The Wageningen University study finds that Irish GHG emissions per kg of milk are the fourth highest in the EU, exceeded only by Poland, Estonia and Bulgaria.
The FAO GLEAM results show that Irish emissions per kg of protein in milk are 50pc higher than the average for producers in Western Europe as a whole.
There are several reasons why different studies might give different results. Some sources are based on activity-based emissions, which is the way emissions are recorded in the annual GHG inventories prepared by each country using IPCC guidelines. Activity-based emissions only consider emissions which are directly associated with the respective activity, such as those associated with rumen fermentation and manure management in cattle.
An alternative approach is called a life cycle assessment (LCA), which examines every step and input during production to calculate a product's total GHG emissions. This is the approach used to calculate a product's carbon footprint.
When consumers are making purchasing decisions, they want to know the total environmental impact and not only emissions at an individual stage of production.
A comprehensive LCA for livestock would not only include direct emissions from animals, but also emissions of fodder production in and outside the country and emissions from fertiliser production and transport.
Some studies also include emissions (or removals) from land use and land use change caused by changes in carbon sequestration rates related to feed production (including grassland and grazing).
Teagasc researchers have shown that including the emissions associated with the production of feed reverses the relative carbon efficiency ranking when comparing confinement and grass-based dairy systems.
On an LCA basis, where the full emissions of producing milk including the emissions from producing the feed, the grass-based system turns out to be more carbon-efficient. International comparisons are model-based studies and are not based on measurements made on individual farms.
Differences in the assumptions used to estimate these models can also explain differences in the results obtained in different studies.
In the EU, around 60pc of beef production comes from the dairy herd, either in the form of calves or cull cows.
In estimating the emissions associated with the production of beef and milk, some way must be found of allocating the emissions associated with dairy cattle to milk and to beef. Different allocation rules will give different results.
Teagasc researchers have shown that the allocation of emissions to dairy cows and dairy calves that enter into the beef production system greatly reduced GHG emissions from beef production in Ireland from 13.0 to 9.8kg CO2eq per kg liveweight.
Studies also differ in the data they use for the emission factors to calculate the emissions intensity of production. The ranking will also differ depending on the functional unit used to present the results. Are we interested in emissions per unit of area, or per kg of product, or per kg of protein?
This article has explained some of the complexities in measuring and comparing the carbon efficiency of beef and milk production across countries.
The JRC study showing that Ireland is a carbon-efficient producer of milk and beef is an important benchmark, but other sources come to opposite conclusions. Further research to reduce this confusion would be very desirable.
International comparisons of average efficiencies should not distract attention from the huge variability in carbon efficiency that exists within the country.
The Teagasc Sustainability Survey shows that the top performing third of farms emitted, on average, 9.8kg CO2 equivalent per kg beef, compared with 16.7kg for the bottom performing third of cattle farms.
Reducing this variability is a real opportunity to make progress in reducing emissions from cattle production in Ireland.
Alan Matthews is Professor Emeritus of European Agricultural Policy at Trinity College Dublin