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Being green is a good thing, but how are we to know what the greenest choices to make are? Why not make up your own mind by analysing some of the following statements yourself?

At the foot of the question we provide you with lots of data taken from the internet. This isn't necessarily all of the data needed to complete the investigation, but a set of data which appears to be relevant to the problem. Some of the sources are given at the foot of the problem. You may, in your work, find that you need more data or question what the data, as presented, might really mean. Use your common sense to try to decide what the most important factors are.

Questions to consider

  1. Would it be better for commuters to catch the bus to work, or to go by car?
  2. Is it better to buy locally produced goods or goods imported from overseas?
  3. Is it better to go on a holiday by boat, train and car or by plane?
Suggested approach
You can make sensible comparisons by working out the mass of carbon needed to complete each activity.

Thoughts on the data
Perhaps the activity might make you question the way in which some of the 'standard' data below is calculated, or what it really means. What questions are raised? To analyse these questions fully, what experiments or measurements would you need to make?

1 gallon of petrol produces $8.87$kg of CO2  
A round trip, economy class, from London to New York produces $760.22$kg of CO2 per person  
Efficiency of a car ranges from about $25$ miles per gallon to $50$ miles per gallon  
An efficient small car produces about 120g per km of CO2  
An efficient large car produces about 140g per km of CO2  
An efficient empty delivery truck produces about 220 g per km of CO2  
Locally produced food travels an average of about 200 miles from farm to shop  
Environmental CO2 output from shipping is twice as much as airlines  
The distance from London to New York is about 2000 miles.  
An empty bus emits seven times more CO2 per km than an empty car  
Short haul air freight emissions are 1580 g CO2 per tonne kilometre  
Long haul air freight emissions are 570 g CO2 per tonne kilometre  
Freight in a medium truck produces 85 g CO2 per tonne kilometre  
Freight in a large transport ship produces 10 g CO2 per tonne kilometre.  
An average man weighs about 80 or 90 kg  
Bus journeys can take longer than car journeys  
On average, rail journeys produce 0.0602 kg of CO2 per passenger km  
On average, bus journeys produce 0.0891 kg of CO2 per passenger km  
Journeys in slow moving traffic can dramatically reduce fuel efficiency.  
Buses, taking into account stops, use an average of $39$ litres of fuel per $100$km  

You might also want to challenge other commonly accepted green ideas or the impact of various green activities. For example, what is the relative impact of these green measures? What mass of carbon do these various common activities use up?

  1. Washing clothes at $30^\circ$ degrees instead of $40^\circ$.
  2. Shutting your computer down every night.
  3. Using low-energy light bulbs instead of regular light bulbs.
  4. Switching off the television instead of leaving it on standby
  5. Boiling only 1 cup of water at a time, instead of a whole kettle.
  6. Disposable nappies are less green then washable nappies.


Making sense of the mass of data available on the internet is immensely tricky. Getting to the truth of the matter is very difficult. Scientists and statisticians need to bring a healthy sceptical view to data. This is why good, scientifically sound documents will contain references in which the exact meaning of the figures presented will be given. The scientist or statistician can then make an informed judgement based on the evidence. Of course, we do not have time to make informed judgements about everything, but it helps to be aware that data and facts are hardly ever simply black or white.


  1. International Civil Aviation Organisation
  6. (National Atmospheric Emissions Inventory)