Kooiti MASUDA said...
> Crop yields will decline, particularly in Africa
I'll start off by pointing out that this comment of the BBC was directly
linked to the environmental impact of climate change (via Stern),
therefore speculation about peak oil and the sustainability of farming
practices is rather tangential to the point they were making.
http://news.bbc.co.uk/1/hi/business/6098362.stm
But anyway...
I think that THIS is likely -- a very unfortunate thing.
There are two very distinct issues are there.
One is my outlook of climate change. It is difficult to predict future
distribution of aridity. But it is almost certain that local, short-term
precipitation can increase in proportional to saturation specific
humidity that is exponential in temperature, and that global mean
precipitation does not increase so much because it must almost match
global mean evaporation which is limited by energy balance at the
surface where the input will increase more or less like log(CO2 conc.).
Thus it is almost sure that precipitation tend to concentrate spatially
and temporally. In some places there will be excess of precipitation
which may lead to floods. Elsewhere precipitation does not increase to
match evaporation locally and the land would be more arid.
Well, as already discussed, the model projections give a worst-case of
~25% reduction in potential yield over significant areas in Africa, with
roughly equivalent increases in other regions:
http://news.bbc.co.uk/1/low/sci/tech/6126242.stm
Even with extremely high emissions growth, this change takes about 80
years, so a 0.5% reduction per year seems like a safe upper estimate for
the rate of potential yield loss due to climate change.
Google suggests that the rate of observed yield increase due to
technological and economic factors is much larger than that, at least
over the last 40 years (I could be really heretical and suggest that the
recent warming might have also contributed to the yield growth, but I'd
probably get lynched):
http://historylink101.com/lessons/farm-city/yeild.htm
and there is still a vast gap between the actual yield in poor regions
and the "potential yield" which was the subject of the model
projections, so it's not as if the yields are pushing up against some
hard climate-dominated ceiling.
The other thing is the capacity of agriculture. The increase of crop
yield in the 20th century owe much to fossil fuel: chemical fertilizers,
pesticides, machines (e.g. tractors), large-scale irrigation, etc. They
do not logically depend on fossil fuel -- just on low-entropy energy
resources. But practically we cannot immediately substitute them with
renewable resources only (or even with nuclear power, for that matter).
We cannot assume either that the future generations will magically find
some novel low-entropy resources that we cannot access now.
Energy use is a rather different matter, and I agree that much modern
agriculture is essentially the conversion of oil into food calories.
However, it is a little soon to say that fossil fuels are actually
running out - especially when you consider coal and oil shales etc. I
don't want to appear to wave too many magic wands but there are plenty
of opportunities for efficiency gains here - eg low-tillage methods are
increasingly popular, which saves on a lot of energy. As far as I can
see, the recent tripling of the cost of oil has had negligible impact on
food prices.
Here is some info on energy use in USA agriculture, including a
substantial increase in efficiency over the past 50 years:
http://www.ers.usda.gov/AmberWaves/April06/Features/Energy.htm
After all, we cannot simply assume that future generations are richer,
or more adaptable.
Well if we do not get richer rapidly and keep on increasing GHG
emissions then the rate of climate change will tail off substantially in
any case... (and certainly will not accelerate, which is the assumption
underlying Stern).
I conclude that neither climate change nor energy costs are going to
prevent crop yields increasing for the foreseeable future over any
reasonably broad scales and would happily bet on that proposition. Any
takers?
James
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