Human activities which contribute to an increase in greenhouse gases in the atmosphere — AQA GCSE Combined Science

Carbon dioxide is a greenhouse gas released by human activities, chiefly burning fossil fuels such as coal, oil and natural gas, and by deforestation. In the atmosphere it absorbs outgoing infrared radiation and re-emits it, warming the Earth's surface: the greenhouse effect. Students should link each human activity to the extra CO₂ it adds. For example, a coal-fired power station burns carbon, forming CO₂; clearing forest removes trees that would otherwise remove CO₂ by photosynthesis, so CO₂ rises further. More CO₂ means more absorption and re-emission of infrared, so average global temperature increases.

Methane is a greenhouse gas released by human activities including farming cattle and other livestock, growing rice in flooded paddy fields, and decomposing organic waste in landfill sites. It absorbs outgoing infrared radiation and re-emits it, contributing to the greenhouse effect and global warming. Students should name a source and explain the mechanism. For example, cattle digestion and anaerobic decay of rubbish in landfill produce methane; when this reaches the atmosphere it absorbs infrared radiation emitted by the Earth's surface and re-emits it, so more energy is retained and average global temperature rises.

Greenhouse gases trap infrared radiation, and human activity raises their atmospheric amounts. For carbon dioxide, burning fossil fuels such as coal, oil and natural gas in power stations, factories and vehicles releases CO₂, while deforestation removes trees that absorb CO₂ and burning cleared vegetation adds more. For methane, rearing livestock such as cattle and sheep produces methane from digestion, and landfill sites release methane as waste decomposes anaerobically; rice paddy fields and extracting fossil fuels also add methane. A strong answer names two distinct activities per gas and links each to the gas released, for example coal-fired electricity generation for CO₂ and cattle farming for CH₄.

Peer-reviewed evidence means research checked by independent experts before publication, giving it greater reliability than unverified claims. Many scientists use such evidence, including global temperature records, ice-core data, satellite measurements and computer models, to conclude that human activities are increasing greenhouse gases and that this will raise surface temperatures. Warmer surface temperatures are expected to drive global climate change, meaning long-term shifts in weather patterns, sea level and ecosystems worldwide. A good answer distinguishes weather from climate, explains that the belief rests on peer-reviewed evidence, and states both parts: surface temperature increase and resulting global climate change.

Global climate is a complex system: the atmosphere, oceans, ice, land and living things interact, and changes feed back on one another. Because so many variables interact over long timescales, computer models must simplify reality, for example by using grid boxes of hundreds of kilometres or by averaging ocean mixing. Such simplifications make predictions uncertain, so scientists compare model outputs with observations and with each other. When this science reaches the media, reports may quote only one study, ignore uncertainty ranges, or select evidence supporting a particular viewpoint, producing speculation or bias. A useful method is to ask: what evidence is used, what is left out, who benefits from the claim, and does the source cite peer-reviewed data?

Evaluating evidence quality means judging how trustworthy a climate report is, not simply summarising it. Work through the evidence systematically: identify the claim, then ask how the data were obtained. Consider sample size, timescale and geographical spread; a conclusion drawn from ten years of records at one weather station is weaker than one from decades of global satellite and ground measurements. Check whether measurements are reliable and repeatable, whether sources are peer-reviewed, and whether the report distinguishes measured data from computer-model projections. Look for bias, selective use of data, missing uncertainties and conflicts of interest. Weigh strengths against weaknesses and reach a justified overall judgement about how much confidence the evidence deserves.

Uncertainty in the evidence base means the extent to which climate data and conclusions may be incomplete, imprecise or open to more than one interpretation. Describe it by naming sources of uncertainty and explaining their effect. Measurement uncertainty arises from instrument accuracy and calibration, for example a thermometer reading of 15.0 °C carries an uncertainty of about ±0.5 °C. Natural variability, such as El Niño years, makes short records hard to interpret. Sparse coverage in oceans and polar regions, gaps in historical records and changing station locations add further uncertainty. Computer models carry uncertainty because future emissions, feedbacks and cloud behaviour are not fully known. Scientists reduce uncertainty with more measurements, better instruments and multiple independent lines of evidence, but some uncertainty always remains.

Science is a social process, not just a set of measurements. When a scientist claims that a human activity is adding greenhouse gases to the atmosphere, other specialists independently scrutinise the method, data and interpretation before the claim is accepted; this is peer review. Reviewers check whether the evidence supports the conclusion, whether the sample and apparatus were suitable, and whether alternative explanations were considered. Results are then communicated to audiences such as other scientists, policymakers, industry and the public, each needing different depth and language. A claim that survives review and is clearly communicated is more trustworthy, and it can inform decisions about emissions.