Very high concentrations of atmospheric CO2 can produce a state of hypercapnia or an excessive amount of CO2 in the blood (Nahas et al., 1968; Brackett et al., 1969; van Ypersele de Strihou, 1974), which typically results in acidosis, a serious and sometimes fatal condition characterized in humans by headache, nausea and visual disturbances (Poyart and Nahas, 1968; Turino et al., 1974).  However, these phenomena do not impact human health until the atmosphere's CO2 concentration reaches approximately 15,000 ppm (Luft et al., 1974; Schaefer, 1982), which is approximately 40 times greater than its current concentration.  Hence, we do not have to worry about any direct negative health effects of the ongoing rise in the air's CO2 content.

But what about positive health effects of atmospheric CO2 enrichment?  Is there any evidence the historical rise in the air's CO2 content has been good for us?

In a lengthy review of research directly related to this question, Idso and Idso (2001) note that elevated concentrations of atmospheric CO2 have been shown to increase the concentrations of vitamins A and C in various fruits and vegetables.  They also note that atmospheric CO2 enrichment increases the concentrations of several disease-fighting substances in certain medicinal plants.  In experiments with the woolly foxglove (Digitalis lanata), for example, in addition to increasing plant biomass by 63 to 83%, a near-tripling of the air's CO2 content increased the concentration of heart-helping digoxin by 11 to 14% (Stuhlfauth et al., 1987; Stuhlfauth and Fock, 1990).  Likewise, in the tropical spider lily (Hymenocallis littoralis), in addition to increasing plant biomass by 56%, a mere 75% increase in the air's CO2 content increased the concentrations of five different substances proven effective in treating a number of human cancers (leukemia, melanoma, brain, colon, lung, ovarian and renal), as well as several viral diseases (Japanese encephalitis and yellow, dengue, Punta Tora and Rift Falley fevers) by 6 to 28% (Idso et al., 2000).

If these types of responses are widespread - and there is no reason to believe they are not - what would one expect to have observed over the past century and a half of rising atmospheric CO2 concentrations?  For one thing, we should have seen increasingly better human health; and, as a corollary of increasingly better health, we should have seen ever-increasing longevity in humans.  So, what has been observed in this regard?

In a study of mortality over the period 1950-1994 in the G7 countries - Canada, France, Germany (excluding the former East Germany), Italy, Japan, the United Kingdom, and the United States - Tuljapurkar et al. (2000) found that "in every country over this period, mortality at each age has declined exponentially at a roughly constant rate."  In commenting on this finding, Horiuchi (2000) notes that the average lifespan of early humans was approximately 20 years, but that in the major industrialized countries it is now about 80 years, with the bulk of this increase having come in the past 150 years.  He also notes that "it was widely expected that as life expectancy became very high and approached the 'biological limit of human longevity,' the rapid 'mortality decline' would slow down and eventually level off."  In light of the findings of Tuljapurkar et al., however, Horiuchi acknowledges that "such a deceleration has not occurred."

"These findings give rise to two interrelated questions," says Horiuchi: (1) "Why has the mortality decline not started to slow down?" and (2) "Will it continue into the future?"

Some points to note in attempting to answer these questions are the following.  First, in Horiuchi's words, "in the second half of the nineteenth century and the first half of the twentieth century, there were large decreases in the number of deaths from infectious and parasitic diseases, and from poor nutrition and disorders associated with pregnancy and childbirth," which led to large reductions in the deaths of infants, children and young adults.  In the second half of the twentieth century, on the other hand, he notes that "mortality from degenerative diseases, most notably heart diseases and stroke, started to fall," with the reduction being most pronounced among the elderly.  Horiuchi says some scientists suspected this latter drop in mortality might have been achieved "through postponing the deaths of seriously ill people."  However, he notes that data from the United States demonstrate that "the health of the elderly greatly improved in the 1980s and 1990s, suggesting that the extended length of life in old age is mainly due to better health [our italics] rather than prolonged survival in sickness."  These observations, says Horiuchi, led many biologists who "used to think that senescent processes might be programmed into the biological clock of the body" to adopt the view that "senescence is mainly due to the body's imperfect systems of maintenance and repair, which allow the long-term accumulation of unrepaired damage in macromolecules, cells, tissues and organs."

To summarize to this point, it appears that in countries with highly developed market economies - where good health care is readily available - deaths of infants, children and young adults have been dramatically reduced over the last century or so, to the point where average life expectancy is now largely determined by what happens to elderly people; and it is evident that the elderly are living longer and longer with the passing of time.  It is further evident that this phenomenon is likely due to ever-improving health in older people, which is likely due to continuing improvements in their bodily systems for repairing cellular damage caused by degenerative processes associated with advancing age.  [See Finkel and Holbrook (2000) and Melov et al. (2000) for more on this point.]  And since these observations are common to all of the G7 nations, they must be the result of an ubiquitous phenomenon that is occurring simultaneously across the entire planet and increasing in importance with the passing of time.  What could that thing be?

The most logical candidate would appear to us to be the historical and still-ongoing rise in the air's CO2 content.  This phenomenon provides a near-perfect fit with both the timing and the nature of the increase in human longevity, which has (1) occurred primarily over the past 150 years, and which has (2) progressed essentially exponentially.  In addition, the increase in the air's CO2 content has been demonstrated to have the capacity to increase both the availability of nutritious food, as well as the concentrations of important antioxidants, vitamins and other health-promoting substances contained therein.

Further support for our hypothesis is provided by the study of Manton and Gu (2001).  With the completion of the latest of the five National Long-Term Care Surveys of disability in U.S. citizens over 65 years of age - which began in 1982 and now extends to 1999 - these researchers were able to discern two most interesting trends: (1) disabilities in this age group decreased over the entire period studied, and (2) disabilities decreased at a rate that grew ever larger with the passing of time.

Specifically, over the entire 17-year period of record, there was an amazing relative decline in chronic disability of 25%, as the percentage of people over 65 years of age who were disabled dropped from 26.2% in 1982 to 19.7% in 1999.  What is more, the rate of disability decline for the periods 1982-1989, 1989-1994 and 1994-1999 was 0.26, 0.38 and 0.56% per year, respectively.  Commenting on the ever-accelerating nature of this disability decline, Manton and Gu say "it is surprising, given the low level of disability in 1994, that the rate of improvement accelerated" over the most recent five-year interval.

To what do Manton and Gu attribute these amazing observations?  The scientists cite many research papers that point to improvements in human nutrition as the underlying cause of the documented improvements in well-being among the elderly; and as we have noted previously, that is exactly what the ongoing rise in the air's CO2 content should be providing, according to our hypothesis, i.e., foods with ever-increasing concentrations of various substances that promote good health.

In spite of all this positive news, there are still those who attempt to paint the rising CO2 content of the atmosphere as a threat to human health.  Wayne et al. (2002), for example, grew common ragweed plants (Ambrosia artemisiifolia L.) in controlled-environment glasshouses maintained at ambient (350 ppm) and enriched (700 ppm) atmospheric CO2 concentrations, finding that "stand-level pollen production was 61% higher in elevated versus ambient CO2 environments."  Hence, they concluded that "the incidence of hay fever and related respiratory diseases may increase in the future."  And in the Harvard University Gazette, as noted in our Editorial of 10 April 2002, it was claimed that these results "highlight the need to reduce carbon dioxide levels."

To truly determine if atmospheric CO2 levels should be encouraged to change (either rise or fall), it is obvious that all of their potential impacts need to be evaluated, as well as the likelihood of those impacts actually occurring in the real world, as opposed to simply occurring in computer models (in the worst of cases) or laboratory or field experiments (in the best of cases).  In our Editorial of 10 April 2002, for example, we discussed the very pressing need to obtain "more crop per drop" of water within this context - which elevated levels of atmospheric CO2 ably facilitate by enhancing plant water use efficiency - while in our Editorial of 29 May 2002 we discussed the ability of atmospheric CO2 enrichment to reduce the worldwide dispersal of debilitating iron-coated dust particles, which often harbor pathogenic fungi, bacteria and viruses.

In concluding our summary of the health effects of atmospheric CO2 enrichment, we must also take note of two other studies.  In the first, Caporn et al. (1999) grew the bracken weed (Pteridium aquilinum) - which is a potential threat to human health in the United Kingdom and several other regions of the world - at atmospheric CO2 concentrations of 370 and 570 ppm for 19 months at normal and high levels of soil fertility.  In both fertility treatments, elevated CO2 did not increase the biomass of any plant parts; and in the normal nutrient regime, it actually reduced the area of plant fronds, suggesting - if anything - that this noxious weed may possibly have less of a deleterious impact as the CO2 content of the air continues to rise.

In the second study, Malmstrom and Field (1997) grew oat plants - one third of which were infected with barley yellow dwarf virus, which plagues more that 150 species of plants, including all major cereal crops - for two months in chambers having atmospheric CO2 concentrations of 350 and 700 ppm, finding that the extra CO2 increased plant biomass by 12% in the healthy plants, but that it boosted vegetative biomass three times more - by fully 36% - in the infected plants.  Hence, it can be appreciated that elevated levels of atmospheric CO2 have a medicinal effect on plants affected with this potent virus, and that they are thus of benefit to humanity as well, as they allow more cereal grains to be produced to feed the world's growing population.

In light of these many and diverse observations, it should be clear that elevated levels of atmospheric CO2 have a host of positive impacts on human health, as well as the health of most of the animate world.  Although we have identified one potential negative impact (the enhanced production of ragweed pollen) - and there could well be others - we feel that the balance of evidence clearly implies a net benefit in terms of overall biospheric health.

References
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