A Warming World: Climate Change Science and Impacts

Climate change refers to long-term shifts in the Earth's climate that are warming the atmosphere, ocean and land, driven overwhelmingly by human emissions of greenhouse gases. It is the master theme of GS-3 environment: it amplifies every other problem, from land degradation to biodiversity loss, and it has already been asked in UPSC mains as a 15-marker on India's Himalayan and coastal vulnerability.

This article sticks to the science and the impacts, leaving negotiations and India's policy response to companion articles. It explains how the greenhouse effect works, takes stock of where the planet stands, unpacks tipping points, traces the damage through oceans, ice and Indian districts, and closes with the twin responses of mitigation and adaptation.

The science: how the greenhouse effect works

The story begins in 1824, when the French physicist Joseph Fourier realised the atmosphere acts like glass: it lets sunlight in but traps outgoing heat. In 1859 the Irish physicist John Tyndall identified the culprits, showing that gases like carbon dioxide and water vapour absorb infrared radiation. In 1896 the Swedish chemist Svante Arrhenius did the maths, calculating that doubling atmospheric CO2 could raise global temperatures by several degrees, a prediction modern models have broadly confirmed.

The mechanism is simple. Incoming shortwave solar radiation passes through the atmosphere and warms the Earth's surface. The surface re-radiates energy as longwave infrared radiation. Greenhouse gases (GHGs) absorb this outgoing infrared and re-emit it in all directions, including back downwards, warming the lower atmosphere. Without this natural greenhouse effect the planet would average around minus 18 degrees Celsius; the problem is the enhanced greenhouse effect from human-added gases.

Four groups of gases matter. Carbon dioxide (CO2), from fossil fuels and deforestation, is the largest driver: long-lived (centuries) and now tracked by the Keeling Curve, the continuous Mauna Loa record begun by Charles David Keeling in 1958 that first proved CO2 was climbing year on year. Methane (CH4), from rice paddies, livestock, wetlands and gas leaks, lives only about 12 years but warms roughly 80 times more than CO2 over a 20-year horizon, which is why cutting it buys fast climate relief. Nitrous oxide (N2O), largely from nitrogen fertilisers, persists over a century with a warming potential near 270 times CO2. Fluorinated gases (HFCs, SF6 and kin), used in refrigeration and industry, are emitted in tiny quantities but carry warming potentials thousands of times CO2. Water vapour is the most abundant greenhouse gas of all, but it acts as a feedback (warming increases it) rather than a forcing (an initial driver), a distinction UPSC loves to test.

The ledger: where the planet stands

The baseline is stark. Global temperatures have risen about 1.1 to 1.2 degrees Celsius above pre-industrial levels, and 2024 was the warmest year on record, touching roughly 1.55 degrees above pre-industrial, briefly breaching the 1.5-degree mark that the Paris Agreement treats as a guardrail. The UNEP Emissions Gap Report 2025 measures the distance between promise and physics: the emissions gap is the difference between expected emissions and the level needed to limit warming.

Its findings are sobering. If current policies continue, warming reaches about 2.8 degrees by 2100; even if every country fully implements its climate pledges, the world still lands near 2.3 to 2.5 degrees. Under the Paris Agreement countries must submit updated Nationally Determined Contributions (NDCs) every five years, yet by September 2025 only about 60 countries covering 63 per cent of global emissions had filed new 2035 targets. The G20 produces about 77 per cent of global greenhouse gas emissions, and among the six largest emitters only the European Union cut emissions in 2024. This is the arithmetic of insufficiency that frames every negotiation.

Why 1.5 and 2 are not just numbers: tipping points

A tipping point is a threshold beyond which a system shifts abruptly and often irreversibly to a new state, like a canoe tipping past its balance point. The climate has several, and the difference between 1.5 and 2 degrees of warming is measured in how many we cross.

Coral reefs are the most vivid example. The IPCC Special Report on 1.5 degrees (2018) warned that at 1.5 degrees of warming, 70 to 90 per cent of coral reefs could disappear; at 2 degrees, losses may exceed 99 per cent, effectively a global wipeout of warm-water reefs. The mechanism is coral bleaching: heat stress forces corals to expel the symbiotic algae (zooxanthellae) that feed them and give them colour; prolonged bleaching starves the coral to death.

The slower tipping elements are just as consequential. The Atlantic Meridional Overturning Circulation (AMOC), the great ocean conveyor that carries warm water north and shapes European and monsoon climates, is at its weakest in over a thousand years; its collapse would rearrange rainfall worldwide. The Greenland and West Antarctic ice sheets hold metres of sea-level rise behind thresholds that sustained 2-degree warming could breach. Permafrost thaw would release methane stores that amplify warming further, a classic positive feedback. The State of India's Environment 2026 warns that warm tropical reefs are already crossing their thermal tipping point, with deaths turning irreversible.

Oceans under stress

The ocean has absorbed most of the excess heat, and it is showing the strain. Marine heatwaves have doubled in frequency since 1982 (IPCC), while the ocean has absorbed more than 90 per cent of the excess heat trapped by greenhouse gases. These underwater heatwaves bleach corals, collapse kelp forests and drive fish stocks poleward, hammering coastal livelihoods.

Two chemical changes compound the heat. Ocean acidification, as atmospheric CO2 dissolves into seawater forming carbonic acid, has raised surface ocean acidity by 30 to 40 per cent since the industrial era; it dissolves the calcium carbonate shells of molluscs and weakens the skeletons corals need to rebuild after bleaching. Meanwhile, warmer water holds less oxygen, expanding deoxygenated zones where marine life suffocates.

The physical consequences reach land. Rising sea surface temperatures are intensifying severe tropical cyclones, loading them with more moisture and energy; warmer air holds more water vapour, so rainfall extremes intensify even where total rainfall barely changes. Sea-level rise, driven by thermal expansion and melting ice, is already eroding coasts and salinising aquifers, a direct threat to delta regions like the Sundarbans, where displacement is accelerating.

The cryosphere is shrinking

The cryosphere, the planet's frozen realm of glaciers, ice sheets, snow and permafrost, is retreating fast. The period 2023 to 2025 recorded the most severe three-year glacier mass loss on record, a milestone with brutal implications: glacial meltwater sustains the great rivers of Asia, and its disruption threatens water security for nearly 2 billion people. For India, this is the fate of the Hindu Kush Himalaya, the water tower feeding the Indus, Ganga and Brahmaputra.

The Arctic is the canary. It is warming four times faster than the global average, a phenomenon called Arctic amplification, and its sea ice is declining by over 13 per cent per decade. When reflective ice gives way to dark ocean, the ocean absorbs more heat and melts more ice, another positive feedback. The consequences travel: Arctic warming disturbs the jet stream, linked to the stuck weather patterns behind recent heatwaves and floods, including in South Asia.

India in the crosshairs

India is among the most exposed large economies. Over 85 per cent of Indian districts are highly vulnerable to multiple climate hazards, heatwaves, floods and droughts striking in combination. Urban heat islands make cities several degrees Celsius hotter than surrounding countryside, turning heatwaves into mass-casualty events for outdoor workers. In 2025, India experienced extreme-weather events on 99 per cent of days, killing 4,419 people and damaging 17.41 million hectares of cropland, the highest toll in four years according to the State of India's Environment 2026.

The geography of vulnerability splits two ways, exactly as UPSC asked in 2017. Himalayan states face glacial lake outburst floods, landslides, shifting snowfall and drying springs as glaciers retreat and permafrost thaws. Coastal states face sea-level rise, cyclone intensification, saltwater intrusion into aquifers and farmland, and the slow submergence of delta islands. Between them, the Indo-Gangetic plains face the monsoon gamble: erratic rainfall, unseasonal downpours and prolonged dry spells that cut agricultural productivity and deepen food insecurity.

The health and human toll follows. Rising temperatures and shifting rainfall expand the range of vector-borne diseases such as dengue and malaria; heat stress cuts labour productivity; and displacement falls hardest on the already vulnerable. Over a billion children live in countries facing extremely high climate risk, women form a large share of the climate-displaced and carry disproportionate water and food burdens, and poor communities, most dependent on natural resources, absorb the deepest shocks. This asymmetry is the moral core of India's climate-justice argument in negotiations.

Two responses: mitigation versus adaptation

Every climate policy is one of two things. Mitigation means attacking the cause: cutting greenhouse gas emissions and enhancing sinks, through renewable energy, energy efficiency, electrified transport, industrial decarbonisation and afforestation. Adaptation means managing the unavoidable consequences: early-warning systems, heat action plans, climate-resilient crop varieties, watershed restoration, seawalls and revised building codes. The two are complements, not substitutes: without mitigation, adaptation eventually gets overwhelmed; without adaptation, today's warming still kills.

A third pillar has now entered the frame: loss and damage, the harms that occur despite mitigation and adaptation, from destroyed homes to lost cultures. Its institutional journey, from the Warsaw International Mechanism (2013) to the dedicated fund agreed at COP27 and operationalised at COP28, belongs to the negotiations story. For the science article, the takeaway is simpler: the ledger shows warming already locked in, which makes adaptation a survival imperative for India even as mitigation remains the global necessity.

Causes in full: natural drift versus human forcing

UPSC tested causes in 2017, and a complete answer separates slow natural drivers from fast human forcing. Natural causes operate over millennia: continental drift and plate tectonics rearrange ocean basins and mountain belts, volcanic activity injects carbon dioxide, dust and aerosols that can warm or cool for years, ocean currents redistribute heat between latitudes and depths, and variations in Earth's orbit change the seasonal and geographic distribution of sunlight. None of these explains the speed of warming since the industrial era.

Human forcing does. Greenhouse gases from fossil fuels, deforestation and industry trap outgoing infrared radiation. Land use change, especially deforestation and intensive agriculture, releases stored carbon while shrinking sinks. Atmospheric aerosols complicate the picture because they push in opposite directions: black carbon and brown carbon absorb sunlight and warm the atmosphere, while sulphate droplets reflect sunlight and cool the surface, masking part of the greenhouse warming while harming health. That masking is one reason cleaning air without cutting gases can briefly unmask warming.

On potency, keep two horizons distinct. Global Warming Potential (GWP) is the heat trapped by a gas over a set period compared with carbon dioxide, whose GWP is 1 by definition. Over 100 years, methane is about 28 to 36 times as potent as carbon dioxide while lasting only about 12 years in the air, and nitrous oxide is about 265 to 298 times as potent while lasting more than a century. High GWP fluorinated gases trap thousands of times more heat per kilogram but are emitted in far smaller volumes. The policy moral is sequencing: cut methane for fast relief, cut carbon dioxide for lasting relief, and never trade one for the other.

Why India has warmed more slowly than the world average

A favourite discriminator question, asked in 2022, notes that global annual mean temperature has risen by about 1.1 degrees Celsius since pre industrial times, while India's annual mean has risen by about 0.7 degrees Celsius since 1900, well below the roughly 1.59 degree rise recorded for land areas globally over a comparable framing (as reported in the source synthesis). Three mechanisms explain the gap, and all three appear in strong answers.

First, large scale circulation moves heat from the tropics toward the poles, so tropical land warms less than high latitude land. Second, the Arctic has warmed at least twice as fast as the global average because melting ice lowers albedo, while changes in clouds, water vapour and lapse rate amplify polar warming, a pattern called Arctic amplification. Third, high aerosol loading over South Asia scatters sunlight and alters clouds, offsetting part of the greenhouse warming locally. The implication is sobering rather than comforting: as air pollution is cleaned and Arctic feedbacks strengthen, India's realised warming can accelerate, so a slower past does not mean a safer future.

Ozone: the other sky problem, and a treaty that worked

Ozone (O3) is a gas of three oxygen atoms with opposite meanings by altitude. In the troposphere it is a pollutant and smog component harmful to lungs and crops. In the stratosphere it forms the ozone layer, which absorbs harmful ultraviolet radiation and protects life. Ozone depletion is most dramatic over Antarctica as the seasonal ozone hole. The mechanism is catalytic: chlorine released from chlorofluorocarbons under ultraviolet light forms chlorine monoxide and destroys ozone repeatedly, while bromine from halons destroys even more ozone per atom.

The response is the closest thing climate governance has to a success story. The Vienna Convention (1985) created the framework for cooperation, and the Montreal Protocol (1987) mandated the phase out of ozone depleting substances, with later amendments accelerating the timetable and the Kigali Amendment addressing hydrofluorocarbons as climate gases. India signed the Protocol in 1992, phased out several substances ahead of schedule, and banned the import of HCFC 141b through the ODS (Regulation and Control) Amendment Rules, 2019, while continuing a phased reduction of hydrochlorofluorocarbons. For answers, Montreal proves that binding schedules plus finance plus available substitutes can work, which is precisely the combination climate talks struggle to replicate.

Carbon capture, utilisation and storage: buying time for hard sectors

Carbon Capture, Utilisation and Storage (CCUS) is a suite of technologies that captures carbon dioxide from large sources such as power plants, steel mills and cement factories before it reaches the air, then stores it permanently in deep geological formations or uses it in products. Utilisation pathways include synthetic fuels and chemicals, enhanced oil recovery, building materials such as concrete, and conversion into plastics and fertilisers.

Its role is specific, not general. For hard to abate sectors such as cement, steel and fertilisers, which cannot easily electrify their core chemistry, CCUS offers one of the few near term routes to deep cuts. Combined with bioenergy as BECCS, it can deliver net removals. It also lets existing fossil infrastructure run cleaner while alternatives scale, bridging the transition. The IPCC Sixth Assessment Report (2022) treats CCUS as essential for limiting warming to 1.5 degrees, contributing up to about 15 per cent of cumulative mitigation effort by 2050 in its pathways. India's coal heavy power and industrial base makes pilots strategically relevant, but cost, storage site characterisation and the risk of prolonging fossil dependence mean CCUS complements renewables and efficiency; it does not replace them.

Key Terms

• climate change: Climate change is the long-term shift in global temperatures and weather patterns, driven since the industrial era chiefly by greenhouse gas emissions from fossil fuels, deforestation and industry. Its features are rising temperatures, sea-level rise and extreme weather. It matters for UPSC because GS-3 covers mitigation and adaptation, India's Panchamrit pledges and climate finance debates, all anchored by IPCC assessments. Example: the Paris Agreement (2015)
• greenhouse effect: The greenhouse effect is the natural warming of the Earth's surface caused by atmospheric gases, such as carbon dioxide, methane, and water vapour, which trap outgoing infrared radiation. Without it, the planet would be far too cold for life; because of it, the Earth stays warm enough to support living things. Human activity has intensified this effect by adding extra greenhouse gases, pushing global temperatures upward. Example: Burning of fossil fuels since the Industrial Revolution has raised atmospheric CO2 from about 280 parts per million to over 420 parts per million, amplifying the natural greenhouse effect.
• tipping points: Tipping points are the multiple thresholds across Earth's climate subsystems, ice sheets, ocean circulation, rainforests, and permafrost, beyond which abrupt and often irreversible shifts occur. Scientists track these interacting tipping elements because crossing one can raise the odds of crossing others, producing a cascade. Keeping warming well below 2 degrees C is framed partly as avoiding these thresholds. Example: Earth system scientists list tipping elements such as the Greenland and West Antarctic ice sheets, the Atlantic Meridional Overturning Circulation (AMOC), the Amazon rainforest, and Arctic permafrost.
• mitigation: In climate policy, mitigation means actions that reduce the severity of climate change by cutting greenhouse gas emissions or enhancing carbon sinks. It includes shifting to renewable energy, improving energy efficiency, electrifying transport, halting deforestation and capturing carbon. Mitigation is paired with adaptation: mitigation tackles the cause of warming, while adaptation manages its unavoidable consequences. Example: India's target of 500 GW of non-fossil electricity capacity by 2030 and its Perform Achieve and Trade scheme for industry are mitigation measures, since they cut the emissions driving climate change.
• adaptation: Adaptation, in the climate context, means adjusting natural or human systems in response to actual or expected climate stimuli and their effects, so as to moderate harm or exploit beneficial opportunities (as defined by the IPCC). It covers structural measures like sea walls and flood embankments as well as non-structural ones like drought-tolerant crop varieties, early warning systems and revised building codes. Adaptation complements mitigation (cutting emissions) and is the core of developing-country negotiating positions on climate justice, since poorer nations bear disproportionate impacts. Example: Growing salt-tolerant paddy varieties in coastal districts and maintaining cyclone early-warning networks are adaptation measures.
• Joseph Fourier: Joseph Fourier (1768-1830) was a French mathematician and physicist who first described what we now call the greenhouse effect. In papers published in the 1820s, he calculated that the Earth, given its distance from the Sun, should be far colder than it actually is, and proposed that the atmosphere acts like a covering that lets sunlight in but traps outgoing heat. He did not know which gases were responsible, a gap filled decades later by John Tyndall, but his insight gave climate science its founding idea. Example: Fourier estimated that without the atmosphere's warming influence, the Earth's surface would be tens of degrees colder, a calculation that framed every later study of the greenhouse effect.
• John Tyndall: John Tyndall (1820-1893) was an Irish physicist who provided the first experimental proof of the greenhouse effect. Beginning in January 1859, using a spectrophotometer he built himself, he showed that gases such as carbon dioxide and water vapour strongly absorb radiant heat (infrared radiation), while the atmosphere's main gases, oxygen and nitrogen, are nearly transparent to it. He concluded that changes in the amount of water vapour or carbon dioxide in the atmosphere could change the climate, laying the physical foundation of modern climate science. Example: Tyndall's famous summary line from his 1861 Royal Society lecture was that the atmosphere admits the entrance of solar heat but checks its exit, tending to accumulate heat at the planet's surface.
• Svante Arrhenius: Svante Arrhenius (1859-1927) was a Swedish scientist who in 1896 first quantified the greenhouse effect, calculating how changes in atmospheric carbon dioxide would alter global temperatures. His work laid the scientific foundation of climate science, and he also won the 1903 Nobel Prize in Chemistry for the theory of electrolytic dissociation. Example: His 1896 paper estimated how doubling atmospheric CO2 would raise Earth's temperature, the first quantitative climate prediction.
• shortwave: In climate science, shortwave radiation is the incoming energy from the Sun, concentrated in the visible and near-infrared parts of the spectrum. Earth's surface absorbs it and re-emits energy as longwave (infrared) radiation, which greenhouse gases trap: this shortwave-in, longwave-out imbalance is the physical basis of the greenhouse effect. Satellites monitor both streams to measure changes in Earth's energy budget. Example: Clouds reflect a large share of incoming shortwave radiation back to space, which is why they cool the surface during the day.
• longwave: Longwave radiation is the infrared heat energy that the Earth's surface emits after being warmed by incoming sunlight. Shortwave solar radiation passes through the atmosphere easily, but the longwave radiation coming back up is partly absorbed by greenhouse gases such as carbon dioxide, methane and water vapour, which re-radiate heat in all directions including back to the surface. This absorption of outgoing longwave radiation is the physical mechanism of the greenhouse effect. Example: Satellite instruments measure outgoing longwave radiation escaping to space; a decline in it over decades, alongside rising surface temperatures, is direct evidence that greenhouse gases are trapping more heat.
• enhanced: In climate science, enhanced describes the strengthening of the natural greenhouse effect by human-added greenhouse gases such as carbon dioxide, methane, nitrous oxide and fluorinated gases. The natural greenhouse effect keeps Earth about 33 degrees C warmer than it would otherwise be (around minus 18 degrees C without it); the enhanced greenhouse effect is the additional warming driven by anthropogenic emissions and is the primary cause of observed climate change.
• Keeling Curve: The Keeling Curve is the continuous record of atmospheric carbon dioxide concentrations measured at the Mauna Loa Observatory in Hawaii since March 1958, begun by scientist Charles David Keeling of the Scripps Institution of Oceanography. It shows two features: a saw-tooth seasonal cycle as Northern Hemisphere vegetation breathes in and out, and an unbroken year-on-year rise driven by fossil fuel burning. Starting near 313 parts per million in 1958, the curve has crossed 425 parts per million in recent years, making it the most iconic evidence that human activity is raising atmospheric CO2. Example: The curve recorded its biggest ever annual jump in 2024, rising by 3.58 parts per million in a single year, a milestone widely cited as proof that emissions are still climbing.
• Fluorinated gases: Fluorinated gases are synthetic gases containing fluorine, including hydrofluorocarbons (HFCs), perfluorocarbons and sulphur hexafluoride, used in refrigeration, air conditioning, aerosols, foams and fire protection. HFCs were adopted as ozone-friendly replacements for CFCs but are potent greenhouse gases, so the 2016 Kigali Amendment to the Montreal Protocol mandated their global phase-down by 80 to 85 percent. India, grouped with countries freezing HFC consumption in 2028, is transitioning to low-global-warming alternatives. Example: HFC-134a, used in car air conditioners, does not deplete ozone but has a high global warming potential and is being phased down under Kigali.
• Water vapour: Water vapour is water in its gaseous state and the most abundant natural greenhouse gas in the atmosphere, responsible for a large share of the natural greenhouse effect that keeps Earth habitable. Unlike carbon dioxide, it is a feedback rather than a forcing: warmer air holds more moisture, and the extra vapour amplifies warming caused by other gases. It also forms clouds, which can both warm and cool the climate depending on their type and altitude. Example: Rising atmospheric water vapour is one reason a warmer world experiences more intense rainfall events, since moist air releases heavier downpours.
• feedback: In climate science, a climate feedback is a process triggered by warming that then feeds back on the climate, either amplifying the initial warming (positive feedback) or dampening it (negative feedback). Positive feedbacks include the ice-albedo feedback, where melting ice exposes darker ocean or land that absorbs more heat, and permafrost thaw, which releases stored methane and carbon dioxide. Understanding feedbacks is essential because they determine how strongly the climate responds to a given amount of greenhouse gas emissions. Example: The ice-albedo feedback in the Arctic: as reflective sea ice melts, the darker ocean absorbs more solar heat, accelerating further warming and melting.
• forcing: Short for radiative forcing, the measure of how much a factor changes Earth's energy balance, expressed in watts per square metre (W/m2). Positive forcing warms the planet by trapping more energy than before, as with added greenhouse gases; negative forcing cools it, as with reflective aerosols. Radiative forcing is the standard metric the IPCC uses to compare the warming influence of carbon dioxide, methane, aerosols, land use change and solar variation. Example: The radiative forcing from increased atmospheric carbon dioxide is roughly 2.2 W/m2, making it the dominant human driver of current warming.
• UNEP Emissions Gap Report 2025: The UNEP Emissions Gap Report 2025, titled Off Target and released on 4 November 2025 ahead of COP30, measures the gap between countries' climate pledges and the emissions cuts needed for Paris Agreement goals. It found that full implementation of current NDCs would still lead to 2.3 to 2.5 degrees of warming this century, and that global emissions must fall 55 percent by 2035 from 2019 levels to stay on a 1.5-degree path. Only 60 parties, covering 63 percent of emissions, had submitted new 2035 NDCs. Example: Its headline finding that a temporary overshoot of 1.5 degrees is now likely unavoidable, making rapid cuts essential to limit its scale and duration.
• emissions gap: The emissions gap is the difference between projected global greenhouse gas emissions under current policies and national pledges, and the much lower emission levels consistent with the Paris Agreement's temperature goals. UNEP publishes an annual Emissions Gap Report tracking this shortfall; the 2024 edition, titled 'No more hot air... please!', found that emissions must fall 42 percent by 2030 from 2019 levels to stay on a 1.5 degrees C pathway, while current policies point toward up to 3.1 degrees C of warming. Example: The 2024 report estimated a gap of about 24 gigatonnes of CO2 equivalent by 2030 between current policies and a 1.5 degrees C pathway.
• G20: G20 is the premier forum for international economic cooperation, comprising 19 countries plus the European Union and the African Union. Born as a finance-ministers' group in 1999 after the Asian financial crisis, it was elevated to leaders' summits in 2008. The African Union became a permanent member in 2023. For UPSC, the G20 is GS-2 staple: India's 2023 presidency showcased its Global South leadership and summit diplomacy. Example: The 18th G20 Summit in New Delhi in September 2023, which admitted the African Union and adopted the New Delhi Declaration.
• tipping point: In climate science, a tipping point is a threshold beyond which a climate subsystem shifts abruptly and often irreversibly into a new state. Crossing it triggers self-reinforcing feedbacks: for example, melting permafrost releases methane, which causes more warming and more melting. The concept matters for policy because it means gradual warming can produce sudden, large-scale changes. Example: The potential collapse of the West Antarctic Ice Sheet, which could commit the world to several metres of sea-level rise even if emissions stopped.
• Coral bleaching: Coral bleaching is the whitening of corals when stressed polyps expel zooxanthellae, the symbiotic algae that give them colour and food, usually because of abnormally warm ocean water. Bleached corals can recover if stress is brief, but prolonged warming kills them. It matters for UPSC because mass bleaching signals climate change impacts on marine biodiversity and on India's reef systems in the Gulf of Mannar and Lakshadweep. Example: the fourth global coral bleaching event confirmed by NOAA in April 2024, the most widespread on record
• zooxanthellae: Zooxanthellae are single-celled photosynthetic dinoflagellate algae that live symbiotically inside the tissues of reef-building coral polyps, supplying the coral with energy from photosynthesis and giving reefs their colour. When stressed by warming seas, corals expel them, causing coral bleaching that can kill the reef if prolonged. For UPSC they are central to GS-3 environment questions on coral bleaching, ocean warming and marine biodiversity conservation. Example: The mass bleaching events on Australia's Great Barrier Reef.
• Permafrost: Permafrost is ground, soil, rock or sediment, that remains frozen at or below zero degrees Celsius for at least two consecutive years, underlying large parts of the Arctic, sub-Arctic and high mountains. It locks away huge stores of organic carbon and methane, so thawing driven by warming releases greenhouse gases, destabilises infrastructure and reshapes landscapes. Example: Thawing permafrost in Siberia has buckled roads and buildings and released methane from ancient organic matter, creating a feedback loop that accelerates warming.
• Marine heatwaves: Prolonged periods, lasting days to months, when sea surface temperatures in a region are anomalously high relative to historical norms. Driven by climate change, they bleach corals, disrupt fisheries, trigger harmful algal blooms and shift marine species ranges. They are to the oceans what heatwaves are to the land. Example: Recurrent marine heatwaves in the Indian Ocean have caused mass bleaching of coral reefs, including in the Lakshadweep islands.
• Ocean acidification: The ongoing decrease in ocean pH caused by the absorption of excess atmospheric carbon dioxide, which reacts with seawater to form carbonic acid and reduces the availability of carbonate ions. Falling carbonate availability weakens the ability of corals, shellfish and plankton to build their calcium carbonate shells and skeletons, threatening reef ecosystems and marine food chains. It is often called the less visible twin of climate change. Example: The Great Barrier Reef is experiencing reduced coral calcification linked to acidifying waters, compounding the damage from marine heatwaves.
• deoxygenated zones: Areas of ocean or coastal water with very low dissolved oxygen, often called dead zones, where most marine life cannot survive. They typically form when nutrient runoff from fertilisers and sewage fuels algal blooms; the algae die, sink and are decomposed by bacteria that consume the oxygen. Climate change worsens the problem because warmer water holds less dissolved oxygen. Example: The Gulf of Mexico dead zone, fuelled by nutrient runoff down the Mississippi, spans thousands of square kilometres each summer.
• sea surface temperatures: Sea surface temperature is the temperature of the uppermost layer of the ocean, measured by satellites, ships and drifting buoys such as the ARGO network. It is a key climate variable because warmer oceans evaporate more moisture, fuel tropical cyclones, and influence phenomena such as El Nino, the Indian Ocean Dipole and monsoon strength. Sustained rises in sea surface temperature are among the clearest fingerprints of global warming. Example: Record global sea surface temperatures in recent years have been linked to intensified heatwaves and coral bleaching events.
• cyclones: Intense rotating tropical storm systems built around a low-pressure centre, with strong winds, heavy rainfall and storm surges. They form over warm oceans (above about 26.5 degrees Celsius) and are called hurricanes in the Atlantic and typhoons in the northwest Pacific. Climate change is expected to make cyclones more intense, with heavier rainfall and higher storm surges even if their total number does not rise. Example: Cyclone Biparjoy (2023) struck Gujarat as a very severe cyclonic storm, while Cyclone Remal (2024) hit West Bengal and Bangladesh.
• Sea-level rise: Sea-level rise is the long-term increase in global mean sea level caused mainly by thermal expansion of warming ocean water and the melting of glaciers and ice sheets. The IPCC's Sixth Assessment Report records about 0.2 metres of rise between 1901 and 2018 and projects 0.28 to 1.01 metres by 2100 depending on emissions. For India it threatens low-lying coasts, delta agriculture, and cities such as Mumbai, Chennai and Kolkata. Example: The IPCC AR6 projection of up to about one metre of rise by 2100 under high-emission scenarios.
• Sundarbans: Sundarbans is the world's largest mangrove forest, spread across the Ganga-Brahmaputra delta in India and Bangladesh. A UNESCO World Heritage Site on the Indian side and a Ramsar site, it shelters the Royal Bengal tiger, estuarine crocodiles and unique tidal flora, while facing sea-level rise and cyclones. It is indispensable for GS-3 environment answers on wetlands, adaptation and human-wildlife conflict. Example: UNESCO World Heritage Site, 1987
• cryosphere: All the frozen-water components of the Earth system: glaciers, ice sheets, ice caps, sea ice, snow cover, permafrost and seasonally frozen ground. It stores the bulk of the planet's freshwater, regulates sea level and influences global climate by reflecting sunlight. Warming is shrinking the cryosphere, with consequences for water security, sea-level rise and polar ecosystems. Example: The Hindu Kush Himalayan glaciers, often called the Third Pole, feed Asia's great rivers and are retreating under warming, as documented in ICIMOD assessments.
• Hindu Kush Himalaya: The Hindu Kush Himalaya is the vast mountain system stretching 3,500 km across eight countries, from Afghanistan in the west to Myanmar in the east. Often called the Third Pole or the Water Tower of Asia, it holds the largest ice reserves outside the polar regions and is the source of ten major Asian river systems, including the Indus, Ganga and Brahmaputra. It sustains about 240 million people in the mountains and water supplies for nearly 2 billion downstream, and is warming faster than the global average. The Kathmandu-based ICIMOD coordinates research and policy for the region. Example: The ten river systems rising in the Hindu Kush Himalaya provide water to nearly a fourth of humanity, making its glacier retreat a continental water-security issue.
• Arctic amplification: Arctic amplification is the phenomenon by which the Arctic warms two to four times faster than the global average. The main driver is the ice-albedo feedback: melting sea ice exposes dark ocean water that absorbs more solar radiation, which causes more warming and more melting. It also involves increased atmospheric moisture and heat transport from lower latitudes. It matters for India because Arctic changes influence mid-latitude weather and monsoon variability. Example: Arctic sea-ice extent has declined sharply since satellite records began in 1979, the clearest observable signature of the phenomenon.
• Urban heat islands: An urban heat island is the phenomenon where a city is significantly warmer than its surrounding rural areas because concrete, asphalt and glass absorb and re radiate solar heat, while the loss of tree cover removes natural cooling. The effect is worsened by waste heat from vehicles, air conditioners and industry, and it raises heat related illness, energy demand for cooling, and peak electricity loads. Mitigation measures include urban greening, cool roofs, permeable pavements and preserving water bodies. Example: Delhi routinely records night time temperatures several degrees higher than surrounding rural Haryana, which intensifies heatwave mortality among outdoor workers and slum dwellers.
• Himalayan states: The Himalayan states are the Indian states and union territories of the Indian Himalayan Region, spanning the western, central and eastern Himalaya. They are among India's most climate-vulnerable regions, exposed to glacial melt, glacial lake outburst floods, landslides, flash floods and shifting monsoon patterns. Dedicated policy attention comes through the National Mission for Sustaining the Himalayan Ecosystem, one of the eight missions under the National Action Plan on Climate Change. Example: The October 2023 glacial lake outburst flood from South Lhonak lake in Sikkim, which devastated downstream areas, illustrates the hazards facing Himalayan states.
• Coastal states: Coastal states are Indian states with a coastline bordering the sea, giving them jurisdiction over coastal waters, marine fisheries, ports and coastal ecosystems. Nine states (including Gujarat, Maharashtra, Goa, Karnataka, Kerala, Tamil Nadu, Andhra Pradesh, Odisha and West Bengal) plus island and coastal Union Territories fall in this category. They bear special responsibilities under the Coastal Regulation Zone notifications, disaster management for cyclones and tsunamis, and marine biodiversity conservation. Example: Gujarat has the longest mainland coastline of any Indian state, about 1,600 km, making it central to both port development and cyclone preparedness debates.
• vector-borne diseases: Vector-borne diseases are infections transmitted to humans through the bite of arthropod vectors, most commonly mosquitoes, ticks, sandflies, or fleas. The pathogen, which may be a virus, bacterium, or parasite, completes part of its life cycle inside the vector before being passed on to the human host, so control depends on interrupting the vector as much as treating the patient. Climate change is expanding the geographic range of many vectors, increasing the risk zone for these diseases. Example: Malaria (spread by the Anopheles mosquito) and dengue (spread by the Aedes mosquito) together account for a major share of India's communicable disease burden.
• loss and damage: Loss and damage refers to the harms from climate change that occur despite mitigation and adaptation: destroyed homes, lost livelihoods, damaged ecosystems and even disappearing cultures. It became the third pillar of climate action after developing countries argued that the poorest, who contributed least to emissions, were suffering irreversible damage. Its institutional journey runs from the Warsaw International Mechanism (2013) to the dedicated Loss and Damage Fund agreed at COP27 in Sharm el-Sheikh (2022) and operationalised at COP28 in Dubai (2023). Example: After the catastrophic 2022 Pakistan floods, which submerged a third of the country, Pakistan's demand for compensation from high emitters became the global symbol of the loss and damage debate that led to the fund's creation.
• Warm-water coral reefs: Warm water coral reefs are shallow tropical reefs built by reef building corals that live in a symbiotic relationship with zooxanthellae algae, which need sunlight and water temperatures roughly between 23 and 29 degrees Celsius. They are among the most biodiverse ecosystems on Earth, supporting fisheries, tourism and coastal protection, but they are extremely sensitive to warming, and even 1 to 2 degrees of excess heat causes mass coral bleaching. They are distinct from cold water corals, which live in deep, dark, cold seas without algal symbionts. Example: India's major warm water reefs are in the Gulf of Mannar, the Gulf of Kachchh, Lakshadweep and the Andaman and Nicobar Islands.
• AMOC ocean circulation: The Atlantic Meridional Overturning Circulation, the system of surface and deep ocean currents that carries warm water northward in the Atlantic and cold water southward, regulating Europe's climate and global heat distribution. Scientists warn it is at its weakest in a millennium and that a collapse would be a major climate tipping point. Example: weakening of the AMOC is associated with harsher European winters and disruptions to tropical rainfall belts. Example: Cited as one of the major climate tipping elements because a collapse would sharply disrupt global heat and rainfall patterns.
• Greenland / West Antarctic ice: The Greenland Ice Sheet and the West Antarctic Ice Sheet are two of the largest potential tipping elements in the climate system. The West Antarctic sheet rests on bedrock below sea level, making it vulnerable to runaway collapse once warm ocean water undercuts it, while Greenland is losing ice at an accelerating rate from surface melting. Full loss of either would raise global sea levels by several metres over centuries to millennia, redrawing coastlines worldwide. Example: West Antarctica's Thwaites Glacier, nicknamed the Doomsday Glacier for its instability, is closely monitored as a bellwether for ice-sheet collapse.
• Amazon rainforest: The Amazon rainforest is the world's largest tropical rainforest, spanning the Amazon basin across Brazil, Peru, Colombia, and six other South American countries. It holds roughly a tenth of known species, stores vast carbon stocks, and drives regional rainfall through moisture recycling, often called flying rivers. Its fate is central to global climate and biodiversity negotiations. Example: Record wildfires and drought in the Amazon in recent years have raised fears that parts of the forest could tip from a carbon sink into a carbon source.
• Key takeaways: Key takeaways is a quick-revision summary box placed at the end of each article, listing the most exam-relevant points in a short numbered list. It distils definitions, dates, figures, provisions and examples into a form suited for last-minute revision before Prelims and Mains. Together with the article's practice questions, it turns every topic into a self-contained study unit.
• enhancement: Enhancement, in the climate-change context, is the anthropogenic strengthening of the natural greenhouse effect through the addition of greenhouse gases to the atmosphere. It results from burning fossil fuels, deforestation, agriculture and industrial processes releasing carbon dioxide, methane, nitrous oxide and fluorinated gases beyond natural levels. This enhancement traps extra outgoing longwave radiation and is the central mechanism of human-caused global warming. Example: Atmospheric carbon dioxide rising from about 280 parts per million in the pre-industrial era to over 420 parts per million today, the clearest measure of the enhancement.
• Mechanism: In climate science, the term denotes the physical mechanism of the greenhouse effect, best understood in three steps: shortwave solar radiation passes through the atmosphere and warms the Earth, the Earth re-radiates energy as longwave infrared radiation, and greenhouse gases such as carbon dioxide and water vapour absorb part of that outgoing radiation and re-emit it, trapping heat. Human emissions intensify this natural mechanism. Example: Venus, whose thick carbon dioxide atmosphere traps heat in a runaway version of this mechanism, has surface temperatures around 460 degrees Celsius.
• The gases: In climate science, the gases are the greenhouse gases that drive the greenhouse effect: carbon dioxide (long-lived, tracked by the Keeling Curve), methane (short-lived but far more potent per tonne), nitrous oxide and fluorinated gases, with water vapour acting as an amplifying feedback. They let shortwave solar radiation in but absorb and re-emit outgoing longwave infrared radiation, warming the planet. Human activity has raised their concentrations far above pre-industrial levels. Example: Atmospheric carbon dioxide crossing 420 parts per million in recent years, the highest level in at least two million years.
• Close: In climate-science discourse, close describes how narrow the margin between 1.5 degrees Celsius and 2 degrees Celsius of warming is, and how consequential that small difference is. Because of tipping points and non-linear feedbacks, half a degree of warming is not an incremental change but the difference between damaged and collapsed systems. The term captures why the Paris Agreement's aspirational 1.5-degree target is treated as a hard guardrail rather than a rounding error. Example: The IPCC's special report on 1.5 degrees finds 70 to 90 percent of tropical coral reefs lost at 1.5 degrees but over 99 percent lost at 2 degrees, showing why the half-degree gap matters.
• National exposure: The extent to which a country's territory, population and economy lie physically in the path of climate and weather hazards, making them liable to be affected. It is distinct from vulnerability, which also depends on sensitivity and adaptive capacity: high exposure plus low adaptive capacity produces the worst outcomes. India's exposure is severe, with about 85 per cent of its districts classified as multi-hazard vulnerable. Example: India experienced extreme weather events on 99 per cent of days in 2025, according to the State of Environment 2026 report.
• Human layer: The human layer is the human-systems dimension of climate impact assessment, covering how climate change affects health, livelihoods, settlements and social groups. It includes heat-related mortality in cities, the spread of vector-borne diseases, urban heat islands that make cities several degrees hotter than their surroundings, and the disproportionate burden on women and the poor. Adaptation in this layer must be region-specific, combining public-health preparedness, cooling infrastructure and social protection. Example: Urban heat islands can make Indian cities 5 to 15 degrees Celsius warmer than nearby rural areas, sharply raising heat-stroke risk during heatwaves.
• India's case: India's case is New Delhi's climate-justice negotiating stance: minimal historical responsibility for emissions combined with maximum exposure to climate impacts, with about 85 per cent of Indian districts classed as vulnerable. From this flows the justice argument of common but differentiated responsibilities, with finance and technology transfer from developed nations treated as preconditions for deeper developing-country action. Example: India invokes common but differentiated responsibilities to argue that developed nations must lead on emission cuts and fund adaptation in vulnerable countries.
• Justice argument: The justice argument is the ethical case in climate negotiations that countries which industrialised early and emitted the most greenhouse gases historically bear the greatest responsibility for global warming. It holds that developed countries should therefore lead in cutting emissions and should provide finance and technology to developing countries, which contributed least to the problem but face its worst impacts. This argument is the moral basis of the principle of common but differentiated responsibilities enshrined in the UN Framework Convention on Climate Change. Example: India and other developing countries have long invoked the justice argument to demand that developed nations deliver the promised climate finance, pointing to the developed world's far larger cumulative historical emissions.
• Greenhouse gases (GHGs): Greenhouse gases are gases that absorb and re-emit infrared radiation, trapping heat in the atmosphere and keeping the planet warm enough for life. The principal ones are carbon dioxide, methane, nitrous oxide, ozone, water vapour and fluorinated gases such as CFCs and HFCs. Human activity has sharply raised their concentrations, especially CO2 from fossil-fuel burning and methane from agriculture and waste, intensifying the greenhouse effect and driving global warming. Because CO2 persists in the atmosphere for centuries, past emissions commit the planet to long-term warming. Example: Atmospheric CO2 has crossed 420 parts per million, compared with about 280 ppm before industrialisation.
• Carbon dioxide (CO2): Carbon dioxide is a colourless, odourless greenhouse gas released by respiration, decomposition, volcanic activity and, most significantly today, the burning of fossil fuels. Although it makes up only about 0.04 per cent of the atmosphere, it is the dominant long-lived greenhouse gas driving human-caused climate change, with atmospheric concentrations now above 420 parts per million. It also acidifies the oceans as seawater absorbs it. Example: India's per capita CO2 emissions are roughly 2 tonnes per year, well below the global average, though its total emissions rank third in the world.
• methane: Methane is a colourless greenhouse gas released from wetlands, livestock digestion, rice paddies, landfills and leaks from oil and gas systems. Though it stays in the atmosphere for only about a decade, it is roughly 80 times more potent than carbon dioxide at trapping heat over a 20-year period, making it a powerful short-term driver of warming. Cutting methane is therefore one of the fastest ways to slow near-term climate change. Example: The Global Methane Pledge, launched at COP26 in Glasgow, commits signatories to cut methane emissions 30 percent by 2030 from 2020 levels; India has not signed it, citing its agricultural and development needs.
• Nitrous oxide (N2O): A long-lived greenhouse gas with a warming potential roughly 265 times that of carbon dioxide over a century, also known as laughing gas. It persists in the atmosphere for well over a hundred years and additionally depletes stratospheric ozone, making it a significant though often overlooked climate pollutant. Example: Agricultural soils treated with nitrogen fertilisers are the largest human-caused source of nitrous oxide emissions.
• Nationally Determined Contributions (NDCs): The plural term for the collection of all countries' individual climate pledges under the Paris Agreement. The global stocktake process reviews the aggregate effect of these contributions every five years and calls on parties to strengthen them, since current pledges collectively still fall short of limiting warming to 1.5 degrees Celsius. Example: At COP summits, developing countries argue that developed countries' contributions remain inadequate given their historical responsibility for emissions.
• IPCC Special Report on 1.5 degrees (2018): The IPCC Special Report on Global Warming of 1.5 degrees, released in October 2018 at the Paris Agreement's invitation, compared climate impacts at 1.5 degrees versus 2 degrees of warming. It warned that at 1.5 degrees of warming, 70 to 90 per cent of coral reefs could disappear, and beyond 2 degrees losses may exceed 99 per cent, marking reefs as the ecosystem closest to a climate tipping point. Example: Its comparison of 1.5 degree and 2 degree outcomes gave scientific weight to the Paris Agreement's push to limit warming to 1.5 degrees.
• Atlantic Meridional Overturning Circulation (AMOC): The Atlantic Meridional Overturning Circulation is the large-scale system of Atlantic Ocean currents, including the Gulf Stream, that carries warm surface water northward and returns cold deep water southward. It redistributes heat globally, keeps Western Europe unusually mild for its latitude, and influences monsoon systems and sea levels. Palaeoclimate evidence shows AMOC slowdowns have triggered abrupt climate shifts in the past. Example: Recent studies warn the AMOC is at its weakest in a millennium and could weaken further under Greenland ice melt, with potential impacts on European climate and tropical rainfall.
• Global Warming Potential : Global Warming Potential (GWP) is the heat trapped by a greenhouse gas over a chosen period compared with carbon dioxide, whose GWP is 1. It matters because it lets policy compare a tonne of methane (about 28 to 36 times carbon dioxide over 100 years) with a tonne of nitrous oxide (about 265 to 298 times) on a single scale, while lifetimes decide how fast the benefit arrives.
• Ozone layer : The ozone layer is the stratospheric band of ozone (O3) that absorbs harmful ultraviolet radiation from the Sun. It matters because its depletion by chlorofluorocarbons raises skin cancer, eye disease and ecosystem harm, and because the Vienna Convention and Montreal Protocol show that a binding phase out can reverse an atmospheric threat.
• Carbon Capture Utilisation and Storage : Carbon Capture, Utilisation and Storage (CCUS) captures carbon dioxide from large industrial sources for permanent geological storage or use in fuels, chemicals and building materials. It matters for hard to abate sectors such as cement and steel, and IPCC pathways assign it up to about 15 per cent of cumulative mitigation effort by 2050 for a 1.5 degree limit.

Practice questions