Greenhouse effect and global warming

Carbon dioxide is a greenhouse gas because its molecules absorb outgoing infrared radiation and re-emit energy within the atmosphere. Its concentration has increased substantially because of fossil-fuel combustion, cement production, and land-use change. Oxygen, neon, and helium do not significantly absorb terrestrial infrared radiation in the same way. Therefore, B is correct.

Global warming refers to the long-term rise in Earth’s average surface temperature, mainly because increased greenhouse gases reduce the rate at which outgoing heat escapes to space. It is a climate trend measured over long periods, not a single hot day or a short-term weather event. Thus option C gives the precise meaning.

Sunlight reaches Earth mainly as shortwave radiation. After absorbing this energy, Earth’s surface becomes warm and emits energy upward as longwave, or terrestrial infrared, radiation. Greenhouse gases absorb and re-emit portions of this longwave radiation, reducing heat loss to space. Therefore, D correctly identifies the radiation involved.

Coal is a carbon-rich fossil fuel. During efficient combustion, its carbon combines with oxygen to form carbon dioxide, and large quantities of CO₂ are released. Coal burning can also produce pollutants such as sulphur dioxide and particulate matter, but among the given choices carbon dioxide is the relevant greenhouse gas. Thus A is correct.

Forests act as carbon sinks because trees remove carbon dioxide from the atmosphere during photosynthesis and store carbon in biomass and soil. Deforestation reduces this removal capacity and may also release stored carbon through burning or decay. Consequently, atmospheric CO₂ can rise and increase warming. Therefore, option B is correct.

The natural greenhouse effect traps part of Earth’s outgoing infrared energy and maintains a temperature suitable for liquid water and living organisms. Without it, Earth would be much colder and life as we know it would be difficult or impossible. The enhanced greenhouse effect is different because human activities intensify the natural process and cause excessive warming. Thus D is correct.

The enhanced greenhouse effect occurs when human activities increase the atmospheric concentration of heat-trapping gases beyond natural levels. Burning coal, oil, and gas, deforestation, industrial processes, livestock, and fertilizer use are important contributors. The additional gases absorb more outgoing infrared radiation and produce extra warming. Therefore, A is correct.

Global warming increases temperatures in many regions and can cause glaciers and ice sheets to lose mass. Glacier melt contributes to sea-level rise and threatens freshwater supplies that depend on seasonal meltwater. The rate and effect vary by region, but melting glaciers is a well-established consequence of warming. Therefore, option B is correct.

An increase in greenhouse gases strengthens the atmosphere’s ability to absorb and re-emit outgoing longwave radiation. This means more heat remains in the Earth system instead of escaping directly to space. The immediate physical mechanism is enhanced heat trapping; rising average temperature and other impacts follow from it. Therefore, D is the best answer.

Solar energy is renewable and produces electricity without directly burning coal, oil, or gas during operation. Replacing part of fossil-fuel use with solar power can reduce carbon dioxide emissions and other air pollutants, thereby supporting climate-change mitigation. Solar technology still has manufacturing and land-use impacts, but option A correctly states its main climate benefit.

Climate mitigation means actions that reduce the causes of climate change or increase the removal of greenhouse gases from the atmosphere. Examples include renewable energy, energy efficiency, public transport, forest protection, and carbon capture. Adaptation is different because it reduces harm from impacts already occurring or expected. Hence B is correct.

Methane, nitrous oxide, and carbon dioxide absorb infrared radiation and contribute to the greenhouse effect. Molecular nitrogen, although it forms most of the atmosphere, does not significantly absorb the relevant infrared wavelengths and is therefore not considered a major greenhouse gas. Abundance alone does not make a gas a greenhouse gas. Thus D is correct.

When many people share a bus, train, or metro, fewer separate vehicles may be needed for the same number of journeys. This can reduce fuel use, traffic congestion, and greenhouse-gas emissions per passenger, especially when the system is efficient and well used. The exact benefit depends on vehicle technology, occupancy, and energy source. Therefore, B is correct.

Water vapour is the most abundant natural greenhouse gas. It absorbs and re-emits part of Earth’s outgoing infrared radiation, helping regulate surface temperature. Its amount is strongly controlled by temperature because warmer air can hold more moisture, so it acts mainly as a feedback in present warming rather than the initial long-term forcing caused by human carbon dioxide emissions. Hence C is correct.

Global warming raises the baseline temperature, so extreme heat events can become more frequent, longer, or more intense in many regions. The exact pattern varies with location and atmospheric circulation, and warming does not mean every day is a heat wave. Nevertheless, increased heat-wave risk is a major climate-health concern because it can cause dehydration, heatstroke, and excess deaths. Thus A is correct.

Ozone depletion and global warming are different environmental processes. Ozone depletion mainly concerns the reduction of stratospheric ozone, which allows more harmful ultraviolet radiation to reach Earth. Global warming refers to the long-term rise in average temperature caused mainly by enhanced greenhouse gases. They may have some connections, but they are not identical.

Wind energy is renewable because moving air is continuously replenished by natural atmospheric processes and does not become exhausted through normal use. Coal, petrol, and diesel are fossil fuels formed over geological time; their reserves are finite, and burning them releases carbon dioxide and other pollutants. Therefore, wind energy is the correct answer.

In the expression global warming, the word global indicates that the phenomenon concerns the Earth as a whole rather than one isolated place. Scientists study changes in the long-term average temperature of the entire climate system, while recognising that warming may differ from region to region. Thus, Whole Earth is correct.

Weather describes short-term conditions, such as today’s temperature or a week’s rainfall, and can vary greatly from day to day. Climate is understood from patterns and averages observed over many years, normally several decades. Long-term records therefore help identify persistent trends and distinguish climate change from ordinary weather variation.

Carbon dioxide contains one carbon atom chemically bonded to two oxygen atoms, so its formula is CO₂. The subscript 2 shows that two oxygen atoms are present for every one carbon atom. Carbon dioxide is also an important greenhouse gas because it absorbs and re-emits some outgoing infrared radiation, contributing to the greenhouse effect and global warming.

The chemical formula of methane is CH₄. It shows that one methane molecule contains one carbon atom and four hydrogen atoms. Methane is a greenhouse gas that can be released from wetlands, livestock, rice cultivation, landfills, and fossil-fuel production. CO₂, NH₃, and N₂ represent different substances.

Nitrous oxide has the chemical formula N₂O, meaning that each molecule contains two nitrogen atoms and one oxygen atom. It is a powerful greenhouse gas and can be emitted from agricultural soils, fertiliser use, and some industrial processes. NO₂ is nitrogen dioxide, while N₂O₅ is dinitrogen pentoxide.

Urbanization often concentrates people, buildings, industries, vehicles, and services in cities. This can raise demand for electricity, heating, cooling, construction materials, and transport, much of which may use fossil fuels. Consequently, carbon dioxide and other greenhouse-gas emissions can increase, although efficient public transport and clean energy can reduce this effect.

Global warming can increase the temperature of ocean water because oceans absorb a large amount of excess heat from the climate system. Warmer oceans contribute to thermal expansion and can affect marine ecosystems, circulation, and storms. The change is gradual and uneven, not an instant disappearance, combustion, or conversion of seawater into stone.

Greenhouse gases mix through the atmosphere, and their effects influence regions beyond the countries where emissions occur. Climate change also affects shared oceans, food systems, health, trade, and vulnerable communities. International cooperation is therefore needed for emission reduction, adaptation, finance, technology, research, and fair implementation. No single country can solve the problem alone.