Carbon Sums Explained
It is relatively easy to point to all the sources and sinks of greenhouse gases on farms (see Fig 1).
Carbon dioxide – symbol CO2 i.e. two molecules of oxygen bound with one molecule of carbon
As shown in Fig.1 CO2 is primarily taken into plants during day time.
Plants 'fix' each CO2 molecule so it is no longer a gas with the help of the energy produced by the sun i.e. via the biological process of photosynthesis.
The CO2 molecule can be released from the plant tissue as a gas back into the atmosphere when the plant respires, rots or is burnt.
Soil organisms also release CO2 into the atmosphere via the biological process of respiration.
Nitrous oxide – symbol N2O i.e. two molecules of nitrogen and one molecule of oxygen
As you can see from Fig. 1 nitrogen can be 'fixed' a bit like the carbon into cells (N fixation) but it is soil bugs rather than the plants which take the gaseous nitrogen and convert it into a non gaseous compounds.
N2O is released again as a gas from the soil by other soil bugs in the biological processes of nitrification and denitrification.
It is also released as a gas from animal dung and urine and when vegetation is burnt.
N2O has a higher residency time in the atmosphere and is therefore considered a more damaging greenhouse gas. Scientists consider that N2O has 310 times the potential to heat the world (i.e. global warming potential - GWP) compared with CO2.
Methane – symbol CH4 i.e. four molecules of hydrogen and one molecule of carbon
There are three main sources of CH4 released into the atmosphere as you can see in Fig1.
- When animals burp or fart often termed enteric fermentation
- By soil bugs in the biological process of methanogenesis – the soil has to be very wet with very little oxygen for bugs to produce CH4, like in a rice paddy field or blanket bog
- When plants or plant material are burnt
CH4 also has a higher residency time in the atmosphere compared with CO2. Methane is considered to have a GWP of 29 i.e. every molecule of CH4 has twenty nine times the potential to heat the world compared with one molecule of CO2
Other gases
The other gases shown in Fig. 1 which are primarily released by burning are NOx, NH3, NMVOC and CO. These are often formed into one of the three main gases described above (N2O, CH4 or CO2). These other gases are therefore considered indirect emissions at this time.
Adding up the parts
While it is relatively easy to point to all the sources and sinks of greenhouse gases on farms (see Fig 1) it is not easy to estimate exactly how much gas is produced by each element. As you can imagine the amount of CO2 a plant respires depends on many things including the ambient temperature, available soil nutrient etc. and the amount of dung a cow produces depends for example on its size and what/how much it has been eating. So how can we calculate the total greenhouse gas emissions for your farm or your so called carbon foot print? Well basically we use the figures which scientists have measured from lots of experiments.
Scientists have been working to estimate the amount of greenhouse gases produced from all the sources and sinks shown in Figure 1 for many years and there have been large international projects which have involved scientists from around the world.
The international effort has been lead by the Intergovernmental Panel on Climate Change (IPCC) which formally started work in 1988.
The IPCC have reviewed all the scientific literature and industry reports and have published equations for all the sources and sinks of greenhouse gases. We decided to use the latest guidelines published by IPCC (2006) as this is the standard for the industry. There are published equations to calculate the emissions of greenhouse gases from all the elements in Fig. 1. The emission data for N2Oand CH4 are converted into global warming potentials (i.e. multiplied by 310 and 29 respectively) and the total expressed as carbon dioxide equivalents (CO2 eq).
IPCC methodology is designed for reporting national inventories, our members and visitors to this site have questioned how appropriate it is at the farm scale. We agree that the emission calculations are correct but question if farmers are getting all the credits they deserve. Working with other research scientists we have produced a discussion document which highlights the issues our members feel are important click here to learn more.