There is a very interesting map of study done on human carbon production vs usage in New Scientist 26/06/04 p9 . The basis of our civilization is vegetation grown in the ground . All else is dependant on it . This study shows that the global average is about 20% . We consume or use about one fifth of the planetary vegetation growth . This is not spread evenly : a large number of people in a dry area will consume a higher percentage of new growth ( crops,firewood) than in lush areas . This uneven distribution is of great interest . Areas in Africa where this happens are prone to political instability ( to put it mildly ) .
It is the typical volkewanderung pattern , but there are no empty spaces . Civil war of the genocidal variety ususally results . Actual examples are the hot spots in a band stretching from the northern parts of Nigeria,Ghana ,Liberia through to the Sudan , Ruanda , Eastern Congo , Ethiopia . Note the recent genocides:they are not madness , but a logical solution to a scarcity of resources . We have seen their like before in Europe's volkewanderungs.
The critical limit in a primitive society seems to be about 30% . About 60% of plant mass is used as fuel and the rest as food .
Note that Gorillas and other plant eaters need a huge gut to keep in the length of intestines necessary to digest plant matter. Humans first lost the big gut by eating meat , then exploded in numbers when they learned to use an external gut : the cooking pot .
Humans cannot survive in the wild without an external gut . (For instance , a human will have to eat about 10 kg of raw vegetation each and every day (that is more than twenty pounds) to just have enough calories to keep going .
An interesting point is that at first the good cooking vessels would have been fragile ceramics . This tied groups to a hearth and family . When copper and bronze was worked , it freed the surplus males from the hearth . Note how Homer extols copper and bronze bowls . They were essential to survival .The second-last thing a Mongol would give up is his cooking bowl . (The last thing was his horse) .
Where fuel is scarce and expensive , wok-style cooking evolved . Lots of
(cheap) human preparation and short , high-temperature cooking ( to break the stubborn chemical bonds - thats why you cook) .
Europe initially had a lot of fuel (the forests) , but as they were consumed , things like pittas and pizzas evolved. Old style peasant slow cooking is now very expensive in terms of fuel and attention ( compared to fast fries).
A deep covered pan of hot oil can cook the food for a small army using very little fuel . The second-last thing a Roman legion abandoned was its cooking pots . (The last thing was the standard . ) Legions were judged on the standard of their cooking .
Bread yeasts were jealously guarded . The Lightning got its name from its spicy food . A good (make that a superb) cook had it made . He never had to fight , had the best of the camp girls and could retire just about anywhere he liked . As usual with really good things , you don't hear about them . Why spoil a good thing .
To get back to the main argument:
If humans consume more than 30% of yearly vegetation for food, they must have an alternative fuel or food source . They do this by trade . As long as there are areas on the planet where the usage of vegetation is less , then manufactured items or raw materials (like oil) can be used to smooth the demand .
But areas like India , China , Saudi-Arabia , Israel , Western Europe are consuming far more vegetation than they produce . They import by paying with manufactured goods . (Oil is a manufactured good)
This is analogous to an entropy system . As long as there is an area where usage is say 20% and an industrialised state where usage is 2000% , trade can flow. But there is a cut-off point . The poor area cannot be be stripped by more than 100%.
This has already happened with cod , for instance .
Most of the population of the planet still depends on firewood for cooking , and is thus trapped in the 60% zone . If they can't cook they die . If they haven't food ,they die .
The grandmother gene kicks in and does what it can to ensure the survival of its grandchildren . Interesting to note that one can predict genocidal civil wars for areas like Northern Liberia,Ghana,Nigeria,Ruanda ,Sudan,Saudi,Israel,Western India,Southern China,South-Eastern Europe .
The shit will really hit the fan if the global average of vegetation usage hits 30% .(Compared to the present 20%) It then means that there is no reason for commerce. One of the parties will die if they trade .
If global trade diminishes significantly , these unstable regions will explode . They will have nothing to eat .
Something similar happened at the end of the Bronze age . They despoiled their environment and then a dry spell overwhelmed their transport capability.The result was the Sea Peoples ,as desperate a bunch of suburbanites as ever survived the collapse of the mall . According to archeological evidence , the mortality rate was 90% . The Med seaboard never recovered . Ever .