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Biological Consequences of Increased Concentrations of Atmospheric CO2

Sherwood B. Idso

For most of the past two millennia, the carbon-dioxide content of earth's atmosphere has been amazingly constant, hovering at a mean concentration of approximately 280 parts per million (ppm). With the dawning of the Industrial Revolution, however, this equilibrium was disturbed: the CO2 content of the air began to rise as humanity embarked upon a course of unprecedented economic development that was fueled by the burning of massive quantities of coal, gas, and oil, which released huge amounts of carbon dioxide into the atmosphere.

The rise in the concentration of atmospheric CO2 that has accompanied this societal transformation is currently viewed with considerable alarm, due to its purported ability to induce what many believe will be catastrophic global warming. But the phenomenon is not without its positive aspects, among which is its ability to stimulate vegetative productivity and it is possible that this demonstrable biological consequence of increased concentrations of atmospheric CO2 may ultimately prove to be of far greater significance than its speculative climatic consequences. Hence, it is important to review what we know about the little-publicized effects of increased concentrations of atmospheric CO2 on the growth and development of plants; for these are the only effects of the rising CO2 content of earth's atmosphere about which we can be truly confident.

Effects of CO2 upon plant growth

Carbon dioxide is the basis of almost all life on earth as it is the primary raw material used by plants to produce organic matter. Rogers et al. (1992) have highlighted this fact by noting that carbon dioxide is the first molecular link between the atmosphere and biosphere, that it is essential for photosynthesis, which sustains plant life, the basis of the entire food chain, and that no substance is more pivotal for ecosystems, either natural or managed. The veracity of these claims is supported by literally hundreds of experiments that have convincingly demonstrated that the more CO2 there is in the air, the better plants perform their many vital functions. In the first major review of this subject (Kimball 1983a), which was based upon 430 individual experimental results, Kimball observed that the productivity of most herbaceous plants rose by about one-third in response to a doubling of the air's CO2 content (330 to 660 ppm); and in a subsequent analysis of 770 sets of measurements, he obtained essentially the same result (Kimball 1983b). Other reviews have revealed similar or even larger CO2-induced growth enhancements (Lemon 1983; Cure and Acock 1986; Mortensen 1987; Lawlor and Mitchell 1991; Drake 1992a; Poorter 1993; Ceulemans and Mousseau 1994; Strain and Cure 1994; Wullschleger et al. 1995, 1997); while in the most comprehensive study of all, which reviewed the results of over 1000 laboratory and field experiments conducted subsequent to the time of Kimball's analyses, Idso (1992a) documented a mean productivity enhancement of 52 percent in response to a 300 ppm increase in the air's CO2 content.

This positive response of plants to increases in the concentration of atmospheric CO2 cuts across all botanical boundaries and is present, to a greater or lesser degree, in all types of vegetation (Poorter 1993). Indeed, it is now realized to be one of the verities of nature that, with more CO2 in the air, plants generally grow larger, have more branches or tillers, more and thicker leaves, more extensive root systems, as well as more flowers and fruit (Idso 1989a); and in consequence of these well established facts, Sylvan H. Wittwer, the father of modern research in this area, stated recently that "it should be considered good fortune that we are living in a world of gradually increasing levels of atmospheric CO2" (1997: 13). As we shall shortly see, we have already reaped immense benefits from this phenomenon and we can anticipate even greater positive consequences in the years ahead.

Effects of CO2 upon the efficient use of water by plants

In addition to enhancing vegetative productivity, an increase in the concentration of atmospheric CO2 tends to reduce the apertures of the small pores or stomates through which water vapor escapes from plant leaves and is lost to the atmosphere (Pallas 1965; Morison 1985). The reduction in rate of evaporation from leaves produced by this phenomenon averages about one-third for a doubling of the air's CO2 content (Kimball and Idso 1983; Cure and Acock 1986); and combining this effect with the simultaneous CO2-induced increase in plant productivity actually doubles the efficiency with which individual leaves utilize water to produce organic matter.

The likely consequences of this phenomenon are truly impressive. As the concentration of atmospheric carbon dioxide rises ever higher in the years ahead, plants should be able to grow where it is presently too dry for them, enabling the most drought-resistant species to reclaim great tracts of land previously lost to desertification (Idso and Quinn 1983; Idso 1989b). Greater vegetative cover should also reduce the adverse effects of soil erosion caused by the ravages of wind and rain (Idso 1991a). And, with greater plant productivity both above-ground (as noted in the previous section) and below-ground (Idso et al. 1988; Rogers et al. 1994; Jongen et al. 1995; Pregitzer et al. 1995; Tingey et al. 1996), there is typically an increase in soil organic matter (Lekkerkerk et al. 1990; Wood et al. 1994; Sombroek 1995; Henning et al. 1996; Batjes and Sombroek 1997; King et al. 1997; Prior et al. 1997), which usually produces even further benefits.

Creatures such as earthworms that live in the soil are greatly stimulated by additions of organic matter (Edwards 1988; Rogers et al. 1994); and an increase in their activity would likely lead to the creation of much new soil (Graham et al. 1988; Johnson 1988) while at the same time improving the fertility, structure, aeration, and drainage of existing soils (Edwards 1997). These improvements, in turn, would likely boost plant productivity even higher, putting still more organic matter into the soil, and so on (Idso 1991b), as the several phenomena reinforce each other to lift the whole biosphere to a new level of activity (Idso 1992b).

The challenge of inadequate resources

In spite of the impressive body of evidence that has established the reality of the many biological benefits of an atmosphere enriched with increased concentrations of CO2, many find it difficult to believe that a gaseous effluent of our industrial society might be good for the biosphere (Gore 1992); and, from time to time, a few scientists have suggested that the limited availability of resources characteristic of unmanaged ecosystems will reduce or even totally negate the growth-enhancing effects of increased concentrations of atmospheric CO2 upon natural (as opposed to managed, i.e. agricultural) plant communities (Kramer 1981; Oechel and Strain 1985; Bazzaz and Fajer 1992). Consequently, in an effort to resolve this issue, Idso and Idso (1994) reviewed the plant science literature of the 10-year period following the original reviews of Kimball (1983a, b) and analyzed the results of all paired sets of CO2-enrichment experiments that were conducted simultaneously under growing conditions that were both ideal and less than ideal. Results of their several findings are summarized in the following subsections.

Low levels of light

Decreasing light intensity had no significant effect upon plants' photosynthetic response to increased concentrations of atmospheric CO2 until the lowest light intensity of the 37 experiments studied was encountered. At that level, contrary to the contentions of those who view inadequate resources as impediments to the positive effects of increased concentrations of CO2, there was a rise in CO2-induced benefits: the mean percentage increase in photosynthesis due to a 300 ppm increase in atmospheric CO2 rose from 68 percent under normal light intensities to 80 percent at the lowest light intensity studied, while the mean percentage increase in photosynthesis due to a 600 ppm increase in CO2 rose from 111 percent to 194 percent.

Studies published subsequent to the review of Idso and Idso (1994) have continued to demonstrate that low light intensities do not negate the beneficial effects of increased concentrations of atmospheric CO2 upon plant growth and development (Maruyama and Huang 1996; Percival et al. 1996; Wang 1996; Kubiske and Pregitzer 1997). In fact, in a study of plants in the forest understory, Osborne et al. (1997) found that elevated CO2 concentrations allowed for a positive net photosynthetic uptake of carbon on days and at locations that typically experienced light intensities so low that they were generally insufficient for positive net photosynthesis under current atmospheric CO2 concentrations, i.e., they found that elevated CO2 concentrations allowed the plants to live where they currently cannot due to a lack of sufficient light. Hence, they concluded that "the potential range of habitats that such species could occupy will expand considerably with rising atmospheric CO2" (1997: 343); similar conclusions are suggested by the work of Caporn et al. (1994); Wang et al. (1995); Kubiske and Pregitzer (1996), and Liang et al. (1996).

Inadequate water

When lack of water posed a limitation to vegetative growth and development, the results of Idso and Idso's (1994) analyses of 55 experiments were even more dramatic than those pertaining to low light levels. The growth enhancement caused by a 300 ppm increase in atmospheric CO2 jumped from 31 percent when water supplies were optimal to 63 percent when they were less than optimal while, for an increase of 600 ppm in the concentration of CO2, the CO2-induced growth enhancement jumped from 51 percent to 219 percent when the availability of water decreased from adequate to less than adequate. Here, too, subsequent studies continue to support these general observations (Liang and Maruyama 1995; Roden and Ball 1996; Goodfellow et al. 1997) and, in a model-driven review of experimentally established principles derived from work on temperate forest species, Thornley and Cannell have independently concluded that the on-going rise in the air's CO2 content "will protect trees from debilitating water stress" (1996: 1343). Kellomaki and Wang (1996) have come to the same conclusion as a result of their own observations and, in much the same vein, Polley et al. (1996) have observed that a doubling of the concentration of atmospheric CO2 can significantly enhance the percentage of seedlings surviving when water is withheld.

Insufficient soil nutrients

In Idso and Idso's (1994) analysis of the effects of limitations of soil nutrients, the growth enhancement due to a 300 ppm rise in the air's CO2 content exhibited a slight decline, dropping from 51 percent to 45 percent when, in a group of 70 experiments, nutrients went from a level that did not limit growth to a level that did. But when the concentration of atmospheric CO2 increased to 600 ppm, this slight negative trend reversed itself, going from a 43 percent CO2-induced growth stimulation when nutrients were present in abundance to a 52 percent enhancement when their supply was sub-optimal. And, with a 1200 ppm increase in atmospheric CO2, growth enhancement jumped from 60 percent when the soil nutrient supply was adequate to 207 percent when it was less than adequate.

Examples from specific ecosystems

Detailed investigations of several managed and unmanaged ecosystems have provided striking examples of how increased concentrations of atmospheric CO2 can endow plants with the capacities they need to overcome the restrictions upon growth that result from limited resources (Koch and Mooney 1996a). In years when the productivities of these plant communities have been unusually high, for example, the effects of elevated concentrations of CO2 have been decidedly moderate; but when environmental factors have combined to curtail their growth and development severely, the effects of increased concentrations of atmospheric CO2 have typically been much more dramatic (Koch and Mooney 1996b).

In the case of a Kansas tallgrass prairie, doubling the atmospheric CO2 concentration enhanced vegetative productivity by only 5 to 10 percent in several years of high productivity but, in a year of intermediate productivity, it increased growth by approximately 40 percent and, in a year of very low productivity, it boosted production by nearly 80 percent (Owensby et al. 1996). Another place where this phenomenon has been observed is in the world's most comprehensive set of Free-Air CO2 Enrichment (FACE) experiments (Hendrey et al. 1993; Dugas and Pinter 1994). In a study where CO2 was injected directly into the air over a wheat crop growing in a field devoid of any alterations to the natural environment, the imposition of a yield-reducing water stress raised the productivity enhancement created by a 180 ppm increase in atmospheric CO2 from 10 percent to 23 percent in two different years (Pinter et al. 1996). Further, in a cotton crop where the wetter of two irrigation regimes reduced the yield resulting from the ambient-air treatment, it was the over-watered plants with lower yield that experienced the greater CO2-induced stimulation to growth in two different years (Pinter et al. 1996).

How high CO2 levels help plants overcome resource deficiencies

One reason that plants are able to respond positively to increased levels of atmospheric CO2 when limited resources significantly curtail their growth is that plants grown at elevated concentrations of CO2 typically have more extensive and active root systems than control plants growing in normal air (Curtis et al. 1990, 1994; Idso and Kimball 1991a, 1992b; Norby 1994; Prior et al. 1995; Gebauer et al. 1996; King et al. 1996; Kubiske et al. 1997), which allows them to explore more thoroughly larger volumes of soil in search of the things they need (Norby et al. 1992; Rogers et al. 1992; Stulen and den Hertog 1993). When more nutrients are encountered in the course of this activity, plants can also acquire them more effectively (Luxmoore et al. 1986; Norby et al. 1986) because the uptake of many essential elements requires the expenditure of metabolic energy (Pitman 1977; Jackson et al. 1980), and the enhanced availability of carbohydrates typically provided by the enrichment of the air with CO2 tends to promote this process (Clement et al. 1978; Rufty et al. 1989; Rogers et al. 1994). A low level of nitrogen in the soil, in particular, is not an impediment to CO2-induced growth enhancement because plants exposed to elevated concentrations of atmospheric CO2 do not need to invest as much nitrogen in their photosynthetic apparatus (Stitt 1991) as it operates so much more efficiently at higher CO2 levels (Nie et al. 1995). Hence, even in the face of severe nutrient deficiencies, plants' photosynthetic rates may be significantly stimulated by increased concentrations of atmospheric CO2 (Norby et al. 1992; Wullschleger et al. 1992), setting in motion still other beneficial phenomena.

One of the most important of these secondary or indirect consequences of CO2-induced growth stimulation in situations where resources are limited is the enhancement of the activity of soil microorganisms (Lamborg et al. 1983; Pregitzer et al. 1995; Tingey et al. 1996) that is provided by enhanced root exudation of organic substances (Norby 1997; Hungate et al. 1997). This enhanced activity of microorganisms typically stimulates a multiplicity of growth-promoting effects beneath the soil surface (Zak et al. 1993). As the "better-fed" hyphae (filamentous structures) (Smith and Read 1996) of more numerous and robust symbiotic fungi extend outward from their CO2-enriched hosts (Ineichen et al. 1995), for example, they lengthen the life of absorptive root hairs (Fogel 1983) and increase the area of root surface available for water and nutrient uptake (Tinker 1984; Clarkson 1985; Smith and Read 1996). The microscopic organisms that live in the plant's root zone or rhizosphere also secrete a number of organic acids that hasten the chemical weathering of soil minerals (Boyle and Voigt 1973; Boyle et al. 1974) and they are especially adept at making phosphorus available to plants by this means (Ortuno et al. 1978; Molla et al. 1984; Babenko et al. 1985). In addition, these microscopic organisms produce a variety of hormones that stimulate root growth (Simmons and Pope 1987, 1988), enhancing the production of lateral roots and root hairs (Umali-Garcia et al. 1980; Kapulnik et al. 1983).

Most important of all, perhaps, is the ability of elevated levels of atmospheric CO2 to stimulate the activity of nitrogen-fixing bacteria directly (Burk 1961; Lowe and Evans 1962). The capacity of these bacteria to remove nitrogen from the atmosphere and make it available to plants appears to be limited by their host plants' rates of carbohydrate production (Sinclair and de Wit 1975; Hardy et al. 1976; Finn and Brun 1982). Consequently, anything that stimulates vegetative productivity, including increased concentrations of atmospheric CO2, generally stimulates bacterial nodule growth and activity (Quebedeaux et al. 1975; Murphy 1986) and several-fold increases in the air's CO2 content have indeed been found to produce several-fold increases in nitrogen fixation in a number of experiments (Hardy and Havelka 1973, 1975; Havelka and Hardy 1976; Phillips et al. 1976; Hardy et al. 1978; MacDowall 1983).

Acting in concert, these phenomena typically allow the growth-enhancing effects of increased concentrations of atmospheric CO2 to be expressed in the face of severe nutritional deficiencies, suggesting that carbon starvation (Svedang 1992; Cizkova-Koncalova et al. 1992; Robinson 1994) may well be a more significant impediment to the growth of the planet's vegetation than is a lack of soil nutrients. In the words of Gunderson and Wullschleger, who have reviewed the subject in depth, "hypotheses that either nutrient or water limitations would limit photosynthetic responses in natural environments have not been adequately supported by the currently available data" (1994: 384). Quite to the contrary, the data demonstrate, as noted by Gifford, that "high CO2 increases light-use efficiency, water-use efficiency and nitrogen-use efficiency," so that "the Ôlaw of limiting factor' concept that such vegetation cannot respond to increased CO2 concentration does not apply to C3 species 
[C3 plant species are distinguished from C4 plant species in that they use the process of photosynthesis first identified by science, a process using an enzyme that reacts with both CO2 and O2 and that is, thus, less efficient in the presence of relatively higher levels of O2 than the process used by C4 plant species, which depends upon a different enzyme that does not interact with O2. Increased concentrations of atmospheric CO2 are, therefore, of much greater benefit to C3 species of plants than to C4 species. The names come from the three-carbon (C3) sugar that is the first product of the photosynthetic process in C3 species and from the four-carbon (C4) organic acid that is the first product in C4 species.]" (1994: 33) which include fully 95 percent of all of earth's plants (Drake 1992b; Bowes 1993).

The challenge of environmental stresses

In addition to limited resources, there are a number of environmental stresses that are typically encountered in natural and agricultural ecosystems and it has been claimed that their debilitating influences reduce or negate the benefits that can accrue to plants from increased concentrations of atmospheric CO2. The review of Idso and Idso (1994) demonstrates once again, however, that such effects are generally not observed in laboratory and field experiments and that, if anything, just the opposite is more likely to occur.

Soil salinity and air pollution

In an analysis of 42 sets of measurements of the reponse of plant growth to increased concentrations of atmospheric CO2 at different values of soil salinity, Idso and Idso (1994) found that the growth-promoting effects of elevated levels of CO2 in the air were found to about the same degree in plants stessed by increased salinity of the soil as in plants in soil of normal salinity, demonstrating that, on average, soil salinity does not reduce the biological benefits of increased concentrations of atmospheric CO2. What is more, in another 20 sets of measurements, when air pollutants were the source of stress, the percentage enhancement in growth brought about by a 300 ppm increase in atmospheric CO2 actually rose in the face of this adversity, from 38 percent when air pollutants were absent to 54 percent when they were present in noxious quantities. And, as was the case where there were limited resources, research conducted subsequent to the review of Idso and Idso (1994) has continued to confirm their basic finding about the interaction between CO2 and air pollution: a doubling of the air's CO2 content was often found to compensate totally for the debilitating effects of air pollutants such as ozone (Rudorff et al. 1996; Volin and Reich 1996; McKee et al. 1997; Mulholland et al. 1997; Fiscus et al. 1997).

Global warming

The environmental stress about which certain scientists-and, therefore, the Press and the public-seem to worry most is that produced by higher air temperatures (Peters and Darling 1985; Paine 1988; Davis 1989; Woodwell 1989; Gear and Huntley 1991; Dobson 1992). They are concerned that future global warming may be so great that plants will need to migrate towards the poles in order to remain within the climatic regimes to which they are currently best suited (Overpeck et al. 1991; Dyer 1995) and, because the warming is predicted to be so rapid, they fear that many plants may not be able to migrate fast enough to avoid extinction (Possingham 1993; Root and Schneider 1993; Pitelka et al. 1997).

Although this scenario may sound reasonable, it is largely contradicted by basic research into plant physiology. In an analysis of 42 different data sets, Idso and Idso (1994), for example, found that the growth enhancement due to a 300 ppm increase in atmospheric CO2 actually rose with increasing air temperature, climbing from nearly 0 percent at 10șC to a full 100 percent at 38șC; for greater increases in the air's CO2 content, the percentage of growth enhancement was greater still, something that ongoing research continues to confirm (Nijs and Impens 1996; Vu et al. 1997).

One reason for this counter-intuitive response is that the optimum temperature for plant growth generally rises as the air's CO2 content rises (Berry and Bjorkman 1980; Taiz and Zeiger 1991; McMurtrie et al. 1992; McMurtrie and Wang 1993). Long (1991), for example, has calculated from basic principles of plant physiology that a 300 ppm increase in atmospheric CO2 should cause the optimum temperatures of most C3 plants to rise by about 5șC; an analysis of the results of 7 studies that have experimentally evaluated this phenomenon (Bjorkman et al. 1978; Nilsen et al. 1983; Jurik et al. 1984; Seemann et al. 1984; Harley et al. 1986; Stuhlfauth and Fock 1990; McMurtrie et al. 1992) reveals a mean optimum temperature rise of 5.9șC with a 300 ppm increase in atmospheric CO2 (Idso and Idso 1994). And, as this rise in optimum temperature is even larger than the rise in air temperature predicted to result from the greenhouse warming caused by such a CO2 increase (IPCC 1990, IPCC I 1996), a CO2-induced warming would not adversely affect the vast majority of earth's plants, 95 percent of which are of the C3 variety (Drake 1992b; Bowes 1993). In addition, the remainder of the planet's species-which may not experience quite as large a rise in optimum temperature (Chen et al. 1994)-are already adapted to earth's warmer environments (De Jong et al. 1982; Drake 1989; Johnson et al. 1993), which are expected to warm much less than the other portions of the globe (IPCC 1990, IPCC I 1996).

In view of these facts, it is clear that a CO2-induced global warming would not produce massive poleward migrations of plants seeking cooler weather, for the temperatures at which nearly all plants perform at their optimum would rise at the same rate as (or faster than) and to the same degree as (or higher than) the temperatures of their respective environments. What is more, the photosynthetic rates of the 7 plants for which these evaluations have been experimentally derived were found to be nearly twice as great at their CO2-enriched optimum temperatures as they were at their optimum temperatures under present CO2 conditions (Idso and Idso 1994), suggesting not only that typically predicted increases in atmospheric CO2 and global air temperatures would not hurt earth's vegetation, but also that they might actually help it, as subsequent investigations have revealed as well (Wang 1996).

Finally, at the highest air temperatures encountered by plants, increased concentrations of atmospheric CO2 have been demonstrated to be especially valuable for they often mean the difference between life and death (Rowland-Bamford et al. 1996; Idso 1997), as they typically enable plants to maintain positive carbon exchange rates in situations where plants growing under present CO2 concentrations exhibit negative rates that ultimately lead to their demise (Idso et al. 1989, 1995). Faria et al. (1996) have studied this phenomenon in detail in seedling oak trees and believe that the life-sustaining function of increased concentrations of atmospheric CO2 at high air temperatures may also be partly due to a stabilization of enzymes susceptible to heat through the increased concentration of sugars generally found in CO2-enriched leaves.

Long-term exposure to increased concentrations of CO2

Most studies of plants' responses to increased concentrations of atmospheric CO2 have been of rather short duration, ranging from hours to days to weeks. Some agricultural investigations have lasted longer, encompassing entire growing seasons; even in the case of long-lived plants, such as trees, however, few experiments have had their durations measured in years. Consequently, there has always been a concern that results derived from short-term experiments may not apply to plants over the longer term; and, in fact, there is an experimental basis for this worry.

Acclimation to high CO2

According to a concept variously referred to as "acclimation" or "down regulation," it is believed by some biologists that long-term exposure to elevated levels of carbon dioxide will result in a reduction of the photosynthetic capacity of plants. This contention is based on the results of a number of experiments that have indeed revealed such behaviour in several plants (Kramer 1981; Pearcy and Bjorkman 1983; Delucia et al. 1985; Cure and Acock 1986; Tissue and Oechel 1987) and, although it is possible that such a phenomenon may regularly occur in certain species (Sicher and Bunce 1997), and that it may occur more generally under certain circumstances (Lewis et al. 1994; Luo et al. 1994; Reining 1994; Ziska et al. 1995; Marek and Kalina 1996; Micallef et al. 1996), there are reasons to believe that it is not expressed-or not as consistently expressed (Sicher and Kremer 1994; Van Oosten and Besford 1996)-in the majority of plants growing in natural situations.

Many of the experiments that have exhibited long-term declines in CO2-induced benefits to plants, for example, have been conducted in controlled environments or greenhouses (Drake and Leadley 1991), where the plants that were studied were grown in pots or some other type of root-restricting container. As a number of studies have demonstrated (Masle et al. 1990; Arp 1991; Hogan et al. 1991; Thomas and Strain 1991; Samuelson and Seiler 1992), root restrictions may sometimes prevent the full expression of the effects of increased concentrations of atmospheric CO2, particularly when the plants grow larger, a natural consequence of longer experiments.

In contrast to the results of studies in controlled environments, long-term field experiments often show no reductions in the photosynthetic capacities of plants exposed to elevated concentrations of CO2 (Radin et al. 1987; Sage et al. 1989; Idso and Kimball 1991b; Norby and O'Neill 1991; Gunderson et al. 1993; Dufrene et al. 1993; Jones et al. 1995; Teskey 1995, 1997; Wang and Kellomaki 1997), or they show an actual increase or "up regulation" in photosynthesis and growth over extended periods of time (Campbell et al. 1988; Conroy 1989; Chen and Sung 1990; Ziska et al. 1990; Arp and Drake 1991; Long and Drake 1991; Drake 1992c; Barton et al. 1993; Vogel and Curtis 1995; Liu and Teskey 1995; Jacob et al. 1995). There has been one field experiment that produced a contrary result (Oechel and Strain 1985; Tissue and Oechel 1987; Grulke et al. 1990) but the Arctic tussock tundra of that study was growing close to the temperature (10șC) at which its response to increased concentrations of atmospheric CO2 truly should have been near zero, according to the analysis of temperature effects in the review of Idso and Idso (1994) and in harmony with the published opinions of several scientists who have analyzed this particular experiment in some detail (Drake and Leadley 1991; Long 1991; Drake 1992c; Kirschbaum 1994; Webber et al. 1994). What is more, in a subsequent three-year study of this same ecosystem, an artificially imposed warming of only 4șC totally eliminated the down regulation observed at current air temperatures (Field 1994; Oechel et al. 1994). Consequently, although not to be ruled out in all instances, down regulation of photosynthesis and growth does not appear to be a major impediment to the long-term effectiveness of the aerial fertilization effect of increased concentrations of atmospheric CO2 in stimulating the productivity of earth's vegetation for, as Woodrow has concluded after reviewing the subject in considerable depth, "C3 plants probably possess the genetic feedback mechanisms required to efficiently Ôsmooth out' any imbalance within the photosynthetic system caused by a rise in atmospheric CO2" (1994: 401) so that, in the words of Amthor, "acclimation . . . of photosynthesis to increasing CO2 concentration is unlikely to be complete" (1995: 243).

The world's longest experiment studying
increased concentration of CO2

In the longest study of its kind ever to be conducted-and where down-regulation would be expected to appear if it were a truly ubiquitous consequence of long-term exposure to increased concentrations of CO2-my colleagues and I planted 8 seedling sour-orange trees directly into the ground at Phoenix, Arizona, in July of 1987 and enclosed them in pairs within 4 open-topped chambers made of clear polyethylene film. Then, in November of that year, we began to pump air with a CO2 concentration of 700 ppm continuously into two of the enclosures through perforated plastic tubes that lay upon the ground while we pumped ambient air with a CO2 concentration of 400 ppm into the other two enclosures (Idso et al. 1991). This protocol we have faithfully maintained to the present day, documenting the course of the effects of increased concentrations of atmospheric CO2 upon the trees as they have progressed from tiny seedlings through the juvenile stage of development and into full maturity, which they appear to have reached about two years ago.

The results of this decade-long experiment have been truly remarkable. Over the entire course of the study, the CO2-enriched trees, which receive 75 percent more CO2 than the control trees-have continually produced over twice as much biomass as the trees growing in normal air (Idso and Kimball 1997); at our last harvest, we picked over three times as much fruit from the trees exposed to the extra CO2. Hence, although we intend to continue the experiment for several more years and do not yet have the final word on the subject, we feel confident that the beneficial effects of increased concentrations of atmospheric CO2 observed to date will likely persist over the entire life span of the trees.

Historical trends in forest productivity

In addition to the experiments of scientists, the biosphere itself is providing evidence for the reality and long-term sustainability of the aerial-fertilization effects of increased concentrations of atmospheric CO2. In the southwestern United States, for example, there is considerable evidence that the woody plants that were present in pre-industrial times, when the air's CO2 content was much lower than now, were somewhat stunted compared to their present-day descendants (Johnston 1963; Scifres 1980). This phenomenon was not fully appreciated, however, until LaMarche et al. published a paper (1984) describing an analysis of annual growth rings obtained from pine trees growing near the timberline in California, Colorado, Nevada, and New Mexico. All of the trees that they studied exhibited large increases in growth rate between 1859 and 1983 and some of them doubled their productivity during this time. The researchers noted that the increased growth rates exceeded those expected from climatic trends but were consistent in magnitude with what would be expected from the historical trend of increasing concentrations of atmospheric CO2, a trend particularly pronounced in recent decades.

Reports of a similar nature followed in quick succession. Along the western coast of North America, conifers growing in the high altitudes of the Cascade Mountains of Washington were found to have increased their growth rates by approximately 60 percent since 1890 (Graumlich et al. 1989). In British Columbia, ring-width measurements of Douglas firs also revealed a marked increase in growth in recent decades and Parker noted that "environmental influences other than increased CO2 have not been found that would explain [the phenomenon]" (1987: 511).

In New England, a study of a 320-year-old Pinus rigida rock-outcrop community showed its trees to have experienced a dramatic increase in growth since 1970 (Abrams and Orwig 1995). Another study of 10 tree species revealed a mean growth enhancement of 24 percent from 1950 to 1980 (Hornbeck et al. 1988). In Georgia, annual growth increments of long-leaf pines began to rise dramatically about 1920, increasing by approximately 30 percent by the mid-1980s (West 1988). In this study, too, it was reported that the increased growth could not be explained by trends in either precipitation or temperature, leaving the rising CO2 content of the air as the most likely cause of the increase in productivity.

In between these regions, the growth rates of several hardwoods in the Great Smokey Mountains also increased over the last half-century (Busing 1989), as did those of other conifers in other regions of both the United States (Jacoby 1986; Graybill 1987; Hornbeck 1987; Kienast and Luxmoore 1988) and Canada (Jozsa and Powell 1987). In one such study, a colleague and I analyzed data collected from long-lived bristlecone, foxtail, and limber pines at high-altitude sites in Arizona, California, Colorado, and Nevada (Graybill and Idso 1993). In almost all of the chronologies we developed, a sharp upward growth trend began about the middle of the 1850s and has continued to the present. No comparable variations were observed anywhere else in the records, which continued back in time almost two millennia, suggesting that the accelerating growth observed in the last century-and-a-half of the several chronologies were truly unique and the result of some major regional or global factor. Furthermore, comparisons of the chronologies with records of changes in temperature and precipitation ruled out the possibility that either of these climatic variables played a significant role in enhancing the trees' growth rates, strongly implicating the rise in the air's CO2 content as the factor responsible for the trees' increasing productivities over the past 150 years.

In Europe, a number of field studies have told much the same story. In forests of silver firs in the mountains of northeastern France, radial growth measurements have revealed a productivity increase of nearly 70 percent from 1830 to 1935 (Becker 1989) and the last decades of the century have seen significant increases in the growth of beech and spruce stands in southern Sweden (Bjorkdahl and Eriksson 1989; Falkengren-Grerup and Eriksson 1990). Similarly, stands of Scots pine in northern Finland have experienced growth increases ranging from 15 to 43 percent between 1950 and 1983 (Hari et al. 1984; Hari and Arovaara 1988). Since CO2 is the only environmental factor that has changed systematically during this century in the remote area of this study, it was thus to this factor that Hari and Aravaara attributed the increased growth of the trees.

Germany presents an even more intriguing situation. In the late 1970s and early 1980s, reports of a large-scale decline in forest productivity were commonplace. When detailed studies based on extensive measurements finally appeared, however, just the opposite proved to be true (Eichkorn 1986; Glatzel et al. 1987; Spelsberg 1987). From the middle of the 1950s at the latest, Norway spruce, silver fir, and beech have all showed remarkable increases in growth rate (Kenk 1989; Spieker 1990), not following the normal age trend (Kenk and Fischer 1988); the growth of pine likewise increased (Pretzsch 1985a, 1985b). By the end of the 1980s, yields of most European forests were clearly above normal (Innes and Cook 1989). In early 1992, for example, scientists from the Finnish Forest Research Institute reported a general increase in forest growth in Austria, Finland, France, Germany, Sweden, and Switzerland (Kauppi et al. 1992) and Weidenbach (1992) reported that annual growth increments in the German state of Baden-WŸrttemberg had risen approximately 20 percent in just the past 20 years.

Possibly the most revealing observations of all are those that come from tropical forests. Noting that the turnover rates of mature tropical woodlands correlate well with measures of their net productivity (Weaver and Murphy 1990), Phillips and Gentry (1994) assessed the turnover rates of forty tropical forests from around the world in order to test the hypothesis that forest productivity is increasing globally. Not surprisingly in view of the material just reviewed, they found that the turnover rates of these highly productive tropical woodlands have indeed been rising ever higher since 1960, if not earlier, and that there seems to have been a worldwide acceleration of this trend since about 1980. They also noted that "the accelerating increase in turnover coincides with an accelerating buildup of CO2" (Phillips and Gentry 1994: 957). And as Pimm and Sugden stated in a companion article, it was "the consistency and simultaneity of the changes on several continents that [led] Phillips and Gentry to their conclusion that enhanced productivity induced by increased CO2 is the most plausible candidate for the cause of the increased turnover" (1994: 933-34).

The breath of the biosphere

Compelling evidence for increasing stimulation of earth's plant life by the ongoing rise in the atmosphere's CO2 concentration is to be found in the air itself. Each spring, when the northern hemisphere's thin veneer of vegetation awakens from the dormancy of winter and begins a new season of growth, it draws enough carbon dioxide out of the air to reduce the concentraton of atmospheric CO2 by several parts per million. In the fall, when much of this vegetation dies, it releases huge quantities of carbon dioxide back to the air, causing the CO2 content of the atmosphere to rise by a small amount. Meticulous measurements obtained over the past four decades have clearly demonstrated that the difference between the high and low points of this seasonal oscillation-this "breath of the biosphere" (Idso 1995)-is growing ever larger. Several groups of scientists have studied the phenomenon in detail and nearly all of them have concluded that it implies that the photosynthetic activity of the earth's plant life is growing greater and greater each year; many suggest that the ever-increasing aerial fertilization effect of the steadily rising CO2 content of the atmosphere is its primary cause.

Working with data for atmospheric CO2 from Mauna Loa Observatory in Hawaii, Point Barrow in Alaska, and Weather Station P in the North Pacific Ocean, Pearman and Hyson (1981) concluded that "it is most probable that there has been an increase in the summer net ecosystem production of the Northern Hemisphere of 8.6 percent over the period 1958-1978," stating that "the results are consistent with the concept of enhanced activity due to increased levels of CO2" (1981: 9842). Two years later, Cleveland et al. (1983) confirmed this finding with additional data from Mauna Loa and a companion record from the South Pole, stating: "we believe it most likely that the CO2 seasonal behaviour reflects an increase in either the seasonally varying biomass or in global photosynthetic activity resulting from the increasing concentration of atmospheric carbon dioxide" (1983: 10,945). Two years later, Keeling et al. (1985) confirmed the earlier results for Ocean Weather Station P, stating that the increase in the amplitude of its annual CO2 cycle "reflects an increase in activity of terrestrial plants" (1985: 10,522). Contemporaneously, after two more years of data were obtained from Mauna Loa, Bacastow et al. (1985) reported that the increase in the seasonal amplitude since 1958 was "approximately 1 ppm, a sizeable fraction of the average amplitude of 6 ppm," adding that "it seems likely that the increase mainly reflects enhanced metabolic activity of the land biota" and that "one obvious factor that might produce this enhancement is the CO2 concentration itself " (1985: 10,529).

Reaffirmations of the reality of this CO2-induced phenomenon continue to appear as more and more data are collected and analyzed. Keeling (1994), for example, observed that the amplitude of the seasonal CO2 cycle increased by as much as 20 percent at high latitudes in both hemispheres between 1988 and 1993, noting that "a preliminary investigation suggests that an increase in the net primary production of plants . . . caused the amplitude increase" (1994: 110). Fourteen months later, Keeling et al. (1995) reported that the amplitude of the seasonal CO2 cycle at Mauna Loa had also risen significantly over this time period, while Okamoto et al. (1995), after analyzing seasonal CO2 amplitude trends at 17 stations stretching from the south pole to Alaska, concluded that "CO2 fertilization exists on the global scale" (1995: 206).

Shortly thereafter, Keeling et al. (1996), working with an ever-expanding data base, reported that the annual amplitude of the seasonal CO2 cycle had increased by fully 20 percent in the latitudinal vicinity of Hawaii and by 40 percent in the Arctic since the early 1960s, noting that "the amplitude increases reflect increasing assimilation of CO2 by land plants" (1996: 146). They also observed shifts in the time of occurrence of different portions of the seasonal CO2 cycle, which suggested that the vegetative growing season had lengthened by about a week during the latter part of the same time period. Then, in an analysis of reflectance data obtained from satellites deployed to monitor various processes on the earth's surface, Myneni et al. (1997) found that terrestrial vegetation between 45șN and 70șN latitudes had grown steadily more productive from 1981 to 1991 and that, simultaneously, the region's active growing season had lengthened by approximately 12 days, a development that Fung has called "the first direct observation of the biosphere that photosynthesis has increased on such a broad scale for such a long time" (1997: 659).

Productivity of plants and biodiversity
in the earth's ecosystems

The evidence for an ongoing CO2-induced rejuvenation of the biosphere that has been reviewed in the preceding pages is irrefutable: the totality of earth's plant life is growing ever more vigorously as we flood the air with ever more carbon dioxide. This development is fortunate indeed, for many of man's activities have consequences that are not nearly so benevolent and that truly do degrade earth's ecosystems, leading to massive reductions in biodiversity (Pimm et al. 1995; Vitousek et al. 1997). This one beneficent side-effect of our industrial activities, however, tends to mitigate the adverse consequences of many facets of industrialization. In some cases, the increase in the concentration of atmospheric CO2 more than compensates: it has clearly resulted in a net increase in the planet's vegetative biomass over the past several decades, as is readily evident from long- term measurements of the air's CO2 concentration and contemporary measurements of the planet's surface reflectance. Even measurements of atmospheric oxygen support this scenario (Keeling and Shertz 1992; Bender et al. 1996; Keeling et al. 1996) for, as Bender (1996) has noted, they suggest "the existence of a large oxygen source, which can only be from photosynthesis associated with the net growth of the land biosphere" (1996: 195).

This CO2-induced increase in vegetative productivity may well be one of the best allies we will ever have in our battle to preserve the planet's biodiversity; in a landmark study of the vascular plant components of 94 terrestrial ecosystems from every continent of the globe except Antarctica, it was found that biodiversity in an ecosystem is more positively correlated with its productivity than it is with anything else (Scheiner and Rey-Benayas 1994). What is more, a major review of interactions between plants and animals in 51 terrestrial ecosystems found that the biomass of plant-eating animals or herbivores was also an increasing function of above-ground primary production (McNaughton et al. 1989). A review of 22 aquatic ecosystems found that the herbivore biomass of watery habitats increased in response to a rise in underwater vegetative productivity (Cyr and Pace 1993) and a number of experiments have demonstrated that the growth of aquatic plants is also enhanced as the air's CO2 content rises (Titus et al. 1990; Raven 1991, 1993; Sand-Jensen et al. 1992; Titus 1992; Madsen 1993; Riebesell et al. 1993; Madsen and Sand-Jensen 1994; Hein and Sand-Jensen 1997; Shapiro 1997). Consequently, it is abundantly clear that earth's animal life-both terrestrial and aquatic- will respond to rising levels of atmospheric CO2 with increases in population that will parallel the increases in the plant kingdom, for "the greater the food base, the greater the superstructure of life that can be supported" (Idso 1995a: 13). Greater populations of individual organisms are clearly required for greater biodiversity, as all species must maintain a certain "critical biomass" to sustain their identities and ensure their long-term viability.

Conclusions

An increase in the concentration of atmospheric CO2 significantly stimulates the growth and development of plants while dramatically enhancing their efficient use of water. These desirable manifestations of carbon dioxide's aerial fertilization effect are rooted in fundamental properties of plants that express themselves almost universally, even in the face of significant resource deficiencies and environmental stresses. Consequently, as the carbon dioxide content of the atmosphere has risen because of the large-scale utilization of fossil fuels, so too has the productivity of the planet's vegetation risen because of the magnitude of mankind's CO2 emissions.

The striking consequences of this mutually beneficial phenomenon are evident in numerous tree-ring records, historical trends in global forest productivity, and the increasing amplitude of the atmosphere's seasonal CO2 cycle. Indeed, the on-going rise in the air's CO2 content is enhancing agricultural productivity the world over (Wittwer 1995) at the same time as it helps to sustain the biodiversity of the planet's natural ecosystems. Thus, Wittwer has written truly of this exceptional consequence of humanity's industrial activity that, "the rising level of atmospheric CO2 is a universally free premium, gaining in magnitude with time, on which we can all reckon for the future" (1997: 13).

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