Provisional figures published by the Environmental Protection Agency (EPA) show that Ireland cut its emissions of gases associated with climate change by 0.9% last year. However, those emissions from agriculture increased by 2.9%. Since 2016, Ireland’s emissions have been running above the targets required to meet 2020 obligations under EU law. How did the EPA come to this conclusion?

Emissions from most economic sectors are easy to measure: they depend on the amount of fossil fuel they burn and the resulting carbon dioxide emissions. This is not so for agriculture: the main greenhouse gases here are methane, released by ruminants and manures, and nitrous oxide coming from nitrogen application to soils. Here is how their emissions are calculated.

Enteric fermentation

This category essentially measures methane belched by cattle and sheep. For cattle, the EPA uses a detailed model of the Irish herd developed in UCD under the direction of Dr Frank O’Mara, now Teagasc’s director of research, partly drawing from research conducted in France and New Zealand. Animals are divided into 11 categories depending on their sex, age and purpose, and 60 different production systems across dairy, suckling and finishing. Data including animal movement statistics, milk and beef output and energy intake in feeding go into the model, which returns corresponding methane emissions. For example, a dairy cow was found to emit 112kg of methane in 2016, while a suckler cow released 74kg.

Sheep and other ruminants such as goats and deer also emit methane, as do – in much smaller proportions – other livestock such as pigs. There aren’t detailed models for these animals and the EPA uses international data for Western Europe, applied to the typical lifetimes of animals on Irish farms. The EPA and Teagasc have plans to develop a more detailed model for sheep.

Manure mangement

This is the methane released from dung and urine excreted by livestock outside, and by slurry and other manures during storage and handling before they are spread on land. Just like enteric fermentation, the model of Irish cattle farming provides detailed estimates of emissions from manures, while a combination of Irish and international data is used for other species. There is also a small amount of nitrous oxide released during slurry handling and storage.

Soils

This category measures the nitrous oxide released by the application of any nitrogen to agricultural land. Within that, the EPA applies a different emission factor to each unit of N coming from synthetic fertiliser, slurry and other organic fertilisers, the nitrogen component of dung and urine from grazing animals, decaying crop residue and changes made to soils such as drainage. The EPA and Teagasc have recently updated methods specific to Ireland to calculate emissions from fertiliser, slurry and sludge. CAN, urea and protected urea are very different: 1kg of N releases 14g of nitrous oxide if applied as CAN, but only 2.5g as urea and 4g as protected urea – which offers the separate advantage of reducing ammonia pollution. Other factors are derived from international guidelines.

Urea, lime and fuel

Burning diesel in machinery releases carbon dioxide. Urea and lime also release some of this gas as they break down. These three categories together account for just 1.5% of greenhouse gases from Irish farms.

Converting to carbon dioxide equivalent

Carbon dioxide, methane and nitrous oxide have different ways of retaining heat in the atmosphere – the so-called greenhouse effect. To measure each country’s contribution to climate change, internationally agreed rules attribute a global warming potential over 100 years to each gas (GWP100).

The GWP100 of methane is currently 25, which means that 1t of methane is considered to cause as much global warming as 25kg of carbon dioxide. The GWP100 of nitrous oxide is 298. There are questions whether new research on methane should lead to a different GWP value, but this has not translated into international rules yet (see ifj.ie/m https://www.farmersjournal.ie/environment-what-we-know-about-methane-417970).

There are other gases used in small quantities across a range of industries, which are thousands of times more damaging to the climate than carbon dioxide. These include HFC gases used as refrigerants, such as R-404A and R-507A used in milk cooling equipment. Restrictions on their use will come into force in 2020.

Uncertainty

The EPA acknowledges a degree of uncertainty in its calculations, worth 3.7% of reported emissions in 2016 and expected to go down for 2017 when final figures are double-checked and published next year. “The most significant contributors to the level of uncertainty are methane emissions from livestock and nitrous oxide emissions from agricultural soils,” the agency said.

This is due to the impossibility of conducting measurements on every farm – as opposed to the direct monitoring of every power station or energy-intensive factory, for example.