Dr. James Glynn:

I thank Deputy O'Rourke. With regard to the Intergovernmental Panel on Climate Change, IPCC, guidelines around how we convert carbon dioxide and other greenhouse gases into carbon dioxide equivalents so that their warming impact can all be added up and understood, to compare apples with apples and oranges with oranges it has come up with different methodologies. I am sure the Deputy has heard of the global warming potential, GWP, 100 and the GWP* methods to aggregate these different greenhouse gases into the same level of warning and the same level of units. The traditional method of GWP 100 is fine when we are talking about annualised targets, in other words, what is the annual impact of this bucket of gases all in the same units? However, when we start to try to create carbon budgets over different timescales, be that five, 50 or 100 years, the impact of warming from each of those gases is different. Both are dependent on each of the gases and on the timescale of that carbon budget. While we can have a five-year carbon budget, and I think we are specifically talking here about biogenic methane, it may make sense to include biogenic methane in a very short carbon budget but it does not make sense to include it in a very long carbon budget because biogenic methane is a short-lived gas in comparison to carbon dioxide, which is a much longer-lived gas. They do not accumulate in the atmosphere at the same rate, nor do they have the same warming impact, which therefore impacts on the temperature increase.

With regard to my point about which gas is to be included in the carbon budget system, it is more to accurately reflect the current state of the science and the state of the art of understanding ultimately the impact on warming for each of these gases and how that warming is measured. If we were adding up different gases over different timescales and accumulating them in different budgets, ultimately, the warming impact can be different. The method that should be used should reflect the state of the art of the science, and the state of the art of the science has probably changed in the past five to ten years. It has been more reflected in the IPCC's special report on 1.5oC. If we were to have long-term carbon budgets beyond the lifespan of some of the short-lived gases, they probably should not be included in five-year carbon budgets and some other method that reflects the state of the art of the science should be used. The appropriateness of that method should be outlined.