The greenhouse effect and enhanced warming (greenhouse gases, radiative forcing, global warming): VCE Environmental Science Unit 4

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.