Greenhouse effect and global warming

Burning coal, petroleum, and natural gas oxidizes stored carbon and releases large quantities of carbon dioxide into the atmosphere. Fuel combustion also produces other pollutants and, in some activities, methane and nitrous oxide. The added greenhouse gases alter Earth’s energy balance by reducing the efficiency of outgoing heat loss, making fossil-fuel use a major driver of current warming.

A carbon sink is a reservoir or process that absorbs more carbon from the atmosphere than it releases over a relevant period. Forests, soils, wetlands, and oceans can act as important sinks by storing carbon in biomass, organic matter, dissolved carbon, or sediments. A source has the opposite balance and releases more carbon than it absorbs.

Plants remove carbon dioxide through photosynthesis and store carbon in vegetation and soils. When forests are cleared, this carbon-sink capacity decreases; burning and decomposition can also release stored carbon. More atmospheric CO₂ strengthens the greenhouse effect and can raise temperature. Deforestation may additionally reduce evapotranspirational cooling, but the option identifies the key indirect greenhouse pathway.

Fresh snow has a high albedo, meaning it reflects a large proportion of incoming solar radiation and absorbs relatively little energy. This helps keep the surface cool. If snow cover melts, darker ground or water is exposed, albedo falls, absorption rises, and additional warming can cause further melting. This is the positive snow-albedo feedback.

Positive feedback occurs when a response reinforces or amplifies the initial disturbance rather than opposing it. For example, warming melts snow and ice, lowering surface albedo; the darker surface absorbs more sunlight and produces further warming. The term positive does not mean beneficial. A process that counteracts the initial change is called negative feedback.

Negative feedback produces a response that counteracts an initial change, thereby tending to reduce its magnitude and move the system toward stability. It does not necessarily eliminate the change completely and can operate for different kinds of disturbances. Option B describes positive feedback because amplification makes the original change stronger and may increase instability.

After sunset, Earth’s surface normally loses energy mainly as longwave infrared radiation. Greenhouse gases and suitable atmospheric conditions absorb and re-emit some of this radiation, slowing the rate of nocturnal cooling. Urban materials and clouds can also influence night temperatures, but the scientifically relevant general explanation in this question is atmospheric heat trapping rather than stronger nighttime sunlight.

Concrete, asphalt, and rooftops generally absorb and store solar energy more effectively than many vegetated surfaces and release it slowly after sunset. Urban growth also removes vegetation, reducing shade and evaporative cooling, while vehicles, buildings, industries, and air conditioners release anthropogenic heat. These combined effects make cities warmer than nearby rural areas, especially at night.

Melting land glaciers and ice sheets transfers water that was stored on land into the ocean. Warming seawater also expands in volume, a process called thermal expansion. Together these are major contributors to observed sea-level rise. Floating sea ice already displaces water, so its melting has a much smaller direct effect, while thermal contraction would lower rather than raise sea level.

During daytime, clouds reflect a portion of incoming shortwave solar radiation back toward space. This reduces the amount of direct energy reaching and warming the surface, although the exact effect depends on cloud type, thickness, and altitude. Thus, option A is correct; the other options describe impossible or unsupported effects.

At night, Earth’s surface loses energy mainly through longwave infrared radiation. Clouds contain water droplets and water vapour that absorb some of this outgoing radiation and re-emit part of it downward. This reduces radiational cooling, so cloudy nights are often warmer than clear nights. Option A correctly explains this greenhouse-like effect.

The natural greenhouse effect is produced by naturally occurring water vapour, carbon dioxide, methane, and other gases, and it keeps Earth warm enough for life. Human activities such as fossil-fuel combustion and deforestation increase greenhouse-gas concentrations, causing additional heat retention and a changed energy balance. Therefore, A is correct.

Solar, wind, hydropower, and other renewable sources generally produce electricity with far lower lifecycle greenhouse-gas emissions than coal, oil, or natural gas. Replacing fossil-fuel use therefore reduces the main sources of warming emissions. Mitigation means addressing the causes of climate change, so option A is the only correct answer.

Long-term sea-level rise is an important indicator of a warming climate. Higher temperatures expand seawater, while melting glaciers and ice sheets add water to the oceans. Tides and storms can cause short-term fluctuations, but they do not explain the sustained global rise. Thus option A identifies the relevant climate processes and is correct.

Proxy data are natural records that indirectly preserve information about past climate. Tree-ring width and density can reflect temperature or moisture, while ice cores preserve bubbles, dust, and isotopes related to past atmosphere and climate. Because they are not direct instrumental observations, they are called proxies. Option A is correct.

Weather refers to short-term atmospheric conditions such as today’s temperature, rainfall, wind, or pressure. A climate trend describes a persistent change in statistical averages or patterns over many years or decades. Therefore, option A correctly distinguishes timescale and meaning; options B, C, and D confuse weather with climate.

Water vapour is a powerful greenhouse gas. On humid nights, it absorbs part of the longwave infrared radiation emitted by the warm surface and re-emits some energy downward. This can reduce nighttime cooling, although clouds, wind, and surface conditions also matter. Thus, option A gives the correct physical relationship.

Earth’s surface absorbs much of the incoming shortwave solar radiation and becomes warm. It then emits energy upward mainly as longwave terrestrial infrared radiation. Greenhouse gases absorb and re-emit part of this outgoing radiation, influencing the energy balance. Therefore, option A is the scientifically precise answer.

A persistent increase in average atmospheric temperature is a component of global warming and broader climate change. It refers to a long-term statistical trend, not a single hot day, local sunshine, or an unrelated physical change. Short-term weather anomalies can occur without changing the long-term climate trend, which is why option A is correct.

Oceans absorb and store large quantities of solar heat and exchange it with the atmosphere through radiation, conduction, evaporation, and condensation. This moderates temperatures and helps drive winds, rainfall, and atmospheric circulation. Since oceans are a major heat reservoir, option A correctly states the broad climatic importance of ocean–atmosphere heat exchange.

Carbon dioxide is a long-lived greenhouse gas that absorbs outgoing infrared radiation and contributes to a positive radiative forcing. When its concentration rises, the climate system tends to warm over time, although natural variability affects individual years. Thus, a sustained increase in average global temperature is a relevant long-term sign, making A correct.

A warmer atmosphere can contain more water vapour before reaching saturation, according to the temperature dependence of saturation vapour pressure. Warm ocean surfaces also increase evaporation, supplying moisture to the air. This added water vapour can strengthen humidity and greenhouse feedbacks, although actual humidity also depends on winds and circulation. Option A is correct.

The natural greenhouse effect keeps Earth substantially warmer than it would be without greenhouse gases by absorbing and re-emitting outgoing infrared radiation. A major decrease would allow more heat to escape to space and could produce strong cooling, potentially threatening ecosystems and liquid-water conditions. Therefore, option A is correct; the other choices do not follow from greenhouse-gas reduction.

Weather changes from day to day, season to season, and place to place. A long-term average smooths out much of this short-term variability and reveals the underlying climate trend. Global warming is therefore assessed through sustained changes in average temperature over decades, not through one unusually hot or cold day.

When the land, ocean or atmosphere absorbs more energy than before, the climate system gains heat. A direct response is usually an increase in surface or near-surface temperature, although the amount and timing depend on circulation, evaporation and storage in the ocean. Warming generally promotes glacier retreat and sea-level rise over time, not immediate sea-level fall or widespread glacier expansion.