Carbon Dioxide as a Pollutant.

The Risks of Rising Atmospheric CO2 Levels on Human Health and on the Stability of the Biosphere.

This is an excerpt from the paper I uploaded on “ArXiv.” You can access the whole paper at this link.

The impact of increasing CO₂ concentrations in the atmosphere was discussed for the first time in 1896 by Svante Arrhenius in terms of its radiative forcing effect as an absorber and emitter of infrared radiation (IR) [1]. Arrhenius didn’t take into account the chemical and biochemical effects of CO₂, which started to be identified only about half a century later. The story is told in detail by Brewer [2], who reports that the first experiments on ocean acidity were carried out in the 1930s, but the term “ocean acidification” didn’t become popular until the turn of the millennium [3]. Ocean acidification occurs when CO₂ dissolves in seawater to form carbonic acid (H₂CO₃), a weak acid that dissociates into bicarbonate (HCO₃^- ) and hydrogen ions (H⁺). Increased H⁺ means increased acidity (lower pH). These high acidity levels have negative consequences, such as coral bleaching and damage to calcifying organisms. Nevertheless, ocean acidification is much less discussed than global warming and there is no agreed limit to ocean acidity equivalent to the internationally set temperature limit of the atmosphere.

The reactivity of CO₂ appears not just in seawater but in all the environments where CO₂ acts as an acid, either in aqueous environments where it generates hydrogen ions or when acting as a Lewis acid as an electron acceptor. Carbon dioxide is the keystone of the two main processes that create the planetary biosphere: photosynthesis and respiration. Without CO₂, there could be no life on Earth, but too much CO₂ is not necessarily a good thing.

The effect of CO₂ on the growth rate of some plants has been known for about a century [2]. This effect is measurable in terms of an increase in growth rates or leaf coverage, and it is considered the leading cause of the “global greening” phenomenon observed in recent years [4], [5]. Although CO₂ may increase agricultural yields, it is known that it does not generate an increase in the nutritional content of the food produced [6], [7].

On the other side of the biosphere cycle, respiration, the effects of high CO₂ concentrations are not easy to measure in quantitative terms but have been known since the 19^th century under the name of “hypercapnia” (from the Greek hyper, "above" and kapnos, "smoke"). Common symptoms are dyspnea (breathlessness), nausea, headache, confusion, lethargy, and other symptoms. These effects are attributable to various factors but have been demonstrated to be related to reduced oxygen flow to tissues and to the brain [8]. It is known that CO₂ concentrations over ca. 50,000 ppm are lethal, while it is normally believed that values up to 5,000 ppm are acceptable for limited periods of time. Values under 1,000 ppm are considered safe inside homes. The effect of lower concentration is less clear, but recent results show that even lower concentrations can have measurable negative effects on the human metabolism, and affect the human brain in terms of the capability of performing complex tasks [9], [10], [11], [12], [13], [14], [15], [16], [17]. The results of these studies have been criticized for internal inconsistencies and other problems [18]. It is clear that we need more and better studies to determine with certainty the effect of CO₂ on human metabolism at these concentrations. But the available data nevertheless point to serious potential problems.

We are introducing into the environment an active substance that we know is lethal at high concentrations. We don’t know what an acceptable lifetime exposure limit could be, and not even if it exists. The only thing we know is that current concentrations have never been experienced by human beings during their evolutionary history of the past few million years. Additionally, nowadays people tend to live in closed spaces where the CO₂ concentrations are typically higher than those in the open, not rarely well above 1000 ppm. The habit of wearing face masks in indoor environments has led to controversial assessments, but even though it may not be a critical problem, it can only increase the concentration of breathed CO₂ [19], [20]. The real problem, though, is that the CO₂ concentration in the atmosphere continues to rise at an increasingly faster rate, now being near 3 ppm per year. If this trend continues, it is clear that we are moving into an unknown territory with risks that cannot be neglected.

The present paper is an exploration of what’s currently known about the metabolic effects of different atmospheric compositions and, in particular, of increasing CO₂ concentrations. It shows how the current atmospheric composition may have been an important factor in the development of the large brains, which are a characteristic of the “homo sapiens” species [21] and of other highly encephalized mammalian and bird species (e.g. dolphins, primates, ravens, and elephants) [22]. The current trends of increasing CO₂ concentrations may make the metabolic requirements of such large brains impossible to maintain.

The biochemical effects of CO₂ are also relevant to the current debate on geoengineering as Solar Radiation Management, SRM. In view of the projected biochemical damage of excessive CO₂ concentrations on the biosphere, radiation shielding makes sense only if coupled with actions aimed at controlling and reducing CO₂ concentrations.

References

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