Explainer Series | Why Biogenic emissions matter

Greenhouse gases (GHGs) such as carbon dioxide, methane, and nitrous oxide are key drivers of climate change. There are many sources of GHG emissions, so how do we know which ones to focus on? With 51% of Aotearoa New Zealand’s gross GHG emissions coming from biogenic sources, and methane making up 71% of New Zealand’s agricultural emissions, biogenic emissions are a significant part of our climate change impact. Understanding their origins and how they interact with the atmosphere can help us accurately measure, report, and reduce these emissions.

What are biogenic emissions?

Biogenic greenhouse gas emissions come from natural sources, such as wildfires, and the decomposition of biomass that are biological or organic in origin (eg wood). These are all examples of non-anthropogenic biogenic emissions. Biogenic emissions can also come from human activities (eg landfills), termed anthropogenic biogenic emissions. The main greenhouse gases associated with biogenic emissions are carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Reducing these emissions is required under the Paris Agreement. In accordance with this, the New Zealand Government has set a target to reduce methane emissions to 10% below 2017 levels by 2030, and 24%-47% below 2017 levels by 2050.

Biogenic and fossil carbon

There are two types of carbon that can be released into the atmosphere: biogenic carbon, and fossil carbon. While both are formed by natural processes, biogenic carbon contributes to the level of GHGs naturally, whereas fossil fuels need to be burnt for fossil carbon to join the atmospheric party.

Biogenic carbon is part of the Earth’s short carbon cycle, and operates on a scale of a few years to decades, even up to half a millennium for vegetation and soils. This cycle sees atmospheric CO₂ absorbed by living cells in vegetation, soils and microbes via photosynthesis. Small amounts of CO₂ are released as the plants “breathe”, and larger amounts released when trees or vegetation decompose. The CO₂ that is released is then absorbed by plant life, continuing the cycle (see Figure 1). Offsetting via tree and forest planting can “lock up” biogenic carbon. However, if they’re cut down, their retention of biogenic carbon is lost.

Deforestation and land use change can alter the balance in the natural carbon cycle, returning the stored biogenic carbon to the atmosphere.

On the other hand, fossil carbon is stored in fossil fuels such as oil, gas, and coal. When this is burnt by driving cars with internal combustion engines, flying a plane, industrial processes, a lot of that carbon dioxide will remain in the atmosphere, contributing to a warming planet (see Figure 1).

Figure 1: Biogenic 'short cycle' carbon vs fossil carbon.

Biogenic methane

Like CO₂, CH₄ can be biogenic or fossil in origin. There are many natural sources of fossil CH₄ such as marine and terrestrial seepages, geothermal vents and mud volcanoes formed over thousands to millions of years. A significant stock of fossil CH₄ exists in frozen hydrate deposits in the ocean and permafrost soils. These may become unstable and be released as global temperatures rise (Ciais et.al., 2013). The fossil CH₄ releases occur during oil and gas extraction, geothermal energy generation and the incomplete burning of fossil fuels.

During decomposition, methane creating microbes (methanogens) feed on the carbon in organic matter, but only do so under low oxygen (anaerobic) conditions. This is why biogenic CH₄ generation occurs in landfill where anaerobic methanogens can feast underground on discarded food and green waste. The same microbes live happily in the human gut, aiding digestion (Kumpitsch et.al. 2020), and also in livestock. This is where ruminants   produce a high amount of methane per unit of feed consumed, within the ruminants’ stomach, known as the rumen (see Figure 2). Even in composting, which requires oxygen to work (the process is aerobic), methane can still be produced where there are pockets of anaerobic activity.

Figure 2: Sheep in Southland. Sheep consume biogenic carbon and emit biogenic CH₄ and biogenic CO₂ during respiration and biogenic N₂O as their excreta degrades. Photo: Andrea Lightfoot/Unsplash

Biogenic nitrous oxide

The other biogenic GHG of significance is biogenic N₂O. Fossil N₂O can come from the manufacturing and degradation of synthetic fertilisers, and fossil fuel combustion. Biogenic N₂O comes from many sources, such as biomass combustion, and typically involves microbial processes (see Figure 3). Such processes in soils and aqueous environments (wastewater plants, wetlands, rivers, lakes, and oceans) give rise to N₂O emissions via nitrification (conversion of ammonium to nitrates) and denitrification (conversion of nitrates to nitrogen gas). Many factors influence the emission of N₂O, including temperature, atmospheric conditions, soil properties, presence of carbon, microbial community, nitrogen species and dissolved oxygen availability.

The observed increase in N₂O concentration is mostly attributed to the reactive nitrogen (N) inputs due to escalated use of synthetic N fertilizer and animal manure, cropland expansion, and processes associated with fossil-fuel combustion and biomass burning. Many influencing variables, such as soil properties and microbial composition, means that the emission factors for N₂O can be highly uncertain (WaterNZ, 2021).

Figure 3: Processes in soils influencing emissions of N₂O. Adapted from Tian et al. 2012.

How do I report biogenic GHGs?

Understanding the reporting requirements for biogenic GHGs allows us to better identify emissions reductions opportunities, as well as increase emissions reporting accuracy. The reporting demands differ depending on the biogenic emission category. Biogenic emissions should be reported within your organisations’ GHG inventory. The biogenic CO₂ is not reported under the scope or categories, but is recorded in your inventory. However, biogenic CH₄ and N₂O have elevated Global Warming Potential (GWP) values relative to CO₂ on the GWP₁₀₀ scale. These emissions must be reported within your scopes or categories, and an overview of the reporting requirements according to ISO 14064-1:2018 is shown in Table 1 below:

Table 1: Reporting requirements for biogenic GHGs

You can also report your reductions from biogenic carbon. It can be sequestered in materials such as wood products and other biomass derived products such as wool or hemp. These are incorporated into products, sometimes for decades, before final disposal. The effect of temporary carbon storage and delayed emissions (discounting of the emissions for less time in the atmosphere), are generally not considered in GHG accounting under the ISO 14067:2018 standard. It is recommended that these be reported separately. Carbon storage is discussed in more detail in one of our helpful explainers.

Why is fossil CO₂ reported differently than biogenic CO₂?

As a greenhouse gas, the warming effect from a molecule of biogenic CO₂ is identical compared to a molecule of fossil CO₂. However, the accounting convention is that the biogenic CO₂ does not contribute to organisations’ ‘in-scope’ measurement, whereas fossil CO₂ does. The justification being biogenic CO₂ is part of the short cycle. It’s also why we don’t ask you to quantify how much CO₂ is emitted through respiration by your employees, or by livestock on your farms.

Why has fossil CH₄ got a different GWP₁₀₀ than biogenic CH₄?

In the IPCC sixth assessment report the GWP₁₀₀ is 27.2 for non-fossil CH₄ and 29.8 for fossil CH₄. Why are they different? When the CH₄ is in the atmosphere it is slowly oxidised and is converted to CO₂. This CO₂ is accounted for differently depending upon whether it is biogenic or fossil in origin (Equations 1 and 2).

What about fertilisers?

The production and use of synthetic nitrogen fertilisers, such as urea, can increase CO₂ and N₂O emissions. Both are long-lived fossil-derived GHGs, with N₂O mainly from fertiliser-rich livestock urine. Theoretically, a lower carbon urea could be manufactured from biomethane feedstocks. Other synthetic nitrogen fertilisers may also release fossil-N₂O during use. However, the use of manures would lead to a release of biogenic GHGs, including biogenic CO₂ and biogenic CH₄.

So, how do we best reduce these biogenic emissions?

That depends on the source! We can try and prevent the organic carbon being converted to methane in the first place, by keeping food waste out of landfill, or convert organic matter to less degradable forms of carbon such as biochar. Ongoing research is looking to breed lower emission animals and introduce vaccines and inhibitors to reduce enteric methane. However, once methane is made, we should find ways to capture it and combust it to recover energy. This could be by covering farm effluent ponds, degrading waste in anaerobic digestors, or capturing and burning the gas at landfill. We can also make better use of methanotrophs, a group of microbes that like to eat methane.

Nitrous oxide can be tougher to mitigate. Nitrification inhibitors can reduce N₂O by suppressing the nitrifying bacteria in soils. A broad combination of environmental and management factors can contribute to N2O emissions so there are levers to reduce emissions. Equally, these factors bring complexity and introduces the further challenge as to how to measure and verify these reductions.

To address the nuanced nature of agricultural emissions, the Government announced a plan in late 2022 to price emissions from both methane and nitrous oxide. Farmers would also receive payments for planting trees, which suck carbon dioxide from the atmosphere. The plan incorporates proposals from the He Waka Eke Noa partnership, and other proposals from the Climate Change Commission. If implemented, the levies are expected to see Aotearoa meet its target of reducing biogenic methane emissions by 10 per cent below 2017 levels by 2030. This plan is currently out for consultation, closing November 18.

Conclusions

• Biogenic GHGs such as CO₂, CH₄ and N₂O come from a range of natural sources, such as decomposed wood and wildfires, which are contributing to a warming planet
• Biogenic CO₂ is part of a shorter carbon cycle (decades to hundreds of years), with carbon captured in vegetation and soils, before being released back into the atmosphere. This differs from biogenic CH₄ and N₂O, which are produced from decaying organic matter or combustion of biomass
• Understanding these differences not only gives us emissions reduction opportunities, but also leads us to reporting accuracy for biogenic emissions. And this really matters. Biogenic CH₄ and biogenic N₂O are reported under your organisations’ emissions scopes or categories. This differs from biogenic CO₂ emissions and removals, which are reported separately within your inventory
• The accounting convention is that the biogenic CO₂ does not contribute to organisations’ ‘in-scope’ measurement, whereas fossil CO₂ does. The justification being biogenic CO₂ is part of the short cycle, such as you and me breathing.

Glossary

Anthropogenic biogenic GHG emission - GHG emission from biogenic material from human activities

Biomass - material of biological origin, excluding material embedded in geological formations and material transformed to fossilized material

Biogenic carbon - carbon derived from biomass

Biogenic CO₂ - CO₂ obtained by the oxidation of biogenic carbon

Enteric methane - methane produced in in the digestive tract, or rumen, of ruminant animals

GWP - global warming potential. An index measuring the radiative forcing following a pulse emission of a unit mass of a given GHG in the present-day atmosphere integrated over a chosen time horizon (typically 100 years), relative to that of CO₂

Microbial process - degradation processes undertaken by microbes, or microorganisms, such as bacteria

Non-Anthropogenic biogenic GHG emission - GHG emission from biogenic material caused by natural disasters (e.g. wildfire or infestation by insects) or natural evolution (e.g. growth, decomposition)

Photosynthesis - the process where plants absorb sunlight and carbon dioxide, which is converted into sugars and oxygen. The plant locks up some carbon, uses the sugars for growth, and releases oxygen into the air

Ruminant - a ruminant is a suborder of mammal with typically a four-chambered stomach and two-toed feet. They include cattle, sheep, goats, deer, and antelopes. They can acquire nutrients from plant-based food by fermenting it in their specialised stomach

Urea - a nitrogen containing compound, used widely as a fertiliser, having the formula CO(NH₂)₂. It is the end product found in urine from the metabolic breakdown of proteins in all mammals and some fishes. It is produced industrially from synthetic ammonia and carbon dioxide