Bazz,
Thanks for the feedback. Glad to see that someone on OLO is at least thinking about the argument in a considered way.
You are right to point out that energy is the key. In my views about population growth are tied to those about the future of our energy supplies. For the future to be sustainable in any meaningful way, it would be imperative that we wean ourselves off these sources of energy. I think you would agree that this would be the case regardless of what happened with population.
Even there, it is foreseeable that these demands could be met in a sustainable way. The potential energy in nuclear is approaching limitless, considering that the current nuclear power plants really only scratch the surface of what is possible. The reason uranium 74 isotope is currently used to generate the majority of the world's nuclear energy is that of all the naturally occurring isotopes, it is the one which produces the LEAST amount of energy and therefore the easiest to control. We need to get over our ignorant fear of nuclear energy and continue to improve it rather than abandoning it.

I firmly believe that as oil declines we can continue to feed ourselves. In answer to your question, diesel and petrol do not contribute greatly to my cost of production DIRRECTLY. In fact, they do not even appear in my top 10 cost categories. However, they (along with electricity) are closely linked to my major costs. (Transport, packaging, fertiliser, chemical, water) All of these costs have a large energy component built into their cost profile.
To be clear, my main argument on this subject is that we have the resources on this earth to sustain the predicted population growth. Certainly not that we can all merrily continue to live the way that we do without making any changes.

Man O' Land;
Some while back I read an article by someone who had tested ploughing
by tractor versus horse.
I can't remember the exact figures but it was based on 100 hectares I think.
It took him three hours to plough that land with a four furrow plough
with a tractor and three days with a horse and a single furrow plough.
His conclusion was that in absence or unaffordability of diesel we
will need about 40 times the number of farmers that we now have.
Another problem will be the affordability of natual gas derived fertilisers.

Nuclear power of course produces electricity not liquid fuels.
Therefore we would have to fit all paddocks with overhead wires, ala
trolley busses.
Incidentally Siemens and Mercedes are experimenting
with electric trucks on autobahns, so the idea is not that wild.

The more important problem is the relative energy efficiency of
hydrocarbons versus uranium, steam, electricity, transmission.
Diesel is very very energy dense.

Bazz,
The inference that we would have to fit paddocks with overhead wires is as ridiculous as returning to the horse and plough (although that might give all the Africans something to do).

You are correct in saying that diesel is energy dense in terms of MJ per L. But hydrogen has an energy density per kg, many times that of diesel. Further more, the advancement of hydrogen fuel cells in the last ten years has been significant. The answer may also lie in cellulose ethanol. It is not easy to predict what the future transportation fuel will look like, but suffice it to say that there ARE indeed alternatives. I have thought for some time now that the next Bill Gates will be someone who comes up with a significant energy solution or advancement.

This is precisely the type of area that I feel we should be focussing on, rather than bleating about controlling the population of the earth. Is your argument based on the assumption that if we reduce the population in the world, then we can all carry on as we are, with little need to worry? Do you think that if we were to halve the population, then we would have enough hydrocarbons to last forever? Of course not. I can't predict what the sources of energy will be, I merely make the observation that we have enough resources to sustain this level of population.

Man O' Land said;
I merely make the observation that we have enough resources to sustain this level of population.

Ah but for how long ?
Re the interference with poles & overhead wires, yes I agree a problem
but currently trolly busses can move out a couple of lanes so it may be
possible to have quite wide cultivation "lanes".
An alternative might be to do what was done from age of steam with
traction engines and cables to pull ploughs etc. They were quite
successful so we might end up there again.
Re hydrogen, I was keen on that idea a while ago but the opinion was
changed in the reading I did and it is now basically dead as a motive fuel.

A couple of Christmases ago I was talking to my son's father in law
who is managing director of the largest bus and fire engine
manufacturing company in the UK.
They had tried hydrogen fuel cells in busses but their lifetime was uneconomic.
There are also other drawbacks, such as the weight of the tanks and
that they can never be stopped from leaking.
The result is any vehicle using hydrogen has to have special garaging
with roof ventilation and they cannot be parked underground.
The loss of hydrogen unless they are in continuous use makes them
expensive to run. The distribution infrastructure will probably never
be financed.

>The answer may also lie in cellulose ethanol.

Possibly but so far it has like ethanol fallen over because of the
EROEI and net energy. There really are no easy solutions on the
horizon for mobile energy, except perhaps steam.

Bazz,
Sorry for the delayed response. I am not running from the debate I assure you.

With regard to the hydrogen technology, I agree that this problem looks difficult at the moment. However, the trend looks encouraging. In 2002, the production cost of fuel cells was around $275/kW, while in 2010, this figure had dropped to $51/kW.
The target here is $30/kW which would make it around $3000 for 100kW (134hp). This would be approximate parity with internal combustion engines. Some significant advancements have also been made with the problems of leaking, reliability and service life you mentioned.
You have to consider the amount of research resource which has been poured into combustion engines in the last 100 years and compare that with the relatively new research effort into hydrogen. Ethanol falls into the same boat.

I also wouldn't discount someone finding more effective ways of storing electricity. Lithium ion batteries have proven to be a significant step forward in this area, although they don't provide the long term solution we are looking for. But the point remains - find an effective way to store electricity and you have solved the worlds problems.

Aside from these points, the way our entire societies are structured with regard to transport, may require some out of the box thinking

Man O' Land;
If they do manage to produce a long life fuel cell there is still the
problem of ERoEI. There is no efficient way to produce hydrogen that
gives a good eroei. There seems to be only electrolysis and natural gas.

Natural gas probably would be better directly used in engines.
Electrolysis by generating electricity from the sun would ultimately
be very neat, but can it be scaled up enough ? Could we afford it ?
Could we build enough solar cells & would the shade area cause problems ?

As you say if we can store electricity on a large enough scale we
would be home & hosed. Unfortunately there does not seem to be
anything remotely on the horizon to do the job.