Greenhouse effect

In the syllabus and the exam

The greenhouse effect is first explained as a natural and necessary process: short-wave radiation from the sun passes through the atmosphere and warms the Earth's surface, which then re-radiates the energy as longer-wave heat; greenhouse gases such as carbon dioxide, methane and water vapour absorb some of this outgoing heat and radiate part of it back toward the surface, keeping average global temperatures high enough for life to exist. Candidates commonly lose marks by describing the greenhouse effect itself as something harmful, when the syllabus requires distinguishing the natural greenhouse effect, which is essential and has existed for millions of years, from the enhanced greenhouse effect, which is the additional warming caused by human activities that raise greenhouse gas concentrations above their natural level, mainly the burning of fossil fuels and large-scale deforestation. Structured questions often ask candidates to name specific greenhouse gases and their human sources, such as carbon dioxide from vehicle exhaust and power stations, or methane from livestock and paddy fields, rather than only stating 'pollution' in general terms. Candidates should also be able to sequence the mechanism correctly: sunlight in, surface warms, heat radiates out, greenhouse gases trap part of that heat, since describing the gases as blocking sunlight from entering, rather than trapping heat leaving, is a frequently tested misconception. Candidates should also be able to compare the relative warming contribution of different greenhouse gases, noting that methane traps more heat per molecule than carbon dioxide even though carbon dioxide is far more abundant in the atmosphere overall.

Example

Carbon dioxide and methane in the atmosphere absorb heat radiated from the Earth's surface and re-emit some of it back downward, a natural process called the greenhouse effect that keeps the planet warm enough to support life.

Related terms

Global warmingSustainability

Source:SRC-DSKP-EN