Climate Change & Global Warming — PPSC previous year questions

A carbon credit is a tradable permit to emit one tonne of CO2 (or its equivalent), and markets such as the EU Emissions Trading System trade them, so statements 1, 2 and 4 are correct. Statement 3 is wrong: under the Clean Development Mechanism, developed countries earn credits only by funding projects in developing countries; projects in other developed countries come under a separate route called Joint Implementation.

Emissions trading (cap-and-trade) is a market mechanism for controlling greenhouse gases, so A is correct. R states the trade backwards: firms that cut emissions by more than required earn surplus credits and sell them to firms that cannot easily meet their cap. So R is incorrect.

Several greenhouse gases - CFCs, HCFCs, HFCs, PFCs and SF6 - contain halogens (fluorine, chlorine, bromine), so A is correct. Halogens such as chlorine and iodine are indeed used as disinfectants, and methyl bromide as a fumigant, so R is also correct. But these gases reach the atmosphere because they are used as refrigerants, propellants and insulators; their use as fumigants does not explain A.

Flooded rice paddies release methane by anaerobic decomposition. Decaying garbage in landfills releases it too. Its level in the air has risen by roughly 145–150% since pre-industrial times, from about 700 ppb to over 1,800 ppb. So statements 2, 3 and 4 are correct. Statement 1 is wrong. The human digestive system produces only trace methane. The large enteric emissions come from ruminant livestock such as cattle.

Common but Differentiated Responsibilities (CBDR) is a principle of international environmental law. It was adopted at the 1992 Rio Earth Summit and written into the UN Framework Convention on Climate Change. All countries share responsibility for protecting the global climate. But developed countries caused most historical emissions and have greater capacity, so they must take the lead. The Kyoto Protocol's differing obligations for Annex I and non-Annex I countries (developed and developing countries) flow from this principle.

Sea level rises mainly through two direct mechanisms: the addition of water from melting land ice (polar ice sheets and glaciers) and the thermal expansion of sea water as the oceans warm. Retreating glaciers also contribute indirectly: exposed dark rock and water reflect less sunlight than ice (lower albedo), so more heat is absorbed, accelerating warming and further melt and expansion. So all three factors, 1, 2 and 3.

Greenhouse gases are called radiatively active gases because they absorb the infrared (long-wave) radiation given off by the earth. The earth's surface does radiate energy back to the atmosphere as terrestrial radiation. So statements 1 and 2 hold. Statement 3 is wrong. Methane survives in the atmosphere for only about 12 years. CFCs persist for roughly 50 to 100 years or more.

Statement I is correct. Methane is a potent greenhouse gas that absorbs long-wave (infrared) radiation. Enteric fermentation (digestion in the gut of cattle and sheep) in the expanding cattle and sheep population is a major source. Statement II is incorrect. Flooded paddy fields are anaerobic, and methanogenic bacteria in the waterlogged soil release large amounts of methane. So the expansion of rice cultivation is genuinely linked to rising methane, not falsely. So only Statement I is correct.

Aerosols that scatter sunlight raise the Earth's albedo (the share of sunlight it reflects), so less energy reaches the surface and it cools; III is right and I (which says the albedo falls) is wrong. Aerosols that absorb radiation, such as black carbon, warm the air locally; the warmer layer radiates more energy back to the ground and weakens the convective transfer of heat upward from the surface, so II is right and IV is its reverse.

Ocean acidification is caused by seawater absorbing excess carbon dioxide, which forms carbonic acid and lowers pH; it has nothing to do with nitrogenous compounds, so statement 1 is wrong. Lower pH reduces the carbonate ions corals need to build skeletons, so their productivity falls rather than rises, making statement 2 wrong. Seagrass meadows take up CO₂ and can locally raise pH, buffering nearby reefs, so statement 3 is correct.