Human activity changes carbon flows

Explanation

Burning fossil fuels transfers carbon from long-term geological stores into atmospheric CO2. Increased energy use over recent centuries has increased this transfer. Greenhouse gases absorb outgoing infrared radiation and alter Earth's energy balance; they do not mainly work by making a physical lid or blocking all incoming sunlight.

Clearing forests can release carbon through burning and decomposition of biomass and disturbed soils. It also removes photosynthetic organisms that would otherwise take up carbon. The net effect depends on regrowth and subsequent land use, so count both stock loss and future changes in uptake.

Food production contributes through land conversion, energy use and methane sources. Ruminant digestion and manure management can release methane, while growing feed adds resource demands. Increasing consumption of high-emission meat products can therefore increase emissions through several linked processes.

Methane can also arise from anaerobic decomposition in waterlogged agricultural settings and waste. CO2 and methane differ in atmospheric behaviour and warming effect, so carbon-footprint comparisons often express a common CO2-equivalent measure with a stated time horizon. The syllabus emphasis here is the causal contribution of CO2 and methane, not memorising a current emissions league table.