Currently, CO2 levels are slightly above 400 ppm.
Below 200 ppm plants plants are essentially starved for CO2.
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During the past 100,000 years, human societies have witnessed the vast change in climate that has occurred as we have transitioned from a glacial period that ended about 20,000 years ago, into the current interglacial period. During the early stages of this period, human lived as hunter/gatherers, relying on a diet that was very heavily weighted towards meat from a wide variety of wild animals, but also included eggs, nuts, fruits and grasses, to the past 10,000 years or less, when agriculture became a much more dominate feature in society and allowed human populations to remain in the one area and create towns and small hamlets. With agriculture came the domestication of many of the wild meat sources.
As the agricultural model was perfected, much larger town and cities were created, society became more complex and the overall standard of living increased. In the past, it was assumed that societal changes were the prime driver in the change from the hunter/gatherer life style to the agricultural based society, but with the recent research into the change in the composition of the atmosphere over time from the various ice core research programs, another possibility emerges. From the ice core data we can now track the quantity of CO2 present in the atmosphere at various points in history, back to about 500,000 years ago. CO2 is one of the key ingredients in the photosynthetic process.
Numerous studies have shown the benefits from increasing CO2 levels on plant growth, but until recently, few studies have been conducted to understand how plants respond to the lower CO2 concentrations that were the norm during the glacial periods. Currently, CO2 levels are slightly above 400 ppm. However, during the last glacial period, ending about 20,000 years ago, CO2 levels were as low as 170 to 180 ppm. Below 200 ppm marks the very lowest level for CO2 since plants evolved and at these levels most plants are essentially starved for CO2.
The purpose of this paper will be to show that it is very likely that it was the increase in the CO2 content of the atmosphere, to levels above 250 ppm, that created the conditions for the plants that served as food sources to humans to thrive, and that made it worthwhile for humans to spend the time and energy in cultivating crops, which then allowed for the creation of cities and much more complex social orders. Prior to reaching this turning point, it was more efficient for humans to let wild animal species forage for the more limited vegetative offerings available, convert the plant material to protein and fat, and harvest the animals. Until about 8000 years ago, the human diet was composed primarily of meat from wild animals, supplemented with nuts, berries, mushrooms, and fungi in the local area. The transition from hunter/gatherer to agriculturalist was not necessarily a one-way process. Climatic changes could have necessitated a return to past methods when necessary for survival, such as occurred during the Younger Dryas.
However, as the following graphs will show, the viability of numerous plants drops considerably as CO2 concentrations diminish. Biomass and seed yield were only about 40% of current plant production rates during the glacial periods; levels that would make agriculture an inefficient use of time and energy in those early cultures.
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