Transport is a significant source of greenhouse gas emissions worldwide. The carbon footprint of a journey depends on the mode of transport used, the number...

(a) The mode of transport with the highest CO₂ emissions per passenger per km is short-haul flight, at 255 g per passenger per km. [1]

Short-haul flights produce more CO₂ per passenger-km than long-haul flights because a disproportionate amount of fuel is consumed during take-off and climbing, and these phases make up a larger fraction of a short journey.

(b) Comparison of single-occupant and four-occupant car emissions: [3]

• A private car with one occupant produces 170 g CO₂ per passenger per km, while the same car with four occupants produces only 43 g per passenger per km. The four-occupant car therefore produces roughly four times less CO₂ per person. [1]
• This difference occurs because the total CO₂ output of the car is approximately the same regardless of how many people are inside it. With four passengers, that fixed total is divided among four people: \( 170 \div 4 \approx 43 \) g per passenger per km. [1]
• Car sharing (increasing vehicle occupancy) is therefore one of the simplest ways to reduce per-person transport emissions without requiring any change in vehicle technology or fuel type. [1]

(c) How public transport and cycling reduce carbon emissions: [3]

• Public transport vehicles (buses and trains) carry many passengers simultaneously, so the CO₂ emissions are spread across a large number of people. A bus at 89 g per passenger-km is roughly half the emissions of a single-occupant car. [1]
• Electric trains are particularly efficient, producing only 14 g per passenger per km, which is about 12 times less than a single-occupant car (170 g). This is because electricity can be generated from low-carbon sources and electric motors are more efficient than internal combustion engines. [1]
• Cycling produces zero direct CO₂ emissions, making it the lowest-carbon transport option. For short journeys, replacing car trips with cycling would eliminate transport emissions entirely and bring additional public health benefits through physical activity. [1]

(d) Challenges of reducing CO₂ emissions from aviation: [3]

• There are currently no commercially viable zero-emission alternatives to kerosene-based jet fuel for long-distance flights. Electric batteries are far too heavy relative to their energy storage capacity to power large aircraft over long distances. [1]
• Sustainable aviation fuels (SAFs), made from biomass or synthesised from captured CO₂, exist but are still expensive to produce and are manufactured in very small quantities compared to the volume of conventional jet fuel consumed globally. [1]
• Global demand for air travel is growing rapidly, particularly in developing countries where a rising middle class increasingly expects to fly. Even when individual aircraft become more fuel-efficient, the overall growth in the number of flights offsets these efficiency gains, a phenomenon known as the rebound effect. [1]