Implications for mitigating methane emissions in agriculture

Since I was discussing methane in yesterday’s post, I thought I would highlight a paper on [u]nderstanding the implications for mitigating methane emissions in agriculture (H/T Miles King). The reason I found it interesting, is that it uses GWP* (which I try to explain here), rather than the more standard GWP100, or GWP20, metrics.

The results are nicely explained at the beginning of the paper. When considering agricultural emission scenarios from 2020-2040:

• A sustained ~0.35% annual decline is sufficient to stop further increases in global temperatures due to agricultural CH4 emissions. This is analogous to the impact of net-zero CO2 emissions.
• A ~5% annual decline could neutralize the additional warming caused by agricultural CH4 since the 1980s.
• Faster reductions of CH4 emissions have an analogous impact to removing CO2 from the atmosphere.

What this illustrates is that fairly modest reductions in agricultural methane emissions (~0.35% per year) can largely stop future agricultural methane-driven warming, stronger methane emission reductions (~5% per year) can reverse the agricultural methane-driven warming since about 1980, and even faster reductions would be analagous to negative CO2 emissions.

The reason these results might seem at odds with previous estimates is that GWP* better estimates methane-driven warming than GWP100, or GWP20.

However, even though modest methane emission reductions can have a big impact on future methane-driven warming, the paper also points out that

a 1.5% annual increase in CH4 emissions would lead to climate impacts about
40% greater than indicated by GWP100.

In other words, if methane emissions continue to increase, then they will lead to substantial future warming, which the standard metrics may under-estimate (GWP100, at least).

Also, even though this indicates that modest reductions in methane emissions can have a substantial impact, the remaining carbon budget for 1.5C is small enough that if we really do want a good chance of limiting warming to 1.5C, then we’d probably still need to make substantial cuts to methane emissions, along with rapidly reducing CO2 emissions. However, I do still think it’s worth pointing out that even modest reductions in methane emissions can have a big impact on future methane-driven warming.

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47 Responses to Implications for mitigating methane emissions in agriculture

1. The article I discussed above focusses mostly on agricultural methane emissions, but you’d get a similar result for methane releases associated with fossil fuels. However, there is one key difference between agricultural methane emissions and fossil fuel-related methane emissions. The latter is actually re-introducing a carbon into the system that has been sequestered in a long-term carbon sink. When the methane decays to CO2 and water, this is essentially equivalent to the emission of fossil fuel CO2 and will contribute to long-term warming. Hence, there is slightly more benefit to reducing fossil fuel-related methane emissions, when compared to agricultural emissions. However, this is a relatively small effect when compared to the direct emission of CO2.

At the end of the day, if we can modestly reduce methane emissions, then that could stop further methane-driven warming, and if we could reduce methane emissions slightly faster, then that could reverse some methane driven warming.

2. Chubbs says:

Noticed that livestock is by far the biggest Ag methane source. Need to re-evaluate my view of the paper discussed in the last post. Perhaps a groundswell to eat less meat is the quickest way to reduce methane, would be a win/win for most. Do disagree with prior paper though on prospects for reducing methane from natural gas, should be a cost of doing business.

3. Ben McMillan says:

Further on the question of metrics, it seems like the main point of a metric is to give a single-figure ‘badness value’ for different forcing agents.

Of course, GWP* doesn’t do this at all, because it is a (albeit very simple) integro-differential operator that takes a time series and returns another one. Effectively, you convolve the emissions time-series with an approximation to the time-derivative of the forcing curve associated with each gas; this reduces to just a lagged difference operator for CH4.

In some sense the whole idea of GWP100 is to remove the complexity of when the gas has an impact by making a specific choice for the time-interval of concern.

It seems like the objective of reinjecting time into the problem, and removing the simplification that metrics like GWP100 give you, is to make claims like “short-lived agents emitted now don’t have much impact e.g. late in the 20th century when you might have peak forcing, and thus worse damage”.

But you then have to start talking about damage curves, and somehow integrate damage over time (using a discount rate?), so to quantify anything you need to make a bunch of somewhat arbitrary choices and a DICE-like model (e.g. vanilla DICE using GWP* would tend to tell you that CH4 is a very big problem).

I guess I’m just pointing out that GWP* is in some sense a deliberate backward step away from a policy-ready metric that allows people to regulate various forcing agents.

4. Ben,
That’s a fair point. GWP100 and GWP20 are nice simple metrics and there can certainly be value in such metrics. I did have a Twitter discussion with someone who was trying to estimate damages due to methane emissions and had considered GWP* but couldn’t (I think) work out how to incorporate it into their assessment. As I understand it, if you did integrate the impact of methane emissions using GWP* over a long enough time horizon, then the standard analsysis might suggest that it’s negligible, when this isn’t the case. I suspect that there is some way to do this, but it may be the case that sticking with a simpler metric is preferable. Of course, it’s still useful to be aware of the limitations of such metrics.

5. Ben McMillan says:

The ‘obvious’ thing to do is to do what economists do, and use a discount rate for future damages, but at that point you are essentially making the same kind of choice (based on ethics/economics) of time-period that GWP20/100 make.

6. Ben,
I’m probably illustrating my ignorance, but my understanding of the complication of using GWP* is that eventually the warming due to a pulse of methane reverses, and so if one integrates over a long enough time horizon, they seemed to suggest that the damage was zero. I was slightly confused by this because I didn’t think that the period when the methane-driven warming reverses would somehow balance the period when it’s warming. I thought the damage estimate was based on $\Delta T$, not on whether or not it’s increasing, or decreasing.

7. Ben McMillan says:

I think that the procedure is to use GWP* to convert CH4 emissions to an ‘equivalent’ CO2, so a pulse of CH4 turns into a positive pulse of CO2eq emissions, plus a negative pulse 20 years later.

The concentration profile is just the time integral of the CO2eq emissions time profile (really there is a factor here).

So the concentration (or forcing) profile due to a pulse of CH4 looks like a ‘top hat’: zero except for the 20 year period after a CH4 emissions pulse. For CO2, profile looks like a Heaviside function, zero for t=0.

Assume instantaneous (marginal) damage is proportional to the temperature deviation due to this pulse, which is proportional to concentration.

The total damage is then the time-integral of the CO2eq concentration profile (times a dimensional scalar).

If you integrate for long enough, damages due to CO2 will secularly increase and thus eventually dominate those due to CH4. But the CH4 damages are always positive.

Unless I’m also confused…

8. Ben McMillan says:

sorry, I meant “Heaviside function, zero for t=0”

9. Ben McMillan says:

Yeah, obviously the problem is that wordpress is treating less than/greater than symbols as HTML!

10. Ben,
What you describe is what I thought, which I why I was a little confused by the discussion I had on Twitter where they seemed to suggest that they couldn’t use GWP* in their calculation.

As far as maths symbols go, I just use Latex (which involves a dollar sign, followed by the word latex – no space – then the maths symbols, or latex commands, and then finished with a dollar sign).

11. Ben McMillan says:

Try again in words, text should be “Heaviside function, zero for t less than or equal to zero, positive constant for t greater than zero”.

Obviously feel free to edit/delete posts!

12. Ben McMillan says:

Yeah, I think that they probably just got confused integrating a pair of delta functions twice.

To be fair, I think that would probably stump a fair proportion of undergraduate physics students.

Will hopefully remember to use latex in future…

13. Russell says:

At the end of the day, attention should be paid to mitigating the methane footprint of what’s already in the oven,

As animal emission rise as they grow to adulthood, and accumulate over time, it would be helpful if climate economists were to calculate the degree to which modifiying carnivore behavior might slow CH4 emissiions .

If beef, muttion and roast boar consumers were to shun them in favor of calves, lambs, and suckling pigs, livestock lifetime emissions would plummet.

A pigouvian carbon tax exemption might encourage this volum=ntary behavioral cahnge, but free range fowl should be exempted in the first place , as they take a bite out of methanogenic insects ,as well as those that un-capture carbon by devouring plants.

14. Ben McMillan says:

England’s new national food strategy is interesting:
https://www.carbonbrief.org/qa-will-englands-national-food-strategy-help-tackle-climate-change
e.g. “The combined land area for rearing beef and lamb for UK consumption is larger than the UK itself”

15. Above, Ben McMillan said, and you agreed that:
“I think that the procedure is to use GWP* to convert CH4 emissions to an ‘equivalent’ CO2, so a pulse of CH4 turns into a positive pulse of CO2eq emissions, plus a negative pulse 20 years later.”

Now, I may be misunderstanding BM or you overall, but the above is incorrect or may mislead others. A pulse of CH4 is not, simply, converted to a pulse of CO2 – that’s what standard GWP or GTP metrics do, so they entirely fail to show the warming reduction available from

GWP* converts a *change* in methane *flow* to a “step” or pulse of CO2 and adds a small “stock” value reflecting longer-term warming due to methane emissions. The GWP* formula does two things. First, it takes the change in annual CH4/yr value of the current year relative to 20 years previously averaged over 20 years; this gives the warming equivalent “step” or pulse) of CO2 of the change in methane flow by tCO2we = 85 x CH4_GWP100 x tCH4/yr_change. Second, the formula then adds 0.283 x CH4_GWP100 x tCH4/yr_current to give the small, cumulative stock = pulse warming value for each year’s CH4 emissions.

Importantly for climate action, GWP* offers very useful measure of the maximum available mitigation opportunity from current methane emissions level, then as part of carbon budgeting we can look at comparing agri CH4 warming reduction versus any warming reduction requirement via CDR by using the same CO2 basis.

For example, UN reported 2019 UK agricultural CH4 emissions were 1.02 MtCH4/yr. Therefore,
using the GWP* basis for flow and stock calculation, the max technical available mitigation from cutting agri CH4 flow to zero would be this value multiplied by the step of 85 multiplied by the AR6 CH4_GWP100 of 27.2 for bio source methane.

So sustaining the current UK agri flow at the 2019 rate is sustaining a level of warming equivalent to a one-off release of about 2,300 MtCO2, about 6 years of total current UK CO2 emissions. Cutting that by a quarter eg by 2035 would have a rapid warming reduction effect of removing 575 MtCO2 from the atmosphere by 2055 (i.e. net zero CO2 in AR6 SPM 1.5ºC pathway).

In addition to cutting CO2 fast, cutting all CH4 (agri and fossil) has a substantial carbon budgeting effect in aligning action with cutting fossil fuel CO2. Economically, not cutting agri CH4 substantially could be considered an opportunity cost. Given GWP* logic, could tax agriculture CH4 emitters now on the above one-off CO2 equivalent basis and they get the money back relative to the CH4 reduction achieved.

16. * end second para end, should read, “standard GWP or GTP metrics do, so they entirely fail to show the warming reduction available from”…sustained and permanent reductions in annual methane emissions.

17. Ben McMillan says:

I think, ignoring the small correction to GWP* to account for slow processes, the formula for calculating the GWP* contribution from CH4 is equation (2) in
https://doi.org/10.1088/1748-9326/ab6d7e
which is
$4*[ E(t) - E(t-20) ]*G$
where E(t) is the methane emissions as a function of time, and G is the GWP100 factor for methane (numerically, 85).

I’m probably expressing things in a nonstandard way, though, which is perhaps why you thought I’d missed the point of GWP*.

18. @Ben. Note that the equation you give is equation (3), in Lynch et. al. 2020, which is a simplification of, but identical in outcome, to equation (2).

Equation (2) is more informative of what GWP* actually comprises, which is as I stated above, a rate term including the r factor (r for rate aka flow aka step) and stock term including the s factor (s = stock aka pulse). Therefore (3) does include your ‘small correction to GWP* to account for slow processes’, the stock term.

In both (2) and (3), the GWP100 in Lynch is taken to be 32 (*not* 85 as you say) – see last para in 2.Methods. This GWP100 value is somewhat confusing in papers from the Oxford team because it appears from the Smith et al. 2021 update https://www.nature.com/articles/s41612-021-00169-8 that the GWP100 of 28 is used but a new factor, g = 1.13, is then applied to the equation, effectively making a G value in your equation above 28 x 1.13 = 31.64, so similar to the value of 32 used in Lynch.

The value of 85 that I refer to is not the GWP100 value. It comes from the rate term in Eqn (2) which is r x ∆Eslcp/∆t x H, where r = 0.75, H = 100, so, the term becomes 75 x ∆Eslcp/∆t.
Using Smith et al. the 75 is multiplied by g= 1.13, to become 84.75, rounded to 85. So Lynch 2020, Eqn (2), using Smith 2021 can be written as:

E_CO2we = { 84.75 x ∆E_slcp/∆t + 0.2825 x E_slcp } x CH4_GWP100,
where CH4_GWP100 = 28 or 27.2 if we use the AR6 biogenic value

If we hypothetically cut current emissions to zero then total step, the available mitigation adding up the rate term averaging over 20 years is approx
= 84.75 x current_tCH4/yr x CH4_GWP100
= 2373 x current_tCH4/yr, using CH4_GWP100 = 28
or
= 2305 x current_tCH4/yr, using CH4_GWP100 = 27.2

It’s not surprising people are misunderstanding GWP*. Eqn (3) may be useful simplification but it unhelpfully obscures the inclusion of both stock and flow terms. It’s also not made clear enough by the Oxford team in many of the public-facing comms that as formulated in the above equations, GWP* only shows rate change relative to 20 years previously, ∆E_slcup. So it does NOT usually show the full “step” of warming that is being maintained by current emissions which is what is important for national or global carbon budgeting to include methane with CO2 and N2O.

The “paper” (an info note) quoted by ATTP unhelpfully fails to clarify this critical issue and obscures important equity considerations.

19. Paul,

A pulse of CH4 is not, simply, converted to a pulse of CO2 – that’s what standard GWP or GTP metrics do, so they entirely fail to show the warming reduction available from

GWP* converts a *change* in methane *flow* to a “step” or pulse of CO2 and adds a small “stock” value reflecting longer-term warming due to methane emissions.

Indeed, you’re quite correct that GWP* converts a changes in methane flow to a pulse of CO2 and adds a small stock value.

You’re probably right that much of the comms has failed to highlight that there is a small stock term. I think we did discuss this briefly on Twitter. There is a time period over which the stock term essentially asymptotes to zero, but it’s long enough that we’d certainly want to included it in any near, or medium, term targets.

20. Paul,

So sustaining the current UK agri flow at the 2019 rate is sustaining a level of warming equivalent to a one-off release of about 2,300 MtCO2, about 6 years of total current UK CO2 emissions. Cutting that by a quarter eg by 2035 would have a rapid warming reduction effect of removing 575 MtCO2 from the atmosphere by 2055 (i.e. net zero CO2 in AR6 SPM 1.5ºC pathway).

In addition to cutting CO2 fast, cutting all CH4 (agri and fossil) has a substantial carbon budgeting effect in aligning action with cutting fossil fuel CO2. Economically, not cutting agri CH4 substantially could be considered an opportunity cost. Given GWP* logic, could tax agriculture CH4 emitters now on the above one-off CO2 equivalent basis and they get the money back relative to the CH4 reduction achieved.

Just so that I understand this, the 2300 MtCO2 is the total amount of methane-driven warming that would persist if we sustained methane emissions at 2019 levels? If we tax methane emissions on this basis, is this consistent with how a carbon tax is normally applied? Isn’t it normally based on the additional warming from a pulse of emission, rather than the total warming associated with these emissions. I may well be confused, though.

21. Yes the stock term is worth considering, but my main point is that the comms fails to identify that GWP*, _as formulated_ in the equations that are put forward, only shows change since 20 years ago so does not show the available mitigation step.

As I noted above for current (2019) UK agri emissions, using the GWP* rate term we can see that if this is maintained it sustains a GWP* warming equivalent to a one-off release of 2,350 MtCO2.

But if we use the GWP* equation, we only “see” the reduction of 0.06 MtCH4/yr from 2001 t0 2021 = 138 MtCO2we warming reduction if we assume stable emissions at each level.

So the full amount of available mitigation is obscured whereas that is what we want to know if we are looking at carbon budgeting and cost effectively meeting a national Paris-aligned target equitably.

Here in Ireland the situation is far worse than the UK. IE 2019 agri methane emissions are 0.549 MtCH4/yr up by 0.073 MtCH4/yr since 2010, effectively bringing the level of warming sustained (mostly by cattle) to 1265 MtCO2we up by 168 MtCO2we (compared to annual CO2 for 41 MtCO2 including land use). On the plus side that does give Ireland a very large lever to cut total warming impact, but only if the agri-expansionary policy since 2010 is reversed, crucially requiring limits on total nitrogen input into animal agriculture, especially ruminants.

As per AR6 SPM scenarios, for full effect by about 2060, need to achieve most methane of the cut by 2030, otherwise full impact is delayed.

22. Paul,
Okay, I think I’m starting to get what you’re highlighting. I wasn’t quite sure what you meant by this, though.

But if we use the GWP* equation, we only “see” the reduction of 0.06 MtCH4/yr from 2001 t0 2021 = 138 MtCO2we warming reduction if we assume stable emissions at each level.

What do you mean by stable emissions at each level?

23. Paul,

Yes the stock term is worth considering, but my main point is that the comms fails to identify that GWP*, _as formulated_ in the equations that are put forward, only shows change since 20 years ago so does not show the available mitigation step.

Okay, I might be getting what you’re saying. The standard formalism makes it looks as though all that matters is the past 20 years of emissions. However, if the formula is

$E_{CO2,we} = (4 \times E_{SLCP(t)} - 3.75 \times E_{SLCP(t-20)}) \times GWP_{100},$

then there’s not requirement for $E_{SLCP(t)}$ to be greater than $E_{SLCP(t-20)}$. So, doesn’t the formula allow for emission reductions to reverse some methane-driven warming?

24. ATTP re. 8:47 pm

“Just so that I understand this, the 2300 MtCO2 is the total amount of methane-driven warming that would persist if we sustained methane emissions at 2019 levels?”

Yes, based on GWP* and UK agri-only CH4 emissions.

“If we tax methane emissions on this basis, is this consistent with how a carbon tax is normally applied? Isn’t it normally based on the additional warming from a pulse of emission, rather than the total warming associated with these emissions.”

Yes, a carbon tax is applied on the pulse of emission. GWP* enables a flow of CH4/yr to be equated to a (one-off) CO2 pulse emission, therefore it provides a possible basis to tax and incentivise CH4 emissions within carbon budgeting. Ongoing CH4 emissions could be taxed accordingly up front with the money repaid if mitigation (flow reduction) is achieved. For CO2 can only tax all ongoing emissions; and pay for CDR.

Carbon budgeting to meet a cumulative national CO2we target equitably aligned with a global CO2we budget would require a combination of gross positive emissions and gross negative emissions from a base year onward. If we start a nation’s depletion of its CO2we quota fair share of a Paris ºC CO2we budget from say the year 2015 (Paris Agreement) then wealthy nations are running out of quota very fast now. For them CH4 reduction (as per AR6 scenarios) is critical to limit overshoot, of course in addition to radical CO2 reduction.

GWP* provides a basis for global CO2we budget estimation. GWP* allows national carbon budgeting on an explicitly defined Paris-consistent basis, but equity basis must be made clear. “No additional warming” is only achieved if a Party reverses any overshoot of its own fair share quota.

Much of GWP* comms, including that info note, entirely fails to make clear that it’s society-wide action that has to meet such a quota globally or nationally – certainly not just net zero for agri as a sector globally or in a nation. Agri, forestry and land use can supply annual net zero very soon and contribute substantial annual net negative CO2we for decades, to limit overshoot in the near-term and to limit dependence on tech CDR or other uncertain geo engineering. Wealthy nations with high input intensive animal agri have a large mitigation lever to achieve “Paris ºC target net zero”, not just “overshoot peak net zero” (the latter amounts to climate action failure).

That’s why methane mitigation is not an issue for poor nations, unlike the wealthy they are not about to overshoot their quota and tacitly commit to costly and uncertain CDR to meet the actual net zero target required by Paris. Cutting agri, fossil and waste CH4/yr quickly is far more certain, delivers sooner and is likely much less costly than any warming reduction by CDR.

25. Paul,

Yes, a carbon tax is applied on the pulse of emission. GWP* enables a flow of CH4/yr to be equated to a (one-off) CO2 pulse emission, therefore it provides a possible basis to tax and incentivise CH4 emissions within carbon budgeting. Ongoing CH4 emissions could be taxed accordingly up front with the money repaid if mitigation (flow reduction) is achieved. For CO2 can only tax all ongoing emissions; and pay for CDR.

Okay, I think I get what you’re suggesting. I’ll have to think about it a bit more, but it’s starting to make sense.

To clarify what you’re saying in the latter part of your comment, are you suggesting that just aiming for net zero means that we’re essentially not taking advantage of the fact that reductions in SLCP emissions reverses some warming and, hence, makes it more likely that we can meet the Paris goals without overshoot? Additionally, this is more relevant for wealthier nations who have a much smaller relative quota that they are going to use up much more quickly if they don’t take advantage of reductions in the emission of SLCPs.

26. Ben McMillan says:

I don’t think it really indicated a fundamental error in the earlier discussion, but indeed GWP100 of CH4 is 32, not 85 as I incorrectly stated, and it is obviously eq 3, not 2 in the paper.

27. ATTP re. 8:53pm

By ‘stable emissions at each level’ I just meant comparing stable emissions at the 2021 level with stable emissions at the 2001 level, 20 years previously. That way the step in warming *change* is evident.

To show the absolute value of the step between the current year’s tCH4/yr emissions and zero – the level of warming being sustained by a current year’s emissions if kept stable – we can simply multiply the tCH4/yr value by 2305 to get tCO2we, the one-off CO2 release warming equivalent.

28. Paul,
Okay, thanks, I do get that. I’m still not sure that I get your issue with using the past 20 years. As I understand it, that’s because that’s the period over which an emission change is expected to have some impact. If you want to assess what would happen under various different changes in future emission, you can still do so using the standard GWP* formalism.

Having said that, I do quite like your suggestion of using the one-off CO2 release warming equivalent as a basis for taxing methane emissions.

29. ATTP re. 9:18pm:
“However, if the formula is
E_{CO2,we} = (4 \times E_{SLCP(t)} – 3.75 \times E_{SLCP(t-20)}) \times GWP_{100},
then there’s not requirement for E_{SLCP(t)} to be greater than E_{SLCP(t-20)}. So, doesn’t the formula allow for emission reductions to reverse some methane-driven warming?”

Yes, of course it does indeed, which is why GWP* is very useful indeed for policy analysis by approximate a climate model outcome based only on existing UNFCCC by-gas reporting. GWP* reflects methane behaviour, including warming reduction if emissions go down. Though it is still being refined – particularly the choice of ∆t = 20 years makes sense for forcing but not I think for warming –GWP* is a huge improvement on GWP100, and hugely welcome for that.

It is the comms that have caused a big problem as we have seen in GWP* being taken up by vested interests and wealthy countries, especially those with substantial emissions from high input ruminants, by claiming that levelling off at a current high emissions level or cutting minimally over time is somehow a net zero relevant to national or global sectoral Paris goals. No such grandfathering rights assertion regarding global equity comes from the science underlying GWP* equation. How could it? Fair share budget allocation is normative, values choices need to be stated.

The main source of this misrepresentation comes from the GWP* formulation being based on last 20 years only, and annual or cumulative analyses that start at zero in 1970 or 1990 very often without pointing out that the zero in these analyses is not zero warming from CH4/yr in that year, Hence, there is a whole cumulative CO2we step of warming that’s being sustained that may be ignored unless this step amount is made explicit.

One can, as Barry McMullin and I have done, start the equity global CO2we budget depletion clock for nations from the Paris Agreement year of 2015, but we are very clear to say what we are doing and our basis in global equal per capita allocation: see Ch 7 in https://bit.ly/3h8UTfr Others of course can use a different “fair share” methodology but they need clearly stated their basis otherwise how can we assess fairness or even begin to assess Paris-consistent costs.

As Rogelj and Schleussner 2019 http://iopscience.iop.org/article/10.1088/1748-9326/ab4928 state:
“The outcomes of the emission accounting using metrics such as grandfathering GWP* or eGWP* variations are thus not scientifically neutral, but the result of value judgements and considerations of equity and fairness that need to be made explicit.”

Too many analyses we are seeing using GWP* assume a net zero of “no further warming” for their preferred nation or sector is a Paris-aligned goal. It is not. Given Paris commitments, the question must be what is a CO2 or CO2we “fair share” quota of the global ºC target-related total for the national Party in question, that needs to be explicit. If not then therein lies bias that needs to be made clear, else what appears a “scientific” analysis is open to critique on its inherent normative choices such as grandfathering emissions rights in favour of historic or current high emitters.

Many analyses claiming to use GWP* in climate action terms are even failing to appreciate that it is cumulative CO2we using GWP*, not annual, that matters in showing current warming impact and mitigation potential in carbon budgeting terms. The “info note” here is a case in point, showing only annual values as in Fig. 3. Cumulative CO2 we is what is needed to show carbon budget and warming (increase or decrease) impact. For three of the info note scenarios I get this chart in cumulative terms (noting my value for current global agri CH4 is lower, using PRIMAP data for IPC3A livestock CH4): https://bit.ly/3h2dJVt.

Notice that, not shown on this chart, if the annual mass emissions level out from 2050 onward, the lag inherent in GWP* takes another 20 years to level out the cumulative CO2we. Notice also that it does not take only 10 years (pulse “lifetime”, more like half-life) for CH4/yr reduction starting now to have most of its effect, it’s more like 35 years. A sustained CH4 flow’s warming impact, or change in such a flow, is very different to that of a single CH4 pulse.

The PRIMAP data indicates 2018 global agri CH4/yr emissions for IPC3A of 0.110 GtCH4/yr. Multiplying by 2305 tCO2we per tCH4/yr = 254 GtCO2we. (The chart value of 335 GtCO2we in 2018 also includes accumulated annual pulse emissions from the stock term of GWP*.) This is the total “step” of warming being maintained by global agri CH4. Aiming to cut that by a fraction limits CDR requirement compared to maintaining these emissions. In the context of remaining global carbon budgets of that same order that option of cutting agri CH4/yr substantially in the near-term needs to be on the table, else CDR also with big land use implications.

Of course that assumes good faith equitable action to limit to 1.5C/wb2C as per Paris Agreement Art. 2. If not, then a policy analysis or national policy might as well be clear in saying that they do not accept that as the goal.

30. Paul,
Thanks, I’m getting it now (bit slow today). By only considering the change in emissions over the past 20 years potentially suggests that this is all that matters with respect to methane-driven warming and ignores that the baseline isn’t really emission levels 20 years ago.

I completely agree with you that many of the assumptions being made include value judgements and that people aren’t always very good at making this clear.

I must admit to often thinking in terms of what would be required to get “net-zero” methane, but this has mainly been to compare with what would be required for net-zero CO2. You do make a good point that reducing methane emissions can play a big role in setting what sort of levels of CDR we’d need if we wanted to meet some of the stated targets.

31. JCH says:

Cloven hooves. Ruminants. They burp vast quantities of methane. They’re both domesticated and wild. Wild America, preindustrial, probably produced a lot of methane because the bison and deer populations were huge. If ruminants are not eating the grass, it still produces methane. Is it the same quantity?

32. JCH,
My understanding (and Paul can correct me, if I’m wrong) is that we’re increasing the number of ruminants, hence methane emissions are increasing. There is also land use emissions associated with this.

33. Our World in Data has detailed pages on prehistoric up to current animal and ruminant biomass and numbers:

https://ourworldindata.org/mammals
From here, mammal biomass 100,000 years ago was about 20 MtC.
Today wild mammals are about 3 MtC, livestock >100 MtC not incl. poultry, and humans 60 MtC.

https://ourworldindata.org/meat-production#global-meat-production
From here, ruminent numbers have gone up quickly since 1900: cattle 3x; sheep 2x. Still going up steadily.

A key point is that what matters NOW is mitigation now. Appeals to what was in prehistory are not useful. We need to assess and act on delivering maximum mitigation from all sources, asap if our Paris commitments are serious. If we are not serious then let’s be clear about failure to date, and get serious.

The recent CH4 trend is worse than animal numbers suggest because average CH4 per animal per year has been going up fast for dairy over the past 30 years. As a result, in some nations, particularly those with high input intensive agriculture, like the USA total cattle or dairy numbers have gone down but total cattle or dairy CH4/yr has gone up. In others, like Ireland, CH4/head/yr has gone up AND numbers have gone up.

Major driver is reactive nitrogen in fertiliser and feed. After a fairly minimal level of per head production very little extra CH4/unit_production is possible, so more CH4 is coupled to production. Not so for poor nations with limited N input. Most “efficient” global outcome [in the current system] would be to redirect some of rich producers N to poor nations.

Basic problem is that animal food production is highly N and GHG inefficient compared to crop food production. In changed systems toward sustainability we could still have ruminants in system to recycle N but fewer of them overall, no synthetic N fertiliser and no N feed imports e.g. see Billen et al. 2021 for Europe https://bit.ly/3DMhAQh = More net nutrition and less GHGs with no imports and much reduced agri N pollution: NH3, NO3- as well as climate N2O pollution..

Cut total N input through animal systems esp cattle is crucial to cutting ruminant CH4. Totally possible in theory, in practice big national and corporate vested interests.

34. David B Benson says:

Related is this scary University College London study:
Global warming probably much worse…

35. Ben McMillan says:

If I’m understanding correctly, the GWP*-based accounting proposed would allow countries with large current CH4 emissions rate to treat reduced emissions rates of CH4 as negative emissions, unlike in GWP100, where any CH4 emissions are positive emissions. And thus countries like Ireland/NZ could emit more CH4+CO2 given the same quota (hence the need for less CDR).

It seems likely that this unfair shift of mitigation burden from rich to poor countries would overwhelm any benefits from GWP* better capturing time-dependence of forcings.

Of course, you could design a GWP*-based accounting method where the baseline rate (used when pre-2015 emissions enter the formula) of CH4 emissions was the world per-capita average. Which wouldn’t perversely reward countries for high pre-2015 emissions. So it is possible to deal with fairness issues and still accurately track temperature-based targets.

But a lot of the interest in GWP* indeed seems to be from places with high ruminant emissions who see it as a way to get a sweeter deal. And you can play games with the metric/accounting to do this.

36. I am pleased to see a serious discussion about the need to reduce methane emissions as quickly as possible. I have no argument against that. I have some concerns that discussion of methane emission reduction do not suggest that we are buying time on CO2 emission reductions or otherwise take away from our efforts to reduce CO2 emissions with their long term impact.

I believe we need to hit emission reduction of all greenhouse gases with our absolute best efforts. I think we are currently not doing that or even coming close to it.
I read this morning that we spend more  globally to support fossil fuel production than we do to improve global air quality. It’s from the Guardian and I didn’t read it closely, but the idea that we are spending bucks to support fossil fuel production at this late date strikes me as rather absurd. https://www.theguardian.com/environment/2021/sep/07/more-global-aid-goes-to-fossil-fuel-projects-than-tackling-dirty-air-study-pollution
Cheers,
Mike

37. Dave_Geologist says:

This is a bit of a shoehorn but since letting methane run riot, if not from agriculture then from a tipping point, is one way for us to go properly Pliocene: there was an under-appreciated extinction of marine megafauna in the Pliocene. The Pliocene marine megafauna extinction and its impact on functional diversity

Perhaps surprisingly, warm-blooded creatures were the worst hit (although of course the sea is unlike the land because temperatures are buffered within a small range). It was during a period of cooling not warming but it’s the change that does it, the adapted become maladapted.

The authors attribute it to habitat loss (shrinking of shelves due to sea level fall, and a transition from stable sea level to falling and rising sea level with Milankovitch cycles), although their own analysis (Fig. 4) finds habitat loss to be a small effect with thermoregulation the biggie. They argue that the high food demands of homeothermy were a force multiplier on habitat loss. Although even then the habitat loss was driven by climate change.

OTOH here is a report on phenotype changes in homeotherms in response to today’s warming. All okey-dokey then?

“Shapeshifting does not mean that animals are coping with climate change and that all is fine,” Sara says.

“It just means they are evolving to survive it, but we’re not sure what the other ecological consequences of these changes are, or that all species are capable of changing and surviving.

“The climate change that we have created is heaping a whole lot of pressure on them, and while some species will adapt, others will not.”

Shape-shifting: changing animal morphologies as a response to climatic warming

Bird bill size is an interesting one, as they grow and wear down like our fingernails. There was a study of oyster-catchers IIRC where zoologists observed long- and robust-beaked morphs, which ate different food. They thought they were perhaps witnessing speciation in action, did the DNA, and were surprised to find that the same individual could have a different beak morph year-on-year, presumably in response to food availability and competition for it. So there is some plasticity either from usage or from grinding it against a stone to shape it like a tool.

38. Chubbs says:

NOAA has just published the May methane value. Still running 1% higher than last year, continuing this year’s spike. This years increase is similar to that observed in the 1980s before the methane rise slowed. Nina tends to elevate methane by increasing rainfall on land, still the methane trend is going in the wrong direction.

https://gml.noaa.gov/ccgg/trends_ch4/

39. Ben McMillan says:

One useful thing that putting a price on methane would do (even if it were smaller than GWP100 would suggest) is create much better inventories on methane emissions, because it would force governments to measure where it is being emitted, and create an incentive to do so (revenue).

Still isn’t clear why methane emissions have been increasing so much recently.

40. If we wanted to get the most bang for our bucks on reducing global warming, where is the low-hanging fruit? I think don’t limit or rank by the gas itself, just a simple idea about what sources of which greenhouse gas could be managed quickly and cheaply?

Please resist the impulse to jump all over me about how long various gases remain in action after emission, let’s just look at this in terms of the kind of cost-effectiveness that might even appeal to a Republican or two in Congress and lead them to vote in favor of climate legislation.

Cheers

Mike

41. JCH says:

Landscaping with fossil fuels could easily be ended altogether. I have the yard of month. I would hate to give up fertilizer, but the equipment can go.

42. Susan Anderson says:

I have a new friend who wrote this fine article on soil restoration, well worth a read. She runs a company called “Soilmaker” in Indiana (Mike Pence’s home state, mixed red/blue). An heroic effort, and a lot of wisdom and knowledge about soil’s potential for carbon sequestration.
https://www.resilience.org/stories/2019-08-01/drawing-down-atmospheric-carbon/

43. Susan,
Thanks, I’m not an expert on this, but I do wonder if that article oversells the drawdown aspect of what is suggested. Land use change has contributed about 25% of our emissions, so there is a big limit to how much trying to reverse some of this can do in terms of drawing down atmospheric CO2. I suspect there are other reasons why what is being suggested would be good to do anyway, and it will have some effect, but my suspicion is that it won’t do all that much in terms of capturing and sequestering CO2.

44. Ben McMillan says:

Even moderate success on the big ticket items (like 80% decarbonisation of electricity) is going to make smaller things like land use look relatively much more important.

What worries me most about the land-use stuff is how hard it will be to measure what is going on. Current offsets industry is doing some dubious things, and all the carbon stored is liable to just go up in smoke.

KSR’s ‘Ministry for the future’ was an interesting read in parts, on these topics.

45. Russell says:

The EU has appoved deployment of a technological milestone in cutting the carbon footprint of agriculture

Studies in 13 countries have shown that adding enzyme inhibitors toanimal feed can halve r methane emission by ruminant livestock

https://vvattsupwiththat.blogspot.com/2021/09/coenzyme-inhibitor-takes-bite-out-of.html