are to be achieved".

Geo-engineering 1: removing CO₂

• Direct air capture machines pull CO₂ from the air. Climeworks' Mammoth plant in Iceland, switched on in May 2024, is designed to capture up to 36,000 tonnes of CO₂ a year.
• It runs on geothermal energy, and the captured CO₂ is pumped underground, where it reacts with basalt rock and turns to stone.
• Evaluate: 36,000 tonnes is a design figure from the company. Climeworks reported that Mammoth removed 675 tonnes in the first half of 2026, far below that capacity (as reported by Bloomberg).

Geo-engineering 2: solar radiation modification (SRM)

• SRM would deliberately reflect a little sunlight back to space. The most developed idea is stratospheric aerosol injection, copying the Pinatubo effect (Mission 1).
• UNEP: SRM technologies "do not solve the climate crisis because they do not reduce greenhouse gas emissions". CO₂ would keep acidifying the ocean (Mission 4).
• IPCC: SRM could offset some warming, but with "substantial residual and overcompensating climate change at the regional scale"; a sudden, sustained stop in a high-emissions world "would cause a rapid climate change" (termination shock).
• UNEP: large-scale deployment is currently "not necessary, viable, prudent or sufficiently safe".

Mission 11: In 2021 Germany's highest court partly upheld complaints against the national climate law brought by people including the very young and some living in Bangladesh and Nepal. How can people outside government change climate action?

Non-governmental stakeholders

• Civil society: citizens, NGOs, young people, community groups and cooperatives.
• Corporations: energy companies, manufacturers, banks and investors.
• Strategies include litigation (going to court), campaigning, community-owned energy, corporate targets and changing investments.
• This mission's case study is Germany, whose emissions EDGAR estimates fell from 1,227 to 660 million tonnes CO₂-eq between 1990 and 2025, a 46% fall (computed from).

A constitutional complaint

• Complainants, some of them very young and some living in Bangladesh and Nepal, challenged Germany's Federal Climate Change Act of 12 December 2019.
• On 24 March 2021 (published 29 April 2021), the Federal Constitutional Court found the complaints "partially successful". The Act's provisions were partly "incompatible with fundamental rights", because they "irreversibly offload major emission reduction burdens onto periods after 2030".
• For the complainants in Bangladesh and Nepal, the court found no violation of a duty of protection; two environmental associations had no standing.

The government's response

• The Act had required at least a 55% cut by 2030, compared with 1990. The court set a deadline of 31 December 2022 for more specific provisions after 2030.
• After the ruling, and with a view to the new EU 2030 target, the government amended the Act in June 2021: a 65% cut by 2030, at least 88% by 2040, and greenhouse gas neutrality by 2045.
• Cause and effect: civil society litigation → court ruling → stronger national law, alongside EU pressure.

A Peruvian farmer sues a German company

• Saúl Luciano Lliuya, a farmer and mountain guide from Huaraz, Peru, sued the energy company RWE in Germany in 2015 (Essen Regional Court, case 2 O 285/15), supported by the NGO Germanwatch.
• Glacial Lake Palcacocha, above Huaraz, reached 17.4 million m³ in February 2016.
• He asked the court to declare that RWE must bear a share of his flood-protection costs equal to its estimated share of global emissions: 0.47% when he sued, reduced to 0.38% at the 2025 hearing.
• Sources and impacts are spatially distant: emissions in Germany, flood risk in the Andes.

The verdict (AO3)

• On 28 May 2025 the Higher Regional Court of Hamm dismissed his appeal against the Essen court's 2016 dismissal, and did not allow a further appeal on points of law.
• It held that, on his account of the facts, RWE could in principle be liable as a co-cause of the risk (Section 1004 of the German Civil Code). But court-appointed experts put the chance of a flood damaging his property within 30 years at about 1%, which the court held insufficient.
• Germanwatch called the reasoning "a significant breakthrough"; the claimant still lost. A strong Discuss answer gives both.

Corporate strategy: RWE

• RWE, a German energy company that generates power from lignite (brown coal), says it is "bringing the exit from coal forward to 2030".
• It says it has closed coal plants one by one since 2020, and that two of its three remaining opencast mines will stop mining lignite in 2029.
• Evaluate: this is the company's own statement. Check it against independent data, and keep it separate from Germany's national legal deadline for coal.

Civil society: citizens' energy cooperatives

• In 2024, DGRV surveyed 951 energy cooperatives. Their 220,000 members have invested about €3.6 billion in renewable energy.
• They generated about 8 terawatt-hours of clean electricity in 2023.
• Community ownership can build local support for wind and solar, and share the profits locally.

Why perspectives differ (synthesis)

• Need: impacts are felt unevenly. In a 2024 UNDP survey of 77 countries, 80% of people wanted their governments to take stronger action; among 20 of the biggest emitters polled, support ranged from 66% in the United States and Russia to 93% in Italy.
• Practicality: costs and jobs. The Dutch Delta Programme expects costs of about €38 billion against a €29 billion budget; coal regions depend on mining jobs.
• Urgency: the remaining 1.5 °C budget equals about four years of emissions; the German court ruled that delay offloads burdens onto the years after 2030.
• Exam move: name each stakeholder, say what they want and why, and weigh their evidence.

How Unit 2 is examined (Paper 2)

• Paper 2: 1 hour 15 minutes, 50 marks. That is 40% of the grade at SL and 25% at HL.
• Section A: a structured question on each core unit (30 marks in total). Section B: an infographic or visual stimulus with structured questions (10 marks). Section C: one 10-mark extended answer from a choice of two.

What the top band rewards (Section C, 9–10 marks)

• "In-depth and question-specific (topic and command term)".
• Examples, statistics and details are integrated and explained, not listed.
• Evaluation is "well-developed": cause and effect, other perspectives, strengths and weaknesses of evidence. In this unit: attribution versus anecdote, projections versus measurements, territorial versus consumption-based emissions.
• A balanced conclusion that matches the evidence; diagrams annotated and explained.

Planning a Unit 2 answer

• Evaluate means "make an appraisal by weighing up the strengths and limitations"; To what extent asks you to "consider the merits or otherwise of an argument" with "empirical evidence and sound argument".
• Example: "Evaluate government-led strategies for responding to climate change." Plan: global scale (Paris, the gap to 2.3–2.8 °C) → carbon trading (EU and China, its limits, offsets) → a national example (UK Act; Netherlands) → technology (removal and its scale) → judgement on effectiveness, for whom and at what scale.

The big ideas

• The energy balance: shortwave in, longwave out; the natural greenhouse effect makes Earth habitable. Volcanoes dim the Sun for a few years; the Sun's cycle is small.
• Feedbacks amplify change: ice–albedo and snow (the Arctic), water vapour, methane from wetlands and permafrost (uncertain in size and timing).
• Responsibility depends on the measure: total, per person, cumulative, consumption-based (the UK paradox).
• Consequences: rising seas (expansion and land ice), weakening sinks, more frequent heat extremes (2021 heatwave), drought from evaporative demand (Horn of Africa), shifting biomes, migrations and crops, health, displacement and Arctic shipping.
• Risk comes from hazard, exposure and vulnerability (the Netherlands and Tuvalu; age in British Columbia).
• Responses: global agreements, carbon markets and offsets, technology, and civil society and companies (Germany). Judge each with evidence.

Final screen. Score, the mission list ticked, and "Next: test yourself in the Verify stage" (links to the Verify quiz).