Monday, May 28, 2007

jumbo electricity

Ages ago, Jim Bliss sat down and did some proper pencil-chewing maths to figure out if - and by how much - it was worse for him to travel from Dublin to London by plane than by bus.

Out of interest, he found it was just over 31 times worse, which is about the average seat occupancy of a coach - get a single plane seat or a coach to yourself, it's the same!

We hear a lot of figures bandied about in climate change things without knowing where they're from or how true they are, so I rather liked his DIY approach.

I saw two things separately that set me off to do one myself.

In researching green electricity I read a piece in The Ecologist by someone who was with a green electricity supplier.

I did the maths. By being with Good Energy last year, I had, according to the firm's calculator, saved the equivalent of a flight in a jumbo jet from London to… well, London: 1.496 tonnes of carbon is what a jumbo emits in a flight lasting just less than eight miles.


So, by this reckoning, a jumbo in a minute is worse than your electricity for a year. That sounded so flabbergasting that I've done some maths to check. It is more or less true.

First, let's get the less disputed figures.

Carbon emissions for UK domestic energy consumption


Electricity

Total UK domestic electricity use: 116,811,000,000 kw/h
[source: ‘Digest of UK Energy Consumption’ 5.2, Department for Trade and Industry, 2005]

UK population: 58,789,194
[2001 census]

1 kw/h electricity = 0.43 kg/CO2
[source: ‘Guidelines for Company Reporting on Greenhouse Gas Emissions’, DEFRA, July 2005]

Electricity use divided by population = average individual domestic electricity use.
Multiply by 0.43 = average individual CO2 emissions for domestic electricity.

This give us the average annual CO2 emissions for domestic electricity for a UK citizen: 854.38kg/CO2

Gas

Total UK domestic gas use: 381,879,000,000 kw/h
[source: ‘Digest of UK Energy Consumption’ 4.2, DTI, 2005]

1 kw/h gas = 0.19 kg/CO2
[source: ‘Guidelines for Company Reporting on Greenhouse Gas Emissions’, DEFRA, July 2005]

Gas use divided by population = average individual domestic gas use.
Multiply by 0.19 = average individual CO2 emissions for domestic gas.

This give us the average annual CO2 emissions for domestic gas for a UK citizen: 1234.19kg/CO2

Combined

Total average annual individual gas and electric emissions = 2088.57kg/CO2.

Now the trickier bit.

Climate impact emissions from flights

Let's take a London-Miami flight, mid range, not short haul but by no means the biggest long haul.

Distance of return flight: 14,207km
Journey time 9 hours 40 minutes (580 minutes) each way (average for London-Miami [source] )

For the CO2 emissions per passenger, I used a 1997 Dutch study 'Energy and emissions profile of aircraft and other modes of transport over European Distances' by Centre for Energy Conservation and Environmental Technologies, which gives 0.17 kgCO2/km.

Then there's the radiative forcing factor; that's how much worse it is to emit at altitude rather than on the ground. Basically, the hot wet exhausts appear to give rise to high cirrus clouds, which reflect back some of what the sun's throwing at us, but also trap some heat. On balance, the trapping is greater. This isn't contested.; exactly what the amount has caused some discussion.

As is explained here, the multiplication factor has changed. This isn't so much to do with any change in science as in accounting. The standard way of measuring the climate impact of an activity is its impact over the next 100 years. Aviation hasn't been measured like this, but now it is being, and the multiplication factor is 1.3.

If we assume -
CO2 emissions: 0.17kg/km per passenger
Seat occupancy: 370 passengers (A Boeing 747 has a maximum of 416)
Uplift factor of 1.3 for emission at altitude.

0.17 x 370 = 62.9kg CO2/km for the whole plane.
14,207 divided by 580 = 12.25km/min (average speed of 735 km/h, or 457mph)
62.9 x 12.25 = 770.525 kgCO2/min

Chuck in the radiative forcing factor

770.525 x 1.3 = 1001.68kgCO2/min equivalent.

Essentially, half the 2088.57kg/CO2 figure for average gas and electricity consumption.

We must note that the maths is approximate. I can't find per-plane emissions figures. Using the per-passenger CO2 is actually counter-productive, as the higher the number of passengers, the higher the emissions we attribute to the plane. So if a Boeing 747 were flying at maximum capacity maximum (416), we’d be saying 866 kg/min, equivalent to 1,126 kg/min, considerably worse than the same plane flying with 46 people less!

But even without that, it's not so simple. Different people come up with different numbers.

Whilst Centre for Energy Conservation and Environmental Technologies say a passenger emits 0.17 kgCO2/km, the Department of Transport issues standard figures of 0.15kgCO2/km for short-haul and 0.11kgCO2/km for long-haul flights. (Cited in George Monbiot, ‘Heat’, p261 footnote 12).

Let’s do the maths again, being more generous to aviation:

Assume -
as above but
CO2 emissions: 0.11kg/km per passenger

0.11 x 370 = 40.7kgCO2/km for the whole plane.
14,207 divided by 580 = 12.25km/min (average speed of 735 km/h, or 457mph)
40.7 x 12.25 = 498.58 kgCO2/min
498.58 x 1.3 = 648.154kgCO2/min equivalent

This is still more than 75% of the average electricity emissions of 854.38kg/CO2.

Using the generous-to-aviation figures, we get 10.806kg/CO2 per second equivalent. That uncontroversial reading can also be expressed like this:

In 79 seconds it's the same as an individual’s domestic electricity for a year.

In 3 minutes 13 seconds it's the same as an individual’s domestic gas and electricity for a year.

So, as you watch one jumbo from your garden it's worse than your electricity and gas for the house behind you in a year.

Of course, it's not strictly like with like; the jumbo has 370 people on board. If we divide that up for every individual passenger, we can still express it in brain-bomb terms; two minutes aboard a plane is the same as a day's worth of your electricity.

Imagine watching a clock showing that as you sit on the plane. Two months flip past in the time it takes to watch some in-flight Meg Ryan rom-com.

Then see a clock for the person next to you doing the same. And then for everyone else on board. Then for every person on every plane, everywhere.

2 comments:

Alex said...

I feel that the achievement of your post was not to shock the reader about how much CO2 that next holiday flight will emmit, but the general confussed and erroneous infomration that is being mixed into the entire discussion on actual emissions.
If we are all to try to cut our emissions, the place to start is to measure them with a certain degree of accuracy. The 'opportunity cost' of a change of behaviour must always be factored in.
The hairy issue seems to always be in such calculations, where are the gross errors lying? Is it in the factoring of prevailing winds on each flight? or is it in the efficiency of your home gas heater and your insulation!
I can only think at the moment that there must be some standard 'top down' independant guides set up that we all follow in these calculations so that we are all working with the same assumptions.
I would love to hear some feedback on that!

CU and thanks for the stimulating calculations!

A

merrick said...

I can only think at the moment that there must be some standard 'top down' independent guides set up that we all follow in these calculations so that we are all working with the same assumptions.

Wouldn't that make everything a lot easier? But who produces them? I thought the IPCC, being not one organisation or scientist but a federation of many, was a good place to start until I looked into the radiative forcing thing a bit more.

As with any science, we're never at an endpoint and there will always be new discoveries that revise the way we understand things.

Which is why, on reflection, I'm more tempted to go with the more recent (and, I'm assured by people who've delved deeper, very rigorous) report from Oxford University.

The safest thing I suppose is to go with the most conservative estimate; it will then be uncontroversial. However, that runs the risk of underestimating the impact, which could well be disastrous.