Contributed by Lindsay Howell ©2018
According to the Merriam-Webster dictionary, “pollutant” is defined as a substance that contaminates an environment. Going further, the definition of “contaminate” is to make said environment unfit for use by the introduction of unwholesome or undesirable elements. So, the question becomes, is CO2 an unwholesome and undesirable molecule that renders our atmosphere “unfit for use”?
When I hear the term “CO2 pollution”, I think of the equation for photosynthesis. For billions of years, the vast majority of Earth’s plant life has been completely dependent on CO2. If it weren’t for plants’ amazing ability to create sugars based on this equation, we oxygen-breathers would simply not exist. It strikes me that CO2 is not a contaminant, but rather a desirable molecule that makes our atmosphere “fit for use”.
For billions of years, the dominant gases in the atmosphere were N2, CO2, NH3 and CH4. Photosynthesis first appeared ~ 3.5 B years ago, and atmospheric O2 was practically nonexistent until ~ 2.4 B years back. At this point in time, the Great Oxygenation Event occurred, which resulted in one of the most significant mass extinctions in Earth’s history. Though it has become essential to life on our planet, oxygen imposes oxidative stress on all animals, slowly “rusting” them from the inside out. Based on these facts, one could facetiously label O2 as an environmental “pollutant”.
Image retrieved from http://elte.prompt.hu/sites/default/files/tananyagok/AtmosphericChemistry/ch01.html
Today’s atmosphere may be considered “carbon-starved” as the CO2 concentration is currently at a relatively low 400 ppm. This is not an ideal concentration for plant growth, as photosynthesis typically shuts down at 200 to 150 ppm. Compare this to the fact that atmospheric CO2 concentration has ranged much higher in Earth’s history (e.g. over 300,000 ppm when life first appeared, 7,000 ppm 500 M years ago, 1,000 – 2,000 ppm 240 – 65 M years ago). During these periods, life has managed to flourish and evolve into countless species. Studies have confirmed that higher CO2 concentrations lead to increased plant growth, which is supported by the stoichiometry of the photosynthesis process. Consider the monstrous, expansive forests that existed during the time of the dinosaurs, for instance.
Recent investigations have determined that increased atmospheric CO2 concentrations have benefited the terrestrial biosphere. Deserts are shrinking, forests are expanding and crop yields are reaching new records. A recent article in Nature Climate Change, summarized by NASA, reports a significant increase in the leaf area (a.k.a. “greening”) of Earth’s vegetation over the past 35 years. This change is largely attributed to increased CO2 concentration in the atmosphere. A blog post by the esteemed Dr. Judith Curry further explores the topic.
Below is a fascinating time-lapse video that documents the effect of elevated CO2 concentration on the growth of a cowpea plant.
For peer-reviewed scientific studies that report the growth responses of plants to atmospheric CO2 enrichment, CO2 Science is one place to start.
In the field of biology, toxicity is determined by the dose size of the substance in question. For CO2 to be considered toxic to most forms of oxygen-breathing life, the concentration would need to be 60,000 ppm, or 150 times higher than today’s concentration. It has been billions of years since Earth’s atmosphere contained a similar concentration. And throughout history the Earth has demonstrated a tremendous capacity to absorb atmospheric CO2 in its crust, oceans, and biomass. Considering all this, I reckon we will be safe for a long, long time.
Meanwhile, we should be celebrating CO2 as a gas of life.