No excuse me. First of all you are right that no one technology by itself will solve the problem. But if you are talking living standard rather than the same toys then we can easily support 9 billion people with a living standard as good or better than the U.S.. (Better if you don't buy the GDP=well being equation) What are the resource limitations? Energy, water (especially for agriculture and industrial use - lack of water for domestic use is pure cruelty - not a problem of resources), topsoil, and soil fertility - plus wildlife habitat. Well energy is technically easy. The politics of winning those changes are difficult, but as a criticism of our present system it is worth noting that we if we wished we could very easily meet energy needs to bring 9 billion people up to U.S. per capita GDP at the same market costs as fossil fuel (say coal) - sustainably. Now there are good arguments that once you have equality of distribution (i.e. socialism) a lower GDP would equal a better standard of living - but I'm going to stick with U.S. GDP because it is the less favorable assumption to my case. To people who think I'm wasting my time making a technical rather than poltical argument. please note that I am answering a techical point - the assertion that without Star Trek technology no political system could provide U.S. level of GDP to everyone on this planet. Since this is a common statement, it is worth answering.
Energy
Tthe key here is energy efficiency -not because it neccesarily reduces overall consumption, but because with increased efficiency you can afford to pay more for affordable sources. If we can squeeze five times as much GDP out of a unit of energy as the U.S. currently does, then we can afford to pay a great deal more for that unit of energy.
Examples - electric light bodied cars (eight times more efficient than U.S. standard) electric ultra-light trains (http://www.cybertran.com) 19 times more efficient than U.S. standard - inexpensive enough tracks and stops to be practical suburban communties.
Sources - solar energy. Solar thermal electricity can be produced more cheaply than photovolatics. More importantly it can be stored in molten salts comparatively cheaply - making it full dispatchable useful for both base and peaking. Once you have a dispatchable base (other sources includes geothermal and existing hydroelectric) you can mix in variable sources such as wind which are much cheaper. Also between 25% and 33% of energy world wide is used for low temp heating and cooling of air and water. Evacuated tube collectors can tap solar for these purposes almost everywhere - even in rainy climates. It is competivie now with fossil fuels when providing about 65% of such needs. Add storage in the form of natural zeolites and you could get all of your low temp needs for the sun in most of the world. (I exclude Alaska and Siberia.)
Water
1)Efficiency - low flush toilets, low flow shower heads, drip irrigatation, low energy high precision sprinklers no till rotational agriculture (builds soil and reduces water use), planting crops appropriate to climate, massive increases in industrial water efficiency through know process substitutions (controlling spills, smaller vessels, longer rinsens etc
Also rainwater capture
2) conservation - more veggie protein less meat protein. Not veggetarian, just using meat more as flavoring and less as a staple. Also raise meat via managment intensive rotational grazing rather than feeding it on grain. 3) recycling and reuse. For example waste water from least dirty step in industrial process can provide rinse water for next dirtiests and so on.
These means alone can mutliply the economic benefit per unit of water by four.
4) Reverse osmosis and other distillation. Distillation lends itself well to variable energy sources since you can store the product (clean water) rather than the energy. The water is more expensive than natural sources; but again if you are gettin 3 times the GDP out of unit of water you can afford to spend more on that unit.
Agriculture We already have most of this other agriculture. Rotational no-till agriculture can drastically lower ecological footprint of row crops. Not disturbing the roots cultivates the fungi that fix glomalin which is a key glue that fixes carbon, and build a soil structure that gives plants access to water and nutrients. Mono-culture no-till and short rotation no-till are often used as excuses for soaking fields in pesticides. But you rotation a fiber crop, a legume, a grain and a leguminous green manure and combine that with integrated pest management you eliminate nitrogen fertilzers, greatly reduce the use of other fertilizers, and cut pesticde and herbicide use in half - while also greatly reducing the use of heavy farm machinery. Rodale has even demonstrated that you can even combine no-till with true organic farming.
Currently we produce plenty of food for everyone on this planet, and about 75% of what we will need in 2050. World hunger at present is due to the hideous cruelty of our system, not to any absolute shortage. The sustainable means I outlined would not only reduce the ecological costs of farming, but increase absolute production. (No-till produces more per acre than conventional farming.) A slight reduction in absolute meat production (which would translate into a fairly large reduction in per capita meat production) would fill the remains of that gap. ============= Lastly there is the question of habitat. As I pointed out, with a reduction in meat consumption and sustainable food production we can feed 9 billion people on current land devoted to agriculture (though there is eroded and damaged land and land held in reserves that could be converted ). If agriculture is done more sustainably with less run-off and less polluted run-off this will be a tremendous preservation of wildlife. Similarly new buildings for the increased human populaton could be done via in-fill - putting them in existing cities, and in-filling existing suburbs. Also harvesting of forests for fiber could be greatly reduced by deriving fiber from straw - which is a row crop waste, only half of which may sustainably be harvested from the soil. Similarly substituting renewable energy for fossil fuels will greatly reduce air and water pollution that destroys habitats. Lastly we can use metal recycling and a general lowering of material intensity to reduce mineral mining.