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The definitive CO2/CH4 comparison post

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There is a new push to reduce CH4 emissions as a possible quick ‘win-win’ for climate and air quality. To be clear this is an eminently sensible idea – as it has been for decades (remember the ‘Methane-to-markets’ initiative from the early 2000s?), but it inevitably brings forth a mish-mash of half-remembered, inappropriate or out-of-date comparisons between the impacts of carbon dioxide and methane. So this is an attempt to put all of that in context and provide a hopefully comprehensive guide to how, when, and why to properly compare the two greenhouse gases.

Historical comparisons

First of all, let’s be clear about the relative magnitude of the gas concentrations. In 2020, CO2 was at ~410 parts per million, while CH4 was around 1870 parts per billion (or 1.87 ppm, a factor of more than 200 smaller). However the relative rise since the pre-industrial is three times larger for CH4, around 150%, compared to the 50% increase in CO2.

The radiative forcing from these changes in concentrations can be easily calculated using standard formulas (from Etminan et al, 2016 which supersede the slightly simpler ones from IPCC TAR), as about 2 W/m2 for the CO2 change and 0.65 W/m2 for CH4.

But methane’s role in atmospheric chemistry and as a source of stratospheric water vapour means that it has a bigger effect on climate than just the direct effect of its concentration. Methane emissions have a feedback on its own lifetime, serve as an ozone precursor, and reduce the production of sulphate and nitrate aerosols (and consequently indirect cloud-aerosol effects), all of which amplify its net warming effect to about 1.2 W/m2 (to about 60% of the CO2 effect since 1750). There is also a very small impact of the CH4 oxidation to CO2 itself for any fossil-fuel derived methane.

(a) Historical radiative forcing by emissions (W/m2) (IPCC fig TS.15a).(b) Historical concentrations of CO2, CH4 and N2O (IPCC AR6 fig TS.9b)

This implies that if you convert the impacts of each set of emissions into temperatures, as was done in the IPCC AR6 report, you get about 0.75ºC from the changes in CO2 and 0.5ºC for CH4 (from the late 19th C, see figure below) or 1ºC and 0.6ºC, respectively, from 1750. Thus despite the smaller concentrations and changes in methane compared to carbon dioxide, the impacts are comparable.

Historical contributions to global warming by emissions (IPCC AR6 SPM)

Stocks and flows

Before we go any further though, we need to understand that the effective perturbation time for CO2 and CH4 in the atmosphere are very different. CO2 emissions embed themselves in the atmosphere/biosphere/upper-ocean carbon cycle and have very long-term impacts (under natural conditions, some 15% of the CO2 perturbation will still be in the atmosphere thousands of years from now). In contrast, methane has a perturbation time-scale of about 12 years. This implies that the impact of CO2 on temperature is cumulative (a function of the total emitted CO2 or stock), while the impact of CH4 is a function of current (~decadal) emissions (the flows). Stabilizing temperature effects from CO2 means getting down to net-zero anthropogenic emissions, while stabilizing temperature effects from CH4 means simply stabilizing emissions.

The impacts of emissions of CH4 compared to CO2 then will have a time-varying component. Over a short time, the enhanced effectiveness of methane will be important but on very long time scales the effects of CO2 will be dominant. This is the source of the difference between the “Global Warming Potential” (GWP) numbers calculated at 20 years or 100 years which have been used for decades. You might recall that GWP is defined as the ratio on per-kg basis of the temperature impact of other greenhouse gases compared to CO2 over a specific time period. But as is clearly stated in AR6, the suitability of comparative emission metrics depends on your end goal or values.

For instance, if you use GWP-100 to trade off emissions on the way to a temperature stabilization scenario, it simply doesn’t work (since you can’t balance any net CO2 emissions with a particular level of CH4 emissions – you would need to have constantly decreasing CH4). Hence, newer concepts like GWP* have been developed that take that into account. Nonetheless, the UNFCCC (and the EPA) use the GWPs from IPCC AR4 for calculating CO2eq emissions and have not updated them as the science has progressed.

Forward-facing comparisons

People tend to be most interested in comparisons related to future choices, and it’s worth bearing in mind that while there are many ways to do this, most don’t relate to real choices that people have, nor do they clearly match up with a consistent set of values. I’ll return to that issue below. So let’s go:

  • Molecule-to-molecule concentrations: On a per-ppm basis, methane is 25 times more effective as a direct greenhouse gas. Including the indirect effects, increases that to 45 times as effective.
  • kg-to-kg: On a mass basis, methane is 70 times more effective as a greenhouse gas. This takes into account of the different molecular weights of the molecules. That would mean 126 times as effective including indirect effects.
  • kgC-to-kgC: an equal amount of kgC as CH4 or CO2 gives rise to the same ppm change, so kgC-to-kgC, methane is again 45 times more effective as a greenhouse gas.
  • kg to kg emitted: This is where it starts to get hairy because of the different timescales. Current (AR6) estimates for fossil-sourced methane are ~83 for GWP-20 and ~30 for GWP-100 (AR6 Table 7.15). (It’s slightly smaller than this for biogenic (non-fossil) methane since the oxidation product of CO2 in that case is carbon neutral). The assessed uncertainties in these values (largely related to direct and indirect aerosol effects) are ±25 and ±11. The AR4 value for methane GWP-100 was 25.
  • kgC emitted to kgC emitted: For some applications, for instance judging the impact of flaring natural gas vs. releasing it directly into the atmosphere, the kg-to-kg comparisons are not relevant, since the same amount of carbon is being emitted, rather than the same total mass. For that, the GWP-like value over 100 years, choosing to release methane directly would be 30*16/44 = 11 times worse than flaring [Corrected 9/20/21].
  • Emissions for temperature stabilization: Each additional GtC of carbon dioxide contributes to about 0.00165ºC of eventual warming (the TCRE), while a sustained TgCH4/yr of methane emissions (0.00075 GtC/yr), leads to ~3 ppb increase of methane concentrations (AR6 Table 5.2), about 0.0024 W/m2 in total radiative forcing, and, assuming a median climate sensitivity of 3ºC for 2xCO2, roughly 0.002ºC of equilibrium global warming. That implies you need a sustained reduction of 0.8 TgCH4/yr (0.0006 GtC/yr of methane) to compensate for a one-off GtC pulse of CO2.

Whatever way you slice this it implies that CH4 reductions have an outsize effect on climate, as well as an undeniably positive impact on air pollution, crop yields and public health (mainly through ozone reductions). It is therefore not a complicated decision to pursue methane reductions, taking care not to assume that doing so gets you off the hook for reducing CO2, whatever the EPA says.

I’d like this page to be useful and current, so if you think I should add an additional comparison, or use case, or if you think I’ve got something wrong, please let me know in the comments.

References

  1. M. Etminan, G. Myhre, E.J. Highwood, and K.P. Shine, "Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing", Geophysical Research Letters, vol. 43, 2016. http://dx.doi.org/10.1002/2016GL071930

We are not reaching 1.5ºC earlier than previously thought

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Guest commentary by Malte Meinshausen, Zebedee Nicholls, and Piers Forster

Of all the troubling headlines emerging from the release of the Intergovernmental Panel on Climate Change (IPCC) WG1 report, one warning will surely dominate headlines in the next days and weeks: Earth is likely to reach the crucial 1.5℃ warming limit in the early 2030s.

In 2018, the IPCC Special Report on 1.5C warming  stated in its summary for policy makers that the world was likely to cross the 1.5℃ threshold between 2030 and 2052, if current warming trends continue. 

In this latest AR6, a more comprehensive assessment was undertaken to estimate when a warming level of 1.5℃ might be reached. As a result, some early media reports suggest 1.5ºC warming is now anticipated 10-years earlier than previously assumed (AFR, THE TIMES). 

We want to explain here why that is not backed up by a rigorous comparison of the SR1.5 and AR6 reports. In fact, the science in the previous SR1.5 report and the new AR6 report are remarkably consistent.

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The IPCC Sixth Assessment Report

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Climate scientists are inordinately excited by the release of a new IPCC report (truth be told, that’s a bit odd – It’s a bit like bringing your end-of-(seven)-year project home and waiting anxiously to see how well it will be received). So, in an uncharacteristically enthusiastic burst of effort, we have a whole suite of posts on the report for you to read.

If/when we add some more commentary as we digest the details and we see how the report is being discussed, we’ll link it from here. Feel free to discuss general issues with the report in the comments here, and feel free to suggest further deep dives we might pursue.

Climate adaptation should be based on robust regional climate information

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Climate adaptation steams forward with an accelerated speed that can be seen through the Climate Adaptation Summit in January (see previous post), the ECCA 2021 in May/June, and the upcoming COP26. Recent extreme events may spur this development even further (see previous post about attribution of recent heatwaves). 

To aid climate adaptation, Europe’s Climate-Adapt programme provides a wealth of resources, such as guidance, case studies and videos. This is a good start, but a clear and transparent account on how to use the actual climate information for adaptation seems to be missing. How can projections of future heatwaves or extreme rainfall help practitioners, and how to interpret this kind of information? 

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Rapid attribution of PNW heatwave

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Summary: It was almost impossible for the temperatures seen recently in the Pacific North West heatwave to have occurred without global warming. And only improbable with it.

It’s been clear for at least a decade that global warming has been in general increasing the intensity of heat waves, with clear trends in observed maximum temperatures that match what climate models have been predicting. For the specific situation in the Pacific NorthWest at the end of June, we now have the first attribution analysis from the World Weather Attribution group – a consortium of climate experts from around the world working on extreme event attribution. Their preprint (Philip et al.) is available here.

Trends in Tmax globally
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The Rise and Fall of the “Atlantic Multidecadal Oscillation”

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Two decades ago, in an interview with science journalist Richard Kerr for the journal Science, I coined the term the “Atlantic Multidecadal Oscillation” (AMO) to describe an internal oscillation in the climate system resulting from interactions between North Atlantic ocean currents and wind patterns. These interactions were thought to lead to alternating decades-long intervals of warming and cooling centered in the extra-tropical North Atlantic that play out on 40-60 year timescales (hence the name). Think of the purported AMO as a much slower relative of the El Niño/Southern Oscillation (ENSO), with a longer timescale of oscillation (multidecadal rather than interannual) and centered in a different region (the North Atlantic rather than the tropical Pacific).

Today, in a research article published in the same journal Science, my colleagues and I have provided what we consider to be the most definitive evidence yet that the AMO doesn’t actually exist.

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Laschamps-ing at the bit

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A placeholder to provide some space to discuss the paper last week (Cooper et al, 2021) on the putative climate consequences of the Laschamps Geomagnetic Excursion, some 42,000 yrs ago.

There was some rather breathless reporting on this paper, but there were also a lot of sceptical voices – not of the main new result (a beautiful new 14C dataset from a remarkable kauri tree log found in New Zealand), but of the more speculative implications – both climatically and anthropologically.

On twitter there were some good threads covering multiple aspects of the paper (and the lead author):

But let me make a couple of different points. We have occasionally discussed the Laschamps event here as a counter-example to the notion that changes in galactic cosmic rays have a major impact on climate. A reversal or near-reversal of the geomagnetic field would be expected to greatly increase the GCR getting to the lower atmosphere – in far greater amounts than over a solar cycle, or grand solar minimum (like the Maunder Minimum). So if people want to postulate a big role for GCR there, they needed to explain why there wasn’t a much bigger signal at 42kya too. These authors are thus not the only people to have looked for significant climate impacts at this time. They are perhaps the first to claim to have found them…

To be clear, the modeling that was done in this paper was good (if extreme) and suggested that the geomagnetic event combined with a severe grand solar minimum (much bigger than the Maunder minimum) would cause significant depletion of the ozone layer and some shifts in the annular modes. But the ozone depletion is less than we’ve seen due to anthropogenic ozone depletion since the 1980s, and the surface climate changes don’t seem very significant at all – especially compared to the massive variability exhibited in the ice cores throughout the last ice age (particularly in Marine Isotope Stage 3 – the Dansgaard-Oeschgar events). At best these are nuanced and subtle climate effects, and certainly not anything apocalyptic (despite Stephen Fry’s dulcet tones).

Finally, it should be called the Laschamps event (with a final, and etymologically correct, ‘s’) after the village in the Auvergne where it was first identified. There is unfortunately 50 years of legacy references to the “Laschamp” excursion, but hopefully it isn’t too late to fix!

References

  1. A. Cooper, C.S.M. Turney, J. Palmer, A. Hogg, M. McGlone, J. Wilmshurst, A.M. Lorrey, T.J. Heaton, J.M. Russell, K. McCracken, J.G. Anet, E. Rozanov, M. Friedel, I. Suter, T. Peter, R. Muscheler, F. Adolphi, A. Dosseto, J.T. Faith, P. Fenwick, C.J. Fogwill, K. Hughen, M. Lipson, J. Liu, N. Nowaczyk, E. Rainsley, C. Bronk Ramsey, P. Sebastianelli, Y. Souilmi, J. Stevenson, Z. Thomas, R. Tobler, and R. Zech, "A global environmental crisis 42,000 years ago", Science, vol. 371, pp. 811-818, 2021. http://dx.doi.org/10.1126/science.abb8677

Regional information for society (RifS) and unresolved issues

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It’s encouraging to note the growing interest for regional climate information for society and climate adaptation, such as recent advances in the World Climate Research Programme (WCRP), the climate adaptation summit CAS2021, and the new Digital Europe. These efforts are likely to boost the Global Framework for Climate Services (GFCS) needed as a guide to decision-makers on matters influenced by weather and climate. 

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Update day 2021

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As is now traditional, every year around this time we update the model-observation comparison page with an additional annual observational point, and upgrade any observational products to their latest versions.

A couple of notable issues this year. HadCRUT has now been updated to version 5 which includes polar infilling, making the Cowtan and Way dataset (which was designed to address that issue in HadCRUT4) a little superfluous. Going forward it is unlikely to be maintained so, in a couple of figures, I have replaced it with the new HadCRUT5. The GISTEMP version is now v4.

For the comparison with the Hansen et al. (1988), we only had the projected output up to 2019 (taken from fig 3a in the original paper). However, it turns out that fuller results were archived at NCAR, and now they have been added to our data file (and yes, I realise this is ironic). This extends Scenario B to 2030 and Scenario A to 2060.

Nothing substantive has changed with respect to the satellite data products, so the only change is the addition of 2020 in the figures and trends.

So what do we see? The early Hansen models have done very well considering the uncertainty in total forcings (as we’ve discussed (Hausfather et al., 2019)). The CMIP3 models estimates of SAT forecast from ~2000 continue to be astoundingly on point. This must be due (in part) to luck since the spread in forcings and sensitivity in the GCMs is somewhat ad hoc (given that the CMIP simulations are ensembles of opportunity), but is nonetheless impressive.

CMIP3 (circa 2004) model hindcast and forecast estimates of SAT.

The forcings spread in CMIP5 was more constrained, but had some small systematic biases as we’ve discussed Schmidt et al., 2014. The systematic issue associated with the forcings and more general issue of the target diagnostic (whether we use SAT or a blended SST/SAT product from the models), give rise to small effects (roughly 0.1ºC and 0.05ºC respectively) but are independent and additive.

The discrepancies between the CMIP5 ensemble and the lower atmospheric MSU/AMSU products are still noticeable, but remember that we still do not have a ‘forcings-adjusted’ estimate of the CMIP5 simulations for TMT, though work with the CMIP6 models and forcings to address this is ongoing. Nonetheless, the observed TMT trends are very much on the low side of what the models projected, even while stratospheric and surface trends are much closer to the ensemble mean. There is still more to be done here. Stay tuned!

The results from CMIP6 (which are still being rolled out) are too recent to be usefully added to this assessment of forecasts right now, though some compilations have now appeared:

CMIP6 model SAT (observed forcings to 2014, SSP2-45 scenario subsequently) (Zeke Hausfather)

The issues in CMIP6 related to the excessive spread in climate sensitivity will need to be looked at in more detail moving forward. In my opinion ‘official’ projections will need to weight the models to screen out those ECS values outside of the constrained range. We’ll see if other’s agree when the IPCC report is released later this year.

Please let us know in the comments if you have suggestions for improvements to these figures/analyses, or suggestions for additions.

References

  1. Z. Hausfather, H.F. Drake, T. Abbott, and G.A. Schmidt, "Evaluating the Performance of Past Climate Model Projections", Geophysical Research Letters, vol. 47, 2020. http://dx.doi.org/10.1029/2019GL085378
  2. G.A. Schmidt, D.T. Shindell, and K. Tsigaridis, "Reconciling warming trends", Nature Geoscience, vol. 7, pp. 158-160, 2014. http://dx.doi.org/10.1038/ngeo2105