Explain human sources of greenhouse gases and evaluate the evidence for anthropogenic warming

What this dot point is asking

SCSA wants you to identify the human sources of greenhouse gases and evaluate the evidence that they are causing warming. A strong answer pairs the sources with the multiple, independent lines of evidence and explains how natural causes are excluded.

Human sources of greenhouse gases

• Burning fossil fuels for electricity, transport and industry is the largest source of carbon dioxide, transferring ancient geosphere carbon into the atmosphere.
• Land clearing and deforestation release stored carbon and remove a sink that would otherwise absorb carbon dioxide.
• Agriculture adds methane from livestock and rice and nitrous oxide from fertilised soils.
• Cement production and industry release further carbon dioxide.

Methane and nitrous oxide are present in smaller amounts than carbon dioxide but trap heat far more strongly per molecule.

The lines of evidence

The case for human-caused warming rests on several independent strands that agree.

• Rising greenhouse gases. Direct measurements show carbon dioxide and methane have risen sharply since industrialisation, and ice cores show today's carbon dioxide is far above the natural range of the last hundreds of thousands of years.
• Isotopic and oxygen fingerprints. The chemistry of the added carbon dioxide points to fossil fuels.
• Observed warming. Instrumental records show rising global average temperatures, warming oceans, melting ice and rising sea levels.
• Pattern of warming. The lower atmosphere is warming while the upper atmosphere cools, the pattern expected from greenhouse trapping rather than from a brighter Sun.

Ruling out natural causes

Evaluating the evidence means showing natural forcings cannot explain the recent trend.

• Solar output has not increased over recent decades, so the Sun is excluded.
• Volcanic eruptions cause brief cooling, not sustained warming.
• Orbital cycles are far too slow and currently favour gradual cooling.
• Ocean cycles redistribute heat but add no net energy over the long term.

With natural causes excluded and multiple human fingerprints present, the evidence points strongly to human activity as the cause. This consensus, supported by the weight of independent evidence, is the conclusion the syllabus expects.

Why methane and nitrous oxide matter despite low concentrations

A common oversight is to treat the problem as carbon dioxide alone, but SCSA expects awareness of the other gases and the idea of global warming potential. Methane is present at far lower concentrations than carbon dioxide, yet over a century each molecule traps many times more heat, and over the first two decades after release its warming effect is dozens of times stronger before it breaks down. Nitrous oxide is rarer still but extremely long-lived and a very potent absorber. This matters for evaluating sources and solutions: agriculture (livestock and rice paddies releasing methane, fertilised soils releasing nitrous oxide) and fossil-fuel extraction (leaking methane) contribute warming out of all proportion to the gases' tiny concentrations. It also shapes mitigation priorities, because cutting short-lived but potent methane can slow warming quickly, while carbon dioxide cuts are essential for the long term. Recognising that warming impact depends on both concentration and per-molecule potency is the nuance that lifts an answer.