Greenhouse effect and global warming

Forests remove carbon dioxide during photosynthesis and store carbon in trunks, roots, leaves and soils. Protecting them maintains this carbon sink, preserves biodiversity and helps limit the growth of atmospheric greenhouse gases. Forest protection does not guarantee an immediate global temperature change, and the other options describe no valid general benefit.

Warmer air and surfaces generally increase evaporation from oceans, lakes, soil and vegetation. The extra water vapour can alter cloud formation, rainfall intensity, seasonal timing and the likelihood of droughts or heavy precipitation. Evaporation does not mean condensation stops, and temperature rise does not make every river dry or permanently end rainfall everywhere.

Flooded paddy fields often contain little oxygen. In these anaerobic conditions, microorganisms decompose organic matter and release methane, which is a potent greenhouse gas. Rice cultivation can also involve other emissions, including nitrous oxide from fertilisers, but the characteristic gas associated with waterlogged rice fields is methane. Therefore option A is correct.

The greenhouse effect occurs because gases such as water vapour, carbon dioxide and methane absorb some infrared radiation emitted by Earth’s warmed surface and re-emit it in different directions. This retains heat in the surface–lower-atmosphere system. It does not block all incoming sunlight, arise only from clouds or stop atmospheric winds, so B is the only correct statement.

Forests absorb atmospheric carbon dioxide through photosynthesis and store carbon in vegetation, woody biomass, litter, and soils. They are called carbon sinks when their total absorption exceeds their release. Forests are not perfectly emission-free because respiration, decomposition, and fires can release CO₂.

Permafrost stores organic material that has remained frozen for a long time. When thawing occurs, microorganisms can decompose this material under oxygen-poor conditions and release methane. Methane is a powerful greenhouse gas and can create a positive climate feedback. Carbon dioxide may also be released, but methane is the expected answer.

Cement production releases carbon dioxide in two important ways. Burning fuel supplies heat for making clinker, while heating limestone chemically breaks calcium carbonate into lime and CO₂. Because of these process and energy emissions, cement is a major industrial source of greenhouse gases. Therefore, option A is correct.

Global warming raises the temperature of the atmosphere and oceans and changes moisture, circulation, and energy patterns. These changes can intensify or increase some heat waves, heavy-rain events, droughts, and storms, although impacts differ by region. Permanent snowfall everywhere is scientifically impossible under general warming.

Buses, trains, and metros carry many people in one vehicle. When occupancy is reasonably high, fuel use and greenhouse-gas emissions per passenger can be lower than when each person travels separately in a private car or motorcycle. Public transport can also reduce congestion and energy demand, although its benefit depends on operation and occupancy.

Permafrost contains organic matter preserved in frozen soil. When rising temperatures thaw it, microorganisms can decompose the newly available material, especially in oxygen-poor conditions, and release methane. This methane can strengthen warming, creating a positive feedback in the climate system. Therefore, permafrost is the best answer.

Higher concentrations of carbon dioxide, methane, and other greenhouse gases absorb more outgoing longwave radiation from Earth. This reduces the immediate loss of heat to space and disturbs the planet’s energy balance, causing warming until a new balance develops. The other choices are local or unrelated processes, not the main global cause.

Urban trees and vegetation shade roads and buildings and cool the air through evapotranspiration. These processes reduce the urban heat island effect, in which built-up areas become hotter than surrounding rural areas. Green spaces may also store carbon and improve stormwater management, but reduced urban heat is the direct benefit asked here.

Forests absorb carbon dioxide through photosynthesis and store carbon in trunks, roots, leaves, dead organic matter, and soils. When this uptake is greater than the carbon released by respiration, decomposition, or fire, the forest acts as a carbon sink. Coal plants and vehicles are carbon sources; oceans can also act as sinks but are not the intended terrestrial example.

Waterlogged paddy fields are a major agricultural source of methane. In flooded rice fields, water fills the spaces between soil particles and limits the supply of oxygen. Under these oxygen-poor conditions, anaerobic microorganisms decompose organic matter and release methane. The gas can then escape to the atmosphere through water, soil, and rice plants. Dry sand, mountain rocks, and icy roads do not normally provide this waterlogged, oxygen-free environment. Therefore, option A is correct.

The greenhouse effect is the natural warming process in which gases such as water vapour, carbon dioxide, methane, and nitrous oxide absorb part of the outgoing longwave radiation emitted by Earth’s surface. These gases re-emit energy in all directions, including back toward the surface, thereby keeping the lower atmosphere and surface warmer than they would otherwise be. Human activities can strengthen this natural effect by increasing greenhouse-gas concentrations. Thus, option A gives the correct definition.

Carbon dioxide absorbs outgoing infrared, or longwave, radiation emitted by Earth’s warmed surface. It then re-emits energy, including toward the surface, reducing the rate at which heat escapes to space. Human activities such as fossil-fuel burning and deforestation increase its concentration and strengthen warming.

Water vapour is a strong natural greenhouse gas because its molecules absorb outgoing infrared radiation. Its amount is controlled mainly by temperature and the water cycle, so it acts largely as a feedback: warming permits more vapour, which can cause additional warming. It is not simply a dust-producing substance.

Earth’s temperature depends on a balance between incoming solar energy and outgoing infrared energy. Higher greenhouse-gas concentrations absorb more of the outgoing infrared radiation and re-emit part of it downward. Until the surface and lower atmosphere warm enough to restore balance, the retained energy produces a warming trend.

Compared with the same mass of carbon dioxide over a specified time period, methane absorbs infrared radiation more strongly and therefore has a higher global-warming potential. Methane is present in a much smaller concentration, but potency describes warming effect per unit amount, not total atmospheric abundance.

Nitrous oxide is a long-lived greenhouse gas that absorbs outgoing infrared radiation and contributes to global warming. In the stratosphere, its chemical products can also participate in reactions that destroy ozone. Therefore its increase creates both climate and ozone-layer concerns, especially from fertiliser use and combustion.

Forests remove carbon dioxide from the atmosphere through photosynthesis and store carbon in vegetation and soils. Deforestation reduces this carbon-sink capacity and often releases stored carbon through burning or decomposition. Consequently, more carbon dioxide remains in the atmosphere and strengthens radiative warming.

Earth’s surface continuously emits infrared, or longwave, radiation, including after sunset. When greenhouse gases and clouds absorb more of this radiation, they re-emit infrared energy in all directions. A portion is directed back toward the surface, reducing the rate at which the ground loses heat during the night. This atmospheric back-radiation is why clear, dry conditions often allow stronger cooling, while greater infrared absorption helps maintain warmer nights. Hence, option A is correct.

Albedo is the fraction of incoming sunlight reflected by a surface. When albedo falls, less radiation is reflected and more solar energy is absorbed by land, water or the exposed dark surface. That extra absorbed energy raises temperature and can produce further ice loss, creating a warming feedback.

Snow and ice have high albedo and reflect much incoming sunlight. When warming melts them, darker ocean or land is exposed, absorbing more solar energy. The added absorption causes further warming and more melting. Because the initial change reinforces itself, this is called a positive ice–albedo feedback.

Weather describes short-term atmospheric conditions such as today’s temperature, rainfall or wind. Climate describes long-term averages, patterns and trends over many years. The greenhouse effect changes the long-term energy balance, so global warming is assessed through persistent climate trends rather than one unusually hot or cold day.