Much has been said in recent times about ‘new science’ that changes the way we should think about how methane is assessed and its impact on climate change. To understand this, you first need to have a basic understanding of what the scientists are trying to do.

Global warming is obviously concerned about the science of what is happening in our atmosphere. This is a relatively new science so it is unreasonable to expect that every assumption would be 100% correct from the word go. One of the basic assumptions of global warming is that gases which trap more heat are becoming increasingly important in the atmosphere.

Much has been said about the role of carbon dioxide (CO2) in this regard, as it is the dominant gas. It is produced during the respiration of most living things and is taken from the atmosphere by photosynthesis to fuel plant growth. But it is also released from the burning of fossil fuels, trees and other biological materials. In addition, it can be produced by a range of chemical reactions, such as the manufacture of cement.

Nitrous oxide (N2O) is a much more potent GHG and mainly arises from human activities

Methane (CH4) is a well-known greenhouse gas (GHG) in agriculture because biogenic emissions occur from livestock. But CH4 emissions also occur during the production and transport of coal, natural gas and oil. Methane is also emitted from the decay of organic waste in municipal solid waste landfills.

Nitrous oxide (N2O) is a much more potent GHG and mainly arises from human activities, such as agriculture, fuel combustion, wastewater management and other industrial processes. It is very potent and long lasting in the atmosphere.

Another major group are the fluorinated gases. These are very powerful GHGs, mainly emitted from industrial processes, with a global warming impact 23,000 times higher than CO2.

Global warming potential

Different gases have varying characteristics which influence their impact on global warming. The net impact depends on the individual gas, how much of each gas is present and how long it is likely to have an effect before it is broken down.

For many years, scientists have used the 100-year global warming potential (GWP) of each gas to estimate its impact on atmospheric temperature.

This was done initially on the basis that all gases were stable and that they had an infinite lifespan. Gases with a higher GWP absorb more energy than gases with a lower GWP, and thus contribute more towards global warming.

However, recent scientific developments have led scientists to challenge the GWP metric and many now regard it as unrealistic for methane.

In a recent article by Professor Ray Bates of UCD, titled New Scientific Developments Require a Change in Methane Accounting, the author explains that the science used in the current methane accounting system fails to make the required distinction between situations in which methane emissions are increasing, stable or declining.

In all three situations, the GWP system assigns a CO2-equivalent for methane based solely on its current year emissions. This implies a continued contribution from methane to global warming. This is unrealistic, because a declining trend in methane emissions can contribute to global cooling. This happens when the warming caused by methane emitted in earlier decades is declining faster than the warming caused by new methane is increasing.

Prof Bates goes on to explain that a revised methane assessment process or metric, called GWP Star (GWP*), was developed by researchers based at Oxford University. The GWP* metric remedies this important flaw by including the current year’s methane emissions and the trend for the most recent 20-year period. Prof Bates agrees with the leader of the Oxford group Prof Myles Allen, who referred to the GWP as “an outdated accounting system”.

Prof Bates states that when the GWP* metric is applied to Irish agricultural methane emissions, it indicates that they had a much smaller global warming impact than the GWP metric suggests over the past 20 years. Indeed, GWP* indicates that our methane emissions contributed to global cooling in recent years, due to their decline since the late 1990s.

Given these findings, Prof Bates argues that Ireland would have appeared in a much more favourable light in recent EU emissions tables if the new accounting system had been used.

Methane is recycled carbon

Other scientists form a similar opinion on the outdated handling of methane assessment. Prof Bates notes that Prof Frank Mitloehner of the University of California, Davis, also supports the use of GWP*.

This researcher stressed that the CO2 that results from the decay of ruminant methane is not new to the atmosphere, but merely recycled. The carbon was absorbed from the atmosphere at an earlier point in time by forage plants, which were subsequently eaten by ruminants.

Mitloehner went on to say that even under a change to the GWP* metric, methane produced from ruminants should be treated separately to methane derived from fossil fuels.

Prof William van Wijngaarden from Canada, confirmed that while methane has high initial potency relative to CO2 on a tonne-per-tonne basis

While the latter might also be regarded as part of a recycling system, it is many thousands of times slower than the agricultural system and so contributes to the release of additional carbon to the atmosphere.

Another scientist, Prof William van Wijngaarden from Canada, confirmed that while methane has high initial potency relative to CO2 on a tonne-per-tonne basis, the low rate of global increase in methane in the atmosphere is so small comparatively that its annual contribution to a warming influence is only one-tenth that of CO2.

If only ruminant methane is considered, this warming-potential factor is further decreased to only 1/77 that of CO2, making it almost irrelevant in terms of global warming.

Time for change

These are very different scientific findings to those indicated by the previous research.

Prof Bates argues that these recent scientific advances on the climate impact of methane are now sufficiently well established to allow them to be used. He concludes that the Government now needs to end the use of the outdated GWP system in national methane accounting, as failure to do so could result in unjustified material damage to Irish agriculture.

The fact that different gases have different warming potential necessitated the development of a common unit – carbon dioxide equivalent or CO2-e – to add them together.

Carbon dioxide (CO2) is given a global warming potential (GWP) of one, while nitrous oxide (N2O) is currently calculated as being 265 to 298 times that of CO2 for a 100-year timescale.

This article shows that the situation with methane (CH4) is far more complex and does not now lend itself to the use of a single coefficient.

Up to recently, the coefficient used for CH4 was 28 times that of CO2, but recent science is indicating that this could be a negative value in a situation where the rate of biogenic methane is declining over time.

While use of CO2-equivalent may continue, the contribution of methane needs to be handled differently.