For the paper by Tom Wigley, to be published in the journal Climatic Change entitled “The relationship between net GHG emissions and radiative forcing with an application to Article 4.1 of the Paris Agreement.”
The Paris Agreement on climate is the international agreement on how to respond to the threat of global warming. It was agreed to at a meeting of the United Nations Framework Convention on Climate Change (UNFCCC) held in Paris in November 2015. The Agreement entered into force on 4 November 2016.
From a climate science perspective, the most important Article in the Paris Agreement is Article 2, which specifies a global warming goal that Parties to the Agreement should strive to achieve in order to avoid dangerous interference with the climate system … i.e., a goal to keep the temperature of the planet within tolerable limits.
Article 2 quantifies this warming goal. It states that the increase in global average temperature since pre-industrial times (defined as the period 1850 to 1900) should be held “to well below 2°C”, while “pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels”.
The scale of the challenge to keep warming within tolerable limits is evident when one realizes that we have already warmed the planet by about 1.2°C. The present warming rate is about 0.2°C per decade. No matter what we do, this rate will almost certainly continue for two or more decades into the future. It is clear, therefore, that we are virtually committed to passing the 1.5°C threshold some time in the mid to late 2030s.
This paper presents a devastating critique of Article 4.1 of the Paris Agreement on climate change.
Article 4.1 suggests that to meet the temperature targets of Article 2 net anthropogenic GHG emissions must fall to zero some time in the window 2050 to 2099. Many countries have decided to aim for the low end of this range, but the present paper shows that even this is insufficient:“net-zero” needs to be reached even earlier than this, as early as 2036. The paper, furthermore, shows that the goal of reaching net zero GHG emissions per se is of minimal value because (1) it ignores the effects of non-GHG forcing agents, and (2) it fails to consider what might be required after the net-zero point is reached.
This new paper is concerned with Article 4.1 of the Paris Agreement, the primary goal of which is to provide guidance on how “to achieve the long-term temperature goal set out in Article 2”. To this end, Article 4.1 states that we need to decrease greenhouse-gas (GHG) emissions so that net anthropogenic GHG emissions fall to zero some time in the window 2050 to 2099. In other words, to meet the temperature goal of Article 2, net-zero anthropogenic GHG emissions must be reached some time in this window. Is the 2050 to 2099 window correct? … or, more importantrly, is it even relevant? My paper shows that the answer to these questions is, first, “no”, and second, “only marginally”.
The purpose of my paper is to provide a critical assessment of Article 4.1.
There are three aspects of the wording of Article 4.1 that are crucial here: (1) how to define “net emissions”; (2) the significance of the word “anthropogenic”; and (3) the focus on GHG emissions (i.e., not just the emissions of CO2).
- The term “net emissions” is shorthand for the difference between sources and sinks. This is generally accepted. So the question is … what emissions?
- Article 4.1 restricts emissions to anthropogenic emissions only (i.e., those arising from human activities). Is this enough? Again, the answer is “no”. In modelling the growth of CO2 and other GHG concentrations, it is essential to consider all sources and sinks, not just anthropogenic sources and sinks.
- The focus on GHGs alone in Article 4.1 is an even more limiting restriction. Temperature changes depend on the effects of all factors that affect the climate, not just GHGs. The most important missing factors are aerosols, particularly sulfate aerosols, and tropospheric ozone.
Article 4.1, with its restriction to anthropogenic GHGs, falls at the first jump, because information on anthropogenic GHGs alone is insufficient to fully understand how to meet the Article 2 temperature goals.
There is a more fundamental problem with Article 4.1, and this is the issue of how to combine, or aggregate, the emissions of different GHGs. To do so we need to make the emissions of different GHGs directly comparable. The UNFCCC recommends that we do this by applying appropriate scaling factors to the emissions of different GHGs; and they further tell us to use 100-year Global Warming Potentials (100-year GWPs) as the scaling factors. Applying these scaling factors converts the emissions of non-CO2 gases to “equivalent CO2 emissions”… after which we can calculate the aggregated effect of all GHGs simply by adding their equivalent CO2 emissions.
In the first part of my paper I test this GWP-100 scaling method using a reduction in methane emissions as an example of an emissions perturbation for a non-CO2 gas. I calculate temperature changes for this scenario in two ways. Firstly, I use a climate model to calculate the effects of the reduction in methane emissions directly. Secondly, I use the GWP-100 scaling method to convert the methane emissions reduction to the equivalent CO2 emissions reduction, and then use the climate model again to calculate the implied change in temperature. The results of these two methods differ markedly. The UNFCCC scaling method fails.
We therefore need a more accurate method for aggregating GHG emissions. I derive a method that uses radiative forcing as the aggregation metric. This allows me to define the required net-zero GHG point accurately: the net-zero point corresponds to the point at which total GHG radiative forcing maximizes.
Using emissions scenarios that meet the 1.5°C warming target specified in Article 2, I find that the net-zero GHG point must be reached as early as 2036. In other words, if the claim implied by Article 4.1 that the net-zero GHG emissions point is what controls whether we meet the Article 2 warming targets is correct, then we need to reach this point much earlier than the window stated in Article 4.1. But this claim is manifestly incorrect. Whether or not we reach an Article 2 warming goal depends as much on what we do after reaching the target net-zero date, as it does on the target date per se.
The term “net-zero” has become a buzz-word for many politicians and environmental groups, but I suspect that few of these people understand what the term “net-zero emissions” means, or just how relevant the net-zero emissions point is to meeting the warming goal of Article 2.
My work shows, first, that the target for reaching net-zero GHG emissions given in Article 4.1 is much too lenient. This is important because it means that we must reduce GHG emissions much more rapidly than is implied by Article 4.1. However, in terms of meeting the ultimate warming goal set in Article 2, the net-zero emissions point is of only marginal relevance at best. This is because GHG emissions are only part of the reason for future warming. To understand and project future warming we must consider all climate-forcing agents … and Article 4.1 fails to do this. Furthermore, it is likely that many of those who advocate for “net-zero” fail to realize that this is not the end. Even when the net-zero point is reached, net emissions and both GHG and total radiative forcing will have to continue to be reduced for at least a further century if we are to successfully meet the Article 2 target.
In my paper I consider, as well as GHGs alone, the effects of the full suite of climate forcing agents. Scientifically, it is true that GHG forcing results are interesting. However, in the context of the issue of knowing “how to get there from here” (with “there” being the Article 2 warming targets) GHG-only target dates are, as already noted, of only marginal value. Yes, it is important to realize that the 2050 to 2099 window is far too weak a target … we must reduce GHG emissions much more rapidly than implied by this target if we are ever to meet the Article 2 warming goal. But we already have far more relevant and useful information for this purpose, to wit, the many detailed, multi-gas emissions scenarios that have been constructed for the IPCC SR-15 report, the focus of which is to assess the implications of the 1.5°C warming target. It is these data that provide scientifically credible insights into how to meet the Article 2 targets.
The important bottom line from my paper is that one cannot provide useful guidance for achieving the Article 2 (or any) temperature goal using information about GHGs alone (or, more crucially, net anthropogenic GHG emissions alone). Essentially, with the present wording, Article 4.1 is conceptually flawed, incomplete and mathematically incorrect.
As a final aside, it is my view that the above-mentioned SR-15 scenarios do not cover the full spectrum of possibilities for meeting the Article 2 targets, but that is a discussion for another day.
Tom M.L. Wigley, 14 Oct, 2021
School of Biological Sciences,
University of Adelaide,
South Australia, 5005.
Tel. +61 (0) 438 662 399