Carbon cycle - Practice

What you will learn

name the major carbon reservoirs (atmosphere, biosphere, hydrosphere, lithosphere) and their relative sizes,
describe the key natural processes that move carbon between reservoirs: photosynthesis, respiration, combustion, dissolution, weathering, volcanism, fossilisation,
explain how human activities (burning fossil fuels, deforestation, cement production) alter the balance,
connect an increase in atmospheric CO $_2$ with the enhanced greenhouse effect and ocean acidification,
interpret flow diagrams and quantitative comparisons of carbon fluxes.

Worked example 0 Real-world example: where does the CO$_2$ from a car go?

A petrol car emits about $2.3$ kg of CO $_2$ per litre of petrol burned. Over a year a typical Australian car burns $1\,500$ L. Estimate the annual CO $_2$ emission, and explain where that CO $_2$ eventually ends up.

Annual CO $_2$ : $1\,500 \times 2.3 = 3\,450$ kg $\approx 3.5$ tonnes per car.
Of the global CO $_2$ released each year, about $25\%$ is absorbed by the ocean, $25\%$ by plants and soils, and the remaining $\sim 50\%$ stays in the atmosphere — which is why atmospheric CO $_2$ keeps rising.
Over many years, ocean CO $_2$ becomes carbonate-rich seawater; some plant-absorbed carbon is buried as soil organic matter.

Key idea: burning fossil fuels moves carbon from a deep geological reservoir (oil) into the atmosphere in seconds. Removing it back takes centuries to millions of years.

1. Carbon reservoirs

Carbon is stored in four main places. Approximate sizes in gigatonnes (Gt) of carbon:

Reservoir	Where	Approx. size (Gt C)
Atmosphere	CO $_2$ and small amounts of CH $_4$	$\sim 870$
Biosphere	plants, animals, microbes, soil organic matter	$\sim 2\,000$
Hydrosphere	dissolved CO $_2$ , bicarbonate, carbonate ions in oceans	$\sim 38\,000$
Lithosphere	limestone, chalk, fossil fuels, deep carbon	$\sim 100\,000\,000$

Most carbon sits in rocks for millions of years. The atmosphere is actually the smallest reservoir — which is why even small flux changes noticeably shift its CO $_2$ level.

2. Natural processes that move carbon

Key processes linking carbon reservoirs. Arrows indicate the direction of carbon flow.

Photosynthesis: plants absorb CO $_2$ , using sunlight to build glucose and release O $_2$ . $6\text{CO}_2 + 6\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ .
Respiration: plants, animals and microbes break down glucose using O $_2$ and release CO $_2$ (reverse of photosynthesis).
Decomposition: microbes break down dead matter, returning CO $_2$ to the atmosphere.
Combustion: burning organic matter or fossil fuels releases CO $_2$ rapidly.
Ocean dissolution: atmospheric CO $_2$ dissolves in seawater, forming carbonic acid and bicarbonate.
Sedimentation and fossilisation: dead marine organisms sink; over millions of years their shells form limestone, their soft tissue forms oil or gas.
Weathering of rocks: CO $_2$ dissolved in rain reacts with silicate rocks — a slow but powerful long-term sink.
Volcanism: eruptions release CO $_2$ from deep in the lithosphere back to the atmosphere.

Worked example 1 Balancing photosynthesis and respiration

Over a forest’s day-night cycle, photosynthesis (daytime) removes CO $_2$ and respiration (all day) releases it. Why does atmospheric CO $_2$ still rise steadily from year to year despite this?

Forest photosynthesis - respiration is roughly balanced on short timescales.
The net biosphere-atmosphere flux is small compared with the huge annual release of CO $_2$ from burning fossil fuels (~ $10$ Gt C/year).
Additional carbon from the deep lithosphere enters the atmosphere; natural sinks can only absorb about half of it.
Net result: atmospheric CO $_2$ accumulates.

Key idea: the cycle can absorb shocks only if the fluxes stay in balance. Fossil-fuel combustion adds a one-way flux that nature cannot match.

3. Human impact on the carbon cycle

Key human fluxes:

Fossil fuel combustion: $\sim 10$ Gt C/year released. Coal, oil, natural gas.
Deforestation: $\sim 1$ - $2$ Gt C/year. Trees burned or cleared release their stored carbon; cleared land also absorbs less CO $_2$ in future.
Cement production: CaCO $_3$ is heated to produce CaO, releasing CO $_2$ . $\sim 1$ Gt C/year globally.
Agriculture: methane from livestock and flooded rice fields; N $_2$ O from fertilisers (though not strictly carbon cycle, they add to greenhouse forcing).

Worked example 2 Why the atmosphere is so sensitive

Atmospheric carbon is $\sim 870$ Gt. Humans add $\sim 10$ Gt of carbon each year. What percentage rise in atmospheric carbon does one year of emissions represent, if no carbon was removed?

Percentage: $\dfrac{10}{870} \times 100 \approx 1.15\%$ .
About half is absorbed by natural sinks, so actual annual atmospheric rise is roughly $0.5$ - $0.6\%$ .
Over $50$ years at this rate, atmospheric carbon rises by $\sim 25$ - $30\%$ — matching the observed increase from $280$ to over $420$ ppm.

4. Consequences

Enhanced greenhouse effect: CO $_2$ absorbs outgoing infrared radiation. More CO $_2$ means more warming.
Ocean acidification: $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$ . Ocean pH has dropped from $\sim 8.2$ to $\sim 8.1$ — a $30\%$ increase in hydrogen-ion concentration.
Ecosystem stress: coral bleaching, shifting agricultural zones, loss of ice cover.

Worked example 3 Ocean acidification arithmetic

A $0.1$ unit drop in pH sounds small. Show that it actually represents a $\sim 30\%$ increase in H $^+$ concentration.

pH is a log scale: $\text{pH} = -\log_{10}[\text{H}^+]$ .
$[\text{H}^+]_1 = 10^{-8.2}$ mol/L; $[\text{H}^+]_2 = 10^{-8.1}$ mol/L.
Ratio: $\dfrac{10^{-8.1}}{10^{-8.2}} = 10^{0.1} \approx 1.26$ .
Increase: $\approx 26\%$ . Even a small pH shift has a large chemical meaning for shell-building organisms.

Practice: Year 10

Fluency

Reservoirs and processes

List the four main carbon reservoirs, in order from smallest to largest.
Write the balanced equation for photosynthesis.
Write the balanced equation for aerobic respiration.
Name two natural processes that move carbon from the atmosphere to the biosphere.
Name two processes that move carbon from the biosphere to the atmosphere.
Give one process that moves carbon from the biosphere to the lithosphere.

Fluency

Concepts

Why is the atmosphere considered sensitive to carbon emissions even though it is not the largest reservoir?
In what chemical forms is carbon stored in the ocean?
How does limestone store carbon? (Give the chemical formula.)
Name three fossil fuels and explain briefly how each formed.
What is the difference between a carbon flux and a carbon reservoir?

Reasoning

Human impact

Explain how burning coal in a power station transfers carbon from one reservoir to another.
A forest is cleared and replaced by pasture. Describe two ways in which this increases atmospheric CO $_2$ .
Cement production releases CO $_2$ in two ways — through fuel burning and through the chemical reaction $\text{CaCO}_3 \to \text{CaO} + \text{CO}_2$ . Explain why cement is a significant climate concern.
“Planting trees absorbs CO $_2$ .” Explain why (a) this is true in the short term and (b) it is not a permanent solution.
Why are methane emissions from cattle and wetlands a concern even though CH $_4$ lasts only about a decade in the atmosphere?

Problem solving

Apply and calculate

A car emits $2.3$ kg CO $_2$ /L. If petrol costs $1.85/L and a driver does $12\,000$ km at $8$ L/100 km, calculate (a) annual fuel cost and (b) annual CO $_2$ emissions.
If global emissions are about $10$ Gt C/year and half stays in the atmosphere, how much carbon is added per year on average? Over a decade?
Pre-industrial atmospheric CO $_2$ was $280$ ppm; today it is $\sim 420$ ppm. Calculate the percentage rise.
Ocean pH fell from $8.2$ to $8.05$ . Find the percentage increase in H $^+$ concentration. (Hint: pH scale is base $10$ .)
One large tree absorbs about $20$ kg of CO $_2$ per year. How many trees are needed to offset the $3\,500$ kg CO $_2$ /year of one car?

Challenge

Reasoning

Harder reasoning

Explain, step by step, why atmospheric CO $_2$ continues to rise even though the ocean and land absorb about half of human emissions each year. Use the idea of a reservoir in (dis)equilibrium.
The carbon cycle has natural negative feedbacks (e.g. warmer temperatures speed up silicate weathering, removing CO $_2$ ) and positive feedbacks (e.g. warmer tundra releases stored methane). Discuss two of each and argue which are more significant on a human timescale.
A chemistry-minded student proposes solving climate change by growing vast algae blooms that capture CO $_2$ . Identify three practical obstacles, using ideas from carbon-cycle flux balance.
Oil takes tens of millions of years to form but can be burned in seconds. Explain how this timescale mismatch is the crux of the anthropogenic climate problem.