Carbon cycle - Answer key

Fluency

Atmosphere < biosphere < hydrosphere (oceans) < lithosphere (rocks/fossil fuels).
$6\text{CO}_2 + 6\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ .
$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O}$ .
Photosynthesis; ocean dissolution (into phytoplankton).
Respiration; decomposition; combustion.
Fossilisation (dead organisms buried and compressed over millions of years); sedimentation of shells into limestone.

Fluency

It is the smallest reservoir, so small flux imbalances produce large percentage changes; it also controls the greenhouse effect directly.
Dissolved CO $_2$ , carbonic acid (H $_2$ CO $_3$ ), bicarbonate ion (HCO $_3^-$ ), and carbonate ion (CO $_3^{2-}$ ).
As calcium carbonate, CaCO $_3$ , formed from the shells and skeletons of marine organisms.
Coal (from compressed plant matter in ancient swamps); oil (from compressed marine plankton and algae); natural gas (similar origin, deeper/hotter conditions favour methane).
A reservoir is a store of carbon; a flux is a flow of carbon per unit time between reservoirs.

Reasoning

Carbon stored in coal (lithosphere) is burned, producing CO $_2$ that enters the atmosphere. A flux from lithosphere to atmosphere.
Burning trees releases their stored carbon as CO $_2$ ; the cleared land stops absorbing CO $_2$ through photosynthesis, so the atmospheric balance shifts upward.
Cement production releases CO $_2$ both from burning fuel in the kiln and directly from the decomposition of limestone — even with clean energy, CaCO $_3$ $\to$ CaO + CO $_2$ is unavoidable.
(a) Growing trees lock carbon into wood and soils. (b) Mature trees reach a steady state; if the forest burns or is cleared, carbon returns to the atmosphere. Reforestation buys time but does not remove carbon permanently.
CH $_4$ is $\sim 25$ times more potent per molecule than CO $_2$ at trapping heat over a century; large ongoing emissions keep concentrations high even though each molecule is short-lived.

Problem solving

Fuel used: $12\,000 \times \dfrac{8}{100} = 960$ L. (a) Cost: $960 \times 1.85 = 1\,776$ dollars. (b) CO $_2$ : $960 \times 2.3 = 2\,208$ kg.
$5$ Gt C/year; over a decade, $\sim 50$ Gt C added.
$\dfrac{420 - 280}{280} \times 100 = 50\%$ .
$[\text{H}^+]$ ratio $= 10^{(8.2 - 8.05)} = 10^{0.15} \approx 1.41$ . So about a $41\%$ increase.
$\dfrac{3\,500}{20} = 175$ trees per car.

Reasoning

Human emissions continually push the atmosphere away from equilibrium. Sinks (ocean, land) absorb a fraction but cannot keep pace because absorption depends on the difference from equilibrium — even as that gap widens, the remaining $\sim 50\%$ of emissions accumulates, further shifting equilibrium upward.
Negative: silicate weathering increases with temperature and removes CO $_2$ ; more CO $_2$ enhances plant growth which absorbs CO $_2$ (limited by water/nutrients). Positive: warming melts permafrost, releasing methane; warmer oceans hold less dissolved CO $_2$ . On human timescales (decades), the positive feedbacks act faster than silicate weathering (millennia), making them more urgent.
Scale — algae biomass would need to exceed current ocean phytoplankton many times over. Nutrients — huge nitrogen and iron inputs would be required. Permanence — dead algae decompose, returning CO $_2$ ; only if they sink and are buried does the carbon leave the fast cycle.
Fossil fuels accumulated over tens of millions of years; burning them in ~200 years releases that carbon ~ $10^5$ times faster than it was captured. Natural sinks have no capacity to absorb at that rate, which is why atmospheric CO $_2$ keeps rising.

Year 10 answers