Thursday, May 02, 2013

The Wool Sock’s Carbon Footprint


Four years ago, I blogged about the cognitive disconnect between the ecological perceptions of wearing wool and eating beef.  However, I did not actually calculate out exactly what the carbon footprint of a wool sock is.  Here it goes:
According to Wikipedia’s wool bale article, a bale contains about 60 fleeces, and weights 150 ± 50 kg.  This gives a fleece weight of about 2.5 kg.

This wool sock weighs about 100g, meaning that you can get about 25 socks per fleece.  A sheep produces one fleece per year.

A ballpark estimate from the NSW department of primary industries suggests that a medium sized (45 kg) adult sheep in warm weather needs about 500g of dry feed per day to survive.  If this feed is mostly cellulose, it will metabolize to produce about 800g of CO2 per day, or 297 kg/ year. Assuming 25 socks per year, that gives about 12 kg of respired CO2 per sock.

However, in addition to respiration, sheep also produce a fair amount of methane, which is generally considered to be 25 times more potent a greenhouse gas than carbon dioxide.   This paper estimates a methane yield of about 20 grams / day/ sheep, or about 7.3 kg of methane per year.  Using the 25 times multiplier, we get a CO2 equivalence for that methane of about 180 kg / sheep/ year, which is a bit over half the direct respiration emissions.  Dividing by 25 socks/sheep gives is a CO2 equivalent of 7.3 kg per sock (300 grams methane).  In total, our CO2 equivalent emissions from the sheep are about 19 kg of CO2 per wool sock- 12 from respiration, and 7 from methane.  This figure only includes the CO2 footprint for growing the wool.  It does not include additional emissions from shearing, transporting the wool, spinning it into yard, and manufacturing the sock.  This is the same amount of CO2 released by burning about 8 liters of gasoline (which is enough to drive a mid-size car 100 km), or one sixth the emissions of a top fuel drag race (with 2 cars in it).  So a hackey sack game with more than three pairs of new socks in it is worse for the atmosphere than this.

In contrast, a 50 gram synthetic sock (synthetics weigh less than wool) probably has a carbon footprint of 10-25 grams*.  It production is one THOUSAND times less carbon intensive than a wool sock.  So the next time some green evangelists starts looking down their noses at your car or your plate, check out their feet.

* In both the case of the plastic sock and the wool sock, the carbon in the sock itself is sequestered in the sock drawer for the lifetime of the sock, and in a landfill for several decades afterwards.  Unless you burn your old socks, which smells, or recycle your used synthetic socks into drink bottles, which is disgusting.

3 comments:

Anonymous said...

I find that typical wool socks lasts severql years longed than synthetics. Also , good wool socks can be worn multiple times before laundering whereas synthetics need to be washed every wearing. So, there's some balance restored over time.

Robert Schenck said...

Above Anon-
Please note that the wool sock is 19 KILOgrams of CO2, the synthetic is 25 grams. The wool is 3 orders of magnitude worse. You'd have to wear 1,000 synthetic socks in the time it takes to wear the wool one.

So, my conclusion is that you should definitely wash your socks each time you use them, because anything less is gross. And if you can't do that, use synthetic socks as disposables and you'll still be ahead.

Robert Schenck said...

This reminds me of the joke about how animal rights people will throw paint onto old ladies in fur coats, but not bikers in leather jackets. Never heard of PETA interrupting the Sturgis rally, for example.