10.5 - Greenhouse Gases & Climate Change

The greenhouse effect

• Gases in the atmosphere trap some of the heat radiated from the Earth's surface, keeping the planet warm enough to support life. This warming is called the greenhouse effect.
• The main greenhouse gases are carbon dioxide, methane, and water vapour.
• Methane is released into the atmosphere by:
  • Decomposing vegetation
  • Digestion in animals

More greenhouse gas means more warming

• An increase in carbon dioxide or methane traps more heat in the atmosphere.
• This drives climate change: as average global temperatures rise, wider weather and climate patterns change with them.

How the greenhouse effect works

1. Energy from the Sun reaches the Earth mostly as short-wavelength radiation, which passes through the atmosphere and warms the surface.
2. The warmed surface radiates energy back out as longer-wavelength infrared radiation.
3. Greenhouse gas molecules absorb this longer-wavelength radiation and re-emit it in all directions, including back towards the surface.
4. This reduces the thermal energy lost to space, so heat energy stays in the atmosphere and keeps the Earth's surface warmer.

Effects of climate change

A rise in average global temperature brings wide-ranging effects:

• Sea levels are rising - Polar ice and glaciers melt, and seawater expands as it warms, increasing coastal flooding.
• Rainfall patterns are changing - Some regions face more droughts, while others get heavier rainfall and flooding.
• Extreme weather is more frequent and severe - Storms, heatwaves, droughts, and floods happen more often.
• Species and habitats are affected - Shifting temperatures and rainfall threaten species that can't adapt or migrate fast enough, putting populations at risk.

Weighing the effects

• Effects differ in scale, likelihood, and environmental impact.
• Sea-level rise and changing rainfall affect entire regions, are already being measured, and are near certain (large scale, high risk).
• Loss of a species or habitat may be local, but it's permanent.

Reducing climate change

• The effects of climate change can be reduced, or mitigated, through deliberate action.
• Deciding what action to take means weighing the scale of the measure against its risk and wider environmental implications.

Strategies that reduce climate change

• Decreasing fossil fuel use - Cuts the carbon dioxide released from burning it.
• Increasing hydrogen and renewable energy - Generates energy without releasing carbon dioxide.
• Planting trees - Increases the carbon dioxide absorbed by plants as they grow.
• Reducing livestock farming - Lowers the methane released from animal digestion.

Photosynthesis as a carbon dioxide sink

• Photosynthesis is the reaction plants use to make glucose.
• It takes place in the presence of light, which is absorbed by the green pigment chlorophyll to supply the energy needed.
• Reactants are carbon dioxide and water; products are glucose and oxygen.
• carbon dioxide + water → glucose + oxygen (using light energy absorbed by chlorophyll)
• 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)

Why planting trees helps

• Photosynthesis removes carbon dioxide from the atmosphere.
• Planting more trees increases the total carbon dioxide taken up, helping to reduce climate change.