Dr Gemma Miller, SEFARI fellowship's Blog - 3 December 2019

There has been some publicity recently around how methane is accounted for in greenhouse gas (GHG) inventories writes Gemma Miller, the SEFARI fellowship with NFU Scotland as COP25 opens in Madrid.

The issue is around the ‘permanence’ of methane in the atmosphere as compared to carbon dioxide and nitrous oxide.

Carbon dioxide and nitrous oxide stay in the atmosphere for long periods (>100 years).  This means that even if emissions were at a constant rate, atmospheric concentrations would continue to increase.

Methane has a half-life of 12 years and so, at constant emission rates, atmospheric concentrations remain stable over time (removals keep pace with emissions).

These three GHGs also have different impacts on the climate. Within the Paris Agreement international reporting framework, emissions are expressed relative to the global warming potential of carbon dioxide over a 100-year time frame (GWP100), where carbon dioxide = 1 carbon dioxide equivalent (CO2e); methane = 28 CO2e, and nitrous oxide = 265 CO2e.

If we also consider the impact that increasing temperature has on large carbon stores (e.g. melting permafrost increasing methane emissions from northern peatlands), these values increase to 1, 34 and 296 CO2e, respectively.

The argument is that methane doesn’t remain in the atmosphere for 100 years and so this method of accounting for it is flawed. What happens, in reality, is that methane produces a large temperature response (84 CO2e) relatively soon after it’s emitted.

Climate scientists at the University of Oxford at the University of Oxford have adapted the GWP100 method to take this into account. They argue that methane emitted at a constant rate has a constant impact on the climate (concentrations staying the same), whereas CO2 emitted at a constant rate has an increasing impact (concentrations increasing). It follows that methane only has an additional impact on warming if its emission rate increases.

Because methane has to fit into the CO2e metric, any increase in the rate of methane emission is treated like a one-off pulse of carbon dioxide – which increases the overall warming effect in the long term. This simple approach allows the model to more accurately simulate the actual temperature response.

However, the downside of this method is it doesn’t take into account that when methane degrades in the atmosphere, it produces carbon dioxide. It’s also unlikely that the international framework for reporting GHG emissions will adopt this approach any time soon.

Finally, this is not an excuse for apathy. It highlights the importance of increases or decreases in rates of methane emissions. Increased emission rates result in rapid warming. Conversely, decreasing emission rates result in a quick and significant slowing of global warming in the short term, allowing additional time to bring atmospheric levels of longer-lived gases down before irreversible changes to the climate occur.

Schematic diagram of methane (CH4) and carbon dioxide (CO2) emissions and the associated impact on warming. Diagram from the University of Oxford website

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Robert W Balfour

1630 days ago

This is real progress. We need now to measure any carbon we store in the soil. In my view we will not increase soil carbon without huge changes in how we farm eg regenerative agriculture and pasture fed livestock
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