How much carbon?

Carbon dioxide is a major contributor to global warming, and is released by many human activities. This article compares the amount of carbon that is released per kilometer for various modes of travel and the corresponding cost per kilometer. It shows that the least environmentally friendly way of getting around is in a large car, and the most is via long-haul coach, followed by train. An airplane falls somewhere in the middle. I've also included electricity and natural gas in the table for comparison.

The major environmental problem with air travel is that you cover so much distance, even if you cover that distance relatively efficiently. For example, a 3000km return flight from London to Alicante releases about 3000km * 86g/km = 258kg of carbon into the atmosphere. In electricity terms, this is about 2715 kWh, enough to run a 10W energy efficient light bulb for for about 30 years!

Mode Carbon Cost How measured How calculated
Small car 130g/km 6p/km A Vauxhall Astra 1.4i does a combined 51.4 miles to the UK gallon which is 5.5 litres every 100km. When burned a litre of petrol releases 2.3kg of CO2. So 2.3kg/L * 5.5L/100km = 0.127kg/km. The most efficient small cars range from 90-110g according to a Which report, so 130g seems reasonable. With petrol costing about £1.10 (March 2015) the cost of driving 1km in this car is about 110p/L * 5L/100km = 6p.
Large car 310g 14p A Chevrolet Suburban 2500 gets 12 miles to the US gallon or 19.6L/100km, which works out to 450g/km. Some SUVs get up to 30mpg which works out as 180g/km. I'll use a value in the middle of about 310g. This is a bit more than double the CO2 of a small car, so the cost per km is a bit more than double as well.
Train 53g 16p The emissions calculator reckons a train journey produces 53g of CO2 every km. A 2005 UK Parliamentary report shows 182 miles per gallon per passengers = 292 km per 4.54 L = 0.016L/km. If they ran on the same petrol as a small car, this would mean 2.3kg/L * 0.016L/km = 37g/km per passenger. But they often run off of electricity, so I'll use the figure above as this one is hard to verify. Brighton-London return, 90 miles (140km) costs £22 (March 2015), depending on city, route, time of day, etc.
Bus 90g 48p Bus fuel consumption seems to be about 7 mpg (US) or 34L/100km (RITA). Buses use diesel which emits 2.7kg/L, a bit more than petrol. 2.7kg/L * 34L/100km = 918g/km for a whole bus, so it very much depends on how many people the bus is carrying. A UK parliamentary report calculated an average of 9 passengers/bus. A USA Today article which uses this data quotes 8.8 passengers/bus. Using 9 passengers/bus would give 102g/km. This is more than the parliamentary report which quotes 98 miles per UK gallon per passenger = 0.03L/km * 2.7kg/L = 81g/km. Transport for London's 2013 report (page 28) estimates 77g/km per passenger. So I'll use an average of these figures of 87g/km. A single journey in Brighton costs £2.40 (March 2015), so if that was for 5km, it would cost 48p/km, though other places in the world are much cheaper.
Coach 30g 10p A motor coach is like a bus, but for longer journeys. The National Express website claims 29g/km per passenger. The American Bus Association's 2005 Census (page 6) gives 5.52 miles per US gallon or 42 L/100km, which is less than the average for buses above, which is odd because I thought they would be more fuel efficient as they stop less. A 2006 report claims 148 miles per gallon per passenger, which would mean the average coach was carrying 27 passengers. This is 3 times the average for a bus, and explains why coach travel releases 3 times less carbon than buses, but I would like to verify this figure. The 2005 report also said that 595 million passengers were made on coaches which covered 2390 million miles. That would mean an average journey length of 108 miles (2390m miles * 27 passengers)/(595m passengers) or 173km, which seems plausible. A 2009 European coach transport report gives the number of passenger journeys and distance in different European countries. In Germany (page 7) they estimate journey lengths of 400-543km. In the UK (page 75) it gives an average of 50-100 miles per journey. For all these countries togther 77579 million km were travelled by 1349 million passengers journeys, for an average of 57km each journey, three times less than the 2006 report would indicate, but that was for the US. Based on a £14 return ticket for a Brighton to London Victoria return (90 miles, 140km), this method of travel works out at 10p/km.
Plane 86g 8p The Carbon independent website estimates 37g per passenger per km. A Boeing 747 uses about 12L of fuel every km according to this How Stuff Works article. Occupancy rates on a flight seem to vary from 50-80% according to a Carbon Offset Research & Education article. If the 747 has 568 seats and is 70% full, that's about 400 people, so 0.03L/km per passenger. A US Energy Information Administration article gives the carbon figures for jet fuel as 9.6kg per US gallon = 9.6kg/3.78L = 2.53kg/L, about the same as car petrol. Multiplying 2.53kg/L times 0.03L/km = 76g/km per passenger, about double the figure above. The UK Parliamentary report lists 40-60 miles per UK gallon for air travel. Using 50 mpg = 50 miles per 4.54 litres = 80km/4.54L = 0.057L/km = 144g/km which is four times the figure above. I'll average the three. A return flight from London to Alicante covers 1800 miles (3000km) and ranges from £100 to £400 depending on when you want to go. If we use £250, then it works out about 8p per km.
kWh of electricity 95g 15p Electricity consumption is measured in kilowatt-hours. So if you run a 1kW appliance (such as a fan heater) for 1 hour, then you have used 1kWh. How much carbon is created to do this depends on how the electricity is generated. If it is by burning coal, then the US Energy Information Administration estimates about 2.1lbs of carbon per kWh = 0.95kg. This cost comparison website says electricity in the UK costs about 15p per kWh. One kWh is enough for 1kW heater to run for an hour, a computer for about 10 hours or a light bulb for 100 hours.
kWh of natural gas 55g 6p The US Energy Information Administration estimates that natural gas releases 1.22lbs of carbon per kWh. This 2012 Shrink That Footprint graph compares natrual gas prices in different countries. This and my gas bill show a natrual gas price of around 6p per kWh. This is the gas used in about 20 minutes of gas hob, or 4 minutes of central heating (when it is running my boiler uses 15kW per hour - I need to verify this).