Guest essay by Don Healy
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.
To clarify, there are three different photosynthetic pathways: C3, C4 and CAM. For the purposes of this discussion, only the C3 and C4 pathways are of concerned. CAM is utilized by cacti and similar plants that are not a large component of the human diet. The C3 process evolved first, over 400 million years ago when CO2 levels where many times current levels and are utilized by about 85% of the existing plant species today. The C4 process evolved much more recently, about 30 to 40 million years ago, when CO2 levels had dropped to levels still above todays levels, but much lower than when C3 plants evolved.
It is believed that the C4 process was a natural adaptation to lower atmospheric CO2. With CO2 levels rising from levels of 180 ppm during the last glaciation to about 400 ppm currently, the C3 plants show a larger response, but the C4 plants also benefitted to a considerable degree, due to increased drought resistance and mycorrhizal colonization of plant roots. Examples of C3 plants are beans, rice, wheat, barley, rye, oats, soybean, peanut, cotton, sugar beets, spinach, potatoes, all woody trees and most lawn grasses. The C4 plants include corn, sugarcane, sorghum, millet, Bermuda grass and poa.
A more graphic display of the effect of various CO2 levels on plant growth follows:
Higher concentrations of CO2 also have another beneficial effect upon plant growth. As CO2 levels increase up to certain limits, plants are able to use water more efficiently. The reason for this is that the stomata, the pores on the leaves of plants, must remain open longer at low CO2 concentrations to allow sufficient CO2 to enter the plant. CO2 is one of crucial ingredients in the photosynthetic process. While the stomata are open, water vapor escapes as transpiration. The longer the stomata remain open to allow sufficient CO2 to enter the leaf for photosynthesis to occur, the more water escapes. Thus, plants are more drought resistant at higher CO2 concentrations.
Many plants species evolved at much higher CO2 concentrations than we are experiencing currently. The predecessors to Gymnosperms, or evergreens, evolved about 360 million years ago when CO2 levels were about 4000 ppm, 10 times today’s levels. The Angiosperms, or flowering and deciduous trees, evolved about 160 million years when CO2 levels were about 2200 ppm, over five times current levels. So at the levels experienced during the last glacial period of 180 ppm, the plant kingdom was clearly under great stress. We are all quite aware that the survival of the more advanced members of the animal kingdom which includes humans, are clearly dependent upon the well-being of the plant kingdom. So it would appear that during recent glacial periods, much of the life on earth was in jeopardy.
Will rising CO2 levels enhance the growth of many plant species existing today? The answer is clearly yes. A note of caution is warranted here in that the burning of coal, oil and wood have been responsible for most of the increase in CO2 concentration in the modern era, and the burning of these fuels also releases many other toxic substances and pollution into the atmosphere, such as soot, nitric oxides, sulfur dioxides, and trace amounts of radioactive material. However, it would appear that within certain limits the increase in atmospheric CO2 has been beneficial to date, and very likely will continue to be for some time into the future. Please examine the graphs on the next page.
Greening of the Globe: If the studies cited above are correct, then it would stand to reason that we should be able to detect a growth response in the vegetated portions earth and possibly see an expansion in the overall area occupied by vegetation. The abstract below indicates that this is occurring.
“Global environmental change is rapidly altering the dynamics of terrestrial vegetation, with consequences for the functioning of the Earth system and provision of ecosystem services1, 2. Yet how global vegetation is responding to the changing environment is not well established. Here we use three long-term satellite leaf area index (LAI) records and ten global ecosystem models to investigate four key drivers of LAI trends during 1982–2009. We show a persistent and widespread increase of growing season integrated LAI (greening) over 25% to 50% of the global vegetated area, whereas less than 4% of the globe shows decreasing LAI (browning). Factorial simulations with multiple global ecosystem models suggest that CO2 fertilization effects explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%). CO2 fertilization effects explain most of the greening trends in the tropics, whereas climate change resulted in greening of the high latitudes and the Tibetan Plateau. LCC contributed most to the regional greening observed in southeast China and the eastern United States. The regional effects of unexplained factors suggest that the next generation of ecosystem models will need to explore the impacts of forest demography, differences in regional management intensities for cropland and pastures, and other emerging productivity constraints such as phosphorus availability.”
During the 400 million or so years that plants have existed on earth, the average CO2 level has been about 1100 ppm, with a high near 4000 ppm when gymnosperms first evolved about 360 million years ago,, to a low of 180 ppm during the last glacial period. Agriculture did not become a practical enterprise for humans until 8,000 to 10,000 years ago when CO2 levels finally moved above 250 ppm during the current interglacial period. At today’s levels, just over 400 ppm we are seeing a significant increase in both the growth of individual plants and in their global distribution. The last 2.5 to 3 million years comprise an ice age in which the pattern has been 100,000 years of glacial advance, interspersed with interglacial periods of 10,000 to 20,000 years, give or take.
During the recent glacial advance, when CO2 levels dropped to 180 ppm, mark the very lowest levels of CO2 during the past 500 million years, and probably much longer. It should be noted, that below levels of 180 ppm, things become extremely dire. Were we to return to levels much below 250 ppm we would probably lose 70 to 80 percent of the human population to starvation and the societal turmoil that would ensue as we have to forgo the benefits of agriculture and go back to being foragers.
The IPCC warns us that at CO2 levels above 300 ppm we face dire consequences. It appears that the quandary we are facing is this: Do we allow CO2 levels to rise, face a modestly warming earth, but one with abundant plant growth, or try to lower CO2 levels which could have much more disastrous consequences for mankind? Ironically, if past geologic history is any indication, we could be approaching the end of the current interglacial and will then have to deal with the glacial narrative.
So, the question I put to you is this: After reviewing the information above, and perhaps doing your own research, what would be the ideal concentration of CO2?