The relationship between humans and the state of the ecosystem is not only dependent upon how many people there are, but also upon what they do. When there were few people, the dominant factors controlling ecosystem state were the natural ones that have operated for millions of years. The human population has now grown so large that there are concerns that they have become a significant element in ecosystem dynamics. One of these concerns is the relationship between human activities and climate, particularly the recent observations and the redictions of global warming, beginning with the alarm sounded by W. Broecker (1975).
The relationships among humans, their activities and global temperature can be assessed by making the appropriate measurements and analyzing the data in a way that shows the connections and their magnitudes. Human population can be closely estimated and the consequences of their activities can be measured. For example, the volume of carbon dioxide, methane and nitrous oxide emissions is an indicator of human’s energy and resource consumption.
An examination of opulation size, atmospheric concentrations of these gases and global temperature relative to time and with respect to each other is presented here to demonstrate the relations among these factors. Many of us have seen linear graphs of human population showing the enormous growth in the last two centuries. However, significant changes in population dynamics are lost in the exponential growth and long time scales. If the data are replotted on a log-population by log-time scale, significant population dynamics emerge.
First, it is apparent that population growth has occurred in hree surges and second, that the time between surges has dramatically shortened (Deevey, 1960). <Picture>Figure 1. Population (Log-population verses log-time since 1 million years ago). Time values on x-axis, ignoring minus sign, are powers of 10 years before and after 1975 (at 0). Vertical dashed-line at 1995. Filled circles for known values are to left of 1995 and open circles on and to right of 1995 are for projected values. (Data updated from Deevey, 1960). Deevey’s 1960 graph has been brought up to date in Figure 1 to reflect what has een learned since then. The data have been plotted relative to 1975 with negative values before 1975 and positive values thereafter. The reason for this will become clear below. The values of the time scale, ignoring the minus signs, represent powers of 10 years. It has been argued that a population crash occurred about 65,000 years ago (-4. 8, Fig. 1), presumably due to the prolonged ice-ages during the preceding 120,000 years (Gibbons, 1993).
Humans came close to perishing and Neanderthal became extinct. However, by 50,000 years ago (-4. , Fig. 1), humans had generated population mini-explosions all around the planet. Deevey’s data for population size since 500 years ago have been replaced with more recent estimates taken from The World Almanac, (1992 – 1995) including population projections out to 2025. A vertical dashed-line has been placed at 1995. Filled symbols for the known values are to the left of it and open symbols on and to the right of it are for values projected into the short-term future.
The first surge coincides with the beginning of the cultural revolution about 00,000 years ago, interrupted by the population crash 65,000 years ago. Population size rebounded 50,000 years ago and then growth slowed considerably. The second surge began with the agricultural revolution about 10,000 years ago and was followed by slow growth. Deevey argued that moving down the food chain was the underlying cause of this large and rapid spurt. The timing of the present surge matches the rise of the industrial-medical revolution 200 years ago. A relation between innovation and population growth is embedded in the log-log plot.
There was rapid growth at the start of each surge. Then, growth rate slowed as people adapted to the precipitating innovations. Each surge increased the population more than 10-fold. It appears that we are nearing the end of the present surge as recent growth rates have declined. After the initial spurt, subsequent innovations did not perpetuate growth rates. The only significant innovations were those that produced the next surge. However, accumulated innovations during the surges may have played a role in the eventual decline in population growth rates.
Starting with high birth and death rates, death rate declines and longevity increases, but birth rates stay high. Some time later, birth rates decline so that eventually, net births minus deaths produces slow growth. The result is a spurt in population size. When referring to the industrial revolution, this phenomenon has been called the “demographic transition”. It appears that this dynamic may have occurred twice before. The decreases in time between surges suggests that, if past behavior is the best predictor of future behavior, we are due for another surge.
It may have already egun, as indicated by the upturn in the projections at the right end of the curve in Figure 1. What might the basis for another surge be? One can think of several possibilities, including the “green revolution” and the “global economy”. A dominant element in past surges has been innovations in energy use (e. g. , fire, descending the food-chain, beasts of burden, fossil fuels, high-energy agriculture). Thus, the development of an abundant and cheap energy source would have a profound effect. Another 10-fold (or more) surge would produce a population of 60 to 125 billion.