Dear Editor

It is a simple fact that methane is a powerful greenhouse gas. Every tonne of methane emitted causes the same amount of warming regardless of its source.

Your article of 25 January (Promised methane move would be ‘big change’ – Teagasc) adds to confusion already caused by the widely promoted but now scientifically discredited suggestion that GWP* is an alternative to GWP100 (global warming potential 100) for greenhouse gas accounting.

Unlike GWP100, GWP* does not model the warming caused by greenhouse gas emissions.

Instead, it models the effects of changes in emissions. To use a motoring analogy, GWP100 measures speed while GWP* models the effect of acceleration and deceleration.

Therein lies a trap for many Irish farmers: with GWP*, emissions from those with growing farm businesses (those accelerating) will be penalised as causing additional warming while ‘contracting’ businesses with identical but declining emissions (deaccelerating) will be regarded as causing ‘cooling’.

Warming impact

Given that the misleading use of GWP* has been highlighted in recent years, including in a paper in Environment Research Letters that I co-authored, it is remarkable that GWP* is still presented by Teagasc as a metric that leads to “significant reductions” in warming impact.

The article’s distinction between biogenic and fossil sources reflects serious misunderstanding, as does the statement that GWP100 reflects a 100-year lifespan in the atmosphere. As far as the Paris Agreement is concerned, your report reflects distorted arguments from industry-supported scientists that in effect set Article 2 (to limit global warming) against Article 4 (to achieve net-zero emissions).

The Teagasc statement on its website that “using GWP* in a herd with stable methane emissions will show a much lower warming effect coming from livestock” is false. It even contradicts a presentation from David Styles and others on the same website that shows that GWP* models marginal change and is not suitable for use below the global level.

The idea implicit in the article that the challenge to mitigate methane emissions can be reduced by switching from GWP100 to GWP* borders on climate change denial and serves neither farmers nor citizens.