Environmental Effects of Increased Atmospheric Carbon Dioxide
Arthur B. ROBINSON**, Sallie L. BALIUNAS*, Willie Soon*, and Zachary W. ROBINSON**
**Oregon Institute of Science and Medicine, 2251 Dick George Rd., Cave Junction, Oregon 97523 [info@oism.org]
*George C. Marshall Institute, 1730 K St., NW, Ste 905, Washington, DC 20006 [info@marshall.org]
January 1998
ABSTRACT
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 dimtic effects due to future increases in minor greenhouse gases like CO2 are in error and do not conform to current experimental knowledge.
World leaders gathered in Kyoto, Japan, in December 1997 to consider a world treaty restricting emissions of "greenhouse gases," chiefly carbon dioxide (CO2), that are thought to cause "global warming" - severe increases in Earth's atmospheric and surface temperatures, with disastrous environmental consequences.
Predictions of global warming are based on computer climate modeling, a branch of science still in its infancy. The empirical evidence - actual measurements of Earth's temperature - shows no man-made warming trend. Indeed, over the past two decades, when CO2 levels have been at their highest, global average temperatures have actually cooled slightly.
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 concentration of CO2 in Earth's atmosphere has increased during the past century, as shown in figure 1 (1).
Fig. 1, Atmospheric CO2 eonccntradons in parts per million by volume, ppm, at Mauna Loa, Hawaii. These measurements agree well with those at other locations (1). Periodic cycle is caused by seasonal variations in CO2 absorption by plants. Approximate global level of atmospheric CO2 in 1900 and 1940 is also displayed (2).
The annual cycles infigure 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).
Fig. 2. Surface temperatures in the Sargaso Sea (with time resolution of about 50 years) ending in 1975 as determined by isotope ratios of marine organism remains in sediment at the bottom of the sea (7). The horizontal line is the average temperature for this 3,000 year period. The Little Ice Age and Medieval Climate Optimum were naturally occurring, extended intervals of climate departures from the mean.
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.
Fig. 3. Moving 11-year average of terrestrial Northern Hemisphere temperatures as deviations in øC from the 1951-1970 mean - left axis and darker line (8,9). Solar magnetic cycle lengths - right axis and lighter line (10). The shorter the magnetic cycle length, the more active, and hence brighter, the sun.
In any case, what effect is the rise in CO2 having upon the global enviroment? The temperature of the Earth varies naturally over a wide range. Figure 2 summarizes, for example, surface temperatures in the Sargaso Sea (a region of the Atlantic Ocean) during the past 3,000 years (7). Sea surface temperatures at this location have varied over a range of about 3.6 degees Celsius (øC) during the past 3,000 years. Trends in these data correspond to similar features that are known from the historical record.
Fig. 4. Annual mean surface temperatures in the contiguous United States between 1895 and 1997, as compiled by the National Climate Data Center (12). Horizontal line is the 103-year mean. The trend line for this 103-year period has a slope of 0.022 øC per decade or 0.22 øC per century. The trend line for 1940 to 1997 has a slope of 0.008 øC per decade or 0.08 ø12 per century.
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 warning" 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 Northem 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. temperaus. These temperatures are now near average for the past 103 years, with 1996 and 1997 having been the 42nd and 60th coolest years.
Fig. 5. Radiosonde balloon station measurements of global lower tropospheric temperatures at 63 stations between latitudes 90 N and 90 S from 1958 to 1996 (15). Temperatures are three-month averages and are graphed as deviations from the mean temperature for 1979 to 1996. Linear trend line for 1979 to 1996 is shown. The slope is minus 0.060 øC per decade.
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 curremly near their 40-year mean (15'), and have been trending slightly downward since 1979.
Fig. 6. Satellite Microwave Sounding Unit, MSU, measurements of global lower tropospheric temperatures between latitudes 83 N and 83 S from 1979 to 1997 (17,18). Temperatures are monthly averages and are graphed as deviations from the mean temperature for 1979 to 1996. Linear trend line for 1979 to 1997 is shown. The slope of this line is minus 0.047 øC per decade. This record of measurements began in 1979.
Fig. 7. Global radiosonde balloon temperature (light line) (15) and global satellite MSU temperature (dark line) (17,18) from figures 5 and 6 plotted with 6-month smoothing. Both sets of data are graphed as deviations from their respective means for 1979 to 1996. The 1979 to 1996 slopes of the trend lines are minus 0.060 øC per decade for balloon and minus 0.045 for satellite.
Since 1979, lower-tmpospheric 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, verities 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 øC per decade, it has been reported that global surface temperatures trended upward by about 0.1 øC 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 øC per decade (12) while global surface temperatures are reported increased by plus 0.03 øC per decade (23). The corresponding weather-balloon and satellite tropospheric 10-year trends are minus 0.4 øC and minus 0.3 øC per decade, respectively.
Figure 8. Tropospheric temperature measurements by satellite MSU for North America between 30ø to 70ø N and 75ø to 125ø W (dark line) (17, 18) compared with the surface record for this same region (light line) (24), both plotted with 12-month smoothing and graphed as deviations from their means for 1979 to 1996. The slope of the satellite MSU trend line is minus 0.01 øC per decade, while that for the surface trend line is plus 0.07 øC per decade. The correlation coefficient for the unsmoothed monthly data in the two sets is 0.92.
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 øC 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 øC 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.
Fig. 9. Qualitative illustration of greenhouse warming. Present: the current greenhouse effect from all atmospheric phenomena. Radiative effect of CO2: added greenhouse radiative effect from doubling CO2 without consideration of other atmospheric components. Hypothesis 1 IPCC: hypothetical amplification effect assumed by IPCC. Hypothesis 2: hypothetical moderation effect.
There is such a thing as the greenhouse effect. Greenhouse gases such as H20 and CO2 in the Earth's atmosphere decrease the escape of terrestrial thermal infrared radiation. Increasing CO2, therefore, effectively increases radiative energy input to the Earth. But what happens to this radiative input is complex: It is redistributed, both vertically and horizontally, by various physical processes, including advection, convection, and diffusion in the atmosphere and ocean.
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 øC 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 knownClick here for the next section