Archive for the 'food' Category

Food Chain Emissions


Friends of the Earth have sent our household a postcard. It says «The meat and dairy industry produces more climate-changing emissions than all the planes, cars and lorries on the planet.» They don’t quote a study, or any other source. Just a bold assertion which, on the face it, seems implausible. Even if you eat a gargantuan 250 g of meat a day (in other words, the typical US diet; Europeans eat about half that), does that really compare to all that driving round? It also seems a little bit mean to exclude trains and ships on the “transport” side. Is the balance between transport and food really so close that those 2 modes make all the difference? In the UK, rail and water account for about 4% of the total transport energy budget, so I would hope that the question isn’t so close that adding them back in tips the scales the other way. For one thing, any reasonable quantification of errors is bound to swamp that.

I think the FoE statement is false, here’s my homework.

David MacKay stacks up the UK’s energy consumption (Sustainable Energy – Without the Hot Air, Chapter 18, page 103), he has (per person): car 40 kWh/d, plane 30 kWh/d, food 15 kWh/d. So with 70 kWh/d (82 if we add the other transport modes) on the side of transport, and 15 kWh/d on the side of food then it does indeed seem implausible that food chain emissions would be higher. Note that we have all food production on one side, I can’t be bothered separating out meat from the rest, clearly meat forms the bulk of the energy consumption anyway. But wait…

As well as emissions related to the energy required to maintain the animals, they produce carbon-dioxide and methane all by themselves. In other words the food industry has emissions not related to its energy inputs (even if all the energy was produced sustainably, there would still be emissions). Non-energy related emissions show a weakness in David MacKay’s book; he neglects them completely. That’s okay, because his focus is Sustainable Energy, but be aware that it’s not the whole picture. Food, concrete, deforestation all have non-energy emissions. For animals I think we can neglect the CO2 emissions because the carbon originally came from the atmosphere anyway (respiration forms part of a close carbon cycle). Methane however is not negligible.

I reckon 1 kg of lamb produced between 60 g and 180 g of methane when it was walking about in the Peak District. That’s equivalent to about 2.4 kg of CO2. Let’s say I eat 100g of lamb a day. That’s (methane emissions equivalent to) emissions of 240g CO2, or about 1kWh of diesel. That’s roughly 0.1 litres; if you fill up 40 litres (about the size of my small car’s tank) every two weeks then that’s 3 litres a day. How often do you fill up? From a personal perspective, It looks like food-related methane emissions are not even close (to transport emissions).

Okay. So much for the ovine. What about the bovine, porcine, and, er, chickens? Well, I’m no veterinarian so this will take a lot of piecemeal research. Bugger that, lets go to a (competent?) summary: The UK’s Fourth National
Communication under the United Nations Framework Convention On Climate Change
. In 2004 UK agriculture (note: not just meat and dairy) emitted 13.8 MtC (megatonnes of carbon equivalent); transport emitted 37.4 MtC. Just what are these Friends of the Earth smoking that makes them think they can claim “The meat and dairy industry produces more climate-changing emissions than all the planes, cars and lorries on the planet” when it is so out of line with the UNFCCC GHG inventory. Is the UK really so atypical?

I suspect that what’s really happening is that the FoE are doing some clever accounting. There’s probably a little bit of double accounting (example, counting transport of feed on both sides), and I suspect some land use change. Perhaps they include chopping down ancient forest to grow soya beans for animal feed as an emission on the food change? I just don’t know, because they don’t show their homework. But I have a couple of points to make anyway. The first is that it’s not at all clear that the beef industry is too blame. If there was less demand for beef (and hence soya beans to feed the cows), then I think it’s likely that the same companies would have chopped down the same forest to grow something else. Miscanthus perhaps. The second is that while this land use change will be an emission (the UNFCCC recognises land use and land use change as a carbon source / sink), this emission occurs only once. Once the forest is cleared to grow soya, there will be no land use change emissions. So the emissions from the single land use change should be amortised over all future soya bean seasons. I think.

So FoE, how do you make the sums add up?

Appendix for the pedantic

«250g of meat a day … the typical US diet»

A quote from USDA Agriculture Factbook 2001-2002, Chapter 2, “Profiling Food Consumption in America”, :

“In 2000, total meat consumption … reached 195 pounds … per person”. That’s 242 g per person per day (2000 was a leap year).

«rail and water account for about 4% of the total transport energy budget»

Department for Transport, TSGB Chapter 3:

«1kg of lamb produced between 60g and 180g of methane»:

One 60 kg ewe produces about 20 litres methane a day (see below). Boned and trimmed meat is about 2/3 of the animal’s weight, so 0.5 litres / kg (boned). Lamb is generally defined as less than 12 month’s old or less than 18 month’s old for export. 360 days × 0.5 litres = 180 litres. × (the density of methane gas) 0.717 g/l = 129 g. 60 g to 180 g gives a range around this (to account for younger and older lambs, for one thing).

«one 60 kg ewe produces about 20 litres methane a day»

See Proceedings of the Nutrition Society, Volume 41, page 9A, meeting of 1981-07-17, “Methane production in lambs fed high- and low-roughage diets”. It depends on their diet: about 23 litres for high roughage; about 9 litres for low roughage. Two things: 1) when did you last see sheep being fed lucerne hay? 2) using 20 litres per day favours the FoE case anyway.

«equivalent to about 2.4 kg of CO2»

In terms of greenhouse gas warming potential, per kilo, methane is 20 times more potent than CO2. So 120 g methane equivalent to 2.4 kg CO2.


Putting the Heat on Wheat


Wherein I play with the lovely Google Charts API and expose my total incompetence in statistics, economics, agriculture, and geography. And quite possibly other things too.

So I was reading the Open Knowledge Foundation blog and came across this article featuring US wheat production, which points to this dataset of wheaty goodness. My recent work on Clear Climate Code had made me already aware of the availability of GISTEMP’s summary data products.

So it occurred to me that this could be used to answer the question “when the weather is warmer, does more wheat grow?”.

So the wheat data is US wheat production, including yields in bushels/acre, sigh. GISTEMP even do a dataset that shows the temperature anomaly for the US. I think this is incredibly parochial, but it happens to be just what I want.

So the wheat yield (volume of wheat per harvested unit area) has a general upward trend. At least from the mid 1930’s or so. Because I’m only interested in the local variation I have detrended the wheat data:

My hypothesis is that any deviation of the temperature from the long term average will lower wheat yields. I think this because I would expect that over the thousands of years of selection humans will have cultivated a variety of wheat that is optimised to grow at the average temperatures and it will do less well when temperatures deviate.

So what do we see? Here’s wheat yields and temperatures together:

Well, there’s no obvious correlation to eyeball. Scattergram:

(which is almost just changing ‘cht=lc’ to ‘cht=s’ in the above chart URL)

Bit of a blurry mess. If anything a slight negative trend, which would mean that colder temperatures gave a higher wheat yield. And indeed Pearson’s correlation is about -0.3 (assuming my calculations are correct) indicating a weak negative correlation.

There are problems. One problem is that I have no p-value. That’s partly because I haven’t read that far on the Wikipedia page (I’m not using some fancy stats package for my analysis; everything is hand-coded in Python), and partly because I have a degrees of freedom problem. Temperature is autocorrelated, so whilst I have 128 samples, that’s fewer than 128 degrees of freedom, so the standard assumption of independent variables is incorrect.

The other problem is that it looks like the detrending might have introduced a bit of an alarming feature into the wheat anomalies. There’s a gentle hump from 1866 to about 1940 and a similar one from about 1940 to 2000. This is almost certainly because I’ve used a cubic polynomial to fit to the data to detrend it. It looks like a two-leg linear fit would be better (with a kink around 1942), but I haven’t found how to do that. I have a sneaking suspicion I have some FORTRAN code lying around here to do it, but I’m too scared to look.

Final tiny problem almost too small to be worth mentioning: the wheat data is for the entire US, whereas the temperature data is for the contiguous 48. I’m guessing that Alaska and Hawaii make so little wheat contribution that it doesn’t matter.

In any case it doesn’t really look like fixing these problems would ever indicate a strong positive trend between temperature anomalies and wheat yields. So we can reject the notion that warmer weather means higher wheat yields. Of course warmer weather might mean we can grow more of something else (possibly just a different variety of wheat); it also might mean that the available belt of land for growing wheat is larger (but this is unlikely since it probably means the available belt of land for growing wheat has moved North).

Slicing Soreen


I think Soreen is easier to slice and butter after it’s been frozen and defrosted. Slightly.

I tend to only buy Soreen when there’s a 2-for-1 offer on, so one unit gets eaten, the other gets frozen.