Climate Change

Evidence for Climate Change

Temperature Records

Instrumental temperature records, compiled from weather stations, ships, and buoys, show that Earth's average surface temperature has increased by approximately $1.1^\circ$C since the pre-industrial period (1850--1900). The rate of warming has accelerated: the warmest decade on record was 2014--2023, and each of the last four decades has been successively warmer than any decade that preceded it since 1850.

Key data sources include:

• HadCRUT5 (Hadley Centre and Climatic Research Unit): a gridded dataset of near-surface temperature anomalies since 1850, combining land station and sea surface temperature data.
• GISTEMP (NASA Goddard Institute for Space Studies): a global temperature dataset since 1880, incorporating corrections for urban heat island effects and changes in measurement practices.
• NOAA Global Temperature Anomaly dataset: maintained by the US National Oceanic and Atmospheric Administration.

All three datasets show consistent warming trends despite differences in methodology and spatial coverage, providing robust confirmation of the warming signal.

Ice Cores

Ice cores drilled from ice sheets in Greenland and Antarctica provide a continuous record of atmospheric composition and temperature spanning the last 800 000 years (the EPICA Dome C core in Antarctica). By analysing the ratio of oxygen isotopes ($^{18}\mathrm{O}$ / $^{16}\mathrm{O}$) in the ice, scientists reconstruct past temperatures: during glacial periods, $^{18}\mathrm{O}$ is preferentially retained in the oceans, so ice deposited during glacial periods has a lower $^{18}\mathrm{O}/^{16}\mathrm{O}$ ratio than ice deposited during warm periods.

Ice cores also preserve air bubbles containing samples of past atmospheres, allowing direct measurement of greenhouse gas concentrations. The Vostok and EPICA ice cores show that atmospheric $\mathrm{CO_2}$ concentrations have fluctuated between approximately 180 ppm (during glacial periods) and 280 ppm (during interglacial periods) over the last 800 000 years. The current atmospheric $\mathrm{CO_2}$ concentration, approximately 420 ppm (2023), exceeds any level in this period and is rising at an unprecedented rate (approximately 2.5 ppm per year).

Sea Level Rise

Global mean sea level has risen by approximately 20 cm since 1900, and the rate of rise is accelerating. Satellite altimetry (since 1993) shows a rate of approximately 3.4 mm per year, compared to approximately 1.3 mm per year during the twentieth century.

Sea level rise is caused by two processes:

1. Thermal expansion: as ocean water warms, it expands. Thermal expansion accounts for approximately 42% of observed sea level rise since 1970.
2. Melting of land ice: melting glaciers and ice sheets (Greenland, Antarctica) add water to the oceans. The Greenland Ice Sheet alone lost approximately 280 billion tonnes of ice per year during the period 2012--2016.

Ocean Acidification

The ocean absorbs approximately 30% of anthropogenic $\mathrm{CO_2}$ emissions. Dissolved $\mathrm{CO_2}$ reacts with seawater to form carbonic acid ($\mathrm{H_2CO_3}$), which dissociates to release hydrogen ions ($\mathrm{H^+}$), reducing ocean pH. Since the pre-industrial era, average ocean pH has decreased from approximately 8.2 to approximately 8.1, representing a 26% increase in acidity (because pH is logarithmic).

Ocean acidification impairs the ability of calcifying organisms (corals, shellfish, pteropods) to build and maintain their calcium carbonate shells and skeletons. Coral bleaching, driven by both warming and acidification, threatens coral reef ecosystems that support approximately 25% of marine biodiversity and provide livelihoods for approximately 500 million people.

The Greenhouse Effect

Natural Greenhouse Effect

The natural greenhouse effect is a physical process that warms Earth's surface. Solar radiation (shortwave radiation, predominantly in the visible spectrum) passes through the atmosphere and is absorbed by Earth's surface. The surface re-radiates this energy as longwave infrared radiation. Greenhouse gases in the atmosphere -- primarily water vapour ($\mathrm{H_2O}$), carbon dioxide ($\mathrm{CO_2}$), methane ($\mathrm{CH_4}$), nitrous oxide ($\mathrm{N_2O}$), and ozone ($\mathrm{O_3}$) -- absorb and re-emit this longwave radiation in all directions, including back toward the surface, thereby warming the lower atmosphere.

Without the natural greenhouse effect, Earth's average surface temperature would be approximately $-18^\circ$C rather than the observed $+15^\circ$C. The natural greenhouse effect is therefore essential for maintaining habitable conditions on Earth.

Enhanced Greenhouse Effect

The enhanced greenhouse effect refers to the additional warming caused by human activities that have increased the atmospheric concentrations of greenhouse gases beyond their natural levels.

Greenhouse Gas	Pre-industrial Concentration	2023 Concentration	Main Sources	Global Warming Potential (100-year)
$\mathrm{CO_2}$	280 ppm	420 ppm	Fossil fuel combustion, deforestation, cement production	1 (reference)
$\mathrm{CH_4}$	722 ppb	1923 ppb	Agriculture (rice paddies, livestock), fossil fuel extraction, landfills	28
$\mathrm{N_2O}$	270 ppb	336 ppb	Agricultural fertilisers, industrial processes, fossil fuel combustion	265
Fluorinated gases (CFCs, HFCs, SF$_6$)	Negligible	Variable	Industrial processes, refrigeration, air conditioning	Up to 23 000 (SF$_6$)

Global Warming Potential (GWP) is a measure of how much energy a greenhouse gas absorbs relative to $\mathrm{CO_2}$ over a specified time horizon (typically 100 years). Methane has a GWP of 28, meaning that 1 tonne of methane traps 28 times as much heat as 1 tonne of $\mathrm{CO_2}$ over a 100-year period, though methane has a much shorter atmospheric lifetime (approximately 12 years) than $\mathrm{CO_2}$ (approximately 100--1000 years).

Causes of Climate Change

Natural Causes

Volcanic activity. Volcanic eruptions release aerosols (particularly sulphur dioxide, $\mathrm{SO_2}$) into the stratosphere, which reflect incoming solar radiation and produce a short-term cooling effect. The 1991 eruption of Mount Pinatubo in the Philippines reduced global average temperatures by approximately $0.5^\circ$C over the following two years. However, volcanic $\mathrm{CO_2}$ emissions are negligible compared to anthropogenic emissions (volcanic emissions are approximately 0.1% of annual anthropogenic $\mathrm{CO_2}$ emissions).

Solar variability. The Sun's energy output varies over approximately 11-year cycles (sunspot cycles) and over longer timescales. Changes in solar irradiance can influence Earth's climate, but satellite observations since 1978 show that variations in solar output are too small to account for the observed warming trend.

Orbital variations (Milankovitch cycles). Variations in Earth's orbit (eccentricity, axial tilt, and precession) alter the distribution and intensity of solar radiation received at different latitudes and seasons, driving glacial-interglacial cycles over timescales of tens of thousands of years. These cycles cannot explain the rapid warming observed since 1850, which has occurred over decades rather than millennia.

Anthropogenic Causes

The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (2021--2023) states with very high confidence that "human influence has warmed the atmosphere, ocean and land." The primary anthropogenic causes are:

Fossil fuel combustion. Burning coal, oil, and natural gas for energy, transport, and industry is the single largest source of anthropogenic $\mathrm{CO_2}$, accounting for approximately 73% of total anthropogenic greenhouse gas emissions. Global $\mathrm{CO_2}$ emissions from fossil fuels reached approximately 36.8 billion tonnes in 2023.

Deforestation. Forests store approximately 860 billion tonnes of carbon. Deforestation releases stored carbon as $\mathrm{CO_2}$ when trees are burned or left to decay, and reduces the capacity of the land surface to absorb $\mathrm{CO_2}$ through photosynthesis. Deforestation and land-use change account for approximately 11% of total anthropogenic greenhouse gas emissions.

Agriculture. Agricultural practices produce $\mathrm{CH_4}$ (from rice paddies and enteric fermentation in livestock, particularly cattle) and $\mathrm{N_2O}$ (from synthetic fertilisers and manure). Agriculture accounts for approximately 12% of total anthropogenic emissions, and when land-use change (primarily deforestation for agriculture) is included, the food system accounts for approximately 26--34% of total emissions.

Industrial processes. Cement production (calcium carbonate is heated to produce calcium oxide, releasing $\mathrm{CO_2}$), chemical manufacturing, and fluorinated gas production contribute approximately 5% of total emissions.

Impacts of Climate Change

Global Impacts

The IPCC projects that under current policies, global mean surface temperature is likely to reach $2.5$--$2.9^\circ$C above pre-industrial levels by 2100. Under the Paris Agreement pledges (Nationally Determined Contributions), warming of $2.0$--$2.8^\circ$C is projected. Limiting warming to $1.5^\circ$C (the aspirational target of the Paris Agreement) would require global $\mathrm{CO_2}$ emissions to reach net zero by approximately 2050.

At $1.5^\circ$C of warming, projected impacts include:

• Sea level rise of approximately 26--77 cm by 2100 (relative to 2000)
• 70--90% decline in coral reefs
• Increased frequency and intensity of extreme heat events (events that occurred once per decade in the pre-industrial climate are projected to occur 4.1 times per decade at $1.5^\circ$C)
• Increased intensity of precipitation (approximately 7% more intense rainfall per degree of warming)
• Reduced crop yields in tropical and subtropical regions (maize yields projected to decline by approximately 10% at $1.5^\circ$C)

At $2.0^\circ$C, impacts are significantly more severe:

• Sea level rise of approximately 36--87 cm by 2100
• 99% decline in coral reefs
• Extreme heat events 5.6 times per decade
• Additional 420 million people exposed to extreme heat
• Maize yield declines of approximately 15%

Regional Impacts

The Arctic. The Arctic is warming at approximately 2--4 times the global average rate (Arctic amplification), driven by several feedback mechanisms: reduced ice and snow cover decreases surface albedo (reflectivity), exposing darker ocean water that absorbs more solar radiation; thawing permafrost releases $\mathrm{CH_4}$ and $\mathrm{CO_2}$, further amplifying warming. Arctic sea ice extent has declined by approximately 13% per decade since satellite observations began in 1979. Some models project ice-free Arctic summers (sea ice extent below 1 million km$^2$) as early as the 2030s.

Small island developing states (SIDS). Low-lying atoll nations (Maldives, Tuvalu, Kiribati, Marshall Islands) face existential threats from sea level rise, saltwater intrusion into freshwater aquifers, increased storm damage, and coral reef degradation. At current rates, some atoll islands could become uninhabitable by mid-century.

Africa. Africa is the continent most vulnerable to climate change despite contributing the least to global emissions (approximately 3.8% of cumulative historical $\mathrm{CO_2}$ emissions). Projected impacts include: reduced agricultural yields (particularly for rain-fed agriculture in Sub-Saharan Africa), increased water stress (an additional 350--600 million people in Africa projected to face water scarcity by 2050), increased frequency and intensity of droughts and floods, and expanded geographic range of vector-borne diseases (malaria, dengue fever).

Economic impacts. The Swiss Re Institute estimates that global GDP could decline by up to 18% by 2050 if global temperatures rise by $3.2^\circ$C, compared to a scenario with no climate change. The economic costs are unevenly distributed: low-income countries, which have contributed least to emissions, face the highest economic losses as a proportion of GDP.

Social and Environmental Impacts

• Health: heat-related mortality, expanded range of vector-borne diseases, respiratory illness from wildfire smoke and air pollution, food and water insecurity
• Migration: the World Bank estimates that up to 216 million people could be internally displaced by climate change by 2050 (in Sub-Saharan Africa, South Asia, and Latin America)
• Biodiversity loss: the IPCC estimates that a significant fraction of species face increased extinction risk under warming exceeding $1.5^\circ$C
• Ecosystem disruption: shifts in species distributions, phenological changes (timing of flowering, migration, breeding), and increased wildfire frequency

Mitigation Strategies

Carbon Capture and Storage (CCS)

CCS involves capturing $\mathrm{CO_2}$ from point sources (power plants, industrial facilities) or directly from the atmosphere (direct air capture, DAC), compressing it, and transporting it to a storage site (typically deep saline aquifers or depleted oil and gas reservoirs), where it is injected and permanently sequestered underground.

• Current capacity: approximately 40 million tonnes of $\mathrm{CO_2}$ captured per year globally (2023), representing a tiny fraction of annual emissions (approximately 0.1%)
• Limitations: high cost (DAC is estimated to cost USD 250-600 per tonne of $\mathrm{CO_2}$, compared to approximately USD 50-150 for point-source CCS); energy-intensive; storage site availability and long-term liability for sequestered $\mathrm{CO_2}$; public acceptance concerns

Renewable Energy

Technology	Description	Global Capacity (2023)	Key Advantages	Key Limitations
Solar photovoltaic (PV)	Converts sunlight directly to electricity using semiconductor cells	Approximately 1200 GW	Rapidly declining costs (approximately 90% decline since 2010); modular; low operating costs	Intermittent; land use; requires energy storage or backup
Wind (onshore and offshore)	Converts kinetic energy of wind to electricity	Approximately 1000 GW	Low operating costs; declining costs	Intermittent; visual impact; noise; bird and bat mortality
Hydropower	Uses gravitational potential energy of falling water	Approximately 1400 GW	Reliable; provides baseload power; storage capability	Environmental impacts (habitat destruction, sediment disruption); displacement of communities
Nuclear	Generates electricity through nuclear fission	Approximately 440 GW	Low-carbon; reliable baseload; high energy density	High capital cost; radioactive waste; safety concerns (Chernobyl, Fukushima)
Geothermal	Uses heat from Earth's interior	Approximately 15 GW	Reliable baseload; small land footprint	Limited to tectonically active regions; drilling costs

The International Energy Agency (IEA) estimates that achieving net-zero emissions by 2050 would require annual clean energy investment to increase from approximately USD 1.8 trillion (2023) to approximately USD 4.5 trillion by 2030.

International Agreements

The Kyoto Protocol (1997). The first legally binding international agreement to reduce greenhouse gas emissions. It imposed emission reduction targets on developed (Annex I) countries but exempted developing countries. The Protocol was weakened by the withdrawal of the USA (the world's largest emitter at the time) and the failure of some signatories to meet their targets. The Protocol's first commitment period (2008--2012) was followed by a second commitment period (2013--2020), but the latter covered only a small fraction of global emissions.

The Paris Agreement (2015). Adopted by 196 parties at COP21, the Paris Agreement aims to limit global warming to "well below $2.0^\circ$C" above pre-industrial levels, with efforts to limit it to $1.5^\circ$C. Key features:

• Nationally Determined Contributions (NDCs): each country sets its own emission reduction targets, submitted every five years, with a "ratchet mechanism" requiring progressively more ambitious targets
• Global Stocktake: a periodic assessment (first completed in 2023) of collective progress toward the Agreement's goals
• Climate finance: developed countries committed to mobilising USD 100 billion per year in climate finance for developing countries (a target that was not met until 2022, two years late)

Effectiveness. As of 2023, current NDCs are insufficient to limit warming to $2.0^\circ$C, let alone $1.5^\circ$C. The UN Environment Programme's Emissions Gap Report (2023) estimates that implementing current NDCs would result in warming of approximately $2.5$--$2.9^\circ$C by 2100. The Agreement's reliance on voluntary national pledges, without enforcement mechanisms, is its principal weakness.

Adaptation Strategies

Adaptation involves adjusting to the actual or expected effects of climate change. Adaptation strategies vary by context and include:

Physical Adaptation

Strategy	Description	Example
Sea walls and coastal defences	Hard engineering structures to protect against sea level rise and storm surges	The Netherlands' Delta Programme, investing over €20 billion in coastal protection; the MOSE barrier in Venice
Flood management	Improved drainage, floodplain restoration, early warning systems	Bangladesh's Flood Action Plan, which has reduced flood fatalities despite increasing flood frequency
Drought-resistant crops	Development and deployment of crop varieties that tolerate heat and water stress	Drought-tolerant maize varieties developed by the CGIAR for Sub-Saharan Africa
Water management	Desalination, rainwater harvesting, improved irrigation efficiency	Singapore's Four National Taps strategy (see Freshwater Issues)
Building codes	Requiring climate-resilient construction standards	Japan's earthquake and tsunami-resistant building codes; Caribbean building standards for hurricane resistance

Social and Institutional Adaptation

• Early warning systems: the WMO reports that early warning systems can reduce damage from extreme weather events by 30%. However, approximately one-third of the world's population, primarily in Least Developed Countries and Small Island Developing States, is not covered by early warning systems.
• Migration planning: planned relocation of communities from areas at risk of permanent inundation (e.g., Fiji's planned relocation of coastal villages).
• Insurance and risk transfer: climate risk insurance schemes (e.g., the Caribbean Catastrophe Risk Insurance Facility) provide financial compensation after extreme events.
• Ecosystem-based adaptation: using natural systems (mangroves, wetlands, forests) to reduce climate risks. Mangrove restoration can reduce wave energy by 66% and is significantly cheaper than constructing sea walls.

Climate Justice

The Equity Problem

Climate change raises profound questions of equity and justice because:

1. Historical responsibility. Approximately 60% of cumulative anthropogenic $\mathrm{CO_2}$ emissions since 1850 have been produced by the USA (approximately 25%), the EU (approximately 17%), China (approximately 11%), and Russia (approximately 7%). Yet the countries most vulnerable to climate change (Small Island Developing States, Least Developed Countries) have contributed least to cumulative emissions.
2. Per capita emissions. Per capita $\mathrm{CO_2}$ emissions vary enormously: the USA emits approximately 14 tonnes per person per year, China approximately 8 tonnes, India approximately 2 tonnes, and many Sub-Saharan African countries less than 0.5 tonnes.
3. Vulnerability. Developing countries face the greatest impacts (reduced agricultural yields, water stress, extreme weather, sea level rise) but have the least capacity to adapt, due to limited financial resources, institutional capacity, and technological infrastructure.

The Principle of Common But Differentiated Responsibilities (CBDR)

The UNFCCC (1992) establishes the principle of "common but differentiated responsibilities and respective capabilities," recognising that all countries have a shared obligation to address climate change but that developed countries bear a greater responsibility due to their historical contributions and greater capacity.

Loss and Damage

"Loss and damage" refers to the irreversible impacts of climate change that cannot be adapted to -- for example, the permanent loss of territory due to sea level rise, or the extinction of species. Developing countries, particularly Small Island Developing States and Least Developed Countries, have advocated for a dedicated loss and damage finance mechanism for decades.

At COP27 (2022), parties agreed to establish a Loss and Damage Fund, and at COP28 (2023), the fund was operationalised with initial pledges of approximately USD 700 million. However, this sum is negligible relative to estimated loss and damage costs: the UN estimates that annual loss and damage in developing countries will reach USD 290-580 billion by 2030.

Case Study: The Maldives

The Maldives is an archipelago of 1192 coral islands in the Indian Ocean, with a population of approximately 520 000. The average elevation is approximately 1.5 m above sea level, making it one of the world's most vulnerable countries to sea level rise.

• Vulnerability: at projected rates of sea level rise (approximately 3--5 mm per year), many Maldivian islands could be submerged by the end of the century. Saltwater intrusion is already contaminating freshwater aquifers on some islands. Coral bleaching (mass bleaching events in 1998, 2016, and 2020) threatens the tourism industry, which accounts for approximately 28% of GDP.
• Adaptation strategies: the Maldives has constructed artificial islands (Hulhumale, built by dredging sand from the sea floor to create land 2 m above sea level), sea walls around vulnerable islands, and desalination plants. Former President Mohamed Nasheed held an underwater cabinet meeting in 2009 to draw attention to the country's vulnerability.
• Loss and damage: the Maldives has been a vocal advocate for loss and damage finance in international climate negotiations, arguing that it faces permanent territorial loss due to emissions it did not produce.

Case Study: Bangladesh

Bangladesh (population approximately 170 million) is one of the most climate-vulnerable countries in the world. Approximately 80% of its land area is low-lying floodplain, and the country is regularly affected by cyclones, flooding, and sea level rise.

• Impacts: the 2007 Cyclone Sidr killed approximately 3400 people and caused approximately USD 1.7 billion in damage. Sea level rise of approximately 3--8 mm per year (higher than the global average) is causing saltwater intrusion into the Ganges-Brahmaputra delta, reducing rice yields and contaminating drinking water. An estimated 20 million people in Bangladesh could be displaced by climate change by 2050.
• Adaptation: Bangladesh has invested in cyclone shelters (approximately 4000 built since the 1990s, which reduced Cyclone Sidr fatalities by over 90% compared to similar-magnitude cyclones in the 1970s), early warning systems, embankments, and floating agriculture (baira -- rafts of water hyacinth that support crops in flood-prone areas). Bangladesh's community-based adaptation programmes are widely regarded as models of good practice.

Case Study: The Netherlands

The Netherlands, with approximately 26% of its land below sea level and 59% susceptible to flooding, has one of the world's most sophisticated flood management systems.

• Historical context: the Netherlands has been managing water for over 1000 years, through polders (land reclaimed from the sea or lakes by diking and draining), windmills (later steam and electric pumps), and dunes and dikes.
• Delta Programme: established after the devastating North Sea flood of 1953 (which killed 1836 people and inundated 200 000 hectares), the Delta Programme has constructed a comprehensive system of storm surge barriers (the Maeslantkering, the world's largest movable storm surge barrier), dikes, and dams. The current Delta Programme (2010--2050) invests over €20 billion in climate-adaptive water management.
• Room for the River (2006--2015): a programme that widened river channels, lowered floodplains, and created water storage areas, allowing rivers more space to flood safely. This approach represents a shift from hard engineering (fighting water) to working with natural processes.

Common Pitfalls: Confusing Mitigation and Adaptation

Mitigation reduces the causes of climate change (greenhouse gas emissions); adaptation reduces the impacts. Both are necessary, but they are distinct. A solar panel is a mitigation measure (it reduces $\mathrm{CO_2}$ emissions). A sea wall is an adaptation measure (it protects against the consequences of sea level rise, which is driven by warming already locked into the system). Some strategies have both mitigation and adaptation co-benefits: mangrove restoration sequesters carbon (mitigation) and provides coastal protection (adaptation). Be precise in distinguishing the two categories in examination responses.