CO2 history & lifecycle
This section of All about CO2 provides insights on the sources of and processes that emit CO2 emissions, the history of CO2 emissions from the earliest stages of earth, and specific stages CO2 passes over its lifecycle.
Knowing more about CO2’s origins and an understanding of its evolution in earth’s historical context can enable readers to get a better contextual perspective of what CO2 really is, beyond just a gas.
CO2 emissions can be a result of natural phenomena or human-induced activities, also called anthropogenic emissions.
Natural sources of emissions
Human & animal respiration - humans and animals inhale oxygen and give out CO2. Cellular respiration converts ingested nutrients and oxygen to energy. CO2 is produced as a byproduct of this reaction.
Plant respiration - Plants undergo respiration at night and exhale the excess CO2 stored in them
CO2 emissions from dead and decaying biomass
Natural phenomena such as volcano eruptions
CO2 emissions from the use of energy for electricity, heating and transport fuels.
Industrial processes (chemical reactions in most cases) that release CO2 as a by-product.
Agricultural activities - activities such as soil tilling during farming can release CO2 stored in the soil.
Land use changes - emissions from human induced changes in the use of land, for instance, changing forest land to agricultural land.
Our cellular respiration converts food and oxygen taken in by the body to energy in the form of adenosine triphosphate (ATP) through a series of reactions.
CO2 is produced as a byproduct of these reactions.
Here is the process is In a bit more detail. The entire process of inputs to energy conversion takes place in four steps: Glycolysis stage, Intermediate stage, Citric acid stage and the electron transfer stage. Each stage has its own role and each cycle in the process results in a net production of about 28-30 ATPs. CO2 is released in the second and third stages of this process viz., Intermediate stage and Citric stage.
During daylight hours, plants take in carbon dioxide and release oxygen through photosynthesis, and at night about half that carbon is then released through respiration. The total amount of CO2 given out by plants during respiration is much higher than CO2 emitted by all human breathing - as much as ten times higher. While that's a lot of CO2 given out, the plants are taking in twice the amount at the start.
All appears to be well.
But perhaps we should start getting a bit more concerned because, as the world becomes warmer, plants and trees could respire more, releasing more CO2 and even higher temperatures, triggering a hazardous feedback loop.
During combustion, the carbon from a hydrocarbon fuel combines with oxygen from the air to produce CO2. Complete combustion of any hydrocarbon will thus always lead to emissions of CO2 and H2O.
How can we make combustion processes emit less CO2? Given the unalterable fact that combustion of a hydrocarbon will always emit CO2, there are only two ways to reduce CO2 emissions from combustion. One is to reduce CO2 emissions from hydrocarbon combustion by increasing the efficiency of combustion such that we recover more energy from a certain mass of hydrocarbon fuel than otherwise. The other way is to use a fuel that is not a hydrocarbon in the first place - a prominent example is combusting hydrogen for energy, which results only in water as the by-product.
Note however that incomplete combustion can result in products that are other than CO2 - carbon monoxide for instance. Two prominent incomplete combustion processes - pyrolysis and gasification - in fact result in an organic mixture of gases that comprise much more carbon monoxide than carbon dioxide.
Between 65% and 80% of CO2 released into the atmosphere dissolves into the ocean over a period of 20–200 years.
The remaining CO2 is removed by slower processes that take up to several thousands, or even hundreds of thousands of years. These processes include chemical weathering and rock formation.
All these imply that once CO2 enters the atmosphere, at least some portions of it will continue to affect climate for many hundreds of years.
While many avenues are available to utilize CO2 by converting it into products or using it in applications - some of which could even sequester it for a very long time - this is not the same as recycling.
Strictly speaking, CO2 recycling refers to a concept in which CO2 is captured at source from a production process emissions and is utilized back in the process in some manner. For instance, in the steel industry, it is possible to use syngas (a mixture of CO and H2) as the reducing agent for iron ore, in place of coke. In the process, where both H2 and CO act as reducing agents, CO2 is emitted. It is possible to capture the CO2 and through reaction with green hydrogen produced from renewable powered electrolysis, produce syngas (a mixture of CO and H2) which can be fed back into the iron ore reduction process, and thus the carbon gets infinitely recycled in this process. Note that such a CO2 recycling, though technically possible, is only being currently piloted in the steel industry (as of 2021).
Another almost perfect example of CO2 recycling happens in the plant domain. Biomass - which is derived from CO2 captured from air - can be converted to biochar which can be used to enhance soil fertility by helping soil retain nutrients like nitrogen and phosphorus, resulting in better plant growth and thus increased CO2 capture from the atmosphere, resulting in a virtuous cycle.
It might be more challenging to find many such avenues for perfect recycling of CO2. However, if one were to broaden the definition of “recycling” and allow for carbon emitted to be converted to a product that sequesters the carbon but could be used in a different location, many recycling possibilities emerge. CO2 emitted from power plants could be used to cure concrete and thus get sequestered for long periods. CO2 captured from breweries and wineries could be converted to durable plastics that can sequester carbon for long time periods.
As a greenhouse gas, CO2 traps the heat radiated from the earth and reflects some of it back to earth.
At high levels of CO2 in the atmosphere, this phenomenon creates global warming. While high levels of CO2 is a problem, having very low levels of CO2 or no CO2 will result in a much colder world because some heat had not been sent back to earth. So very low levels or zero CO2 is not an acceptable solution either!
Very low levels or no CO2 in the atmosphere will also mean plants will not be able to produce their - and our - food.
It's all about maintaining a balance.