The Numbers

Though the scientific and moral case against flying is robust, the reaction to making it has made me lose faith in the power of individual action. I think the only solution is a carbon price: see why in my new introduction.

We need to cut our carbon emissions. By exactly how much is open to question, but most people you speak to could probably agree on somewhere between a factor of two and ten. This means slashing the eleven tonnes of carbon dioxide each UK resident emits per annum down to between one and five tonnes. Even short-haul flights can set you back by tonnes (a return to Spain—emitting over half a tonne per person—is the same as two months commuting by car to work, three months’ gas heating or 33 years lighting your home!), and a full-blown return to Australia can emit a whopping ten tonnes all in one go. There just isn’t anything else, save setting light to an oil depot, which emits that much carbon that quickly.

In depth

The UK emitted 671,000,000 tonnes of carbon dioxide equivalent—or CO₂e—in greenhouses gases last year1. Given a population of 60,609,1532, that gives us a footprint of just over 11 tonnes CO₂e per capita.

In order to stabilise levels of greenhouse gases in the atmosphere, we need to cut these levels by around 80–90%3. Even the government has a 60% reduction in emissions in mind. So, we’ve got at best a couple of tonnes of carbon dioxide equivalent each to play with. This obviously means that we need to make cut-backs in a lot of areas if we’re going to have a hope of meeting that target.

CO₂e is a measure of all the different effects a given activity has on the atmosphere, in terms of how much carbon dioxide it would be equivalent to. For example, methane is 21 times more sunlight-trapping than carbon dioxide, so releasing a tonne of methane is said to be 21 tonnes CO₂e4.

The calculation

So how does air travel measure up? Well, I grabbed some data about different types of aircraft (Boeing 747 Jumbo Jet5, Boeing 737-8006 and the brand-spanking new Airbus A3807) and found that, per mile, per passenger, the carbon dioxide emitted was 100–150g. That doesn’t sound like very much. However, the figure has to be scaled up for a number of reasons.

• The aircraft doesn’t just emit carbon dioxide—water vapour and NO_x gases are also given off in large quantities, and these have complicated atmospheric effects. The water vapour forms the familiar contrails which lace the sky on clear days, and the nitrogen oxides catalyse various atmospheric chemical reactions, notably production of ozone which, at the typical altitude of aircraft flight, is a powerful greenhouse gas. These effects in combination are much more damaging than the carbon dioxide alone: the IPCC estimate that the total warming effect, in CO₂e, is 2.7 times greater than the mass of CO₂ emitted8.
• My numbers were obtained by dividing maximum fuel capacity by maximum range; this is quite a good estimate for long-haul flights, but is inaccurate for shorter journeys, where take-off and landing use a massive chunk of the fuel. A 300-mile journey has its fuel use increased by about 25% due to this effect. Shorter journeys, like UK domestic flights, are even worse. There is a sweet spot at just over 2,000 miles where air travel is at its most efficient—which still isn’t very—and then, it gets worse again, because the very long flights spend the early part of the flight carrying all the extra fuel necessary for the second half of the journey9.
• Aircraft don’t usually fly quite full. A common estimate is that 80% of seats will be occupied13, which means that 80% of the capacity number of passengers are responsible for all the fuel usage.

This eventually works out as approaching 500g per person per mile. This is slightly worse than a car doing thirty miles to the gallon which, if you were on your own in it, would use about 400g per person per mile. Thus, flying is about as bad as everyone on the plane driving a car the same distance.

And then, people usually want to come home, so you have to double any distance you may do this calculation with for the return flight…

Considering it on a per person, per mile basis can mask the enormity of aircraft emissions, though—a Jumbo Jet can burn over two hundred thousand litres of fuel in a trip, emitting the equivalent of over five hundred tonnes of carbon dioxide. The new Airbus A380 can top that, with a fuel capacity of 310,000 l, meaning 720 tonnes CO₂e. Yikes.

The bottom line

The bottom line is that air travel uses one Hell of a lot of juice. A short-haul journey like London–Barcelona (700 miles) and back emits over half a tonne CO₂e per person, whilst a return ticket to the States will set each passenger back three or four tonnes CO₂e. About the worst place you can fly to from the UK, Australia, can see you using about five tonnes each way! Recall that we’re trying to reduce emissions to around 1–2 tonnes per person, per annum here: flying to the Antipodean and back could use your sustainable carbon allowance for ten years, meaning you’d be allowed to do literally nothing for the ten years after that to maintain a sustainable level of emissions.

To cut back on air travel is the single biggest and simplest contribution you as an individual can make. There just isn’t anything else you can do which can compare to the reduction which can be made by just deciding to holiday somewhere else this year.

For example, to compensate for taking a plane flight, you could…

• …reduce the mileage of your car by around the same amount of miles as the flight you took. If the flight was a return trip to America, this rapidly becomes rather prohibitive. However, even if it’s a weekend break to Europe, the couple of thousand miles you cover would still be very hard to recoup if you use your car primarily for fairly short ‘essential’ trips, such as commuting to work or going to the shops. Given that the average UK commute is about 8.5 miles10, you’d need to go two months without driving to work to compensate for a 1,200-mile return flight to Spain.
• …not use your heating for a week per hundred miles travelled, based on the average UK gas bill1112. So, popping to Spain and back will set you back three months’ heating, which could make for a cold winter.
• …live in the dark for a day per mile of flight—assuming 0.43 kg/kWh of electricity12 and five 60W light bulbs on for four hours a day. Three 15W energy-saving lightbulbs used for four hours each would increase that to a whopping ten days in the dark for every mile flown! That’s 33 years with no lighting to offset one short-haul return to Spain. (!!!)

These offsets give you an idea of just how inefficient flying a plane is, but they’re not the answer. You could, by combining increased energy efficiency across your lifestyle, offset a short flight once every few years without much inconvenience to yourself. The problem is that we need to do more than cap everyone’s emissions at the current level—we need to cut them by a significant fraction. Once this is taken into account, even a return flight to the continent could set you back a couple of years’ carbon budget; the sort of amount which can’t be recouped by working from home and turning out the lights.

The point is not that other ways of reducing energy consumption aren’t necessary—they are—but that it’s so easy to cancel out what a lot of effort energy saving in other areas by jumping on a plane.

So, it looks like aircraft are a significant source of carbon dioxide. But does it justify a sensationalist pronouncement like “your holiday is killing African children”? Aircraft are damned by the figures; the next stage is word-by-word justification of that statement.