The CO2 theory of the agricultural revolution
April 14, 2026
Farming has been the most common occupation on earth for all of written history. The technological progress of the modern world may owe more to agriculture than to any other cultural development, since reliable harvests create the leisure time necessary for innovation. Yet it began only around 10,000 years ago, very recently in the long history of Homo sapiens.
Agriculture emerged independently in numerous places around the world between 10,000 and 3,000 years ago. Because these populations were largely isolated, agriculture cannot be explained by a sudden genetic change that enabled humans to understand plant cultivation. In any case, humans have been exceptionally intelligent for much longer than 10,000 years. Even a child can observe that if a piece of fruit drops from a tree and is left to rot on the ground, it will eventually produce another tree. Hunter-gatherers, immersed in a world of plants, undoubtedly knew that water and sun were the key ingredients for plant growth, and their intuitions about plant growth were probably much stronger than those of modern humans. They likely had immense knowledge of the nutritional and medicinal qualities of hundreds of plant species in their ecosystems, and they would have been well aware of the conditions under which they grew and thrived. Plant cultivation was likely common knowledge among most pre-agricultural populations.
So why, then, did agriculture emerge when it did? The simplest answer is that it was not a sudden discovery, but a shift in behavior — a choice shaped by changing conditions.
There is evidence of plant cultivation as early as 23,000 years ago. Yet these early efforts did not lead to sustained or widespread farming, suggesting that agriculture was not initially a major success.
Things began to change between 20,000 and 10,000 years ago. During this period, atmospheric carbon dioxide (CO2) levels rose by roughly 50 percent, from about 185 to 270 parts per million (ppm). CO2 is a limiting reactant for plant photosynthesis, and hundreds of studies have shown that plants generally grow larger, more quickly, and more water-efficiently under conditions of higher atmospheric CO2. Today, it's not uncommon to directly pump CO2 into horticultural greenhouses as a form of fertilizer. Many plants are particularly sensitive to atmospheric CO2 changes in the 170-300 ppm range, including all the earliest domesticated crops. The lower end of this range is near the lowest level at which most land plants evolved, and many plants may have been near carbon-starvation levels 20,000 years ago, with significantly limited photosynthesis as a result.
As CO2 was rising, the most recent glacial period was ending. This brought longer growing seasons and more rain, so plant life was more abundant. In regions with abundant edible plants, this may have simply made foraging easier. But elsewhere, a global megafauna extinction was underway, possibly driven by overhunting. As hunting was becoming less reliable, farming was becoming more viable, creating a practical incentive for many populations to augment their food resources with agriculture[1].
Agriculture, then, may not have been adopted because humans suddenly discovered how to farm, but because environmental pressures made it increasingly worthwhile to do so.
Notes:
- Comparable climate conditions existed 135,000 years ago during the end of the penultimate glacial period, with rising CO2 and warming temperatures. Though no megafauna extinction occurred then, the environmental conditions for agriculture were present. It's possible that future discoveries will reveal evidence of limited plant cultivation from this earlier period.