Human social development has always — since the domestication of fire and even more so during the Holocene with the process of agrarianization — been synonymous with a process of ‘trophic expansion.’ As a species, culture has allowed us to sequestrate a steadily rising proportion of the primary productivity of the planet

You say at one point:
“this depends on the efficiency of the solar energy collection. (If no or only minor efficiency gains are achieved, there won’t be enough land.)”

The discussion at this point implies that artificial or human-directed solar powered production must involve a zero sum competition the rest of the bioshere. This is certainly true if our use of the sun requires more intensive use of more of the surface area of the biosphere (more land and more ocean). This has always been the case so far in human development. However it is possible that we now have a choice between

(1) trophic imperialism - i.e. a complete take over of the biosphere as a substrate, platform and resource for human growth and development. This option is implied not only by high tech intensive agriculture, including the production of GM biofuels, but also any organic/permacultural scenario that doesn’t include a radical reduction of human population.

(2) trophic detachment - i.e. the develpoment of artificial photosynthesis for production of hydrogen but also organic substrate for a completely synthetic high rise, urban agro-ecosystem.

OPtion 1 would imply that humans would be forced to take responsibility for management of all of the earth’s climate and life support systems in perpetuity (a scenario that Evan Eisenberg, in the Ecology of Eden, refers to as a hostile take over). This is a frightening and daunting prospect. We are already edging towards it with the new emphasis on climate engineering in response to global warming.

OPtion 2 would imply the development of a detached urban based synthetic ecosystem. Three dimensional architectural surface areas at the moment unproductive, would be transformed into photo-generative power plant - and the basis for a disengagement of human social development from the wider ecolutionary ecology of the planet. If this high tech. scenario is possible, it does suggest a very different evolutionary future for non-human nature. If food production is industrialised and urbanised and moved out of the countryside — with invitro synthesis of food substrates and fuel, using a combination of urban architecture and (relatively) biologically moribund areas of desert –then a remarkable prospect opens up: for the first time 10,000 years (agriculture) or a million years (fire)…the tightening grip of humanity on the biosphere might start to relax, with large areas of agriculture reverting back to relatively autonomous wild ecosystems.

This may seem science-fictional. It is!. But it was a prospect that exercised the imagination of the prominent biologist Conrad Waddington and science writer Nicholas Calder during the 1960s. The same idea seems to lie at the heart of the sky-farm idea that has been advanced by Dr. Dickson Despommier at Columbia (and others). It also comes up in a different way in the eco-city ideas of Richard Register and the ‘arcologies’ of Paulo Soleri. It is also a vista that seems to be at least entertained by James Lovelock in his most recent books. Waddington and Calder argued that the prospect of such high density, bio-synthetically self-sufficient cities was 50 years away. And of course they are still 50 years away ( atleast). But if LOvelock’s worst case scenario comes about, and the 21st century sees billions dying and human civilization shifting to the polar regions - then it is possible to at least envisage the creation of self-sufficient cities based on such a process of trophic detachment. This is not a pleasant prospect. But it is useful to sketch out these scenarios. If nothing else they demonstrate that the relationship between human and non-human ecosystems is not straightforward. In the very long term, ‘nature’ may well be best served by its most prodigal child leaving home so to speak.

The references for Waddington and Calder are:

Calder,N (1967) The Environment Game (London: Secler & Warburg)
Waddington,CH (1965) ‘Science and Wisdom’ Vol 2, p 13 in N Calder (ed) ‘The World in 1984’ (Pelican Books, ed N. Calder)

ANd my own commentary on their futurology:

Quilley,S (2004) Social Development as Trophic Expansion: Food systems, Prosthetic Ecology and the Arrow of History, Amsterdam Sociologisch Tijdschrift, September Vol 31(3): 321-348

“This suggests that tampering with plant productivity may be a grave mistake or impossible” Unfortunately, it’s all too possible, at least for a while. Human agricultural practices provide an example, although “tampering” in this case is usually called “breeding”. In many cases, this worked for a while, then the law of unintended consequences kicked in and civilizations suffered and died. (There might be some useful lessons from studying the cases of agricultural practices that have been sustained for millenia, as for example rice cultivation in China.)

Your last quote reminded me of the latest essay by John Michael Greer, “Galloping with Blinkers On” (http://thearchdruidreport.blogspot.com/2009/03/galloping-with-blinkers-on.html). Synchronicity?