I've noticed a few media reports about research into alternative aircraft fuels, although this only relates to turbine engines which are reasonably flexible with fuel requirements. Piston engines can possibly be operated on ethanol or even hydrogen but I'm not aware of any cases where either has been used. In any case, alternative fuels still involve substantial carbon emissions. Whether or not the emissions issue is sufficient to force a dramatic rethink of air travel is another story. Personally I subscribe to the views of David Suzuki / James Lovelock who believe its at least thirty years too late to worry about climate change, consequently there is nothing to be gained by worrying about the relationship between carbon emissions & climate change. On the other hand, many claim that climate change is a natural event & has nothing to do with carbon emissions. Only a few brave souls hold to the view that its not too late to make a difference. Given the obvious financial interests its inconceivable that the last group woll ever achieve their goals so we may as well keep flying until the end of civilization as we know it ... somewhere between five and twenty years down the track.

kadaitcha,
In a time of depleting oil supplies, to use the small amount of
alternative liquid fuels that we will have would not be used for
aviation. They would go to food production.
It comes back to the same problem, could we afford to divert crop land
to feed our cars, aircraft etc or ourselves ?

Basically it is the scale of the problem.
I like to use this simile;
Japan uses 8 Million barrels of oil a day.
A very large tanker carries 2 Mbd.
Therefore 4 tankers have to arrive in Japan every day.
It takes about 25 days sailing time from the middle east.
Therefore there are 100 tankers en route at any time.
Therefore there are 100 tankers sailing empty back to the middle east.

This the scale for just one country.
Aviation is I think about 8% of total from memory.
How many hectares needed to grow that amount of biofuel.

Hydrogen is a no go, aside from producing it, it would require a very
large and heavy tank in the middle of the aircraft.
A 747s capacity would be about 25% of its present capacity.

Would you prefer to fly in a coal fired 747 ?
You could employ all the unemployed as stokers on the air route
to the UK.

What happened to the VFT (Very Fast Train) that was once seriously proposed and investigated as a link between Sydney and Canberra and possibly Melbourne? A lot of work went into assessing this idea but then air travel became so economically cheap that the VFT fell off the agenda. Cheap air travel is unlikely to last given increasingly global oil consumption and the inevitability of 'Peak Oil' etc. Any viable alternatives to AvGas are likely many years away and would very likely be far more economically costly. So.... why not be the 'clever country' that we once claimed to be, and invest in the sort of high-speed rail infrastructure that China is building? Apparently western European high-speed rail is rapidly displacing short-haul air travel. Even if we just did the Sydney, Canberra, Melbourne connection, we'd make a significant dent in our current use of relatively short-haul and therefore especially ineffecient air travel. But no, that would be too progressive for Australia! We'd rather build another airport and more freeways (sigh)!

As I suggested previously, turbine engines (jets) aren't particularly picky about what fuel is used, for example they would probably run on overproof rum or even scotch. Certainly ethanol or dieseline would work & virtually anything else that burns. Unlike piston engines, turbines don't need high octane rated fuel, they could even (potentially at least) run on pulverized coal dust. Licencing for aviation use is another issue but one that could be overcome if necessary. Piston engines are more problematic, only real alternative to avgas is ethanol. Your point about land use is certainly valid if we assume global population will continue to expand, however the climate change scenarios postulated by the likes of David Suzuki & James Lovelock suggest a dramatic reduction in population by way of starvation. Whether or not that transpires before we run out of fossil fuel is another question .........

Avgas is really only unleaded petrol certified for use in aircraft with internal combustion (piston) engines. This means only light aircraft like Cessna, Piper, Jabiru need alternative high octane fuels & ethanol would definitely work if it was properly handled. Contamination and / or oxidation in storage & transport is the main factor requiring avgas to be certified as aircraft fuel, hence the higher cost than unleaded fuel (currently around $1.60 per litre) Turbines are only cost effective at high altitudes, fuel consumption at under 15,000 feet (ceiling of unpressurized light aircraft) is high enough to render them unviable at present.

Commercial aircraft invariably have turbine engines which run on kerosene, mainly because its relatively cheap. Being external combustion engines, they can run OK on anything that burns, however OP Bundy or Scotch is probably a bit more expensive than most airlines can justify :) FWIW, there are small turbines available to suit light aircraft (albeit at much higher cost & with higher fuel consumption than the normal piston engines), but maybe thats something to be addressed if we do run out of fossil fuels & still wish to operate light aircraft.

Many suggestions for aviation fuels have been made but they all seem
to fall down when the real experts put in an appearance.
It would be politically untenable except for defence to have aircraft
flying around on bio fuels that were taking food out of peoples mouths.
It does, unfortunately, get down to that.

At present fuel is about 70% of an airline operating costs according
to a recent report and as the previous CEO of Qantas said at over
$100 a barrel airline's business plans fail.

We don't have high speed trains because the government deliberately
ignores peak oil, just does not want to know.
As a result they will not spend money on straightening out the
Sydney Melbourne line. That line was laid before diesel powered
earth moving machines, horses and scoops were used, so it goes around
the big hills.
They prefer roads for cars and trucks.

If the world oil production falls this year and a tightness predicted
by the IEA occurs then I give the airlines three to five years before
a large number of bankruptcies happen.
Ten, years and no commercial airlines will remain in operation.
VIP flights will all that will be seen overhead.