D4.2 Sustainability and change

The question asks about the mechanism by which CO₂ traps heat — not just what greenhouse gases do, but how they do it.

Earth's surface absorbs solar radiation and re-emits it as longwave (infrared) radiation. Greenhouse gas molecules such as CO₂ have molecular structures that allow them to absorb this infrared radiation and re-emit it in all directions, including back toward Earth's surface.

This is distinct from reflecting incoming solar radiation (that's the albedo effect), ozone depletion (a separate process involving CFCs), or changes in heat capacity. The key is absorption and re-emission of outgoing infrared radiation.

The correct answer is: Carbon dioxide absorbs outgoing longwave infrared radiation emitted by Earth's surface and re-emits it back downward. This accurately describes the enhanced greenhouse effect mechanism.

The question requires identifying the correct physical mechanism by which CO₂ contributes to atmospheric warming.

'Carbon dioxide breaks down ozone molecules...' is incorrect. Ozone depletion is caused by chlorofluorocarbons (CFCs), not CO₂. These are entirely separate atmospheric processes.

'Carbon dioxide molecules reflect incoming shortwave solar radiation back into space' is incorrect. Reflecting solar radiation would actually cool Earth, not warm it. Greenhouse gases primarily interact with outgoing, not incoming, radiation.

'Carbon dioxide binds chemically to water vapour...' is incorrect. CO₂ does not chemically bind to water vapour to create a denser atmosphere. This fabricated mechanism has no basis in atmospheric physics.

The remaining option — absorption and re-emission of outgoing longwave infrared radiation — correctly describes the greenhouse effect mechanism and is the right answer.

The central ecological rule of sustainable resource use is that the rate of harvesting must not exceed the rate of natural replenishment. If more is taken than can be replaced, the population will decline over time.

For cod, natural replenishment occurs through reproduction and population growth. If the annual harvest equals or is less than the net population growth rate, the cod population can maintain itself indefinitely.

Constant fixed catch ignores population size changes and could easily exceed the replenishment rate in poor breeding years. Maximising profit and removing large fish both prioritise short-term gain over long-term population viability.

'The annual harvest does not exceed the annual growth and reproduction rate of the cod population' directly embodies the definition of sustainable harvesting.

The question asks which criterion confirms sustainability — the emphasis is on ecological long-term viability, not short-term output or efficiency.

'Maximises catch efficiency to generate the highest possible profit' is not a sustainability criterion. Maximising extraction typically leads to overexploitation, which is the opposite of sustainability.

'Total catch is kept constant regardless of how the cod population changes' is problematic. If the population declines, a fixed constant harvest could still exceed the replenishment rate, driving the population to collapse.

'Removes all fish above a certain size class' could actually be harmful — large individuals are often the most reproductively productive, and removing them can reduce the population's recovery capacity.

The answer that harvest does not exceed natural growth and reproduction rate is the only option that matches the ecological definition of sustainability.