Greenhouse effect and global warming

Without an atmosphere, Earth’s surface would lose the main medium that redistributes and retains heat. During daylight, the sunlit surface would absorb solar energy directly and could become extremely hot. After sunset, there would be no atmospheric blanket or greenhouse gases to absorb and re-radiate outgoing infrared energy, so the surface would cool rapidly and become extremely cold. The atmosphere also transports heat between regions and moderates temperature differences. Therefore, option B is the best answer.

The stratospheric ozone layer absorbs much of the Sun’s harmful ultraviolet radiation and protects living organisms. Global warming is mainly caused by the enhanced greenhouse effect, in which gases such as carbon dioxide, methane, and nitrous oxide trap outgoing infrared radiation. Ozone depletion and global warming are therefore related atmospheric issues, but they are not the same process.

Many atmospheric dust and aerosol particles act as condensation nuclei. Water vapour can condense more readily on these tiny surfaces to form cloud droplets. If their concentration decreases, cloud formation may become less efficient in some situations, although clouds can still form around other particles and meteorological conditions. Therefore, complete disappearance of rain is not a valid conclusion.

The urban heat-island effect is mainly a local or regional phenomenon in which built surfaces absorb and store heat, vegetation is reduced, and waste heat is released by vehicles and buildings. These factors can make cities warmer than nearby rural areas. This is different from global warming, but emissions from urban activities contribute to global warming and can intensify local heat risks.

Annual tree rings can preserve information about the conditions under which a tree grew. Ring width, density, and chemical characteristics may respond to temperature, moisture availability, drought, and the length of the growing season. By comparing many dated trees and calibrating their patterns against modern observations, scientists use them as proxy evidence for climate variability before instrumental records began.

Ice sheets accumulate snowfall in layers, and compacted ice can preserve tiny bubbles of ancient atmosphere. Scientists analyse these bubbles for gases such as carbon dioxide and methane, while isotopes, dust, and chemical markers provide clues about past temperature and environmental conditions. Ice cores therefore connect atmospheric composition with climate history over thousands of years.

Mitigation addresses the cause of climate change by reducing greenhouse-gas emissions or increasing their removal through measures such as clean energy and carbon sinks. Adaptation addresses present and expected impacts by reducing vulnerability, for example through heat planning, drought-resistant crops, or coastal protection. In a high-emission pathway, mitigation limits future damage while adaptation manages unavoidable effects.

Climate justice emerged because responsibility for greenhouse-gas emissions, exposure to climate hazards, capacity to respond, and access to resources are distributed unequally. Industrialised countries have historically contributed a large share of cumulative emissions, while many poorer communities have contributed less but face severe risks. Justice therefore concerns fairness in responsibility, finance, technology, participation, and protection.

Clouds have two competing effects on planetary radiation balance. During daylight, their bright tops reflect incoming short-wave solar radiation back to space, increasing albedo and producing a cooling influence. At the same time, cloud water and ice absorb and re-emit outgoing long-wave radiation, especially at night, reducing heat loss. The net effect depends on cloud altitude, thickness, type, and duration.

Permafrost contains frozen organic matter accumulated over long periods. When thawing allows microbes to decompose this material, carbon dioxide can be released in relatively oxygen-rich conditions and methane can be produced in waterlogged, oxygen-poor conditions. These greenhouse gases can cause additional warming, which promotes further thawing. This positive feedback makes permafrost loss an important climate risk.

The correct answer is A. More carbon dioxide can increase photosynthesis and water-use efficiency in some plants, especially under suitable conditions. This benefit is not universal and may be limited by factors such as nutrients and water. Indirectly, the warming associated with rising greenhouse gases can increase heat and drought stress, which may reduce plant growth or yields.

Reduced atmospheric transparency means that gases, dust, smoke, or aerosols interfere more strongly with the passage of radiation. Depending on their properties and altitude, they may absorb, scatter, and re-emit energy, altering Earth’s radiation budget. If outgoing heat is retained more than incoming energy is reduced, the surface and lower atmosphere can warm.

Water vapour is itself a greenhouse gas. When warming increases evaporation and allows the atmosphere to hold more moisture, the additional water vapour absorbs more outgoing infrared radiation and can amplify the initial warming. It is mainly a rapid climate feedback rather than the principal long-term human forcing, because its atmospheric amount is strongly controlled by temperature.

Climate sensitivity describes how strongly the climate system responds to a specified radiative forcing, often the forcing from a doubling of atmospheric carbon dioxide. High sensitivity means that a relatively small change in energy balance can produce a comparatively large change in global temperature. Feedbacks involving water vapour, clouds, snow, ice, and vegetation can strengthen or weaken the response, so sensitivity is a measure of response strength rather than weather variability.

A climate system contains balancing processes, or negative feedbacks, that can limit a disturbance. If the ability of these processes to offset change is weak, a forcing may produce a larger and more persistent response. Positive feedbacks can then amplify warming or other changes, increasing the risk of instability, rapid transitions, and difficult-to-reverse impacts. This does not imply changes in Earth’s mass or the formation of gold in air.

Some human-produced aerosols, such as sulfate particles from fossil-fuel combustion, scatter incoming sunlight back to space and increase the reflectivity of the atmosphere. Aerosols can also change cloud brightness, lifetime, and droplet properties, producing additional cooling in some situations. Their effects vary by composition, location, altitude, and lifetime; some black-carbon aerosols absorb sunlight and warm the atmosphere. Thus, aerosol forcing is complex but can be negative overall.

The Intergovernmental Panel on Climate Change (IPCC) reviews and synthesizes a very large body of peer-reviewed scientific research. Its assessment reports explain observed changes, causes, future risks, and possible responses, helping governments and institutions make evidence-based climate policies. The IPCC does not conduct hourly weather forecasting or prepare school examinations.

Warmer nights generally indicate that the surface is losing less longwave infrared energy after sunset. Greenhouse gases, water vapour, clouds, and other atmospheric factors can absorb and re-emit part of this outgoing radiation, keeping the lower atmosphere warmer. The observation does not imply a changed solar orbit, instant salinity increase, or rain becoming solid.

Carbon dioxide absorbs selected wavelengths of outgoing infrared radiation emitted by Earth’s surface and atmosphere. Although it re-emits energy in all directions, the net effect of increased CO₂ is that less energy escapes directly to space until the climate system warms enough to restore balance. This positive radiative forcing drives modern warming.

Methane has a higher global warming potential than carbon dioxide over commonly used short- and medium-term time horizons because each methane molecule absorbs infrared radiation efficiently. However, methane is present in much smaller concentrations and generally has a shorter atmospheric lifetime than CO₂. Thus, option A is correct when comparing per-molecule potency, not total climate influence.

Nitrous oxide, N₂O, is a long-lived and powerful greenhouse gas. It absorbs outgoing infrared radiation and contributes to positive radiative forcing and global warming. Important human sources include agricultural soils, nitrogen fertiliser use, manure management, and some industrial processes. N₂O also participates in stratospheric ozone depletion, but salinity, freezing rain, and gravity are unrelated.

Atmospheric transparency describes how easily radiation passes through the atmosphere. More dust, aerosols, clouds, and radiatively active gases can reduce transparency by scattering or absorbing radiation. Absorbed energy may later be re-emitted, including as longwave radiation, thereby altering the surface-atmosphere energy budget. The effect can vary by particle type and wavelength, but option A is the relevant process.

Warmer air can hold more water vapour, provided moisture is available. Because water vapour absorbs outgoing infrared radiation, the added moisture can strengthen the greenhouse effect and provide a positive feedback: warming increases evaporation, and increased vapour can produce additional warming. This does not directly diagnose ozone depletion, which involves different chemical and radiative processes.

Permafrost contains frozen organic matter accumulated over long periods. When it thaws, microorganisms can decompose this material and release carbon dioxide in oxygen-rich conditions and methane in oxygen-poor conditions. These greenhouse gases increase heat retention and can accelerate further thawing, creating a positive feedback. The main global risk is therefore carbon release, not salinity change or altered gravity.

Sulfur hexafluoride, SF₆, is an extremely potent greenhouse gas because each molecule absorbs infrared radiation very efficiently. It also has a very long atmospheric lifetime. Methane and nitrous oxide are powerful gases too, but their per-molecule warming effectiveness is lower than that of SF₆. The question asks specifically about molecular potency, not total emissions or abundance.