Greenhouse effect and global warming

After sunset, the surface continues to emit longwave infrared radiation. Greenhouse gases absorb part of this radiation and re-emit energy in all directions, including downward toward the surface. Increasing their concentration can therefore reduce the rate at which heat escapes to space at night, raising minimum temperatures relative to an atmosphere with fewer greenhouse gases. It does not increase night-time sunlight.

Water vapour is an important greenhouse gas because it absorbs outgoing longwave infrared radiation emitted by Earth and re-emits energy in different directions, including back toward the surface. This retains heat in the lower atmosphere. It does not destroy incoming solar energy or stop winds, although its amount is strongly influenced by temperature.

Scattering occurs when dust, smoke, aerosols, or droplets redirect radiation from its original path. Reduced atmospheric transparency can therefore change the direction of incoming solar radiation, and some energy may be sent back toward space. Scattering is not the same as complete absorption of outgoing infrared radiation, which is mainly associated with greenhouse gases.

At night, the surface loses energy mainly as terrestrial longwave radiation. Cloud droplets and ice crystals absorb part of this radiation and emit some energy downward, reducing radiative cooling of the surface. Consequently, cloudy nights are often warmer than clear nights. Clouds do not increase sunlight at night or completely stop convection.

Higher carbon dioxide can stimulate photosynthesis in some plants, particularly many C3 crops, and may improve water-use efficiency. However, warming, heat waves, irregular rainfall, pests, and greater evaporative demand can increase water stress and reduce yields. The response varies by crop, temperature, nutrients, and water availability, so no universal benefit occurs.

Tropical cyclones obtain much of their energy from warm ocean water. Higher sea-surface temperatures increase evaporation, and the release of latent heat during condensation can strengthen a suitable storm. Other conditions, including atmospheric circulation and wind shear, also matter, so warmer water does not guarantee intensification of every cyclone or eliminate all uncertainty.

Proxy data are indirect natural records that preserve evidence of earlier environmental conditions when instrumental measurements were unavailable. Tree rings, ice cores, pollen, corals, and lake or ocean sediments can reveal information about past temperature, precipitation, atmospheric gases, and volcanic activity. They reconstruct past climate; they are not exact forecasts of future rainfall.

A persistent rise in nighttime temperatures can indicate that the surface and lower atmosphere are losing less heat overnight. Greenhouse gases, water vapour, clouds, and urban surfaces can absorb or re-radiate outgoing longwave energy, while daytime and nighttime responses may differ. A sustained multi-year pattern is more meaningful than a few warm nights.

A climate scenario is a plausible, assumption-based pathway describing how future emissions, land use, population, technology, energy systems, and policies might develop. Models use different scenarios to explore possible future temperature, rainfall, and other outcomes. A scenario is not a certain prediction or a measuring instrument; it is a structured possibility for analysis.

Water vapour is a powerful greenhouse gas, but its atmospheric amount is controlled mainly by temperature and the hydrological cycle rather than by long-term direct human emissions. When warming increases evaporation, warmer air can hold more water vapour. The additional vapour absorbs more outgoing infrared radiation and can cause further warming. This amplifying response is called a positive water-vapour feedback, although clouds can produce more complicated effects.

Methane is more effective per molecule because its molecular structure allows it to absorb infrared radiation strongly in important atmospheric wavelength bands. Its atmospheric concentration is much lower than carbon dioxide, but molecule for molecule its warming influence over a specified time period is substantially greater. Methane also has a shorter atmospheric lifetime than carbon dioxide, so potency and persistence must be considered separately when comparing greenhouse gases.

Atmospheric transparency describes how freely radiation passes through the atmosphere. When dust, smoke, aerosols, clouds or absorbing gases increase, transparency can decrease. More radiation is then scattered or absorbed along its path, and less direct solar radiation may reach the surface. The exact surface-temperature response can vary because aerosols may also affect clouds and outgoing infrared radiation, but reduced transparency does not mean radiation passes unchanged or that surface sunlight necessarily increases.

Permafrost is permanently or persistently frozen ground containing organic matter accumulated over long periods. When it thaws, microbes decompose the exposed material and release carbon dioxide; in waterlogged, oxygen-poor conditions they may release methane. Both gases strengthen the greenhouse effect, producing a positive feedback that can cause further warming and more thaw. The main risk is greenhouse-gas release, not an immediate direct rise in global sea level.

Greenhouse gases have molecular vibration and rotation modes that interact with wavelengths of Earth’s outgoing thermal infrared radiation. They absorb part of this longwave energy and re-emit it in all directions, including downward toward the surface and lower atmosphere. This slows the loss of energy to space and raises the temperature required for outgoing energy balance. They do not simply reflect all incoming sunlight or act only by forming stratospheric ozone.

CFCs are chemically stable in the lower atmosphere, allowing them to reach the stratosphere. Ultraviolet radiation breaks them apart and releases chlorine atoms, which participate in catalytic reactions that destroy ozone molecules. CFCs are also strong greenhouse gases because they absorb outgoing infrared radiation and contribute to warming. Their dual role in ozone depletion and climate change made them a major environmental hazard and led to international controls under the Montreal Protocol.

Climate justice is based on the fact that responsibility for greenhouse-gas emissions and exposure to climate harm are distributed unequally. Some countries and groups have contributed much more historical emissions, while poorer or vulnerable communities may suffer greater losses from heat, floods, droughts, and sea-level rise. Justice therefore considers responsibility, capacity, vulnerability, and fair support.

Climate models use mathematical equations and observations to represent interactions among the atmosphere, oceans, land, ice, and living systems. Researchers run them under different greenhouse-gas and land-use scenarios to estimate long-term patterns in temperature, rainfall, sea level, and extremes. They do not directly control emissions, replace short-term weather forecasts, or prevent disasters.

The saturation vapour pressure of water rises strongly with temperature. Therefore, warmer air can contain a greater maximum amount of water vapour before reaching saturation. Warm ocean surfaces also enhance evaporation, supplying additional vapour to the air. This relationship does not depend on seawater becoming fresh, on warm air always having higher pressure, or on condensation stopping completely.

Aerosols are tiny solid or liquid particles suspended in the atmosphere. They can directly scatter or absorb incoming and outgoing radiation, changing the energy reaching the surface. They can also act as cloud-condensation nuclei, modifying cloud brightness, lifetime, and precipitation. Their climate effect may therefore be cooling or warming depending on composition, altitude, location, and cloud interactions.

Higher atmospheric carbon dioxide can stimulate photosynthesis and water-use efficiency in some plants, especially when nutrients, temperature, and water are adequate. However, this fertilisation benefit is limited and may be offset by heat waves, drought, nutrient shortages, pests, and shifting seasons. Therefore, rising CO₂ can produce a possible growth benefit while climate-related water stress increases.

A carbon budget is a finite cumulative quantity of carbon dioxide emissions associated with a specified probability of keeping global warming below a chosen limit, such as 1.5°C or 2°C. It concerns total emissions over time, not merely one country’s emissions in one year, annual natural uptake, or money collected through a carbon tax.

Proxy data are indirect records that help scientists reconstruct climate conditions before widespread instrumental measurements existed. Tree rings, ice cores, pollen, corals, and lake or ocean sediments can preserve clues about past temperature, rainfall, atmospheric composition, and environmental change. They do not control winds or remove greenhouse gases, so option A is correct.

Ice cores contain layers of compacted snow and tiny bubbles of ancient air trapped during snowfall. Laboratory analysis of these bubbles reveals past concentrations of gases such as carbon dioxide and methane. Isotopes and impurities can also indicate temperature, dust, and volcanic events. River width, mountain age, and air smell cannot be obtained in this way, so A is correct.

Many trees produce an annual growth ring whose width, density, and chemical composition reflect the conditions of that growing season. Favourable moisture and temperature may produce wider rings, while drought or severe stress may produce narrow rings. By comparing dated rings, scientists reconstruct past climate variability. Thus tree rings are proxy records, making A correct.

Elevated carbon dioxide can enhance photosynthesis and water-use efficiency in some crops, especially under suitable conditions. However, warming, drought, heat waves, pests, nutrient limits, and altered rainfall can reduce or reverse that benefit. Crop responses also differ by species and management. Therefore the scientifically balanced answer is A: a possible growth benefit together with water or climate stress.