How does the greenhouse effect work and why is the climate warming?
the natural greenhouse effect, the main greenhouse gases and their sources, and how the enhanced greenhouse effect from human activity drives global warming
A focused answer to the VCE Environmental Science Unit 4 dot point on the natural and enhanced greenhouse effect, the main greenhouse gases and their sources, and global warming.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
VCAA wants you to explain the natural greenhouse effect as an energy process, identify the main greenhouse gases and their sources, and explain how the enhanced greenhouse effect from human activity causes global warming. A strong answer traces energy in, energy out, and the role of the gases.
The natural greenhouse effect
The Sun delivers energy to Earth mainly as shortwave radiation (visible light and ultraviolet). Some is reflected back to space by clouds, ice and bright surfaces (the albedo effect), but much is absorbed by the surface, warming it. The warmed Earth then radiates energy back out as longwave (infrared) radiation.
Greenhouse gases in the atmosphere absorb a large fraction of this outgoing infrared radiation and re-emit it in all directions, including back toward the surface. This traps heat in the lower atmosphere and keeps the planet about 33 degrees Celsius warmer than it would otherwise be. Without the natural greenhouse effect, Earth's average surface temperature would be roughly minus 18 degrees Celsius and liquid water and life as we know it would not exist.
The main greenhouse gases and their sources
- Carbon dioxide (CO2). The most important human-driven greenhouse gas. Sources: burning fossil fuels (coal, oil, gas), deforestation and land clearing, cement production. Long-lived in the atmosphere.
- Methane (CH4). Far more potent per molecule than CO2 but shorter-lived. Sources: livestock (enteric fermentation in cattle and sheep), rice paddies, landfill, coal and gas extraction, wetlands.
- Nitrous oxide (N2O). A potent, long-lived gas. Sources: nitrogen fertilisers, agriculture, some industrial processes.
- Water vapour (H2O). The most abundant greenhouse gas. It acts mainly as a feedback: warmer air holds more water vapour, which amplifies warming.
- Synthetic gases such as chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) are very potent but present in small amounts.
Gases differ in their global warming potential, a measure of how much heat they trap compared with CO2 over a set period. Methane and nitrous oxide trap far more heat per molecule than CO2, even though CO2 dominates by sheer volume.
The enhanced greenhouse effect and global warming
Since the Industrial Revolution, human activities have raised atmospheric concentrations of these gases well above natural levels. Atmospheric CO2 has risen from about 280 parts per million before industrialisation to over 420 parts per million today. More greenhouse gas means more outgoing infrared is absorbed and re-emitted toward the surface, increasing radiative forcing (the extra energy retained). The result is the enhanced greenhouse effect, which is causing global warming: a rise in average global surface temperatures of more than 1 degree Celsius since pre-industrial times.
In Australia, this is consistent with observed trends: rising average temperatures, more frequent and intense heatwaves, longer fire seasons, marine heatwaves bleaching the Great Barrier Reef, and shifting rainfall patterns reported by the Bureau of Meteorology and CSIRO.
Exam-style practice questions
Practice questions written in the style of VCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2025 VCAA4 marksWith reference to each of the relevant forms of radiation, outline how the natural greenhouse effect supports life on Earth.Show worked answer →
A 4 mark answer traces the energy pathway using the correct radiation types and states the benefit.
1 mark: incoming solar energy reaches Earth mainly as shortwave radiation (visible light); some passes through the atmosphere and is absorbed by the surface.
1 mark: the warmed surface re-radiates energy back out as longwave (infrared) radiation.
1 mark: greenhouse gases in the atmosphere absorb this outgoing infrared radiation and re-emit it in all directions, including back toward the surface, trapping heat in the lower atmosphere.
1 mark: this keeps Earth's average surface temperature about 33 degrees C warmer (around 15 degrees C rather than about minus 18 degrees C), warm enough for liquid water and to support life.
2025 VCAA2 marksNitrous oxide has a global warming potential of 273 over a 20-year period. Describe what is meant by the term 'global warming potential' in relation to nitrous oxide.Show worked answer →
A 2 mark answer defines the term and applies the figure.
1 mark: global warming potential (GWP) is a measure of how much heat a greenhouse gas traps in the atmosphere over a set period, compared with the same mass of carbon dioxide, which is given a reference value of 1.
1 mark: a GWP of 273 means that, over 20 years, one mass unit of nitrous oxide traps 273 times as much heat as the same mass of carbon dioxide, so nitrous oxide is a far more potent greenhouse gas per unit mass.
2022 VCAA2 marksDescribe the role of methane in the natural greenhouse effect.Show worked answer →
A 2 mark answer states what methane does to outgoing radiation and the result.
1 mark: methane is a greenhouse gas that absorbs outgoing longwave (infrared) radiation re-radiated from Earth's surface and re-emits it in all directions, including back toward the surface.
1 mark: this traps heat in the lower atmosphere, contributing to the warming that keeps Earth habitable. Methane is naturally present (for example from wetlands) and, per molecule, traps far more heat than carbon dioxide.