The recent United Nations Climate Change Conference, or COP26, focused greatly on cutting global carbon emissions. A lot of emphasis was on when countries would phase out coal or reach carbon neutral targets.
Researchers have also been looking at the total amount of carbon that can be released into the atmosphere as another way to determine what emission targets should be. In fact, there might be a limit as to how much stored carbon there needs to be to prevent very high levels of atmospheric carbon.
Estimating the Earth’s stored carbon is, therefore, critical to potentially knowing how quickly we might have to switch away completely from carbon emitting activities as well as what we can do to mitigate climate change impacts.
How Much Carbon Does the World Have Stored?
In a nearly ten year long project, scientists from the Deep Carbon Observatory (DCO) estimated in 2019 that the world has about 1.85 billion gigatons of carbon, with over 99% stored underground and the remainder, about 43,500 gigatons, in the atmosphere or oceans.
Maintaining well over 99% of the Earth’s carbon so that it is stored means that the natural carbon cycle needs to be maintained, which has been, fortunately for us, mostly stable for Earth’s history.
Disruptions to the Earth’s Carbon Cycle
There have, however, been five major disruptions to the carbon cycle, particularly during major tectonic events, in Earth’s history, with all of those leading to mass extinctions, ocean acidification, and warmer atmospheres.
Currently, volcanos release about 280-360 million tons of carbon per year, while human activities range between 40-100 times this number on an annual basis. It is this addition of human activity that can unbalance the amount of carbon we need to have stored to maintained a health carbon cycle.
Estimating How Much Carbon is Stored in Plants and Soils
Soils can change how much carbon is stored based on varying conditions. For instance, rising CO2 levels may help some plants grow, but soils then lose their ability to store some carbon, diminishing their ability to act as a sponge for excess carbon emissions.
While some have suggested planting trees may offset increased atmospheric carbon as trees can absorb carbon, the problem is not all biomes would give much benefit.
For instance, grassland soils can increase their storage of carbon by over 8% in cases where carbon levels increased. Changing grasslands to forests may actually lead to a decline in overall stored carbon since trees would have a more limited capability in storing carbon for the long-term.
Plants can increase their demand for nutrients as they are able to absorb more carbon, which may explain why soil storage of carbon in some regions, such as forests, may in turn decline. Better understanding our ecosystems and the total carbon stored by plants and soils might be critical if we are to more accurately estimate how much carbon can be stored in the Earth’s topsoil and soil horizons.
Releasing Carbon into the Atmosphere and Warming
A related question to the storage amount of carbon, which is not fully known but it is a number we are getting better at estimating, is how much carbon we can release in the atmosphere through greenhouse gases.
The Intergovernmental Panel on Climate Change (IPCC) has suggested that about 2,9200 gigatons of CO2 can be emitted and the Earth still maintain less than 2°C of warming. However, even this number is not clear and over half that number had been released already by 2013-2014.
What is clear is we are reaching the critical threshold where the atmosphere will not longer be able to adequately store carbon without having deleterious effects on oceans and temperatures.
Natural Carbon Storage Capabilities
Related to the natural storage capabilities of the Earth are questions about carbon sequestration that may be needed to begin to remove the excess atmospheric carbon. Work by the DCO has also shown that natural systems could be used to accelerate the carbon storage process.
For instance, ophiolite rocks can be pumped with carbon-rich fluids which then begins to form carbon crystals. Carbon can be removed from the atmosphere and transformed into liquid and eventually solid forms, which ultimately removes excess atmospheric carbon.
Microbes can transform atmospheric carbon to a solid state and this natural process could be accelerated to help store carbon.
Overall, it also shows that the Earth has natural capabilities in storing carbon and potentially efforts with minimal intervention could help accelerate that storage process. For us to benefit from this process, we need to better understand how feasible such a process is at large scales needed to remove enormous amounts of atmospheric carbon and how much energy might be required.
Storing Carbon Helps to Rebalance the Carbon Cycle
The storage of carbon is a critical issue to understand if we are to rebalance the carbon cycle so that climate change proves to be less disastrous than many are forecasting. While the exact amount or level of carbon in our planet is still debatable, we do now have a better idea.
Understanding how different parts of the Earth can store carbon will also be important in better addressing climate change adaptation. There is hope also that natural cycles in the Earth can be sped up to use a form of natural carbon sequestration that could help us store excess atmospheric carbon.
 For more on the history of the Earth’s storage of carbon and the Deep Carbon Observatory, see: Suarez, C.A.; Edmonds, M.; Jones, A.P. Earth Catastrophes and Their Impact on the Carbon Cycle. Elements 2019, 15, 301–306, doi:10.2138/gselements.15.5.301
 For more on the role of soils in carbon storage and how this changes, see: Terrer, C.; Phillips, R.P.; Hungate, B.A.; Rosende, J.; Pett-Ridge, J.; Craig, M.E.; van Groenigen, K.J.; Keenan, T.F.; Sulman, B.N.; Stocker, B.D.; et al. A Trade-off between Plant and Soil Carbon Storage under Elevated CO2. Nature 2021, 591, 599–603, doi:10.1038/s41586-021-03306-8.
 A summary on the carbon dioxide release estimate can be found here: https://www.airclim.org/how-much-more-can-be-emitted.
 For more on the ophilites that could be used to naturally remove atmospheric carbon, see: Ali, A.; Abbasi, I.A.; Nogueira, L.B.; Hersi, O.S.; Al Kindi, S.A.N.; El‐Ghali, M.A.K.; Nasir, S.J. Geochemical and C‐O Isotopic Study of Ophiolite‐Derived Carbonates of the Barzaman Formation, Oman: Evidence of Natural CO 2 Sequestration Via Carbonation of Ultramafic Clasts. JGR Solid Earth 2021, 126, doi:10.1029/2020JB021290.
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