This leaves researchers filling in the blanks with their next best guess. You would like to think that any data published by the FAO would be based on pretty good guesswork.
However, Teagasc researchers are adamant that the numbers in this instance are sketchy.
Instead, they point to the copious amount of research produced by their own researchers, including Donal O'Brien in Teagasc Moorepark.
Of his many internationally cited papers, I was particularly taken by the one looking to compare emissions from an Irish grass-based dairy cow with her confined equivalent in the US or the UK. Among the many imponderables that O'Brien attempted to address was the issue of how to account for the emissions that don't come from the cow while she is lactating.
This includes when she's being reared and when she's being fattened for beef at the end of her milking career.
It's only when you start looking into this that you realise how many grey areas there are in an industry built on black and whites.
Is a bull calf from the dairy cow a by-product of milk production or an integral part of the beef industry?
What percentage of his emissions should be allocated to the dairy cow? Should this calculation be based on the kilos of protein from his final carcase weight versus the total protein in the milk, his economic value or on his actual emissions?
Should we allow for the carbon sink that grassland undoubtedly is? If we do so, we are breaking with the current agreed international formula. Given that pasture only becomes a carbon sink after 20 years, is it possible that the recommendation to reseed every 10 years will forever rule out Irish grassland as a carbon sink?
It turns out that if the carbon sequestered by our pastures is not included in carbon emission calculations, then there is very little difference between the carbon footprints of a litre of milk produced in the searing heat of a Californian feedlot or a wet field in Camolin.
That's an extraordinary result given the US cow only lasts for about 2.5 lactations. But the increased emissions associated with all those extra replacements and cull cows is more than diluted by the huge amounts of milk produced by the confined cow (which is double the average Irish equivalent).
Suffice to say that this is an evolving situation and what we think is a positive now may well be turned on its head in the future.
But farmers will have to get familiar with this type of debate not just to be able to defend their own system of production. They also need to know where the scope for big improvements lie.
The sudden spike of interest in protected urea is a perfect example with Irish merchants reporting a massive increase in queries and orders for the product in the last month.
The increase in cost to the farmer per unit of nitrogen is marginal, while the use of the product can reduce ammonia and nitrous oxide emissions by over 70pc.
The availability of this product has only been driven by farmer interest, which in turn has been driven by knowledge.
If farmers knew that the carbon footprint associated with soya from the US was at least three times smaller than that coming from South America (due to the associated forest clearance), would there be a similar shift on buying patterns?
There will be a myriad of other emission-reducing info and technologies coming on-stream for farmers over the coming years. News that Carbery Co-op are trialling a system that squeezes the water out of grass before feeding it to cows in an effort to reduce emissions sounds bizarre, but is an indication of the no-holds-barred-approach that is gradually gripping the sector.
However, the most important piece in this jigsaw is the farmer. Keeping up and understanding all that's involved will be just another task in the already heavy workload facing the modern agriculturalist.
No-one ever said it was going to be easy.
Darragh McCullough farms in Meath and presents RTÉ's Ear To The Ground television programme.