Breakdown of forcing from non-CO2 greenhouse gases

Introduction

Among man-made greenhouse gases, carbon dioxide (CO2) is by far the most important. However, there has also been significant forcing or impact from other gases, such as methane.

Traditionally, the successive reports by the Intergovernmental Panel on Climate Change (IPCC) have been the most authoritative source on the amount of forcing by different greenhouse gases. However, the IPCC’s Assessment Report 5 (AR5), published in 2013, offered historical forcing estimates for an aggregation of greenhouse gases that had usually been considered separately; this made comparison with previous reports difficult. The objective of this article is to break down the forcing from these non-CO2 greenhouse gases into its main components.

Methods of the IPCC

AR5 aggregated the forcing of all well-mixed greenhouse gases (WMGHGs) other than CO2. These are:

· Methane (CH4)

· Nitrous oxide (N2O)

· Halogenated gases, or halogens for short. This category includes roughly two dozen gases: CFCs, HCFCs, HFCs, and more.

The part of AR5 that dealt with the physical science of climate change, contributed by Working Group 1, is here. An Excel document with the report’s key spreadsheets can be found here. The table that lists the effective radiative forcing of different forcing agents, including WMGHGs, is AII.1.2. Specifically, this article deals with the ‘GHG Other’ column. Notice that the data extend only up to 2011.

Before continuing, a clarification. ‘Well-mixed’ greenhouse gases are also often called ‘long-lived’, and indeed their long atmospheric lifetime is the reason they are well-mixed (i.e. spread around the globe). Ozone, by contrast, is a greenhouse gas but its atmospheric lifetime is measured in days, and so it tends to be geographically concentrated. Water vapor is also considered a man-made greenhouse gas, insofar as humanity’s actions have increased the amount of this gas in the stratosphere, but obviously it’s not well-mixed. So the IPCC considers ozone and water vapor separately from WMGHGs.

Methods of Lewis & Curry 2018

I’ve cited this paper in several articles by now, but really, it’s the only study I know of that discloses forcing estimates and the code used to generate those estimates. Here is a link to the original study, which I’ll call LC18 for brevity, and here is the authors’ reply to a comment about the paper. The reply’s journal version includes the computer code (written in R).

LC18 offered estimates of forcing up to 2016, from the same baseline as the IPCC (the year 1750). For the main greenhouse gases, the expressions or formulas that LC18 used come from Etminan et al 2016. Although the LC18 formulas for calculating forcing from CH4 and N2O are different from those of AR5, the paper followed the IPCC practice of aggregating all non-CO2 WMGHGs into a single metric. Nevertheless, the code allows one to see exactly how this was done. LC18 followed these steps:

1. Take the GHG Other forcing from AR5

2. Use the old, AR5 formulas for CH4 and N2O to calculate the IPCC’s forcing figures for these gases

3. Subtract the IPCC’s CH4 and N2O forcing from GHG Other to calculate halogen forcing

4. Use new formulas for CH4 and N2O forcing

5. Aggregate halogen forcing to the updated CH4 and N2O forcing, in order to create a new GHG Other series

(If you’re wondering why LC18 used such a roundabout way to estimate halogen forcing, remember that this category of gases involves dozens of different substances; a bottom-up estimate, aggregating the forcing of each gas, would require a paper all by itself)

For this article, steps 2 and 3 are key. Although AR5’s historical data lumps all non-CO2 WMGHGs together, in Table 8.2 it did offer a breakdown of the forcing from each different gas at one point in time (2011). The numbers provided by AR5 in that table agree with what LC18 calculates from AR5’s formulas: 0.17W/m2 for N2O and 0.48W/m2 for CH4. There was a negligible difference in halogen forcing, which was 0.360W/m2 according to the IPCC and 0.361W/m2 following LC18’s method.

Results

With some modifications to the code in LC18 one gets the following plot. Please keep in mind that the title says ‘according to Assessment Report 5’, but actually the chart shows LC18’s calculation using AR5’s formulas, as described in the previous section.

The match between LC18’s calculation (employing AR5’s formulas) and the actual numbers from AR5 is very good, but not perfect. During the era before halogens were released into the atmosphere, and when all the GHG Other forcing should therefore consist of N2O and CH4, the aggregation of forcing from the two latter gases (as calculated by LC18) is sometimes greater than GHG Other forcing (as stated by AR5). That’s why the chart shows an unphysical negative forcing from halogens some years; such negative forcing is an artifact. In any case, this forcing is very small (at most -0.004W/m2).

For N2O there is virtually no difference between the old and new formulas. However, for methane the change is substantial: forcing by 2016 had reached 0.624W/m2, which is 25% than under the old definition. The effect can be seen in the following chart; notice the difference in the y-axis.

I cannot show the code used to generate the above plots, as it’s (mostly) not mine and the original code has only been published behind a paywall. However, a spreadsheet I created with the values generated by the code is available here.