Agriculture's methane figures solid, scientists say

About half our country's greenhouse gas inventory can be attributed to agriculture, and much of that from methane belched from ruminant livestock like sheep and cows. Photo / File

International researchers have suggested agriculture's methane emissions could be much larger than first thought because of out-of-date data.

But Kiwi scientists say New Zealand's inventory remains accurate.

About half our country's greenhouse gas inventory can be attributed to agriculture, and much of that can be put down to methane belched from ruminant livestock like sheep and cows.

New Zealand also has the world's largest methane emission rate - six times the global average.

A new study published in the journal Carbon Balance and Management has estimated that global livestock methane emissions for 2011 were 11 percent higher than guidelines provided by the UN's Intergovernmental Panel on Climate Change (IPCC) in 2006.

That encompassed an 8.4 per cent increase in methane from digestion in dairy cows and other cattle, and a 36.7 per cent increase in methane from effluent management - including an 71.8 per cent increase in the US - compared to previous IPCC-based estimates.

The study attributes the discrepancy to changes in livestock productivity that have occurred since the IPCC guidelines were published.

These increases in animal productivity meant higher feed intakes, and hence, higher methane production.

The authors of the report did remark that there was notable variability in trends in estimated emissions, and that results will differ from locally derived estimates.

"In many regions of the world, livestock numbers are changing, and breeding has resulted in larger animals with higher intakes of food," said study leader Dr Julie Wolf, of the United States Department of Agriculture's Agricultural Research Service.

"This, along with changes in livestock management, can lead to higher methane emissions."

Methane was an important moderator of the Earth's atmospheric temperature, as it carried about four times the atmospheric warming potential of carbon dioxide, Wolf said.

But New Zealand scientists say the research shouldn't call into question the rigour of our country's agricultural methane accounting.

"This report is a global analysis using a simple methodology," said Dr Harry Clark, director of the New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC).

New Zealand had one of the most detailed inventories in the world, and already took into account the factors incorporated into the new study, he said.

"Any estimates based on fixed emission factors, wherever they come from, will always be wrong over time.

"New Zealand updates its inventory on an annual basis and hence this study says nothing about the accuracy of New Zealand's emissions estimates which are derived in a far more sophisticated way."

NZAGRC deputy director Dr Andy Reisinger said that, as farm animals become more productive around the world, it was to be expected that emissions per head of livestock would also rise.

"The adjustment proposed by this study is only an issue for countries that rely on default values for their emissions per animal - and defaults are, by definition, only an approximation and almost never correct for a specific circumstance," he said.

"The productivity of livestock systems around the world keeps increasing - that's a good thing from the perspective of food security and resource efficiency, but it does mean that emissions per animal are also expected to increase.

"This study simply confirms this expectation, but the numbers it provides will become out of date again 10 years from now."

New Zealand researchers were leading an initiative in Southeast Asia to help countries to implement more comprehensive and up-to-date agricultural greenhouse gas accounting methods.

"All countries now have to provide biennial updates of their greenhouse gas emissions and hopefully this will result in more accurate and continuous updating of agricultural emissions," Clark said.