Fuels, climate and alternativesIB MYP Sciences: Revision notes

Earth is warmed by the Sun. The warm surface gives out infrared (heat) radiation. Some gases in the atmosphere, called greenhouse gases, absorb this radiation and re-emit some of it back towards the surface. This is the greenhouse effect.

Without it, Earth's average temperature would be about -18 °C instead of about 15 °C, so the natural greenhouse effect is essential for life. The main greenhouse gases are carbon dioxide (CO₂), methane (CH₄) and water vapour.

Human activities are increasing the amount of greenhouse gases, so more heat is kept in. This is the enhanced greenhouse effect, causing global warming.

• Carbon dioxide is released by burning fossil fuels and by cutting down forests (trees absorb CO₂)
• Methane is released from cattle, rice fields, landfill sites and leaks from natural gas supplies

Consequences may include melting ice, rising sea levels, more extreme weather and changes to where crops can grow.

A renewable fuel can be replaced quickly, but a non-renewable fuel such as coal, oil or natural gas cannot. We look for alternatives to reduce the pollution and carbon dioxide made by fossil fuels.

Biofuels are made from living things, such as bioethanol (fermented sugar cane) and biodiesel (from plant oils). They are renewable because the crops can be regrown. The carbon dioxide released when they burn is about the same as the carbon dioxide absorbed while the plants grew, so they are approximately carbon neutral. But growing them uses land that might be needed for food.

Hydrogen burns (or reacts in a fuel cell) to make only water: 2H₂ + O₂ → 2H₂O. It releases no carbon dioxide where it is used. But it is usually made from natural gas (which releases CO₂) or by using electricity, and it is hard to store.

Fuels can be compared by the energy released per kilogram and by their environmental effects:

• hydrogen: about 142 MJ per kg, makes only water
• petrol: about 46 MJ per kg, makes carbon dioxide and pollutants
• ethanol: about 27 MJ per kg, renewable but needs land
• coal: about 30 MJ per kg, makes carbon dioxide and sulfur dioxide

A fuel with a lower energy value means you must burn a larger mass to release the same energy. Other factors include cost, storage and safety.

In MYP Criterion D you weigh up benefits and drawbacks of a fuel, thinking about social, economic and environmental effects.

• Social: health, jobs, access
• Economic: cost, energy supply
• Environmental: pollution, climate, land use

A good answer gives points on both sides, compares them, and ends with a justified conclusion, for example: "Biofuels should be used as a short-term step because they are renewable, but must not replace food crops."