Global warming – Carbon dioxide, its story in the atmosphere, 2

BY JOSE MA. EDUARDO P. DACUDAO

PHOTOSYNTHESIS in earnest may have begun 3 billion years ago. Around 2.5 billion years ago, after most of the reduced compounds of the air, sea, and land got oxidized, molecular oxygen began to accumulate in the atmosphere. The CO₂-depleting effect caused by steady photosynthetic activity added up over the eons.

Then there is what could be described as the acute geophysical reason for CO₂ sequestration. When eukaryotic organisms evolved in the seas, some of them began incorporating calcium carbonate (CaCO₃) into their bodies as skeleton and armor, making it ultimately from calcium oxides and carbon dioxide. They only began doing so in a really massive and abrupt scale half a billion years ago, which is relatively recent in terms of our planet’s geological history. The most obvious ones we see today are the shelled mollusks, arthropods, echinoderms, brachiopods, sponges, and corals; but massive amounts were also made by planktonic microorganisms such as Coccolithophores and Foraminiferans. When they died, they sank into the seafloor and transformed into hard-to-dissolve carbonate minerals, each molecule of which represents one CO₂ molecule sequestered out of the atmosphere. The net reaction would be CO₂ (Carbon dioxide) + CaSiO₃ (Calcium silicate, a component of many rocks) = SiO₂ (Silicon dioxide, the main constituent of sand and quartz) + CaCO₃ (Calcium carbonate, a component of carbonate minerals).

Another reason for CO₂ sequestration is more long-term. The naturally occurring long-lived radioactive nuclides in the planet’s interior have been steadily decaying over billions of years. Heat from their radioactive decay, which originally was being produced at least two magnitudes greater than that of the present rate, ultimately causes much of Tera’s volcanic activity. As the planet’s primordial radionuclides decayed, corresponding volcanic activity has also steadily diminished. Carbon sequestered into the crust by photosynthesizing organisms as fossil fuels (and also by the carbonate-producing organisms as carbonate minerals), instead of being rapidly released back into the atmosphere with each volcanic event (which would combust the fossil fuels and calcine carbonate minerals thus releasing CO₂ back into the atmosphere), tended to stay there for longer periods of geological time. In other words, the geological carbon cycle (in particular the combustion of buried fossil fuels and the second part of the carbonate-silicate cycle CaCO₃ + SiO₂ = CaSiO₃ + CO₂) was much slowed down. More and more of the carbon tended to remain in the crust than in the atmosphere over the eons.

While this was happening, land plants began to get more efficient in extracting the decreasing amounts of CO₂ in the air by evolving the C4 and CAM carbon-concentrating mechanisms for carbon fixation and photosynthesis, just several tens of millions of years ago. This lowered the atmospheric CO₂ level even more.

In the last era of the dinosaurs, the Cretaceous, CO2 levels probably exceeded 1000 parts per million. By the Quaternary Period of the Cenozoic Era, our present era, CO₂ had fallen to less than 300 parts per million. Thus CO₂ scarcity had become the prime limiting factor in the growth of photosynthesizing organisms.

But that was before the industrial revolution. The industrial revolution was and is powered by fossil fuels. Fossil fuels are literally fossils of long dead ancient photosynthetic organisms that had locked in Carbon in their bodies. Burning them releases the CO2 that they originally had sequestered from the atmosphere of past geological eras back into our modern atmosphere. Thus from less than 300 parts per million, present CO2 levels have increased to more than 400 parts per million. It is still rising.

In fact, one of the most obvious effects that rising CO₂ levels had in industrializing Earth was that the planet’s flora experienced a more rapid growth rate; and moreover, they spread in more places where they did not grow so well before, especially in drier areas. (This is shown by the fact that the Earth’s surface’s greenery is 20% more than it was in the 1980s, based on satellite pictures.) Leaves’ stomata openings do not have to increase in diameter as much in an atmosphere with more CO₂ in order to let it in. The narrower the stomatal openings, the less water loss out of the stomata. More CO₂ helps plants minimize water loss. More CO2 by itself also causes plants to grow faster, which is why CO2 is pumped into greenhouses. (This has been known for more than century.) (To be continued)/PN