IGCSE Chemistry 0620: Methane Increase and the Greenhouse Effect Mechanism

Researchers monitoring the peat bog at Fenwick Moor use an instrumented weather balloon to sample the gases released as the bog dries out and stored organic matter decomposes. Their data logger recorded the average atmospheric methane concentration above the moor as $1850$ parts per billion (ppb) in 2010 and $2035$ ppb in 2025.

(a) Calculate the percentage increase in the methane concentration between 2010 and 2025. [2]

(b) Explain, in terms of the absorption, reflection and emission of thermal energy, how increased concentrations of methane and carbon dioxide in the atmosphere cause global warming. [4]

(c) State one strategy, other than reducing livestock farming, that could help reduce the levels of greenhouse gases such as methane and carbon dioxide in the atmosphere. [1]

Part (a): Percentage increase in methane concentration

The percentage increase is found using the original (2010) value as the base:

$\text{percentage increase} = \frac{\text{increase}}{\text{original value}} \times 100$

The increase in concentration is:

$2035 - 1850 = 185\ \text{ppb}$

So the percentage increase is:

$\frac{185}{1850} \times 100 = 10\%$

Part (b): The greenhouse effect mechanism

The greenhouse effect works through absorption, emission and reflection of thermal energy:

The Earth’s surface radiates thermal (infrared) energy outwards, back towards space.
Greenhouse gas molecules, such as methane and carbon dioxide, in the atmosphere absorb some of this outgoing thermal energy.
These molecules then emit the absorbed thermal energy again, but in all directions, not just outwards.
Some of this emitted energy is reflected/radiated back towards the Earth’s surface, rather than escaping to space.

The overall effect is that increased concentrations of these gases reduce the amount of thermal energy that is lost from the Earth to space. Because more thermal energy is retained within the atmosphere and at the surface, the average temperature rises. This is global warming.

Part (c): A strategy to reduce greenhouse gas levels

Besides reducing livestock farming (which lowers methane from animal digestion), other strategies include:

Planting trees, since photosynthesis removes carbon dioxide from the atmosphere.
Decreasing the use of fossil fuels, which reduces the carbon dioxide released by their combustion.
Increasing the use of hydrogen and renewable energy (e.g. wind or solar power), which reduces reliance on fossil fuels and the carbon dioxide they produce.

Any one of these, clearly stated, is an acceptable answer.

Final answers

(a) $10\%$ increase
(b) Greenhouse gases absorb outgoing thermal energy from the Earth’s surface, then emit it in all directions; some is reflected back to the surface, reducing the thermal energy lost to space and causing global warming.
(c) Planting trees, or decreasing fossil fuel use, or increasing use of hydrogen/renewable energy.