Climate Change, Ozone and Land Degradation

• Statement 1 is correct. The terrestrial biosphere is at present a net carbon sink, absorbing roughly a quarter of anthropogenic emissions, but that sink weakens and can reverse with warming, because soil and plant respiration respond more strongly to temperature than photosynthesis does, because drought and heat reduce productivity, and because thawing permafrost and increased fire and dieback release stored carbon. Beyond about three degrees the projections converge on the land becoming a net source, which is the classic positive feedback.
• Statement 2 is correct. Reef-building corals bleach when thermal stress causes them to expel their zooxanthellae, and widespread mortality is projected at warming well below three degrees, so at three degrees it follows a fortiori.
• Statement 3 is incorrect. The claim that all global wetlands will permanently disappear is an absolute that no projection supports; wetlands will be altered, and coastal wetlands squeezed by sea level rise, but a total and permanent global loss is not a scientific finding.
• Statement 4 is incorrect for the same reason: cereal yields are projected to fall in the tropics and low latitudes, but poleward shifts and carbon dioxide fertilisation mean that cultivation somewhere in the world is not in question. The official answer is (b). The governing principle is that statements 3 and 4 are both unqualified absolutes, and (b) is the only option omitting both, so the item is decidable without any command of the climate science.

• Statements 2 and 3 are correct and statement 1 is incorrect, so the official answer is (c). Earth Hour is an initiative of the World Wide Fund for Nature, not of the United Nations Environment Programme or UNESCO. It began in Sydney in 2007 and became a global observance, held on a Saturday towards the end of March, in which individuals, businesses and civic authorities switch off non-essential lighting for one hour, landmark buildings being darkened as the visible expression of it.
• Statements 2 and 3 accurately describe the mechanism and the purpose respectively: the switching off is symbolic, the measurable saving in electricity over one hour being trivial, and the object is to build public awareness of climate change and to demonstrate the scale of public concern. The item is decided entirely on statement 1, and the elimination is clean, since rejecting it removes (a) and (d) and statements 2 and 3 are both unobjectionable. The general principle worth carrying is that a large proportion of the best-known global environmental observances are run by non-governmental bodies rather than by United Nations agencies, and candidates routinely over-attribute them to the United Nations.

• Option (d) is correct. IndARC is India's first multi-sensor moored observatory in the Arctic, deployed in July 2014 by the Ministry of Earth Sciences through the National Centre for Antarctic and Ocean Research at a depth of about 192 metres in Kongsfjorden, a fjord off Ny-Alesund in Svalbard. Its sensors record temperature, salinity, currents and ocean acidification through the year, including the polar winter when ships cannot reach the site, and the scientific purpose is to understand Arctic processes and their teleconnection with the Indian monsoon.
• Option (c) is the intended trap and inverts the pole: India's Antarctic stations are Maitri and Bharati, the earlier Dakshin Gangotri having been abandoned to ice, while India's Arctic presence is the Himadri station at Ny-Alesund, established in 2008, of which IndARC is the oceanographic extension.
• Option (a) is wrong: the indigenous radar family carries names such as Rajendra, Revathi and Indra.
• Option (b) is wrong: the regional satellite of that description is the South Asia Satellite, and it postdates this paper. Elimination route: the syllable ARC in the name points at the Arctic and not at the Antarctic, which is the single clue that separates option (d) from option (c).

All three are established geological storage options and the official answer (d) follows.

• Statement 1 is correct. Unmineable coal seams store carbon dioxide by adsorption onto the coal matrix, coal having a higher affinity for carbon dioxide than for methane, which is why injection simultaneously displaces coal bed methane; this is enhanced coal bed methane recovery, and the seam must be uneconomic to mine precisely so that the stored gas is not later released.
• Statement 2 is correct. Depleted oil and gas reservoirs are the best characterised of all storage sites, since they demonstrably held buoyant fluids over geological time, which proves the existence of an intact caprock seal, and their geology, injection infrastructure and well records already exist; carbon dioxide injection here doubles as enhanced oil recovery.
• Statement 3 is correct, and deep saline formations carry by far the largest theoretical capacity of the three. Injected below about 800 metres the carbon dioxide is supercritical and dense, and it is retained by structural trapping under the caprock, then progressively by residual, solubility and mineral trapping as the brine dissolves it and it reacts to form carbonates. Since no statement can be rejected, there is no elimination route and the item requires all three to be known independently. Governing principle common to all three: geological storage needs a porous permeable reservoir beneath an impermeable seal, at a depth sufficient for the supercritical phase. Every option here satisfies that.

• Option (a) is correct. Carbon dioxide is a substrate of photosynthesis, and in C3 plants the carboxylating enzyme rubisco is not saturated at present atmospheric concentrations and also catalyses a competing reaction with oxygen, photorespiration, which wastes fixed carbon. Raising the ambient carbon dioxide concentration therefore increases the rate of net carbon assimilation and suppresses photorespiration, and it additionally allows stomata to open less widely for the same carbon gain, improving water use efficiency. The result is the growth enhancement the term names. The effect is significant for C3 crops such as wheat and rice and much weaker in C4 plants such as maize and sugarcane, which already concentrate carbon dioxide at the site of fixation, and it is limited in the field by nitrogen, phosphorus and water availability, which is why free air enrichment experiments yield smaller gains than glasshouse studies and why the protein and micronutrient content of the resulting grain tends to fall.
• Options (b) and (c) are wrong not because they are false statements but because they name different consequences of the same cause: (b) is the greenhouse effect and (c) is ocean acidification, the dissolution of carbon dioxide as carbonic acid lowering seawater pH.
• Option (d) is wrong as a proposition in its own right, since adaptation of all living beings is not an observed phenomenon and is the opposite of the extinction pressure discussed in question 60 of this paper. Governing principle: the word fertilization points to plant nutrition, so among four true sounding consequences of rising carbon dioxide the correct one must be the one about growth.

Hence (a).

Both techniques named in the stem are climate intervention or geoengineering proposals aimed at the planetary radiation balance, so (d) is correct. Stratospheric sulphate aerosol injection is a solar radiation management proposal that seeks to mimic the cooling that follows a large volcanic eruption, the Mount Pinatubo eruption of 1991 having lowered global mean surface temperature by roughly half a degree Celsius through exactly this mechanism, by scattering incoming shortwave radiation back to space. Cirrus cloud thinning works on the other side of the energy budget, since thin high cirrus is largely transparent to incoming shortwave radiation but traps outgoing longwave radiation, so seeding cirrus to reduce its coverage or lifetime would allow more terrestrial radiation to escape.

• Option (a) is wrong because artificial rain uses cloud seeding of warm or supercooled tropospheric clouds with silver iodide, sodium chloride or dry ice, an entirely different operation on a different cloud type at a different altitude.
• Option (b) is wrong because no mainstream proposal uses stratospheric aerosols against cyclones; the speculative cyclone proposals concern sea surface cooling.
• Option (c) is wrong because solar wind is a stream of charged particles deflected by the magnetosphere and is unaffected by aerosols in the stratosphere. The governing principle is that the stratosphere is the wrong place for any weather intervention, since weather is a tropospheric phenomenon, so the mention of stratospheric injection alone points to a radiative rather than a meteorological objective.

• Statement 1 is correct. Methane hydrate, or methane clathrate, is stable only within a narrow window of low temperature and high pressure, so warming of permafrost and of bottom waters shifts conditions outside that window and can destabilise the deposits, which is the basis of the clathrate gun hypothesis and of concern about a positive feedback in the Arctic.
• Statement 2 is correct. The two great reservoirs are the permafrost of the Arctic tundra and the continental margin sediments beneath the seafloor, the latter holding the far larger share, and India's own prospecting in the Krishna Godavari basin belongs to this second category.
• Statement 3 is correct. Atmospheric methane has a perturbation lifetime of roughly nine to twelve years, being removed principally by reaction with the hydroxyl radical in the troposphere and ending as carbon dioxide and water, which is exactly why methane is described as a short lived climate pollutant with high potency but limited persistence. Since all three hold, the official answer (d) follows. The governing principle is the pressure and temperature stability field of the hydrate, which explains both where the deposits occur and why warming threatens them, and holding that single idea validates statements 1 and 2 together.

The social cost of carbon is the present value of the total future damage caused by emitting one additional tonne of carbon dioxide in a given year, aggregated across agriculture, health, sea level rise, productivity and ecosystem loss and discounted to today. Its purpose is to internalise an externality by putting a number on harm that the emitter does not otherwise pay for, and it is used in regulatory cost benefit analysis and in setting carbon prices. Hence (a).

• Option (b) describes energy demand or fossil fuel intensity, a measure of consumption rather than of damage.
• Option (c) describes adaptation cost borne by a displaced person, which is one component of the damage but not the concept.
• Option (d) describes an individual carbon footprint, which measures emissions in tonnes rather than harm in money. The distinguishing features to teach are the unit of one tonne, the reference to a given year and the long term horizon.

Blue carbon is the organic carbon captured and stored by coastal and marine ecosystems, principally mangroves, salt marshes and seagrass meadows, with the ocean itself acting as the largest active carbon sink.

• Option (b) describes green carbon, the terrestrial biomass and soil pool.
• Option (c) describes fossil carbon, a geological stock that is not an active sink.
• Option (d) describes the atmospheric reservoir, which is the source rather than the sink. Only (a) locates the carbon in the marine and coastal compartment, so (a) is correct. The colour coding of carbon pools, green for terrestrial, blue for marine, brown or black for fossil, is worth teaching as a single set.

Hydrofluorocarbons were introduced as ozone safe replacements for chlorofluorocarbons and hydrochlorofluorocarbons, and their recognised applications are refrigeration and air conditioning, aerosol propellants, foam blowing agents, fire suppression and solvents.

• Items 1, 2 and 3 therefore qualify.
• Item 4 does not. Lubricants used in refrigeration systems are polyol esters and mineral oils that work alongside HFC refrigerants, but HFCs are not themselves lubricant constituents, and the standard UNEP and Kigali listings of HFC end uses do not include lubricants. Three of four gives (c).
• Option (d) requires the lubricant claim, and (a) and (b) understate the well established aerosol, foam and fire suppression uses.

• Statement I is correct: cement production is consistently estimated to contribute somewhere between five and eight per cent of global carbon dioxide emissions, so the threshold of more than five per cent is safely met.
• Statement II is correct as a description of the raw mix: cement manufacture combines limestone as the calcareous component with silica-bearing clay or shale as the argillaceous component supplying silica, alumina and iron, which together form the clinker minerals.
• Statement III is correct and is the chemistry that matters: during clinkerisation in the kiln, calcium carbonate decomposes to calcium oxide and carbon dioxide, and this calcination reaction releases carbon dioxide from the raw material itself, independently of the fuel burnt to heat the kiln. Process emissions of this kind account for roughly sixty per cent of the sector's carbon dioxide, which is why cement is described as hard to abate: even a kiln fired entirely by renewable energy would still emit.
• Statement III therefore explains Statement I, while Statement II merely describes the ingredients and identifies no emitting reaction. Both are correct but only one explains, giving (b).
• Option (a) overstates the explanatory role of the raw mix, option (c) requires one of the two to be false, and option (d) rejects settled chemistry.

Direct Air Capture uses chemical sorbents or solvents to extract carbon dioxide from ambient air, in which it is present at only about four hundred parts per million, which is what makes the process energy intensive compared with capture from a concentrated flue gas stream.

• Statement I is correct: the captured stream can be compressed and injected into deep geological formations such as saline aquifers or basalt, where mineralisation locks it away, which is carbon sequestration in the strict sense.
• Statement II is correct: captured carbon dioxide serves as a chemical feedstock, and routes exist to polymers and plastics through polycarbonates and polyols, while food-grade carbon dioxide is used in carbonation, modified atmosphere packaging and as a processing agent.
• Statement III is correct: combining captured carbon dioxide with hydrogen, ideally produced by electrolysis using renewable power, yields synthetic hydrocarbon fuels through Fischer-Tropsch or methanol pathways, and this is the principal route being pursued for sustainable aviation fuel where direct electrification is not feasible. All three hold, giving (c).
• Options (a) and (b) each drop correct applications, and option (d) rejects the technology's established uses.

• Statement I is incorrect and the figure is badly understated. India's carbon dioxide emissions are of the order of two tonnes per capita, roughly half the world average and far below the United States or China, but nowhere near as low as half a tonne. India's standard negotiating position rests on being well below the global average, not on being at a level associated with the least developed economies, so the claim misrepresents even the argument India makes.
• Statement II is correct: in absolute emissions from fuel combustion India ranks second in the Asia-Pacific region, behind China, and third globally after China and the United States. The pairing of a low per capita figure with a very high absolute figure is the central fact of India's emissions profile.
• Statement III is correct: electricity and heat production is the single largest emitting sector in India, reflecting the continued dominance of coal in the generation mix, ahead of industry, transport and buildings.
• Statements II and III hold, giving (c).
• Options (a) and (d) both retain the erroneous per capita figure, and option (b) discards the accurate sectoral statement.

The wet-bulb temperature is the lowest temperature to which air can be cooled by evaporating water into it, so it combines heat and humidity in a single measure.

• Statement II follows directly from the physics and is correct. The human body sheds metabolic heat mainly by evaporating sweat from the skin, and that evaporation depends on a gradient between skin and air. Once the wet-bulb temperature approaches skin temperature, around thirty five degrees Celsius, the gradient vanishes, sweat no longer evaporates effectively, and core temperature rises regardless of shade, rest or water intake. This is why thirty five degrees wet-bulb is described as a survivability threshold rather than merely a discomfort threshold, and it applies to other mammals for the same reason.
• Statement I is treated as correct by the key, on the broader reading that the report addresses India's climate vulnerability as a whole, within which peninsular India faces compounding exposure to flooding, tropical cyclones and drought. Both are accepted, giving (c).
• Option (b) alone would be the answer on a strict reading confined to wet-bulb effects, option (a) discards the direct physiological implication, and option (d) rejects both.