Carbon cycle and the greenhouse effect: HSC Earth and Environmental Science Module 7

What this dot point is asking

NESA wants you to explain how carbon moves between reservoirs, how the greenhouse effect keeps the planet warm, and how human activity is intensifying that effect. You need to handle reservoirs and fluxes, the difference between the natural and enhanced greenhouse effect, feedback mechanisms, and the Australian contribution to emissions.

The answer

Carbon is constantly exchanged between the atmosphere, oceans, land, living things and rocks. The greenhouse effect is the process by which certain gases in the atmosphere trap heat and keep Earth warm enough for life. The two are linked because carbon dioxide is a greenhouse gas, so changes in the carbon cycle change the greenhouse effect.

Carbon reservoirs and fluxes

A reservoir is a store of carbon; a flux is a transfer between reservoirs. The major reservoirs are the atmosphere (as carbon dioxide), the oceans (as dissolved carbon and in marine life), the biosphere (in plants and animals), soils, and the geosphere (the vast store in rocks, fossil fuels and sediments). Fluxes include photosynthesis, which moves carbon from air to plants; respiration and decomposition, which return it to the air; dissolving of carbon dioxide into the ocean; and, over very long times, the burial of carbon as rock and its slow release by volcanoes and weathering. In a stable climate these fluxes roughly balance.

The greenhouse effect

Incoming sunlight is mostly short-wavelength radiation that passes through the atmosphere and warms the surface. The warmed surface re-emits energy as longer-wavelength infrared radiation. Greenhouse gases, mainly water vapour, carbon dioxide, methane and nitrous oxide, absorb this outgoing infrared radiation and re-emit it in all directions, including back toward the surface. This natural greenhouse effect keeps Earth about 33 degrees Celsius warmer than it would otherwise be, making the planet habitable.

The enhanced greenhouse effect

Burning fossil fuels and clearing land add carbon dioxide and other greenhouse gases to the atmosphere faster than natural fluxes can remove them. The extra gas absorbs more outgoing infrared radiation, trapping additional heat. This intensification of the natural process is the enhanced greenhouse effect, and it is the mechanism driving global warming. Atmospheric carbon dioxide has risen from about 280 parts per million before industrialisation to well over 420 today.

Feedback mechanisms

Feedbacks amplify or dampen warming. Positive feedbacks reinforce change: as ice melts, the bright surface that reflected sunlight is replaced by dark ocean that absorbs it, causing more warming and more melting (the ice-albedo feedback). Warming also releases methane from thawing permafrost and reduces the ocean's ability to absorb carbon dioxide. Negative feedbacks resist change: faster plant growth in a carbon-rich atmosphere can absorb some extra carbon. The balance of feedbacks currently amplifies warming.

The Australian context

Australia is a major player in the global carbon budget. It is one of the world's largest exporters of coal and liquefied natural gas, so the fossil carbon it sells contributes to emissions worldwide even though those emissions are counted elsewhere. Domestically, Australia's electricity has historically relied heavily on coal, giving it high emissions per person. Land clearing, particularly in Queensland, has also released carbon stored in vegetation and soils. At the same time, Australian ecosystems such as forests, mangroves and seagrass meadows act as carbon sinks; the blue-carbon stored in coastal wetlands is increasingly recognised in climate policy. Reducing emissions involves shifting electricity to renewables, restoring vegetation and protecting these sinks.

Try this

Q1. Distinguish between a carbon reservoir and a carbon flux, giving one example of each. [2 marks]

• Cue. A reservoir stores carbon (e.g. the ocean); a flux transfers it between reservoirs (e.g. photosynthesis or combustion).

Q2. Explain how the ice-albedo feedback amplifies global warming. [3 marks]

• Cue. Melting ice exposes dark ocean that absorbs more sunlight than the reflective ice, warming further and melting more ice in a self-reinforcing loop.