Glossary: Environmental Sustainability

This chapter's terms in context

Chapter 25 covers four terms describing environmental problems and the response needed to address them: greenhouse effect, global warming, eutrophication and sustainability. The greenhouse effect is the natural process by which certain gases in the atmosphere, mainly carbon dioxide, methane and water vapour, collectively called greenhouse gases, trap heat radiated from the Earth's surface, preventing it from escaping directly into space and keeping the planet's average temperature high enough to support life; this natural greenhouse effect is not itself a problem, and a frequent exam error is describing the greenhouse effect as inherently harmful, when the syllabus specifically distinguishes the natural process from its human-intensified consequence.

Global warming is the enhanced, human-caused intensification of this natural greenhouse effect, resulting from human activities, mainly the burning of fossil fuels for energy and transport, and large-scale deforestation, which removes trees that would otherwise absorb carbon dioxide, that release additional greenhouse gases into the atmosphere faster than natural processes can remove them, trapping more heat than the natural greenhouse effect alone and causing a measurable rise in the Earth's average surface temperature over time. A candidate is often expected to explain the causal chain correctly: human activity increases greenhouse gas concentration, which intensifies the greenhouse effect beyond its natural level, which causes global warming, rather than treating these three ideas as synonyms for one single vague concept of 'pollution'.

Eutrophication is a distinct environmental process affecting bodies of water rather than the atmosphere: it begins when excess nutrients, particularly nitrates and phosphates from agricultural fertiliser run-off or untreated sewage, enter a river or lake and cause a rapid, excessive growth of algae (an algal bloom) on the water's surface. This algal bloom blocks light from reaching aquatic plants below the surface, which then die and are broken down by decomposers; the decomposers' own respiration rapidly depletes the dissolved oxygen in the water, and this oxygen depletion is ultimately what kills fish and other aquatic organisms, a multi-step chain of cause and effect that examiners specifically expect a candidate to explain in full sequence, rather than jumping straight from 'excess nutrients' to 'fish die' without stating the intermediate steps involving algal bloom, light blockage, decomposition and oxygen depletion.

Sustainability, in this environmental context, is the practice of using natural resources and managing human activity in a way that meets present needs without compromising the ability of future generations to meet their own needs, and it connects directly back to the other three terms in this chapter: sustainable practices such as reforestation, using renewable energy sources, treating sewage before discharge, and applying fertiliser in controlled amounts are the practical responses that address global warming and eutrophication at their respective root causes rather than only their visible symptoms.

Paper 2 questions on this chapter frequently ask a candidate to explain the full eutrophication sequence for a given scenario, such as fertiliser runoff into a nearby pond, and full marks typically require every intermediate step to be stated in the correct order rather than only the initial cause and final outcome.

A further exam angle asks a candidate to explain why deforestation contributes to global warming through two separate mechanisms rather than one: burning or decomposing felled trees releases stored carbon dioxide directly into the atmosphere, while the smaller number of remaining trees also removes less carbon dioxide from the atmosphere through photosynthesis than before, so a full answer must state both the direct release and the reduced future absorption rather than only one of the two.