A. DIRECT HEALTH EFFECTS OF ELEVATED CO2

Almost all trace elements and compounds, even beneficial ones, can be poisonous if ingested or inhaled in large enough concentrations.  So what about carbon dioxide?  Do we have to worry about any deleterious health effects as its atmospheric concentration continues to climb?

Inhaling very high concentrations of atmospheric CO2 can induce a state of hypercapnia in people (Nahas et al., 1968; Brackett et al., 1969; van Ypersele de Strihou, 1974).  Characterized by an excessive amount of CO2 in the blood, which typically results in acidosis, this condition is accompanied by headache, nausea, visual disturbances, and is sometimes fatal (Poyart and Nahas, 1968; Turino et al., 1974).  Several studies have demonstrated, however, that these problems do not seriously impact human health until the air's CO2 concentration reaches approximately 15,000 ppm (Luft et al., 1974; Schaefer, 1982), which is approximately 40 times greater than its current concentration.

Clearly, therefore, we do not have to worry about there being any direct adverse health effects associated with the ongoing rise in the air's CO2 content, even if it were to increase by a factor of ten, which is probably all that could be achieved by burning the entire supply of fossil fuels in the crust of the earth.  In fact, the current CO2 concentration of the air in many homes and buildings is often two to three times greater than the CO2 concentration of outdoor air (Idso, 1997), which in large cities is itself often elevated by several tens of percent above the CO2 concentration of rural air (Idso et al. 1998, 2002).

B. INDIRECT HEALTH EFFECTS OF ELEVATED CO2

The old saying "you are what you eat" suggests that effects of atmospheric CO2 enrichment on food production must also be considered in any assessment of the health effects of the historical and still-ongoing rise in the air's CO2 content.  Hence, we begin our investigation of this subject with a brief review of the well-known aerial fertilization effect of atmospheric CO2 enrichment and how it impacts the human health issue.

1. Quantity of Food

First and foremost, people must have sufficient food, simply to sustain themselves; and the rise in the atmosphere's CO2 concentration that has occurred since the inception of the Industrial Revolution (an increase of approximately 100 ppm) has done wonders for humanity in this regard.

a. The past

In a revealing study of the beneficial impact of mankind's historical CO2 emissions on world food production, Mayeux et al. (1997) grew two cultivars of commercial wheat in a 38-meter-long soil container topped with a transparent tunnel-like polyethylene cover within which a CO2 gradient was created that varied from approximately 350 ppm at one end of the tunnel to about 200 ppm at the other end.  Both of the wheat cultivars were irrigated weekly over the first half of the 100-day growing season, so as to maintain soil water contents near optimum conditions.  Over the last half of the season, however, this regimen was maintained on only half of the wheat of each cultivar, in order to create both water-stressed and well-watered treatments.

At the conclusion of the experiment, the scientists determined that the growth response of the wheat was a linear function of atmospheric CO2 concentration in both cultivars under both adequate and less-than-adequate soil water regimes.  Based on the linear regression equations they developed for grain yield in these situations, we calculate that the 100-ppm increase in atmospheric CO2 concentration experienced over the past century and a half should have increased the mean grain yield of the two wheat cultivars by about 72% under well-watered conditions and 48% under water-stressed conditions, for a mean yield increase on the order of 60% under the full range of moisture conditions likely to have existed throughout the entire real world.

It is also important to note that this CO2-induced yield enhancement was not restricted to wheat.  Based on the voluminous amount of data summarized by Idso and Idso (2000) for the world's major food crops, the calculations we have made for wheat can be scaled to determine what the past 150-year increase in atmospheric CO2 concentration likely did for the productivity of other agricultural staples.  Doing so, we find that the Industrial Revolution's flooding of the air with CO2 resulted in mean yield increases on the order of 70% for other C3 cereals, 28% for C4 cereals, 33% for fruits and melons, 62% for legumes, 67% for root and tuber crops, and 51% for vegetables.

b. The future

Clearly, the historical increase in the air's CO2 content that has been experienced to date has vastly benefited mankind and enabled our numbers to grow considerably.  In fact, the very existence of many of the people who read these words may well be attributed to that phenomenon.  But what of the future?  The population explosion of our species has not yet subsided; and there is real concern about our ability to feed the projected population of the world a mere fifty years hence.

Tilman et al. (2001) address this problem in an analysis of the global environmental impacts of agricultural expansion that may occur over the next half-century.  Based on projected increases in population, and even accounting for expected concomitant advances in technological expertise, they conclude that the task of meeting the global food demand expected to exist in the year 2050 will likely exact a heavy environmental toll and produce great societal impacts.

What are the specific problems?  Tilman and his colleagues report that "humans currently appropriate more than a third of the production of terrestrial ecosystems and about half of usable freshwaters," noting that this usurpation of natural resources will increase even more in the future.  In terms of the amount of land devoted to agriculture, they calculate an 18% increase over the present by the year 2050; but because developed countries are expected to withdraw large areas of land from farming over the next fifty years, the net loss of natural ecosystems to cropland and pasture in developing countries will amount to about half of all potentially suitable remaining land, which would "represent the worldwide loss of natural ecosystems larger than the United States."

The scientists go on to say that this phenomenon "could lead to the loss of about a third of remaining tropical and temperate forests, savannas, and grasslands."  What is more, in a worrisome reflection upon the consequences of these land-use changes for both plants and animals, they remind us that species extinction follows rapidly on the heels of habitat destruction.  Finally, in another acknowledgement of just how serious the situation is, Tilman and his associates report that "even the best available technologies, fully deployed, cannot prevent many of the forecasted problems."

So what can possibly be done to avert this future food production shortfall and its devastating consequences that "even the best available technologies, fully deployed," cannot prevent?  This is the question that was addressed by Idso and Idso (2000) in their treatise entitled Forecasting World Food Supplies: The Impact of the Rising Atmospheric CO2 Concentration; and it was their conclusion that -- after all that man can do -- the aerial fertilization effect of the increase in the air's CO2 content that is expected to occur by the year 2050 would be just barely sufficient, in the mean, to assure the agricultural productivity required to prevent mass starvation in many parts of the globe without usurping what little of the natural world would remain at that time.

In view of these observations, not only is the ongoing rise in the air's CO2 content essential for the future well-being of man, it is essential to the future well-being of the entire biosphere.

2. Quality of Food

Clearly, quantity of food is mankind's number one concern when it comes to survival; but after survival is assured, quality of food rises to the fore.  What role does the ongoing rise in the air's CO2 content play here?

a. Protein content

In a review of the scientific literature related to effects of atmospheric CO2 enrichment on plant constituents of significance to human health, Idso and Idso (2001) cited a number of studies that indicated elevated levels of atmospheric CO2 may at times increase, decrease or have no effect upon the protein contents of various foods.

In the case of wheat -- which according to Wittwer (1995) is "the most widely grown plant in the world today," contributing "more calories and protein to the human diet than any other food" -- Pleijel et al. (1999) were able to bring some semblance of order to this confusing situation by analyzing the results of 16 open-top chamber experiments that had been conducted on spring wheat in Denmark, Finland, Sweden and Switzerland between 1986 and 1996.  In addition to CO2 enrichment of the air, these experiments included increases and decreases in atmospheric ozone (O3); and Pleijel et al. found that when increasing O3 pollution reduced wheat grain yield, it simultaneously increased the protein concentration of the grain.  They also found that when O3 was scrubbed from the air and grain yield was thereby increased, the protein concentration of the grain was decreased.  Moreover, this same relationship described the degree to which grain protein concentrations dropped when atmospheric CO2 enrichment increased grain yield.  Hence, it became clear that whenever the grain yield of the wheat was changed -- by CO2, O3 or even water stress, which was also a variable in one of the experiments -- grain protein concentrations either moved up or down along a common linear relationship in the opposite direction to the change in grain yield elicited by the CO2, O3 or water stress treatment.

In an earlier study of CO2 and O3 effects on wheat grain yield and quality, Rudorff et al. (1996) obtained essentially the same results.  They observed, for example, that "flour protein contents were increased by enhanced O3 exposure and reduced by elevated CO2" but that "the combined effect of these gases was minor."  Hence, they concluded that "the concomitant increase of CO2 and O3 in the troposphere will have no significant impact on wheat grain quality."

Earlier still, Evans (1993) had found similar relationships to exist for several other crops, further observing them to be greatly affected by soil nitrogen availability.  It is highly likely, therefore, that the differing availability of soil nitrogen could have been responsible for some of the differing results observed in the many other studies reviewed by Idso and Idso (2001); and, in fact, that is precisely what the study of Rogers et al. (1996) suggests.  Although the latter investigators observed CO2-induced reductions in the protein concentration of flour derived from wheat plants growing at low soil nitrogen concentrations, no such reductions were evident when the soil nitrogen supply was increased to a higher rate of application.  Hence, Pleijel et al. concluded that the oft-observed negative impact of atmospheric CO2 enrichment on grain protein concentration would probably be alleviated by higher applications of nitrogen fertilizers; and the study of Kimball et al. (2001) confirmed their hypothesis.

Kimball et al. studied the effects of a 50% increase in atmospheric CO2 concentration on wheat grain nitrogen concentration and the baking properties of the flour derived from that grain throughout four years of free-air CO2 enrichment experiments.  In the first two years of their study, soil water content was an additional variable; and in the last two years, soil nitrogen content was a variable.  The most influential factor in reducing grain nitrogen concentration was determined to be low soil nitrogen; and under this condition, atmospheric CO2 enrichment further reduced grain nitrogen and protein concentrations, although the change was much less than that caused by low soil nitrogen.  When soil nitrogen was not limiting, however, increases in the air's CO2 concentration did not affect grain nitrogen and protein concentrations; neither did they reduce the baking properties of the flour derived from the grain.  Hence, it would appear that given sufficient water and nitrogen, atmospheric CO2 enrichment can significantly increase grain yield without sacrificing grain protein concentration in the process.

In some situations, however, atmospheric CO2 enrichment may actually increase the protein concentration of wheat.  Agrawal and Deepak (2003), for example, grew two cultivars of wheat (Triticum aestivum L. cv. Malviya 234 and HP1209) in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm alone and in combination with 60 ppb SO2 to study the interactive effects of elevated CO2 and this major air pollutant on crop growth.  They found that exposure to elevated SO2 caused an average 13% decrease in foliar protein concentrations in both cultivars.  However, when plants were concomitantly exposed to an atmospheric CO2 concentration of 600 ppm, leaf protein levels only decreased by 3% in HP1209, while they actually increased by 4% in Malviya 234.

In the case of rice -- which according to Wittwer (1995) is "the basic food for more than half the world's population," supplying "more dietary energy than any other single food" -- Jablonski et al. (2002) conducted a wide-ranging review of the scientific literature, finding that it too appeared to suffer no reduction in grain nitrogen (protein) concentration in response to atmospheric CO2 enrichment.  Likewise, they found no CO2-induced decrease in seed nitrogen concentration in the studies of legumes they reviewed.  This finding is also encouraging, since according to Wittwer (1995), legumes "are a direct food resource providing 20% of the world's protein for human consumption," as well as "about two thirds of the world's protein concentrate for livestock feeding."  What is more, the biomass of the CO2-enriched wheat, rice and legumes was found by Jablonski et al. to be significantly increased.  Hence, there will likely be a vast increase in the total amount of protein that can be made available to humanity in a future CO2-enriched world, both directly via food crops and indirectly via livestock.

With respect to the specific legume soybeans, Thomas et al. (2003) note that "oil and protein comprise ~20 and 40%, respectively, of the dry weight of soybean seed," which "unique chemical composition," in their words, "has made it one of the most valuable agronomic crops worldwide."  In addition, they say that "the intrinsic value of soybean seed is in its supply of essential fatty acids and amino acids in the oil and protein, respectively;" and in this regard they report that Heagle et al. (1998) "observed a positive significant effect of CO2 enrichment on soybean seed oil and oleic acid concentration," although they could find no such effect in their study.

b. Antioxidant content

Antioxidants are also of great importance to human health; and one of the most prominent of these plant products is ascorbate or vitamin C.  In the early studies of Barbale (1970) and Madsen (1971, 1975), a tripling of the atmospheric CO2 concentration produced a modest (7%) increase in this antioxidant in the fruit of tomato plants.  Kimball and Mitchell (1981), however, could find no effect of a similar CO2 increase on the same species, although the extra CO2 of their study stimulated the production of vitamin A.  In bean sprouts, on the other hand, a mere one-hour-per-day doubling of the atmospheric CO2 concentration actually doubled plant vitamin C contents over a 7-day period (Tajiri, 1985).

Probably the most comprehensive investigation of CO2 effects on vitamin C production in an agricultural plant -- a tree crop (sour orange) -- was conducted by Idso et al. (2001).  In an atmospheric CO2 enrichment experiment begun in 1987 and still ongoing, a 75% increase in the air's CO2 content was observed to increase sour orange juice vitamin C concentration by approximately 5% in run-of-the-mill years when total fruit production was typically enhanced by about 80%.  In aberrant years when the CO2-induced increase in fruit production was much greater, however, the increase in fruit vitamin C concentration was also greater, rising to a CO2-induced enhancement of 15% when fruit production on the CO2-enriched trees was 3.6 times greater than it was on the ambient-treatment trees.

These findings take on great significance when it is realized that scurvy -- which is induced by low intake of vitamin C -- may be resurgent in industrial countries, especially among children (Ramar et al., 1993; Gomez-Carrasco et al., 1994), and that subclinical scurvy symptoms are increasing among adults (Dickinson et al., 1994).  Furthermore, Hampl et al. (1999) have found that 12-20% of 12- to 18-year-old school children in the United States "drastically under-consume" foods that supply vitamin C; while Johnston et al. (1998) have determined that 12-16% of U.S. college students have marginal plasma concentrations of vitamin C.  Hence, since vitamin C intake correlates strongly with the consumption of citrus juice (Dennison et al., 1998), and since the only high-vitamin-C juice consumed in any quantity by children is orange juice (Hampl et al., 1999), the modest role played by the ongoing rise in the air's CO2 content in increasing the vitamin C concentration of orange juice could ultimately prove to be of considerable significance for public health in the United States and elsewhere.

Another important study to assess the impact of elevated levels of atmospheric CO2 on plant antioxidant production was that of Wang et al. (2003), who evaluated the effects of elevated CO2 on the antioxidant activity and flavonoid content of strawberry fruit in field plots at the U.S. Department of Agriculture's Beltsville Agricultural Research Center in Beltsville, Maryland, where they grew strawberry plants (Fragaria x ananassa Duchesne cv. Honeoye) in six clear-acrylic open-top chambers, two of which were maintained at the ambient atmospheric CO2 concentration, two of which were maintained at ambient + 300 ppm CO2, and two of which were maintained at ambient + 600 ppm CO2 for a period of 28 months (from early spring of 1998 through June 2000).  The scientists harvested the strawberry fruit, in their words, "at the commercially ripe stage" in both 1999 and 2000, after which they analyzed them for a number of different antioxidant properties and flavonol contents.

Before reporting what they found, Wang et al. provide some background by noting that "strawberries are good sources of natural antioxidants (Wang et al., 1996; Heinonen et al., 1998)."  They further report that "in addition to the usual nutrients, such as vitamins and minerals, strawberries are also rich in anthocyanins, flavonoids, and phenolic acids," and that "strawberries have shown a remarkably high scavenging activity toward chemically generated radicals, thus making them effective in inhibiting oxidation of human low-density lipoproteins (Heinonen et al., 1998)."  In this regard, they note that previous studies (Wang and Jiao, 2000; Wang and Lin, 2000) "have shown that strawberries have high oxygen radical absorbance activity against peroxyl radicals, superoxide radicals, hydrogen peroxide, hydroxyl radicals, and singlet oxygen."  In their experiment, therefore, they were essentially seeking to see if atmospheric CO2 enrichment could make a good thing even better.

So what did the Agricultural Research Service scientists find?  They determined, first of all, that strawberries had higher concentrations of ascorbic acid (AsA) and glutathione (GSH) "when grown under enriched CO2 environments."  In going from ambient to ambient + 300 ppm CO2 and ambient + 600 ppm CO2, for example, AsA concentrations increased by 10 and 13%, respectively, while GSH concentrations increased by 3 and 171%, respectively.  They also learned that "an enriched CO2 environment resulted in an increase in phenolic acid, flavonol, and anthocyanin contents of fruit."  For nine different flavonoids, for example, there was a mean concentration increase of 55 ± 23% in going from the ambient atmospheric CO2 concentration to ambient + 300 ppm CO2, and a mean concentration increase of 112 ± 35% in going from ambient to ambient + 600 ppm CO2.  In addition, they report that the "high flavonol content was associated with high antioxidant activity."  As for the significance of these findings, Wang et al. note that "anthocyanins have been reported to help reduce damage caused by free radical activity, such as low-density lipoprotein oxidation, platelet aggregation, and endothelium-dependent vasodilation of arteries (Heinonen et al., 1998; Rice-Evans and Miller, 1996)."

In summarizing their findings, Wang et al. say "strawberry fruit contain flavonoids with potent antioxidant properties, and under CO2 enrichment conditions, increased the[ir] AsA, GSH, phenolic acid, flavonol, and anthocyanin concentrations," further noting that "plants grown under CO2 enrichment conditions also had higher oxygen radical absorbance activity against [many types of oxygen] radicals in the fruit."  Hence, they determined that atmospheric CO2 enrichment truly did "make a good thing better."

We note additionally in this regard that elevated levels of atmospheric CO2 also make more of that good thing.  Deng and Woodward (1998), for example, report that after growing strawberry plants in air containing an additional 170 ppm of CO2, total fresh fruit weights were 42 and 17% greater than weights displayed by control plants grown at high and low soil nitrogen contents, respectively; while Bushway and Pritts (2002) report that a two- to three-fold increase in the air's CO2 content boosted strawberry fruit yield by an average of 62%.  In addition, Campbell and Young (1986), Keutgen et al. (1997), and Bunce (2001) report positive strawberry photosynthetic responses to an extra 300 ppm of CO2 ranging from 9% to 197% (mean of 76% 15%); and Desjardins et al. (1987) report a 118% increase in photosynthesis in response to a 600 ppm increase in the air's CO2 concentration.

3. Medicinal Constituents of Plants

Primitive medical records indicate that extracts from many species of plants have been used for treating a variety of human health problems for perhaps the past 3500 years (Machlin, 1992; Pettit et al., 1993, 1995).  In modern times the practice has continued, with numerous chemotherapeutic agents being isolated (Gabrielsen et al., 1992a).  Until recently, however, no studies had investigated the effects of atmospheric CO2 enrichment on specific plant compounds of direct medicinal value.

This situation changed when Stuhlfauth et al. (1987) studied the individual and combined effects of atmospheric CO2 enrichment and water stress on the production of secondary metabolites in the woolly foxglove (Digitalis lanata EHRH), which produces the cardiac glycoside digoxin that is used in the treatment of cardiac insufficiency.  Under controlled well-watered conditions in a phytotron, a near-tripling of the air's CO2 content increased plant dry weight production in this medicinal plant by 63%, while under water-stressed conditions the CO2-induced dry weight increase was 83%.  In addition, the concentration of digoxin within the plant dry mass was enhanced by 11% under well-watered conditions and by 14% under conditions of water stress.

In a subsequent whole-season field experiment, Stuhlfauth and Fock (1990) obtained similar results.  A near-tripling of the air's CO2 concentration led to a 75% increase in plant dry weight production per unit land area and a 15% increase in digoxin yield per unit dry weight of plant, which combined to produce an actual doubling of total digoxin yield per hectare of cultivated land.

Equally impressive was the study of Idso et al. (2000), who evaluated the response of the tropical spider lily (Hymenocallis littoralis Jacq. Salisb.) to elevated levels of atmospheric CO2 over four growing seasons.  This plant has been known since ancient times to possess anti-tumor activity; and in modern times it has been shown to contain constituents that are effective against lymphocytic leukemia and ovary sarcoma (Pettit et al., 1986).  These same plant constituents have also been proven to be effective against the U.S. National Cancer Institute's panel of 60 human cancer cell lines, demonstrating greatest effectiveness against melanoma, brain, colon, lung and renal cancers (Pettit et al., 1993).  In addition, it exhibits strong anti-viral activity against Japanese encephalitis and yellow, dengue, Punta Tora and Rift Valley fevers (Gabrielsen et al., 1992a,b).

Idso et al. determined that a 75% increase in the air's CO2 concentration produced a 56% increase in the spider lily's belowground bulb biomass, where the disease-fighting substances are found.  In addition, for these specific substances, they observed a 6% increase in the concentration of a two-constituent (1:1) mixture of 7-deoxynarciclasine and 7-deoxy-trans-dihydronarciclasine, an 8% increase in pancratistatin, an 8% increase in trans-dihydronarciclasine, and a 28% increase in narciclasine.  Averaged together and combined with the 56% increase in bulb biomass, these percentage concentration increases resulted in a total mean active-ingredient increase of 75% for the plants grown in air containing 75% more CO2.

4. Other Plant Constituents

A number of other plant constituents also perform important functions in maintaining human health, including sugars, lipids, oils and fatty acids, as well as macro- and micro-nutrients.  Although concerns have been raised about the availability of certain of the latter elements in plants growing in a CO2-enriched world (Loladze, 2002), the jury is still out with respect to this subject as a consequence of the paucity of pertinent data.  Literally thousands of studies have assessed the impact of elevated levels of atmospheric CO2 on the quantity of biomass produced by agricultural crops, but only a tiny fraction of that number have looked at any aspect of food quality.  From what has been learned about plant protein, antioxidants and the few medicinal substances that have been investigated in this regard, however, there is no reason to believe that these other plant constituents would be present in any lower concentrations in a CO2-enriched world of the future than they are currently.  Indeed, there is ample evidence to suggest they may well be present in significantly greater concentrations, and certainly in greater absolute amounts.