The greenhouse effect, carbon cycle and sustainability

The greenhouse effect

Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth's surface. The surface emits longer-wavelength infrared radiation. Greenhouse gases (carbon dioxide, methane and water vapour) absorb and re-emit some of this infrared, keeping the planet warm.

Human activity increases these gases, causing an enhanced greenhouse effect and global warming (anthropogenic climate change):

• CO2: burning fossil fuels, deforestation (especially burning forests)
• methane: cattle, rice paddies, landfill sites, leaks from gas extraction

The carbon cycle

• Removing CO2: photosynthesis by plants, algae and phytoplankton; dissolving in the oceans.
• Returning CO2: respiration by all organisms, decomposition by microorganisms, combustion.
• Long-term stores: fossil fuels, limestone, peat, deep oceans.

Burning fossil fuels releases carbon locked away for millions of years, so CO2 is added faster than it is removed.

Ways to reduce atmospheric CO2: use less fossil fuel; reforestation (growing trees take in more CO2 than they release); protect peat bogs and forests; use biofuels from crops.

Sustainable use of resources

Biofuels (such as ethanol from sugar cane or biodiesel from oil crops) release CO2 that the crop recently absorbed, so in principle they are close to carbon neutral. But growing and processing them uses fossil fuels, and land used for fuel crops may compete with food crops or replace forests, which releases carbon and destroys habitats.

Sustainable forestry replants trees as they are cut, keeps a mix of ages and species, and protects habitats. Decisions must balance human needs for food, fuel and land against conservation of biodiversity.

Worked example

Explain why planting a large new forest can lower atmospheric CO2, but a mature forest does little.

1. Young, growing trees photosynthesise faster than they respire, so they store carbon as new biomass.
2. In a mature forest, CO2 taken in by photosynthesis is roughly balanced by respiration and decomposition.

Answer: Growing forests are a net carbon sink; mature forests are roughly in balance.

Check yourself

1. Which type of radiation do greenhouse gases absorb and re-emit?
   • Ultraviolet radiation only
   • Visible light from the Sun
   • Gamma radiation
   • Infrared radiation emitted by the Earth's surface
   Show answer

   Infrared radiation emitted by the Earth's surface. The warm surface emits longer-wavelength infrared.

2. Which human activity is a major source of methane?
   • Planting forests
   • Raising cattle
   • Using solar panels
   • Recycling paper
   Show answer

   Raising cattle. Cattle digestion releases methane.

3. Which process removes carbon dioxide from the atmosphere?
   • Respiration
   • Combustion
   • Decomposition
   • Photosynthesis
   Show answer

   Photosynthesis. Photosynthesis fixes CO2 into organic molecules.

4. Why does burning fossil fuels increase atmospheric CO2 so much?
   • It releases carbon that was locked away for millions of years
   • Fossil fuels contain no carbon
   • It increases photosynthesis
   • It stops respiration
   Show answer

   It releases carbon that was locked away for millions of years. Carbon from long-term stores is added quickly.

5. Why are biofuels described as close to carbon neutral?
   • They release no CO2 when burned
   • They are made from fossil fuels
   • The CO2 released when they burn was recently absorbed by the crops
   • They absorb CO2 when burned
   Show answer

   The CO2 released when they burn was recently absorbed by the crops. Crops take up CO2 as they grow, balancing much of what is released.

6. Which is a disadvantage of growing crops for biofuels?
   • Land may be taken from food crops or forests
   • Biofuels cannot be burned
   • Crops do not photosynthesise
   • They always increase biodiversity
   Show answer

   Land may be taken from food crops or forests. Competition for land and deforestation are key concerns.

7. Which practice makes timber production sustainable?
   • Clear-felling and leaving the land bare
   • Burning cleared forest
   • Replanting trees as others are felled
   • Planting one species and removing all others
   Show answer

   Replanting trees as others are felled. Replanting keeps the resource available for the future.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.