Saturday, December 1, 2007


By Gary W. Harding
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 predictions 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 population 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 three surges and second, that the time between surges has dramatically shortened (Deevey, 1960).
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 been 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 (-4.6, 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 600,000 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 begun, 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.
Figure 2. Greenhouse Gases and Mean Global Temperature (Greenhouse gas concentrations and mean global temperature verses time). Time scale same as in Fig. 1. Gas-concentration data have been normalized to the 0 to 1 scale on left: CO2 (squares) - 190 to 430 ppm; CH4 (triangles) - 600 to 2400 ppb; N2O (diamonds) - 280 to 340 ppb. Mean global temperature (circles) plotted relative to oC on right. Vertical dashed-line at 1995, horizontal dotted line at maximum CO2 concentration and global temperature over human history before 1990. Filled and open symbols same as in Fig. 1. Projections in short-term future are based upon continuation at current growth rates. (Data measured from graphs in Gribbin, 1990 and Khalil and Rasmussen, 1992).
Mean-global-temperature (MGT) is related to the concentration of greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapor and other trace gases) in the atmosphere. The most prevalent greenhouse gas is carbon dioxide (CO2). It has been shown that there is a strong relation between the atmospheric concentration of CO2 and MGT over the last 160,000 years (Gribbin, 1990). It has been suspected that the burning of fossil fuels and the clearing of land has reached such proportions that these activities have precipitated a significant increase in atmospheric CO2 concentration. The concentrations of greenhouse gases in the atmosphere have been directly measured since about 1960 and have been determined over the more distant past from air-bubbles trapped in old Antarctic, Greenland and Siberian ice and from deep-sea sediments. Mean-global-temperature has also been measured directly over the last few decades. Estimates of global temperature in the distant past have been deduced from a variety of sources. From these data, the relation among atmospheric greenhouse-gas concentrations, MGT and time is illustrated in Figure 2.
The time scale in Figure 2 is the same as that in Figure 1. Because CO2, methane (CH4) and nitrous oxide (N2O) concentrations have different scales, the data have been normalized on a 0 to 1 scale on the left. For CO2 (squares; Gribbin, 1990), 0 is equivalent to 190 parts per million (ppm) and 1 is equivalent to 430 ppm. For CH4 (triangles; R. Cicerone in Gribbin, 1990), the range is 600 to 2400 parts per billion (ppb). For N2O (diamonds; Khalil and Rasmussen, 1992), the scale is 280 to 340 ppb. Mean global temperature (circles; Gribbin, 1990) has been graphed relative to the degrees-centigrade scale on the right. The vertical dashed-line is the same as that in Figure 1. The horizontal dotted-line is the highest CO2 concentration and temperature in human history before 1990. Greenhouse-gas concentrations and MGT in the short-term future are based upon continuation at the current growth rates. This will be justified in another context below.
Figure 3. Population and Global Warming (CO2 concentration and mean global temperature verses log-population) CO2 concentration (circles) and mean global temperature (squares) plotted relative to their absolute scales, ppm on the left and oC on the right, respectively. Vertical dashed line at 1995. (Data from Figs. 1 and 2)
It is clear that the concentrations of all three gases have increased exponentially since 1950 (-1.4, Fig. 2) and that MGT has done so since 1975. Carbon dioxide concentration began to rise in conjunction with the use of fossil fuels after 1850. Although methane comes from a variety of sources, including plant decay, termites and bovine flatulence, CH4 concentration rises at the same time as CO2. This is probably due to its association with fossil-fuel production. Nitrous oxide concentration does not begin to rise until 1950. At this time, the use of human-made fertilizers and internal-combustion-engine exhaust increased dramatically. Ten thousand years ago (-4, Fig. 2), MGT increased substantially just as the agricultural revolution got started. Over the previous 200,000 years, the ecosystem was dominated by ice-ages. Projected MGT in 2025 (1.7, Fig. 2) is about 17oC, 1.5oC higher than in human history prior to 1990.
We have seen in Figures 1 and 2 that recent population, atmospheric greenhouse-gas concentrations and MGT have grown exponentially over about the same time-course. The relation of CO2 and MGT relative to population size can be observed by graphing these variables as above. Figure 3 shows this graph, where the log of population replaces log-time and CO2 concentration (circles) and MGT (squares) are plotted relative to their absolute scales, ppm on the left and oC on the right, respectively. The vertical dashed-line denotes 1995, as in Figures 1 and 2. When the population reached 4 billion in 1975, the converging relation between population and the other two variables becomes apparent.
The magnitude of the relations in Figures 2 and 3 can be determined by calculating the correlation coefficient between pairs of variables. Table 1 lists these coefficients for the population, greenhouse-gas concentration and MGT variables that we have been examining. The coefficients for the relations during the industrial revolution, 1800 through 1994, are above the diagonal of the table. The coefficients since 2000 years ago through 1994 are below the diagonal. Over the past 2000 years, there is a nearly perfect correlation between the concentration of greenhouse gases and population and between the greenhouse gases themselves. However, the correlations between both population and greenhouse-gas concentrations and MGT (bottom row) are not as strong. After 1800, the latter correlations increase to near perfection (rightmost column). The conclusion from the graphs and table is that there is a strong relationship among population size since 1800, greenhouse-gas concentrations and MGT.
TABLE 1. Correlation coefficients among population size, atmospheric greenhouse-gas concentrations and mean global temperature (1800 through 1994 above the top-left to bottom-right diagonal, n=10; 2000 years ago through 1994 below the diagonal, n=15).
Pop CO2 CH4 N2O Temp
Pop .996 .984 .977 .916
CO2 .990 .994 .974 .942
CH4 .991 .992 .949 .945
N2O .959 .943 .942 .932
Temp .718 .716 .728 .829
Determining that there is a strong relation between population size and global warming does not tell us what the underlying mechanisms are. However, documentation of the relationship between human activities and the release of greenhouse gases produces a strong inference that population size and global warming are closely related (Gribbin, 1990).
Forecasting the future is risky business. Growth rates for greenhouse-gas concentrations and MGT could decline from those at present due to unanticipated innovations or natural events. For example, volcanoes can spew enough ash into the atmosphere to block sunlight and temporarily reduce MGT slightly. However, short-term continued growth at current rates is probably an underestimate. Although population growth rate has slowed, the population is still growing. The dominating factor is that per-capita energy and resource consumption rates are increasing much faster than the population. This is not only due to anticipated increases in standards of living in underdeveloped countries, but also to future increases in the demand for energy in the developed countries (e.g., air conditioning) as summer temperatures rise. Since most of the energy will come from fossil fuels, at least for the next few decades, we can expect the atmospheric concentrations of greenhouse gases and MGT to rise in the short-term future at a faster rate than they have recently. As MGT rises, water vapor, another greenhouse component, will become a more and more significant factor due to increased evaporation.
Although a 1.5oC increase in MGT above where we were in 1990 (1990 to 2025 in Fig. 2) does not seem like much of a change, it is enough to precipitate major changes in climate. A 1.5oC drop in MGT from where we were in 1990, for example, would put the ecosystem on the verge of an ice-age. Already, there is a suspicion that, since 1975, the persistent El Nino is the first sign of the relation between global warming and climate (Kerr, 1994). As MGT increases further, we can expect more frequent and severe hurricanes and perpetual summertime droughts in many places, particularly in the US Midwest. Paradoxically, more intense winter storms will occur in some places and climatic conditions for agriculture will improve in some areas, such as in Russia (Gribbin, 1990; Bernard, 1993).
There has been considerable debate over the ecosystem's carrying capacity for humans. If we define that carrying capacity as the level that the ecosystem can support without changing state more than it has over the duration of human history, then Figures 2 and 3 indicate that we exceeded that capacity in 1975. This is the point in time where exponential growth began to push MGT along a path which has taken it outside the previous range. This does not necessarily mean that humans could not survive if MGT is about 2oC higher than it has ever been in their history. However, we will have to adapt to a radically different climate pattern and, if MGT goes any higher than that, there could be disastrous problems.
If MGT continues to increase beyond 2025 to 4oC above that in 1990, high-northern-latitude temperatures could be as much as 10oC higher than at the equator. The Arctic ice-cap would begin to melt and the permafrost under the tundra would start thawing out. As a consequence, a thick layer of rotting peat would contribute further to atmospheric CO2 and CH4 concentrations (Gribbin, 1990). With a number of human-made and natural positive-feedback elements in operation simultaneously, a threshold could be crossed (Meyers, 1995; Overpeck, 1996). Are these risks that we should be willing to take for the sake of short-term gains?
Bernard, H. W. Jr., "Global Warming Unchecked", Indiana Univ. Press, Bloomington, 1993
Broecker, W., Science, 189:460, 1975
Deevey, E. S., Scientific American, 203:195, 1960
Gibbons, A. , Science, 262:27, 1993
Gribbin, J. , "Hothouse Earth", Grove Weidenfeld, New York, 1990
Kerr, R. A., Science, 266:544, 1994
Khalil, M. A. K. and R. A. Rasmussen, J. Geophys. Res., 97:4651, 1992
"The World Almanac", Pharos, New York, 1992 - 1995
Meyers, N. Science 269:358, 1995
Overpeck, J. T. Science, 271:1820, 1996
Post Script
After this document was written (about a 2 years ago), two books came out which provide much more detail relevant to some of these issues:
HOW MANY PEOPLE CAN THE EARTH SUPPORT? by Joel E. Cohen; Norton, 1995.
DIVIDED PLANET: THE ECOLOGY OF RICH AND POOR by Tom Athanasiou; Little Brown, 1996.
Both are superbly done and provide a much more comprehensive and up to date treatment of the population and economic topics included here.
Recent evidence (Mora et al.; SCIENCE 271:1105, 1996) indicates that the possibility of a "greenhouse runaway" on Earth is much more remote than indicated at the end of the previous version of this document. Therefore, the former apocalyptic ending has been changed. Although the data presented points to a catastrophic conclusion, this was (perhaps) an overstatement of the case.

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