A Pathway To 350 PPM, Part 1: Carbon Sequestration Is Vital

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Humans are responsible for emitting around ​55 billion tonnes of CO​2e​q/year​, with 2⁄3 of the emissions coming from the burning of fossil carbon. Currently, there are ​414 PPM​ of CO​2​ in the atmosphere, and it’s rising at just over 2 PPM per year. Even if we were to completely stop burning fossil carbon, we would still see an increase in the amount of greenhouse gases (GHGs) entering the atmosphere, largely from agriculture and potentially from ​positive feedback loops​ triggered by a hotter world.

Global greenhouse gas emissions per type of gas & source

Source: PBL Netherlands Environmental Assessment Agency, EDGAR v5.0/v4.3.2 FT 2017 (EC-JRC/Pbl, 2018); Houghton and Nassikas (2017)

Positive feedback loops are arguably the most frightening aspect of climate change because they lead to a phenomenon known as “runaway global warming,” a condition in which the warming planet releases trapped greenhouse gases, causing it to warm even faster, regardless of human intervention. One positive feedback loop that gets a lot of attention is the warming of the arctic permafrost. As the arctic tundra melts, methane, a powerful trapped greenhouse gas is released, which leads to more warming, which, in turn, leads to more methane being released, and so on. Some other examples of positive feedback loops include the melting of the ​polar ice cap​, droughts causing ​forests to burn​ and release more CO​2, a​nd warming oceans causing the gasification of ​methane hydrates​ on the seafloor. Scientists have theorized that the gasification of methane hydrates could have been the cause of the greatest extinction event to have occurred on earth known as the “Great Dying” or the ​Permian–Triassic Extinction Event​.

Photo shows permafrost thaw ponds on peatland on the shores of Hudson Bay, Canada. Photo ​by ​Steve Jurvetson

According to a paper by ​Hansen et al​., a safe level of atmospheric CO​2​ to avert runaway global warming is below 350 PPM. Before our discovery and use of fossil carbon, pre-industrial CO​2 levels hovered around ​280 PPM​. So we have our work seriously cut out for us not only to reduce our emissions, but remove (sequester) GHGs from the atmosphere.

Different types of GHGs naturally remain in the atmosphere for different lengths of time. The three most abundant human caused greenhouse gases are nitrous oxide, methane, and carbon dioxide.

Nitrous oxide​ (N​2O​ ) is the third most abundant greenhouse gas and represents 6.2% of human caused greenhouse gas (anthropogenic) emissions. N​2O​ will persist in the atmosphere for about 114 years before being naturally dismantled by chemical reactions in the stratosphere.

Methane​ is second on the list, representing 16% of total anthropogenic emissions. When looking at the global warming potential averaged over a ​20-year time scale​ as opposed to the standard 100-year, methane represents 1⁄3 of anthropogenic emissions, making it a much more serious greenhouse gas in the short term. Methane is a relatively short-lived gas that survives in the atmosphere for only about 12 years before it is broken down by chemical reactions.

Carbon dioxide​ takes the crown contributing 76% of anthropogenic emissions. It naturally survives in the atmosphere between 20 to 200 years with 20% surviving for many thousands of years.

Carbon Sequestration is vital for overcoming the climate crisis

In order to quickly return atmospheric greenhouse gases to safe levels and avoid triggering positive feedback loops, not only do we need to stop burning fossil carbon, we need to implement carbon sequestration strategies. In other words, we need to take CO​2​ out of the atmosphere and store it away, perhaps underground, or more likely, in material forms, like wood. There are two types of carbon sequestration: Geological Carbon Capture and Storage (GEO-CCS) and Biological Carbon Capture and Storage (BIO-CCS) also known as carbon farming. GEO-CCS uses technology, including sorbents, electricity, and heat, to capture and store the whole CO​2​ molecule ultimately underground. BIO-CCS relies on the natural photosynthesis of plants to separate the oxygen atoms of CO​2​ and store the carbon atoms in the stem and roots of plants. Some BIO-CCS examples include forestry, timber and biomass plantations, and ecosystem protection and restoration.

Geological carbon capture & storage

The most popular GEO-CCS technological solutions currently under development to capture CO​2​ from the atmosphere involve ​giant walls of fans,​ sorbents, and heat to extract CO​2​ from the air. The CO​2 must then be compressed at 300psi to become a liquid form and then transported and pumped underground into porous rock, empty oil wells, and mines. All these steps, including the upstream emissions required to construct an air-sourced carbon capture facility, requires a tremendous amount of energy for each molecule of CO​2​ removed. ​Experts​ in the field believe that money is much better spent on renewables (wind, water, solar) that prevent the CO​2 and non-CO​2​ air pollutants from fossil-carbon combustion from getting up there in the first place.

Until we stop burning fossil carbon and have excess renewable energy, GEO-CCS will remain an inefficient and impractical solution for sequestering atmospheric CO​2.​

Biological carbon capture & storage: Carbon Farming

All green growing plants sequester carbon from the atmosphere by storing it in their roots, stems, and leaves. Unlike GEO-CCS solutions, storing carbon in plants require only sunlight, water and green leaves.

Since BIO-CCS only stores carbon, unlike GEO-CCS that stores the much bulkier CO​2 molecule, the oxygen atoms of CO​2​ are returned to the atmosphere and help reverse declining global ​oxygen levels​. ​​BIO-CCS, or more specifically the planting of trees, is a very ​inexpensive option​ for sequestering carbon. The drawback of using plants to sequester CO​2​ is that they require space and lots of it.

When we stop burning fossil carbon we will still be adding about 27% of our current emissions (14.9 gigatonnes of CO​2e​ q) into the atmosphere each year. Perhaps a realistic goal is that we should aim to sequester at least 20 gigatonnes of CO​2e​ q/year to not only capture the equivalent emissions we cannot avoid but also to start drawing down our atmospheric CO​2​ levels back below 350 PPM.

How much land will be required for carbon farming to sequester 20 gigatonnes CO​2e​q/year? Experts estimate afforestation of land in the USA could sequester ​8.4 tonnes/CO​2/​hectare per year​. To sequester 20 gigatonnes would require the planting of 2.4 billion hectares of land, an area the size of North America.

The values are about 3 times better in the tropics that support a year-round growing season and have suitable rainfall. ​25 tonnes CO​2/​hectare per year​ could be sequestered by tree planting which would require 800 million hectares of land, an area slightly larger than Australia.

With a growing population and the world’s tropical forests being cleared each year for agricultural land to feed the world, it seems unlikely that we can find the land to sequester this much CO​2​ with trees, or is it?

See part 2 of this article, where I discuss our options.

About the author: Ryan Logtenberg is a director of the 2 Degrees Institute. The 2° Institute mission is to develop and support strategies that empower people to make the behavioral and lifestyle changes needed to prevent our planet from warming by 2 degrees Celsius.

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