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Wednesday, 21 November 2007

Carbon footprint

I'm not going to ask you to calculate your carbon footprint. I have not done it for myself, because I can't find an accurate way of doing it, and I don't entirely trust these "take 2 minutes to ask 5 questions and we'll tell you" websites.

However, I was interested that the front of Walkers crisps now has a little logo telling me that 75g Carbon dioxide was emitted for my pack of crisps. This time, before I thought about the accuracy of the figure - which is coming later - I thought to myself, I have NO idea how much that is - let's think about that for a second.

We can work it out scientifically. Do you remember GCSE science? Let's try a quick calculation (it won't hurt, promise). If you remember, we work in moles. A mole is just a number of atoms that is convenient. Carbon dioxide is CO2, so in each mole of CO2 there is one mole of C and 2 moles of O. A mole of C weighs 12 grams and a a mole of O weighs 16 grams, so we have 44 grams for a mole of CO2. My 75g of CO2 is just over one and a half moles. That's about 1,000,000,000,000,000,000,000,000 (10^24) atoms, occupying about 35 litres. OK, so atoms are so small that I can't tell if that's a number that I should worry about, but 35 litres of CO2! Just for a pack of crisps?! That seems like loads! I can only assume that all goods we buy are like that, so we must generate roomfulls each day. To me it seems ridiculous that we can produce so much, how can that possibly work out if everyone is doing that?

How accurate is this figure of 75g then? Maybe it has already been taken into account, maybe not, but apart from producing the crisp and the packet, there is the transport from the factory to the shops, the possibility of people driving to the shops to buy the crisps and the cost of disposal of the rubbish. Cost of disposal includes bin lorries, rubbish sorting and decomposition of the material used in the packaging which may or may not itself produce carbon dioxide(?). Looking in even more detail at the bin lorries, they must deal with producing all of their goods like the lorries themselves and the fluorescent uniforms (although if people wear them each day maybe they buy fewer clothes?), they have to maintain the lorries, buy petrol, the employees have to get to work, they have to send out notices when bin days are changed, and also hand out bins, which must themselves be produced. Where do you stop? How can you say that my bag of crisps has a 75g carbon footprint? Does responsibility for the bag and it's footprint pass to the waste disposal people at some point? The whole calculation is much too large for anyone to find a meaningful figure in my opinion.

According to Gordon Brown yesterday, the UK produces 654 million tonnes of carbon dioxide per year. That's a lot more litres. In fact, it's over 300,000,000,000,000 litres. If you sat in the middle of a ball with that much volume, you would have to go about a marathon distance to get to the edge (43km, 27 miles). Bear in mind that carbon dioxide is naturally found in concentrations of one part in 4000 and you see that the amount of carbon dioxide we are producing will have a major effect on the carbon dioxide in the atmosphere.

No wonder something's going on.


andrewthompson said...

I got confused by that. Hope you don't mind comments from an idiot. Where did 35 litres come from? I understand that CO2 must be light as it is a gas so for there to be 75g of the stuff there must be quite a lot. Gases can compress and expand but are you saying that 35 litres is the smallest volume that 1,000,000,000,000,000,000,000,000 atoms can fit into or is that the space they occupy at atmospheric pressure?

I appreciate that giving me an answer may involve you teaching me GCSE Chemistry so don't worry if I'm too think to understand.

P.S. I heard methane was the next one to worry about - 20 times worse than CO2 for global warming and lots trapped in siberian lakes which are rapidly thawing. Scary.

Grinnyguy said...

35 litres is the volume that it would occupy at atmospheric pressure.
I reached that figure by working out the amount of moles of CO2 (each mole weighs 44g) and then we can either look up the fact that one mole occupies 24 litres at atmospheric pressure, or use ideal gas equation, which is:
Here, P = pressure
V = volume
n = number of moles
R = a number - 8.3145
T = Temperature (in Kelvin)

If you put in the numbers P = 10,000Pa = 1 atmosphere
n = 75/44 = 1.7 moles
R = 8.3145
T = 25 celsius = 298K
then V = 0.042 cubic metres = 42 litres

Oops, 35 litres was a bit out, sorry, that's probably why you were confused!

And yes, methane is a big problem. It sounds weird, but cows farting methane and rice paddy fields producing methane are actually quite serious. And as well as the permafrost releasing large amounts of methane, there is a large amount trapped at the bottom of the oceans too in certain rocks, and it is possible that could get released too (although I forget how)

However, methane is found in smaller quantities than carbon dioxide so the total effect is less at the moment, which is why we focus on carbon dioxide as the greenhouse gas to cut down on primarily

andrewthompson said...

Yeah. If you'd said 42 litres I would have understood completely. Um...

P.S. I love this blog. Actually, it's the only blog I've seen that hasn't left me worried about the amount of anger and illiteracy in modern society.