sparticus said:
<snip> Having said this I expect any production figures / ha make some huge assumptions as to how and what we would produce and how we would adapt in a post carbon world <snip>
Yes, assumptions are obviously made to arrive at Prof. Pimentel's figures. You can find them articulated in great detail in his book 'Food, Energy and Society.'
What he's assuming, roughly, is business as usual, only with sustainable energy and agriculture. So he's not for example assuming big changes in population distribution. This is quite important. In practice, a fairly large amount of the energy used by the average family is spent in putting food on the table, far more than on running cars or heating their homes for example.
Here's a graph of energy use by a typical (Swedish) family.
link
The relatively massive energy use in food production arises because of the process that puts food on the table. Pimentel offers a figure of 10 units of oil energy to put 1 unit of food energy on an average US table. This breaks down as petrochemicals used as fertiliser, drugs for animals and pesticide, transport of fertiliser, drugs for animals and pesticide, harvesting, transport of cereals, vegetables and fruit, transport of animals, transport and disposal of wastes, food processing, packaging, transport to supermarkets etc. See for example
what's in your bag of supermarket salad? for a discussion of just some of these process stages in a bit more detail.
If I eat a tomato grown organically 3m from where I'm sitting right now, then clearly these costs don't apply. I don't want to spend my whole life growing food however (although I don't mind spending some time growing it, in fact I rather enjoy gardening as a recreation and believe that it's also a worthwhile spiritual practice, as a kind of meditation)
So in order to minimise these kinds of costs, but maximise the efficiency of food production and hence radically reduce the energy needs of our way of life, I would suggest that one should cooperate with others to grow food close to where one lives. Eco-villages with a population in the 2-500 range are optimal and experience with these models seems to imply an average of say 8 hrs per cap per week spent growing food.
You can do a fair bit better than Pimentel's assumed 1.5 ha for food, pasture, biodiversity etc, and also reduce his 1.5 ha for energy. You need about 0.25 ha per cap for this system to produce a complete and balanced diet, but you still need some space for energy systems, to heat your home etc. But around 0.5 ha or somewhere around one acre per human, is enough to provide for a decent way of life.
Assuming that acre is decent arable land, that still only implies a sustainable UK population of around 12m though, while we have 60m citizens at present. If you assume only half of it needs to be arable, you get 24m, but in practice, you mostly don't get land that's half arable and half marginal and that matters if were talking about something as localised as an eco-village.
I think that you still need to find some way to recover land that presently isn't viable, in order to sustain our present population in the absence of oil, but assuming big changes in our way of life.
I think it's in the realms of the possible, but not with 'business as usual' assumptions, or population distribution.