Climate Change Science for UPSC: Greenhouse Gases, Sinks, Carbon Removal, CCUS and the Latest WMO Numbers
How UPSC Tests Climate Science
🎯 Exam priority: Tier-2, but it explains every other climate question. UPSC has asked what carbon fertilisation is (2018), what cirrus-cloud thinning and stratospheric aerosol injection aim at (2019, reducing global warming), which carbon-removal activities are discussed (enhanced rock weathering, ocean alkalinity enhancement and mineral carbonation of CO2, 2023: all three), the world's largest tropical peatland (the Congo Basin, 2024), the implication of wet-bulb temperatures above 35 °C (2025) and cement's share of global CO2 (2025). The Mains papers asked India's climate vulnerability (2017), global warming and the Kyoto Protocol (2022), sea-level rise (2023) and CCUS (2025).
Climate-science questions reward mechanism: why a gas warms, what a sink is, how a removal method works and why a technology is controversial. This article gives you the vocabulary and the latest global numbers; treaties are the next article and India's action follows.
Greenhouse Gases, Sources and the Latest Global Numbers
The greenhouse effect and the gases
Greenhouse gases let incoming sunlight pass but absorb and re-emit outgoing heat, warming the lower atmosphere. The Kyoto Protocol listed six gases: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6). Carbon dioxide dominates because of the quantity emitted; methane is far more heat-trapping per molecule but shorter-lived; nitrous oxide and the fluorinated gases are long-lived or extremely potent. The IPCC's reference point is the pre-industrial level (about 1750), and the Paris goal uses 1850-1900 as the temperature baseline.
Where the sources are (all in earlier articles)
Fossil-fuel combustion and cement (CO2); the cement chemistry behind the 2025 question: limestone becomes lime in clinker production and releases CO2, so the industry accounts for more than 5 per cent of global CO2.
Rice paddies, livestock and landfills (methane); fertilised soils (nitrous oxide); refrigerants and foams (HFCs).
Deforestation and drained peatlands remove a sink and release stored carbon: the Congo Basin holds the world's largest tropical peatland, about three years of fossil-fuel emissions worth of carbon (2024 PYQ).
The latest global numbers (use with the year)
Carbon dioxide: the WMO's Greenhouse Gas Bulletin No. 21 (October 2025) gave a global average of 423.9 ppm in 2024, with methane at 1,942 ppb and nitrous oxide at 338.0 ppb, all records. The 2023-24 rise of 3.5 ppm was the largest since modern measurements began in 1957.
Temperature: the WMO's State of the Global Climate 2025 (March 2026) put 2025 at about 1.43 °C above the 1850-1900 average, among the second- or third-warmest years, and 2015-2025 the warmest eleven-year period; it was also the first edition to include Earth's energy imbalance.
The IPCC was set up in 1988 by the WMO and UNEP; it does no research or monitoring itself but assesses published science through three working groups: the physical science basis (I), impacts, adaptation and vulnerability (II) and mitigation (III).
Impacts: Heat, Sea Level, Oceans, Mountains and Carbon Fertilisation
Impacts: the chain of effects
Heat: a wet-bulb temperature above about 35 °C means sweating can no longer cool the body, so prolonged exposure is fatal; the World Bank warned India could be among the first places where this is routinely exceeded (2025 PYQ). Heat waves also raise flood, cyclone and drought risk.
Sea level: rises through thermal expansion of water and melting of land ice, threatening deltas, islands and coastal cities (Mains 2023: impact on India and the Indian Ocean region).
Oceans: dissolved CO2 forms carbonic acid (CO2 + H2O gives H2CO3), lowering pH and the carbonate ions that corals and shellfish need; warming drives coral bleaching, where corals expel their symbiotic zooxanthellae.
Mountains and rivers: glacier retreat in the Himalaya first raises and then lowers river flows; glacial-lake outbursts and landslides increase (the Himalayan part of the 2017 Mains question).
Agriculture and health: heat-sensitive crops (wheat, rice, maize) lose yield; vector-borne diseases expand; saline intrusion hurts coastal farming, fisheries and mangroves.
Carbon fertilisation: a partial offset, not a cure
Higher CO2 raises plant photosynthesis and reduces transpiration in some species, called carbon fertilisation (2018 PYQ); it varies with species, temperature, water and nutrients, and does not offset warming.
Sinks, Carbon Removal, CCUS and Geo-engineering
Sinks and carbon removal
A sink absorbs more carbon than it emits: forests, soils, oceans and peat. NCERT-level vocabulary the exam uses:
Green carbon is stored in plants and soils of natural ecosystems; blue carbon is stored by coastal and marine ecosystems (mangroves, seagrass beds, salt marshes).
Carbon capture, utilisation and storage (CCUS) captures CO2 from industrial flue gas, compresses it, and uses it or stores it deep underground (in basalt, which turns it to carbonate rock, or in depleted oil and gas fields, sometimes to push out remaining oil). Its role is to cut emissions from sectors that are hard to decarbonise (cement, steel); its critics point to cost, energy use and leakage risk (the 2025 Mains question).
Bioenergy with CCS (BECCS): plants capture CO2 as they grow; burning them with capture can produce net-negative emissions; challenges include land, water and the food-versus-fuel debate.
Enhanced rock weathering spreads finely ground basalt on farmland to draw CO2 from the air; ocean alkalinity enhancement adds lime to the sea; mineral carbonation injects captured CO2 as carbonated water into rock: all three are discussed for carbon removal (2023 PYQ, 'all three').
Pyrolysis of biomass without oxygen gives biochar that improves soil water- and nutrient-holding capacity and locks carbon in the soil.
Geo-engineering (solar radiation management)
Stratospheric aerosol injection (sulphate particles in the stratosphere to reflect sunlight) and cirrus cloud thinning (seeding high thin clouds so they trap less heat) are proposed ways of reducing global warming (2019 PYQ). They treat the symptom, not the cause, do not stop ocean acidification and carry serious risks: that is the standard evaluation.
How UPSC Could Ask This Next
Mechanism statements (why methane matters, how carbon fertilisation works, what happens to the ocean when CO2 dissolves).
Technology matching: CCUS, BECCS, rock weathering, biochar, geo-engineering; which is removal and which is radiation management.
Latest indicators: record CO2 ppm, the WMO's temperature figures and the IPCC working groups.
Impact questions on India: heat stress, sea level, glaciers, agriculture.
Further Reading
NCERT Biology (Class 12) and Geography textbooks, the India State of Forest Report, the MoEFCC Annual Report, the Economic Survey and the official lists cited in this note — the primary sources behind every figure above.
Test Yourself: Practice Questions and PYQs
Twelve practice questions on greenhouse gases, impacts, carbon removal and geo-engineering, followed by the real UPSC Prelims questions and exact Mains questions.
Practice Questions
Q1. Which of the following are among the six greenhouse gases listed under the Kyoto Protocol?
1. Carbon dioxide
2. Methane
3. Sulphur hexafluoride
4. Ozone
(a) 1, 2, 3 and 4
(b) 1, 2 and 3 only
(c) 1, 2 and 4 only
(d) 2, 3 and 4 only
Show answer
Answer: (b) — The Kyoto gases are CO2, CH4, N2O, HFCs, PFCs and SF6; ozone is not on the list.
Q2. According to the WMO's Greenhouse Gas Bulletin No. 21 (October 2025), the global average carbon dioxide concentration in 2024 was about
(a) 403.3 ppm
(b) 423.9 ppm
(c) 350 ppm
(d) 280 ppm
Show answer
Answer: (b) — 423.9 ppm in 2024, with the largest annual rise (3.5 ppm) since modern records began in 1957. (280 ppm is the pre-industrial level; 403.3 ppm is the 2016 value.)
Q3. Which of the following statements about the IPCC is correct?
1. It was set up in 1988 by the WMO and UNEP.
2. It carries out original climate research and monitoring.
3. Working Group III deals with mitigation.
(a) 2 and 3 only
(b) 1, 2 and 3
(c) 1 and 3 only
(d) 1 and 2 only
Show answer
Answer: (c) — The IPCC only assesses published science; it does no research or monitoring itself.
Q4. Which one of the following best describes 'carbon fertilisation'?
(a) Rise in Earth's temperature due to greenhouse gases
(b) Increased ocean acidity due to dissolved carbon dioxide
(c) Increased plant growth due to increased atmospheric carbon dioxide
(d) Adaptation of all organisms to climate change
Show answer
Answer: (c) — Rising CO2 raises photosynthesis in many plants, though the effect varies and does not offset warming.
Q5. Consider the following activities:
1. Spreading finely ground basalt rock on farmlands extensively
2. Increasing the alkalinity of oceans by adding lime
3. Capturing carbon dioxide from industries and pumping it into abandoned mines as carbonated water
How many of the above are often considered for carbon capture and sequestration?
(a) All three
(b) Only one
(c) None
(d) Only two
Show answer
Answer: (a) — Enhanced rock weathering, ocean alkalinity enhancement and mineral carbonation are all discussed carbon-removal methods.
Q6. The technique of injecting sulphate aerosols into the stratosphere and thinning cirrus clouds is discussed in the context of
(a) reducing the effects of solar wind
(b) reducing global warming
(c) producing artificial rain
(d) reducing the frequency of tropical cyclones
Show answer
Answer: (b) — Both are solar-radiation-management proposals.
Q7. Which one of the following regions holds the world's largest tropical peatland, storing about three years of global fossil-fuel emissions worth of carbon?
(a) Amazon Basin
(b) Congo Basin
(c) Rio de la Plata Basin
(d) Kikori Basin
Show answer
Answer: (b) — The Congo Basin peatland (shared among central African countries) is the largest tropical peatland.
Q8. Why is a wet-bulb temperature above 35 °C considered dangerous?
(a) It stops photosynthesis
(b) Sweating can no longer cool the body, so survival is threatened
(c) It always causes cyclones
(d) It melts glaciers instantly
Show answer
Answer: (b) — At that humidity and temperature the body cannot shed heat by evaporation of sweat.
Q9. Statement-I: Carbon dioxide emissions from the cement industry are more than 5 per cent of global carbon emissions.
Statement-II: Limestone is converted into lime during clinker production for cement manufacturing, releasing carbon dioxide.
Which one of the following is correct in respect of the above statements?
(a) Both statements are correct but Statement-II does not explain Statement-I
(b) Statement-I is incorrect but Statement-II is correct
(c) Both statements are correct and Statement-II explains Statement-I
(d) Statement-I is correct but Statement-II is incorrect
Show answer
Answer: (c) — The calcination of limestone is the main process source of cement's CO2.
Q10. Which of the following is the effect of dissolved carbon dioxide on seawater?
(a) It increases the number of coral reefs
(b) It removes oxygen from the atmosphere
(c) It forms carbonic acid and lowers the carbonate ions that corals need
(d) It raises the pH
Show answer
Answer: (c) — CO2 + H2O gives H2CO3, which acidifies the ocean.
Q11. Blue carbon refers to carbon stored in
(a) forests and agricultural soils
(b) the atmosphere
(c) fossil fuels
(d) mangroves, seagrass beds and salt marshes
Show answer
Answer: (d) — Forests and soils store 'green carbon'.
Q12. Bioenergy with carbon capture and storage (BECCS) can produce net-negative emissions because
(a) plants capture CO2 as they grow, and the CO2 released on burning is captured and stored
(b) it uses nuclear energy
(c) it burns coal more efficiently
(d) it increases methane emissions
Show answer
Answer: (a) — Biomass is carbon-neutral to burn, and capturing the released CO2 removes carbon from the atmosphere overall.
UPSC Previous Year Questions (PYQs)
Real UPSC Prelims questions, checked against the official paper and answer key.
Q1. (2018) Which of the following statements best describes “carbon fertilization”?
(a) Increased temperature of Earth due to increased concentration of carbon dioxide in the atmosphere
(b) Increased plant growth due to increased concentration of carbon dioxide in the atmosphere
(c) Adaptation of all living beings on Earth to the climate change brought about by the increased concentration of carbon dioxide in the atmosphere
(d) Increased acidity of oceans as a result of increased concentration of carbon dioxide in the atmosphere
Show answer
Answer: (b) — Carbon fertilisation is the extra plant growth that follows from a higher atmospheric CO2 concentration.
Q2. (2019) In the context of which of the following do some scientists suggest the use of cirrus cloud thinning technique and the injection of sulphate aerosol into stratosphere?
(a) Reducing the frequency and intensity of tropical cyclones
(b) Reducing the global warming
(c) Creating the artificial rains in some regions
(d) Reducing the adverse effects of solar wind on the Earth
Show answer
Answer: (b) — Cirrus-cloud thinning and stratospheric sulphate-aerosol injection are proposed geo-engineering ways of reducing global warming.
Q3. (2023) Consider the following activities :
1. Spreading finely ground basalt rock on farmlands extensively
2. Increasing the alkalinity of oceans by adding lime
3. Capturing carbon dioxide released by various industries and pumping it into abandoned subterranean mines in the form of carbonated waters
How many of the above activities are often considered and discussed for carbon capture and sequestration?
(a) Only two
(b) Only one
(c) All three
(d) None
Show answer
Answer: (c) — All three are discussed for carbon removal: enhanced rock weathering with crushed basalt, ocean alkalinity enhancement with lime, and injecting captured CO2 as carbonated water into rock formations.
Q4. (2024) One of the following regions has the world's largest tropical peatland, which holds about three years worth of global carbon emissions from fossil fuels; and the possible destruction of which can exert detrimental effects on the global climate. Which one of the following denotes that region ?
(a) Amazon Basin
(b) Congo Basin
(c) Rio de la Plata Basin
(d) Kikori Basin
Show answer
Answer: (b) — The Congo Basin holds the world's largest tropical peatland, storing roughly three years of global fossil-fuel emissions in its carbon.
Q5. (2025) The World Bank warned that India could become one of the first places where wet-bulb temperatures routinely exceed 35 °C. Which of the following statements best reflect(s) the implication of the above-said report?
I. Peninsular India will most likely suffer from flooding, tropical cyclones and droughts.
II. The survival of animals including humans will be affected as shedding of their body heat through perspiration becomes difficult.
Select the correct answer using the code given below.
(a) II only
(b) Both I and II
(c) Neither I nor II
(d) I only
Show answer
Answer: (b) — A wet-bulb temperature above 35 °C stops the body cooling by sweating, which endangers humans and animals (II). The same warming raises flood, cyclone and drought risk for the peninsula (I). Both statements hold.
Q6. (2025) Consider the following statements :
Statement I : Studies indicate that carbon dioxide emissions from cement industry account for more than 5% of global carbon emissions.
Statement II : Silica-bearing clay is mixed with limestone while manufacturing cement.
Statement III : Limestone is converted into lime during clinker production for cement manufacturing.
Which one of the following is correct in respect of the above statements?
(a) Only one of the Statements II and III is correct and that explains Statement I
(b) Both Statement II and Statement III are correct but only one of them explains Statement I
(c) Neither Statement II nor Statement III is correct
(d) Both Statement II and Statement III are correct and both of them explain Statement I
Show answer
Answer: (b) — Cement accounts for more than 5% of global CO2, most of it from limestone turning to lime in clinker making, so both II and III are true. Only III (the calcination step) explains the emissions in I, so one of the two explains it.
Mains Practice Questions
Use these to frame full-length answers. You don't have to answer one exactly — they show the angles UPSC tests, so let them guide which points you cover.
Climate change is a global problem. How will India be affected? How will the Himalayan and coastal States be affected?
Discuss global warming and its effects on the global climate. Explain the control measures in the light of the Kyoto Protocol, 1997.
The IPCC has predicted a global sea-level rise of about one metre by AD 2100. What would be its impact in India and the other countries of the Indian Ocean region?
What is Carbon Capture, Utilisation and Storage (CCUS)? What is its potential role in tackling climate change?
Evaluate geo-engineering as a response to global warming. Why do scientists caution against relying on solar radiation management?
UPSC Mains — Previous Years’ Questions on this topic (last 10 years)
These are the actual GS Mains questions UPSC has asked on this theme — real proof of how, and when, it is tested. Try to write a full answer to each.
2025 (GS-III): What is Carbon Capture, Utilization and Storage (CCUS) ? What is the potential role of CCUS in tackling climate change ?
2023 (GS-III): The Intergovernmental Panel on Climate Change (IPCC) has predicted a global sea level rise of about one metre by AD 2100. What would be its impact in India and the other countries in the Indian Ocean region?
2022 (GS-III): Discuss global warming and mention its effects on the global climate. Explain the control measures to bring down the level of greenhouse gases which cause global warming, in the light of the Kyoto Protocol, 1997. (Answer in 250 words)
2017 (GS-III): 'Climate change' is a global problem. How India will be affected by climate change? How Himalayan and coastal states of India will be affected by climate change? (Answer in 250 words)