Knowledge for better food systems

Demonstrating GWP* with emissions scenarios

This paper uses several simple emissions scenarios to illustrate how GWP* (as opposed to GWP100) can report the warming created by both short-lived greenhouse gases such as methane (CH4) and long-lived greenhouse gases such as carbon dioxide (CO2). 

The figure below shows the first scenario: sustained emissions of either CO2 or CH4 at a constant rate for 200 years. In the case of CH4, the constant rate of emissions leads (after a few decades) to a new, constant atmospheric concentration level of CH4. Radiative forcing (i.e. the degree to which greenhouse gases affect the Earth’s energy balance) lags slightly behind concentration, and temperature increases rapidly at first but more slowly later on. In contrast, constant emissions of CO2 lead to almost linear increases in CO2 concentration, forcing and temperature.

Image: Lynch et al., Figure 1, A step change to sustained emissions of CH4 (top row, orange) and CO2 (bottom row, purple), and resulting impacts on atmospheric concentration, radiative forcing and temperature. For CH4 forcing, the dotted line shows forcing from methane alone, but the total forcing impact (solid line), is greater than this as a result of the ozone and stratospheric water vapour produced as methane breaks down. Temperature change is modelled from total forcing.

The next scenario shows the impacts of constant emissions of either CH4 or CO2 followed by a steep decline. For CH4, reducing emissions causes a decline in concentration and forcing, leading to a one-off cooling of the climate relative to the elevated baseline. For CO2, the fall in emissions only holds temperature to the point that it has already reached, leading to no further warming.

Image: Lynch et al., Figure 2, Impacts on atmospheric concentration, radiative forcing and temperature for a step change to sustained emissions for 50 years, followed by a decline to 0 emissions over the following 50 years for CH4 (top row, orange) and CO2 (bottom row, purple).

In another scenario from the paper, shown in the figure below, the authors look at the warming caused by a sharp decline in CH4 emissions levels. In the middle panel, the blue line shows the CO2 emissions that are calculated to be equivalent to the methane emissions of the first panel, when using GWP*. The red line shows the CO2 emissions that are calculated to be equivalent to the methane emissions of the first panel when using GWP100. The third panel shows the warming caused by the methane emissions themselves (orange dashed line), the warming caused by equivalent CO2 emissions calculated using GWP* (blue line), and the warming caused by equivalent emissions calculated using GWP100 (red line). As shown in the lowest panel, the warming predicted by GWP* matches the actual warming (shown by climate modelling) caused by the methane much more closely than the warming predicted by GWP100. In other words, GWP* accurately shows that cutting methane emissions causes cooling, relative to the elevated baseline, whereas GWP100 inaccurately predicts that cutting methane emissions only halts further warming.

Image: Lynch et al., Figure 4, A demonstration of (A) a step-change to sustained CH4 emissions for 50 years followed by a decline to 0 emissions over the following 50 years, and (B) corresponding annual CO2-equivalent emissions using GWP100 or GWP* (red and blue lines, respectively), followed by (C) the warming resulting from those CH4 emissions (dashed orange line) overlaid with cumulative GWP100 and GWP* CO2-equivalent emissions (solid red and blue lines, respectively).

The authors make the following concluding remarks:

“Using GWP100 to direct climate change mitigation strategy could be unfair, inefficient, and dangerous. Unfair, as it does not provide a clear link between emissions and climate change contribution, and could lead to an expectation that some actors (long-term methane emitters) have to undo their past warming, while others (CO2 emitters) merely have to limit further temperature increases. Inefficient, as it would overstate the level of action needed to offset long-term sustained methane emissions, while simultaneously undervaluing the potential short-term benefits of reducing these methane emissions. Dangerous, as it can greatly understate the impacts of increasing methane emissions, and obscure the fundamental need for net-zero CO2 emissions as soon as possible, regardless of what mitigations are made to shorter-lived climate pollutants.”

They also argue that basing mitigation strategies on GWP100 could erode trust in climate policies, since GWP100 fails to accurately reflect warming impacts.

 

Abstract

The atmospheric lifetime and radiative impacts of different climate pollutants can both differ markedly, so metrics that equate emissions using a single scaling factor, such as the 100-year Global Warming Potential (GWP100), can be misleading. An alternative approach is to report emissions as 'warming-equivalents' that result in similar warming impacts without requiring a like-for-like weighting per emission. GWP*, an alternative application of GWPs where the CO2-equivalence of short-lived climate pollutant emissions is predominantly determined by changes in their emission rate, provides a straightforward means of generating warming-equivalent emissions. In this letter we illustrate the contrasting climate impacts resulting from emissions of methane, a short-lived greenhouse gas, and CO2, and compare GWP100 and GWP* CO2-equivalents for a number of simple emissions scenarios. We demonstrate that GWP* provides a useful indication of warming, while conventional application of GWP100 falls short in many scenarios and particularly when methane emissions are stable or declining, with important implications for how we consider 'zero emission' or 'climate neutral' targets for sectors emitting different compositions of gases. We then illustrate how GWP* can provide an improved means of assessing alternative mitigation strategies. GWP* allows warming-equivalent emissions to be calculated directly from CO2-equivalent emissions reported using GWP100, consistent with the Paris Rulebook agreed by the UNFCCC, on condition that short-lived and cumulative climate pollutants are aggregated separately, which is essential for transparency. It provides a direct link between emissions and anticipated warming impacts, supporting stocktakes of progress towards a long-term temperature goal and compatible with cumulative emissions budgets.

 

Reference

Lynch, J.M., Cain, M., Pierrehumbert, R.T. and Allen, M., 2020. Demonstrating GWP*: a means of reporting warming-equivalent emissions that captures the contrasting impacts of short-and long-lived climate pollutants. Environmental Research Letters, 15(4).

Read the full paper here. See also the Foodsource building block Agricultural methane and its role as a greenhouse gas.

 

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Comments

Simon Ward's picture
Submitted by Simon Ward (not verified) on

As I am sure all recognise methane has been known to have a short life ever since gases were identified as having a global warming potential and that some were unstable.  The issue is whether conventional treatment (GWP 100) is misleading and in turn this depends on the question being asked. It would be naive to ignore the political dimension.

There is a need to maximise effort and direct investment to reduce greenhouse gas but I am not convinced that in general the proposed treatment is an improvement over current practice.  While methane may not be the cause of global warming it does not mean that reduction couldn't be part of the short term solution to buy time in order to reduce the more persistent gases (in a similar way to tree planting). IF the tipping point is close methane reduction is a high priority and allows longer term measures to be found to reduce carbon. IF the tipping point is close far from reducing the weighting of methane it should be increased.

Assessment method depends on the question asked. Has methane emissions from UK stock contributed to the rise in global temperature - NO. Would a reduction in methane emissions reduce global warming in the short term - YES. What investment should be made in reducing methane emissions compared with carbon dioxide? The treatment depends on how crucial the short term is considered to be in fighting global warming and how close we are to technologies that will address reduction in carbon dioxide emissions. Methane reductions are relatively cheap and easy - the short term impact is relatively large.

The GWP* publicity has allowed many to ignore methane reduction and consider it as a non-contributory factor ("fake news") while the cost of reduction is often small and reduction is an important option. The paper is technically correct and not misleading to a scientific readership - it is misleading for those with a political or partisan interest. It might be useful to emphasise the context.

John Lynch's picture
Submitted by John Lynch (not verified) on

Thanks for the comment Simon. A few thoughts below, but first wanted to flag up that we will have an FCRN piece exploring some possible bigger picture implications of methane and the sustainability of ruminants in the not too distant future. "The issue is whether conventional treatment (GWP 100) is misleading and in turn this depends on the question being asked. It would be naive to ignore the political dimension." I think we show quite convincingly that it can indeed be rather misleading for some quite fundamental questions, such as how do emissions of different gases contribute to global warming, and what would we expect to happen following different mitigations? We do briefly comment on some of the more political points about what the failures of GWP100 to fully answer these questions might mean, but this is a bigger debate. We also show how GWP* can be useful to investigate temporal trade-offs in emission reductions, and hence provide insight into your points about whether to focus on the short- or near-term. Section 3.4 and figure 8 in the paper explore some of the 'buying time' arguments. Regardless of what you conclude on this point, GWP* provides a useful representation of the temporal element in a way that is unclear if trying to use a single GWP equivalence. On your last point "The paper is technically correct and not misleading to a scientific readership - it is misleading for those with a political or partisan interest." This is a bit troubling, as it seems to suggest that to some the paper itself will be directly misleading. We put some things across quite strongly, and some may draw different conclusions from certain points. But I will need some convincing that the content of the paper is misleading!

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