What are greenhouse gases?
Carbon dioxide (CO2)
Carbon dioxide is the greenhouse gas driving most of the global warming seen since the Industrial Revolution started in the 18th century. Today, the big, global sources of CO2 are the combustion of coal, oil and gas, as well as deforestation and soil degradation. There are also sinks of carbon, principally vegetation, soils and the world’s oceans, which dissolved CO2 is making more acidic. In 2018, carbon emissions hit an all-time high of just over 37 billion tonnes and CO2 levels are now about 45 per cent higher than they were in pre-industrial times.
In agriculture, CO2 is produced when bushland and other vegetation is cleared, burned and decays. Of course, plants need CO2 to respire and grow, so growing trees, pasture and crops absorb carbon via photosynthesis. Carbon from organic residues (e.g. dead leaves, roots, manure and urine) is also incorporated into soil. Unless sequestered, CO2 lasts for centuries in the air, where it continues to warm our world.
Atmospheric methane stems from a mix of natural and unnatural sources, such as coalmines, gas wells and leaky pipes, burning and rotting vegetation, landfill, and ruminant livestock (burps and dung, including wastewater ponds in dairies and piggeries), and rice paddies. Methane-producing (methanogenic) bacteria in their forestomachs allow cattle, sheep and other ruminants to digest and extract energy from otherwise indigestible plant matter.
The global warming potential of CH4 is approximately 25 times that of carbon dioxide over a 100-year period, and more in a shorter time. Methane, however, lasts for only about a decade in the air, broken down mainly by sunlight and atmospheric chemistry. Importantly, though, while the warming effect of any given emission of methane is temporary, the total warming impacts will continue for as long as the source of methane continues. Since the start of the Industrial Revolution, methane concentrations in the atmosphere have risen by around 150 per cent. Some of this additional methane is from fossil fuels, some from deforestation, and some from expanding agriculture—mainly livestock, which now far outnumber wild ruminants.
Nitrous oxide (N20)
Nitrous oxide is a very potent greenhouse gas, with a global warming potential more than 300 times that of carbon dioxide in a given century. Nitrous oxide is also emitted when soils are disturbed, through erosion and leaching into waterways and the air, from the application of nitrogenous fertilisers (e.g. urea), and from livestock urine and dung. Legumes use nitrogen-fixing bacteria to draw nitrogen down out of the air and turn it into compounds vital to other plants.
For farmers, wasted nitrogen is wasted money. Improving nitrogen-use efficiency makes sense financially as well as environmentally. Unfortunately, what is an optimal N input (fertilisers and legumes) for pasture isn’t for livestock, with most of the nitrogen consumed by ruminants excreted in urine and dung—two of the largest sources of nitrous oxide emissions from grazing properties.
What can be done on farm?
There are a number of options open to farmers looking to mitigate greenhouse gas emissions on-farm. Below we profile just a few.
Get a handle on the farm’s carbon balance
A number of tools and guides exist to help producers understand their carbon accounts, i.e. the sinks and sources of carbon emissions. A tool should be fit for purpose and scientifically sound. If you’re just starting out you may want something relatively simple that gives you a rough idea of where you can make changes. Entering the carbon market, however, or making claims to consumers will require more rigorous accounting.
Some tools to get you started:
For more on carbon accounting see our carbon storage page.