The method used to calculate greenhouse gas emissions from livestock in NI is not accurate, a climate change scientist has said. According to Professor Frank Mitloehner from the University of California, Davis, the current system does not reflect how methane emissions from livestock are part of a “biogenic cycle”.

“Methane is not quantified in an accurate way,” he said during an online event organised by the Dairy Council NI.

Methane from livestock is not new carbon because it is recycled

Mitloehner explained that methane stays in the atmosphere for around 10 years before it is broken down. This process produces carbon dioxide, which is then removed from atmosphere by vegetation, such as grass. Cattle then graze grass, and the carbon is re-emitted to the atmosphere as methane again.

“Methane from livestock is not new carbon because it is recycled. It should not be confused with carbon from fossil fuels,” Mitloehner said.

He described carbon dioxide emissions from fossil fuels as a “stock gas” as it continually builds up in the atmosphere without being recycled.

“Carbon from hundreds of millions of years of biomass was extracted within 60 to 70 years and we have put all that carbon into the atmosphere by burning it,” Mitloehner said.

By contrast, methane from ruminant livestock was described as a “flow gas” because the carbon in the process it is constantly recycled.

Mitloehner said that the current system used to calculate the global warming potential of different greenhouse gases, known as GWP100, treats methane from livestock as a stock gas, instead of a flow gas.

Warming impact

“There is no question, methane is a potent greenhouse gas and we do not want to increase it because if we do, it has a big warming impact,” Mitloehner acknowledged.

However, he said that a cattle herd which has constant methane emissions is not contributing to global warming, because no new carbon is being released to the atmosphere.

“If we look at a scenario where there is a slight reduction in methane emissions, this leads to negative warming, which is another word for cooling,” Mitloehner said.