Biogenic vs. fossil carbon
By A. Clare
The carbon cycle describes the process in which carbon flows between the lithosphere, hydrosphere and atmosphere (Figure 1). Carbon can either be classified as biogenic or fossil, with the former assigned to carbon stored in, sequestered by and emitted through organic matter. Alternatively, fossil carbon forms when living matter dies and is deposited on the ground. Over time, this material becomes buried and is exposed to extreme heat, a lack of oxygen and atmospheric pressure. This extreme environment causes the molecules to break apart and, following millions of years the material degrades to form fossil fuels. Changes in the carbon cycle that result in fluxes of carbon into the atmosphere result in warmer atmospheric temperatures on earth. Once a carbon-containing molecule has entered the atmosphere, its heating capacity remains the same regardless of whether it is fossil- or biogenic-derived.
Figure 1 - The earth's spheres.
Biogenic emissions are those involved in the natural carbon cycle, as well as those formed during the combustion, harvest, digestion, fermentation, decomposition or processing of biological matter. Common feedstocks include those which absorb carbon during their lifecycles, such as trees, plants and soil. Fossil carbon emissions are also released during combustion. The carbon stored within these fuels has a higher carbon content that has accrued over millions of years. As fossil fuels are burnt, an excess amount of carbon – which has not been exposed to the biosphere-atmosphere system before - is released from the geologic pool, resulting in acceleration of the warming process.
Biomass plays a fundamental part in the global carbon cycle. During photosynthesis, plants help mitigate climate change through the removal of CO2 from the atmosphere. Similarly, biomass can further help mitigate warming effects through the replacement of fossil fuels for energy production. Biogenic carbon is often considered preferable over fossil carbon, as the combustion of biological resources returns carbon to the atmosphere that was once absorbed during plant growth. This carbon is often thought to be recycled through biomass feedstocks because the combustion of biomass does not increase the total amount of carbon in circulation between the atmosphere and biosphere.
At present, anthropogenic impacts are perturbating the natural carbon cycle through the burning of fossil fuels and the clearing of land. Clearing land removes dense areas of plants that store carbon in their biomass. Similarly, the replacement of fossil fuels with biomass is likely to have similar effects if not carried out sustainably. Biomass is treated as a renewable resource because it is assumed the CO2 emitted during combustion will be reabsorbed – or offset – by CO2 sequestration as new biomass grows (Figure 2). However, before the carbon is resequestered, there will be an initial increase in atmospheric carbon because, as biomass is burnt, carbon is emitted all at once in a one-time pulse. There is then a carbon ‘payback period’ before the carbon is resequestered by biomass. Once, and if, the biomass regrows, this initial impact will be reversed. Biomass should, therefore, not be instantly regarded as carbon neutral, as the temporary changes in atmospheric carbon should not be neglected.
Figure 2 - Biogenic- vs. fossil-derived carbon.
Climatic impacts may also be aggravated for alternative reasons. For example, if the world continues to experience deforestation whilst harvest intensity increases, the carbon released will not be resequestered as the depletion rate will be greater than the replacement late. Expansion in the bioenergy sector may result in a reduction in biosphere carbon stocks which could have drastic impacts on the climate mitigation benefits if not undertaken sustainably. Furthermore, not all biomass sources are renewable, such as old-growth forests which have a long rotation length (relative to e.g., perennial grass). The slow process of photosynthesis means many plants take a long time to reach maturity (up to several decades). Therefore, depending on the species, long payback periods may result in sequestration of carbon taking place beyond the critical timeframe outlined in the Paris Agreement for addressing climate change. The critical question remains as ‘what will the atmosphere experience and over what time scale’ as a consequence of switching to bioenergy sources.
Sources of carbon emissions should be assessed based on their atmospheric impacts in the near term and, their potential for resequestration. Biogenic carbon is regarded as preferable over fossil carbon as it can be more readily replenished. Biogenic emissions will occur regardless of whether the biomass is used as an energy source (i.e., through mortality and decay), however, utilising biomass as a fuel source before this natural cycle will speed up the release of carbon into the atmosphere and, therefore, the warming process. As the adoption of biomass as an energy source increases, forests need to be managed sustainably to be classified as carbon neutral and maintain a stable (or increasing) carbon stock. Biomass will be utilised most efficiently where it maximises the removal and minimises the release of carbon to the atmosphere.
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