Under the Paris Agreement, which aims to limit the increase in global mean temperature to well below 2°C, countries make their commitments to address climate change in the form of Nationally Determined Contributions (NDCs). A key element of these contributions is reducing greenhouse gas emissions.
It’s important to be able to calculate the impact of the NDCs on global warming accurately: only then will we know whether we are on track to keep warming to safe levels. Unfortunately, for a number of reasons, this isn’t possible.
One of the causes of uncertainty is the common practice of expressing emissions levels as a single number for all greenhouse gases combined. The contribution of each gas to the greenhouse effect is calculated using an emissions metric and expressed as tonnes of carbon dioxide equivalent emissions.
Unfortunately, metrics have their limitations and the de facto standard in policy circles, the Global Warming Potential (GWP100) – a measure of how much heat a greenhouse gas traps in the atmosphere – doesn’t always do a particularly good job of accounting for the impact of short-lived gases.
We decided to find out whether this matters in practice. Two hundred and twenty-two scenarios from the IPCC Special Report on Global Warming of 1.5oC were assessed using FaIR, a simple climate model developed by researchers at the Priestley Centre.
We made adjustments so that the total carbon dioxide equivalent emissions in each scenario were equal to the mean value across all the scenarios. This enabled us to estimate the uncertainty in temperature increase caused by expressing emissions levels as a total for all greenhouse gases combined. We concluded that it was less than 0.17oC with GWP100.
Does this matter? When compared to the amount by which the 2oC target will be exceeded based on the current NDCs, the answer is: “Not at the moment”. Right now it is far more important to deliver the commitments in the current NDCs and rapidly increase the rate of mitigation in the next round of commitments, due in 2020.
However, the uncertainty is not trivial; it’s a significant fraction of the 0.5oC difference between the warming limits specified in the Paris Agreement. It is likely to be material in the future. So, what are the options for reducing it?
One option is to use a different metric, and there are advocates for (and against) alternatives. However, getting international agreement on a change is likely to require time and resources which are already in short supply.
A simpler option, and the one that we recommend, would be to add supplementary information to future NDCs on the emissions levels of the most significant greenhouse gases, particularly carbon dioxide and methane. This would reduce the uncertainty without impairing each country’s freedom to determine its own contribution to achieving the Paris Agreement temperature target.
Steve Denison is a postgraduate researcher at the University of Leeds. His paper ‘Guidance on emissions metrics for Nationally Determined Contributions under the Paris Agreement’ was published in Environmental Research Letters in October 2019.
Main image by IISD/ENB | Kiara Worth