Environmental Effects of Increased Atmospheric Carbon Dioxide
Oregon Institute of Science and Medicine, 2251 Dick George Rd., Cave Junction, Oregon 97523 firstname.lastname@example.org
George C. Marshall Institute, 1730 K St., NW, Ste 905, Washington, DC 20006 email@example.com January 1998
A review of the research literature concerning the environmental consequences of increased levels of atmospheric carbon dioxide leads to the conclusion that increases during the 20th Century have produced no deleterious effects upon global weather, climate, or temperature. Increased carbon dioxide has, however, markedly increased plant growth rates. Predictions of harmful climatic effects due to future increases in minor greenhouse gases like CO2 are in error and do not conform to current experimental knowledge.
To be sure, CO2 levels have increased substantially since the Industrial Revolution, and are expected to continue doing so. It is reasonable to believe that humans have been responsible for much of this increase. But the effect on the environment is likely to be benign. Greenhouse gases cause plant life, and the animal life that depends upon it, to thrive. What mankind is doing is liberating carbon from beneath the Earth's surface and putting it into the atmosphere, where it is available for conversion into living organisms.
The annual cycles in figure 1 are the result of seasonal variations in plant use of carbon dioxide. Solid horizontal lines show the levels that prevailed in 1900 and 1940 (2). The magnitude of this atmospheric increase during the 1980s was about 3 gigatons of carbon (Gt C) per year (3). Total human CO2 emissions primarily from use of coal, oil, and natural gas and the production of cement are currently about 5.5 GT C per year.
To put these figures in perspective, it is estimated that the atmosphere contains 750 Gt C; the surface ocean contains 1,000 Gt C; vegetation, soils, and detritus contain 2,200 Gt C; and the intermediate and deep oceans contain 38,000 Gt C (3). Each year, the surface ocean and atmosphere exchange an estimated 90 Gt C; vegetation and the atmosphere, 60 Gt C; marine biota and the surface ocean, 50 Gt C; and the surface ocean and the intermediate and deep oceans, 100 Gt C (3).
So great are the magnitudes of these reservoirs, the rates of exchange between them, and the uncertainties with which these numbers are estimated that the source of the recent rise in atmospheric carbon dioxide has not been determined with certainty (4). Atmospheric concentrations of CO2 are reported to have varied widely over geological time, with peaks, according to some estimates, some 20-fold higher than at present and lows at approximately 18th-Century levels (5).
The current increase in carbon dioxide follows a 300-year warming trend: Surface and atmospheric temperatures have been recovering from an unusually cold period known as the Little Ice Age. The observed increases are of a magnitude that can, for example, be explained by oceans giving off gases naturally as temperatures rise. Indeed, recent carbon dioxide rises have shown a tendency to follow rather than lead global temperature increases (6).
There is, however, a widely believed hypothesis that the 3 Gt C per year rise in atmospheric carbon dioxide is the result of the 5.5 Gt C per year release of carbon dioxide from human activities. This hypothesis is reasonable, since the magnitudes of human release and atmospheric rise are comparable, and the atmospheric rise has occurred contemporaneously with the increase in production of CO2 from human activities since the Industrial Revolution.
For example, about 300 years ago, the Earth was experiencing the ''Little Ice Age.'' It had descended into this relatively cool period from a warm interval about 1,000 years ago known as the ''Medieval Climate Optimum.'' During the Medieval Climate Optimum, temperatures were warm enough to allow the colonization of Greenland. These colonies were abandoned after the onset of colder temperatures. For the past 300 years, global temperatures have been gradually recovering (11). As shown in figure 2, they are still a little below the average for the past 3,000 years. The human historical record does not report ''global warming'' catastrophes, even though temperatures have been far higher during much of the last three millennia.
What causes such variations in Earth's temperature? The answer may be fluctuations in solar activity. Figure 3 shows the period of warming from the Little Ice Age in greater detail by means of an 11-year moving average of surface temperatures in the Northern Hemisphere (10). Also shown are solar magnetic cycle lengths for the same period. It is clear that even relatively short, half-century-long fluctuations in temperature correlate well with variations in solar activity. When the cycles are short, the sun is more active, hence brighter; and the Earth is warmer. These variations in the activity of the sun are typical of stars close in mass and age to the sun (13).
Figure 4 shows the annual average temperatures of the United States as compiled by the National Climate Data Center (12). The most recent upward temperature fluctuation from the Little Ice Age (between 1900 and 1940), as shown in the Northern Hemisphere record of figure 3, is also evident in this record of U.S. temperatures. These temperatures are now near average for the past 103 years, with 1996 and 1997 having been the 42nd and 60th coolest years.
Especially important in considering the effect of changes in atmospheric composition upon Earth temperatures are temperatures in the lower troposphere at an altitude of roughly 4 km. In the troposphere, greenhouse-gas-induced temperature changes are expected to be at least as large as at the surface (14). Figure 5 shows global tropospheric temperatures measured by weather balloons between 1958 and 1996. They are currently near their 40-year mean (15), and have been trending slightly downward since 1979.
Since 1979, lower-tropospheric temperature measurements have also been made by means of microwave sounding units (MSUs) on orbiting satellites (16). Figure 6 shows the average global tropospheric satellite measurements (17,18) the most reliable measurements, and the most relevant to the question of climate change.
Figure 7 shows the satellite data from figure 6 superimposed upon the weather balloon data from figure 5. The agreement of the two sets of data, collected with completely independent methods of measurement, verifies their precision. This agreement has been shown rigorously by extensive analysis (19, 20).
While tropospheric temperatures have trended downward during the past 19 years by about 0.05 ºC per decade, it has been reported that global surface temperatures trended upward by about 0.1 ºC per decade (21, 22). In contrast to tropospheric temperatures, however, surface temperatures are subject to large uncertainties for several reasons, including the urban heat island effect (illustrated below).
During the past 10 years, U.S. surface temperatures have trended downward by minus 0.08 ºC per decade (12) while global surface temperatures are reported increased by plus 0.03 ºC per decade (23). The corresponding weather-balloon and satellite tropospheric 10-year trends are minus 0.4 ºC and minus 0.3 ºC per decade, respectively.
Disregarding uncertainties in surface measurements and giving equal weight to reported atmospheric and surface data and to 10 and 19 year averages, the mean global trend is minus 0.07 ºC per decade.
In North America, the atmospheric and surface records partly agree (20 and figure 8). Even there, however, the atmospheric trend is minus 0.01 per decade, while the surface trend is plus 0.07 ºC per decade. The satellite record, with uniform and better sampling, is much more reliable.
The computer models on which forecasts of global warming are based predict that tropospheric temperatures will rise at least as much as surface temperatures (14). Because of this, and because these temperatures can be accurately measured without confusion by complicated effects in the surface record, these are the temperatures of greatest interest. The global trend shown in figures 5, 6 and 7 provides a definitive means of testing the validity of the global warming hypothesis.
When an increase in CO2 increases the radiative input to the atmosphere, how and in which direction does the atmosphere respond? Hypotheses about this response differ and are schematically shown in figure 9. Without the greenhouse effect, the Earth would be about 14 ºC cooler (25). The radiative contribution of doubling atmospheric CO2 is minor, but this radiative greenhouse effect is treated quite differently by different climate hypotheses. The hypotheses that the IPCC has chosen to adopt predict that the effect of CO2 is amplified by the atmosphere (especially water vapor) to produce a large temperature increase (14). Other hypotheses, shown as hypothesis 2, predict the opposite that the atmospheric response will counteract the CO2 increase and result in insignificant changes in global temperature (25-27). The empirical evidence of figures 5-7 favors hypothesis 2. While CO2 has increased substantially, the large temperature increase predicted by the IPCC models has not occurred (see figure 11).
The hypothesis of a large atmospheric temperature increase from greenhouse gases (GHGs), and further hypotheses that temperature increases will lead to flooding, increases in storm activity, and catastrophic world-wide climatological changes have come to be known as ''global warming'' a phenomenon claimed to be so dangerous that it makes necessary a dramatic reduction in world energy use and a severe program of international rationing of technology (29).
The computer climate models upon which ''global warming'' is based have substantial uncertainties. This is not surprising, since the climate is a coupled, non-linear dynamical system in layman's terms, a very complex one. Figure 10 summarizes some of the difficulties by comparing the radiative CO2 greenhouse effect with correction factors and uncertainties in some of the parameters in the computer climate calculations. Other factors, too, such as the effects of volcanoes, cannot now be reliably computer modeled.
Figure 11 compares the trend in atmospheric temperatures predicted by computer models adopted by the IPCC with that actually observed during the past 19 years those years in which the highest atmospheric concentrations of CO2 and other GHGs have occurred.
In effect, an experiment has been performed on the Earth during the past half-century an experiment that includes all of the complex factors and feedback effects that determine the Earth's temperature and climate. Since 1940, atmospheric GHGs have risen substantially. Yet atmospheric temperatures have not risen. In fact, during the 19 years with the highest atmospheric levels of CO2 and other GHGs, temperatures have fallen.
Global annual lower tropospheric temperatures as measured by satellite MSU between latitudes 83 N and 83 S (17, 18) plotted as deviations from the 1979 value. The trend line of these experimental measurements is compared with the corresponding trend line predicted by International Panel on Climate Change (IPCC) computer climate models (14).
Not only has the global warming hypothesis failed the experimental test; it is theoretically flawed as well. It can reasonably be argued that cooling from negative physical and biological feedbacks to GHGs will nullify the initial temperature rise (26, 30).
The reasons for this failure of the computer climate models are subjects of scientific debate. For example, water vapor is the largest contributor to the overall greenhouse effect (31). It has been suggested that the computer climate models treat feedbacks related to water vapor incorrectly (27, 32).
The global warming hypothesis is not based upon the radiative properties of the GHGs themselves. It is based entirely upon a small initial increase in temperature caused by GHGs and a large theoretical amplification of that temperature change. Any comparable temperature increase from another cause would produce the same outcome from the calculations.
At present, science does not have comprehensive quantitative knowledge about the Earth's atmosphere. Very few of the relevant parameters are known with enough rigor to permit reliable theoretical calculations. Each hypothesis must be judged by empirical results. The global warming hypothesis has been thoroughly evaluated. It does not agree with the data and is, therefore, not validated.
The new high in temperature estimated by NASA GISS after 1940 is not present in the radiosonde balloon measurements or the satellite MSU measurements. It is also not present in surface measurements for regions with comprehensive, high-quality temperature records (35). The United States surface temperature record (see figure 4) gives 1996 and 1997 as the 38th and 56th coolest years in the 20th century. Biases and uncertainties, such as that shown in figure 13, account for this difference.
The urban heat island effect is only one of several surface effects that can confound compiled records of surface temperature. Figure 13 shows the size of this effect in, for example, the surface stations of California and the problems associated with objective sampling. The East Park station, considered the best situated rural station in the state (37), has a trend since 1940 of minus 0.055 ºC per decade.
The overall rise of about plus 0.5 ºC during the 20th century is often cited in support of ''global warming'' (38). Since, however, 82% of the CO2 rise during the 20th century occurred after the rise in temperature (see figures 1 and 12), the CO2 increase cannot have caused the temperature increase. The 19th century rise was only 13 ppm (2).
In addition, incomplete regional temperature records have been used to support ''global warming.'' Figure 14 shows an example of this, in which a partial record was used in an attempt to confirm computer climate model predictions of temperature increases from green-house gases (41). A more complete record refuted this attempt (42).
Not one of the temperature graphs shown in figures 4 to 7, which include the most accurate and reliable surface and atmospheric temperature measurements available, both global and regional, shows any warming whatever that can be attributed to increases in green-house gases. Moreover, these data show that present day temperatures are not at all unusual compared with natural variability, nor are they changing in any unusual way.
Historical records show no acceleration in sea level rise in the 20th century (44). Moreover, claims that global warming will cause the Antarctic ice cap to melt and sharply increase this rate are not consistent with experiment or with theory (45).
Similarly, claims that hurricane frequencies and intensities have been increasing are also inconsistent with the data. Figure 16 shows the number of severe Atlantic hurricanes per year and also the maximum wind intensities of those hurricanes. Both of these values have been decreasing with time.
As temperatures recover from the Little Ice Age, the more extreme weather patterns that characterized that period may be trending slowly toward the milder conditions that prevailed during the Middle Ages, which enjoyed average temperatures about 1 ºC higher than those of today. Concomitant changes are also taking place, such as the receding of glaciers in Montana's Glacier National Park.
One reservoir that would moderate the increase is especially important. Plant life provides a large sink for CO2. Using current knowledge about the increased growth rates of plants and assuming a doubling of CO2 release as compared to current emissions, it has been estimated that atmospheric CO2 levels will rise by only about 300 ppm before leveling off (2). At that level, CO2 absorption by increased Earth biomass is able to absorb about 10 GT C per year.
As atmospheric CO2 increases, plant growth rates increase. Also, leaves lose less water as CO2 increases, so that plants are able to grow under drier conditions. Animal life, which depends upon plant life for food, increases proportionally.
Figures 17 to 22 show examples of experimentally measured increases in the growth of plants. These examples are representative of a very large research literature on this subject (49-55). Since plant response to CO2 fertilization is nearly linear with respect to CO2 concentration over a range of a few hundred ppm, as seen for example in figures 18 and 22, it is easy to normalize experimental measurements at different levels of CO2 enrichment. This has been done in figure 23 in order to illustrate some CO2 growth enhancements calculated for the atmospheric increase of about 80 ppm that has already taken place, and that expected from a projected total increase of 320 ppm.
As figure 17 shows, long-lived (1,000- to 2000-year-old) pine trees have shown a sharp increase in growth rate during the past half-century.
Figure 18 summarizes the increased growth rates of young pine seedlings at four CO2 levels. Again, the response is remarkable, with an increase of 300 ppm more than tripling the rate of growth.
Figure 19 shows the 30% increase in the forests of the United States that has taken place since 1950. Much of this increase is likely due to the increase in atmospheric CO2 that has already occurred. In addition, it has been reported that Amazonian rain forests are increasing their vegetation by about 34,000 moles (900 pounds) of carbon per acre per year (57), or about two tons of biomass per acre per year.
Figure 20 shows the effect of CO2 fertilization on sour orange trees. During the early years of growth, the bark, limbs, and fine roots of sour orange trees growing in an atmosphere with 700 ppm of CO2 exhibited rates of growth more than 170% greater than those at 400 ppm. As the trees matured, this slowed to about 100%. Meanwhile, orange production was 127% higher for the 700 ppm trees.
Trees respond to CO2 fertilization more strongly than do most other plants, but all plants respond to some extent. Figure 21 shows the response of wheat grown under wet conditions and when the wheat was stressed by lack of water. These were open-field experiments. Wheat was grown in the usual way, but the atmospheric CO2 concentrations of circular sections of the fields were increased by means of arrays of computer-controlled equipment that released CO2 into the air to hold the levels as specified.
While the results illustrated in figures 17-21 are remarkable, they are typical of those reported in a very large number of studies of the effect of CO2 concentration upon the growth rates of plants (49-55).
Figure 22 summarizes 279 similar experiments in which plants of various types were raised under CO2-enhanced conditions. Plants under stress from less-than-ideal conditions – a common occurrence in nature – respond more to CO2 fertilization. The selections of species shown in figure 22 were biased toward plants that respond less to CO2 fertilization than does the mixture actually covering the Earth, so figure 22 underestimates the effects of global CO2 enhancement.
Figure 23 summarizes the wheat, orange tree, and young pine tree enhancements shown in figures 21, 20, and 18 with two atmospheric CO2 increases – that which has occurred since 1800 and is believed to be the result of the Industrial Revolution and that which is projected for the next two centuries. The relative growth enhancement of trees by CO2 diminishes with age. Figure 23 shows young trees.
Clearly, the green revolution in agriculture has already benefited from CO2 fertilization; and benefits in the future will likely be spectacular. Animal life will increase proportionally as shown by studies of 51 terrestrial (63) and 22 aquatic ecosystems (64). Moreover, as shown by a study of 94 terrestrial ecosystems on all continents except Antarctica (65), species richness (biodiversity) is more positively correlated with productivity – the total quantity of plant life per acre – than with anything else.
We also need not worry about environmental calamities, even if the current long-term natural warming trend continues. The Earth has been much warmer during the past 3,000 years without catastrophic effects. Warmer weather extends growing seasons and generally improves the habitability of colder regions. ''Global warming,'' an invalidated hypothesis, provides no reason to limit human production of CO2, CH4, N2O, HFCs, PFCs, and SF6 as has been proposed (29).
Human use of coal, oil, and natural gas has not measurably warmed the atmosphere, and the extrapolation of current trends shows that it will not significantly do so in the foreseeable future. It does, however, release CO2, which accelerates the growth rates of plants and also permits plants to grow in drier regions. Animal life, which depends upon plants, also flourishes.
As coal, oil, and natural gas are used to feed and lift from poverty vast numbers of people across the globe, more CO2 will be released into the atmosphere. This will help to maintain and improve the health, longevity, prosperity, and productivity of all people.
Human activities are believed to be responsible for the rise in CO2 level of the atmosphere. Mankind is moving the carbon in coal, oil, and natural gas from below ground to the atmosphere and surface, where it is available for conversion into living things. We are living in an increasingly lush environment of plants and animals as a result of the CO2 increase. Our children will enjoy an Earth with far more plant and animal life as that with which we now are blessed. This is a wonderful and unexpected gift from the Industrial Revolution.