Greenhouse effect and global warming

Warmer air can hold more water vapour, and water vapour is itself a strong greenhouse gas. If initial warming increases evaporation, the added vapour absorbs more outgoing infrared radiation and causes further warming. This amplifies the original change, so it is a positive feedback. Water vapour is therefore different from a simple one-way external forcing.

The natural greenhouse effect is produced by naturally occurring gases such as water vapour, carbon dioxide and methane, and it keeps Earth warm enough for life. The enhanced effect is the additional warming caused mainly by human emissions from fossil fuels, industry, agriculture and deforestation. Thus the difference is the human-driven strengthening, not the location or time of occurrence.

After sunset, the surface loses energy mainly as longwave infrared radiation. Clouds absorb part of this radiation and emit infrared energy both upward and downward. The downward component reduces net nighttime cooling, so cloudy nights are often warmer than clear nights. Clouds do not completely stop terrestrial radiation, and they do not increase oxygen or nighttime sunlight.

Water vapour is the most abundant natural greenhouse gas and absorbs infrared radiation across several wavelength bands. Its concentration changes with temperature, so it also acts as an important feedback in the climate system. Helium, neon and argon are noble gases that interact weakly with infrared radiation and do not produce a comparable greenhouse effect under ordinary atmospheric conditions.

Earth’s surface normally loses energy by emitting longwave infrared radiation to the atmosphere and space. If greenhouse gases absorb more of this outgoing energy, the initial loss of heat decreases. Energy accumulates until the surface and lower atmosphere become warmer enough to restore balance. Therefore, increased blocking generally raises surface temperature rather than changing salinity, rainfall instantly or Earth’s orbit.

Relative humidity compares the actual water vapour present with the maximum vapour the air can hold at that temperature. Cooler air has a lower saturation capacity. If the amount of vapour remains nearly constant while temperature falls, it represents a larger percentage of saturation, so relative humidity rises and may reach 100 percent, causing condensation.

A carbon budget is an estimate of the total additional carbon dioxide that can be emitted while retaining a chosen probability of staying below a temperature limit, such as 1.5°C or 2°C above the pre-industrial level. It connects cumulative emissions with a warming target. It is not a measure of salinity, pressure or mountain height.

At night, the surface continues to emit longwave infrared radiation even though incoming solar radiation has stopped. Increased greenhouse gases absorb more of this outgoing radiation and re-emit part of it downward, reducing the rate of heat loss to space. Consequently, nighttime cooling can weaken and minimum temperatures may rise. They do not stop wind or turn night into day.

Radiative forcing is a change in the net energy entering or leaving the climate system, often evaluated at the top of the atmosphere or at the tropopause. Positive forcing adds energy and tends to warm the system, while negative forcing removes energy and tends to cool it. Greenhouse gases generally create positive forcing by reducing outgoing infrared loss.

Carbon dioxide is a long-lived greenhouse gas that absorbs outgoing infrared radiation. If its concentration rises and other conditions are considered, the climate system retains more energy, producing a long-term warming tendency. Observations show a sustained rise in global average temperature alongside increasing greenhouse-gas concentrations. Oceans do not freeze globally, air does not disappear and mountains do not melt as a direct general result.

Stratospheric ozone absorbs much harmful ultraviolet radiation, so ozone depletion increases the amount of UV reaching Earth’s surface. The greenhouse effect concerns the absorption and re-emission of outgoing infrared radiation, which affects Earth’s heat balance. The two atmospheric issues involve different gases, wavelengths and mechanisms, although some substances can influence both systems indirectly.

Carbon sinks such as forests, soils, wetlands and oceans remove part of the carbon dioxide released by human activities. If these sinks are damaged, dried, deforested or otherwise weakened, less carbon is absorbed and a larger share remains in the atmosphere. This atmospheric accumulation strengthens the greenhouse effect and can accelerate warming. The other options are unrelated physical outcomes.

A positive feedback amplifies an initial climate change. Ice and snow reflect much incoming solar radiation. When warming melts them, darker land or ocean surfaces are exposed, absorb more solar energy and cause additional warming. That extra warming melts still more ice, creating a reinforcing cycle. The other examples generally remove heat or carbon dioxide and tend to oppose warming.

Climate justice highlights that responsibility for greenhouse-gas emissions and vulnerability to climate impacts are distributed unequally. Historically, some industrialised countries have emitted much more, while poorer communities and countries may face greater floods, droughts, heat stress, and displacement. Therefore, option A correctly expresses the inequality of responsibility and risk.

Emission standards restrict greenhouse-gas releases from factories, vehicles, and power plants. Promoting solar, wind, and other low-carbon energy sources reduces dependence on coal, oil, and gas. By contrast, expanded coal use and deforestation increase emissions and remove carbon sinks. Thus option A is the most suitable mitigation policy for human-driven warming.

Atmospheric transparency falls when dust, smoke, aerosols, or water droplets scatter and absorb incoming sunlight. Consequently, the direct beam reaching the ground may decrease, although diffuse radiation can increase. Reduced transparency does not make all sunlight disappear, so option D is too absolute. Option A is therefore the scientifically correct answer.

After sunset, Earth’s surface loses energy mainly as long-wave infrared radiation. Greenhouse gases such as water vapour, carbon dioxide, and methane absorb some of this radiation and re-emit it in different directions, including back toward the surface. This reduces nighttime cooling and makes their warming effect especially noticeable. Hence option A is correct.

Warm air has a greater capacity to hold water vapour than cold air. Tropical regions receive strong solar heating, so evaporation is generally high and the atmosphere can contain more moisture. Water vapour is itself a greenhouse gas and also affects clouds, rainfall, and feedbacks in the climate system. Therefore, option A is correct.

Sea-level rise increases the baseline water level along coasts. During high tides, storm surges, or heavy rainfall, water can therefore move farther inland and flood low-lying settlements, wetlands, roads, and farmland. It can also worsen erosion and saltwater intrusion. The other options are unrelated, so the correct answer is A.

Albedo is the fraction of incoming solar radiation reflected by a surface or by Earth as a whole. A higher albedo means that a larger share of sunlight is reflected to space, so less solar energy is absorbed by the surface and climate cooling is favoured. Snow and ice have high albedo, whereas dark oceans and forests generally have lower albedo. Hence A is correct.

Greenhouse gases absorb some of the infrared radiation emitted by Earth’s surface and atmosphere and re-emit part of it downward. This slows the loss of heat, particularly after sunset when incoming solar heating has stopped. Clear, dry conditions often allow faster radiative cooling, while easy escape of infrared radiation weakens the greenhouse effect. Thus A is correct.

Mitigation addresses the causes of climate change by reducing greenhouse-gas emissions or increasing carbon sinks, for example through renewable energy and forest protection. Adaptation addresses actual or expected impacts, such as sea-level rise, heat, floods, and drought, through resilient infrastructure or drought-tolerant crops. Option A states this distinction correctly.

Tropical cyclones draw energy from warm ocean water. Higher sea-surface temperatures can increase evaporation and provide more latent heat when moist air rises and condenses. If other conditions are favourable, this additional energy can support stronger storms and heavier rainfall, although warming does not guarantee that every cyclone will intensify. Therefore option A is the most appropriate answer.

Increasing atmospheric carbon dioxide strengthens the greenhouse effect by absorbing more outgoing infrared radiation. Over long periods, this creates positive radiative forcing and raises the average temperature of the climate system. The resulting warming can influence ice, sea level, rainfall, and ecosystems, but it does not make oceans fresh or wind solid. Hence A is correct.

Climate describes typical atmospheric conditions over long periods, whereas weather refers to short-term changes. Averaging temperature observations over months, seasons, or many years reduces the influence of unusual individual days and reveals persistent warming or cooling trends. A single daily maximum represents weather, not climate. Therefore option A correctly explains the importance of average temperature.