Keith Woodford explains why methane and carbon dioxide need separate accounting systems rather than being lumped together in a single emissions trading scheme

Keith Woodford explains why methane and carbon dioxide need separate accounting systems rather than being lumped together in a single emissions trading scheme

By Keith Woodford*

A key issue for New Zealand is how to meet the Paris commitments for greenhouse gas (GHG) emissions. Fundamental to any analysis is the different attributes of long-lived and short-lived gases. In particular, how should methane be accounted for, and how should it be brought into any emission trading scheme?

Back in 2016, current Commissioner of the Environment Simon Upton raised the importance of placing short-lived gases in a different regulatory ‘basket’ from long-lived gases. Remarkably, our rural leaders appear to have failed to pick up on the importance of this issue. 

More than any other country in the world, NZ’s gross emissions are influenced by methane-producing ruminant animals. No other developed country has a comparable emission profile, with the arguable exception of Uruguay. 

Accordingly, the issue of methane accounting, which is crucial to us, really does not matter to almost everyone else. So, no-one else will lead on this one. It is up to New Zealand to lead the debate. 

That means bringing some intellectual grunt to the issue.

At the heart of the issue is the concept of flows and stocks of the various gases.

In the case of methane from all New Zealand sources (but predominantly ruminant agriculture), the flow of emissions into the atmosphere is now lower than 20 years ago, having peaked in 2006 at 35,915 kt of CO2e, declining to 33784 kt of CO2e in 2016.  

The other key metric for determining the net flows and pools of methane is that the atmospheric resident time of methane is 12.4 years (calculated, for those who are mathematically minded, as a logarithmic decay function). This means that half of the warming caused by methane as a greenhouse gas occurs within 12.4 years, and about 94 percent within 50 years.

Bringing these metrics together, the current situation for methane from New Zealand sources (largely pastoral agriculture) is that the amount of methane entering the atmosphere is approximately equivalent to the amount that is leaving via conversion to carbon dioxide and then back into grass and related feeds via the carbon cycle. Given that inflows match outflows, then atmospheric heat sources are being lost as fast as they are being gained.

In contrast, carbon dioxide is largely a stock resource. Every time we produce more carbon dioxide, largely from burning fossil fuels, it stacks up in the atmosphere in amongst all the carbon dioxide that is already there. It takes some hundreds and even thousands of years for the extra carbon dioxide to be dissolved in the oceans or converted into inert forms, such as new coal or oil.

This means that if we keep burning fossil fuels at the current rate, then the world will continue to get hotter. Even if we reduce the burning of fossil fuels, the world will, all other things being equal, continue to get hotter as the stock of carbon dioxide in the atmosphere continues to increase. And even if we stopped all use of fossil fuels, then it would probably be many decades before we would see a meaningful decline of atmospheric carbon dioxide.

And there lies the nub of the issue. Methane from ruminant nutrition is essentially a flow resource, which flows in and out of the atmosphere, whereas carbon dioxide is a stock resource that keeps on building up.

For those who like bathtub analogies, in the New Zealand methane bathtub the tap and the plughole are in balance. For the carbon dioxide bathtub, the tap keeps flowing faster while there is still only a dribble coming out the bottom.

These concepts of flows and stocks are embedded within modelling techniques called system dynamics. Developed originally in the 1960s by American Jay Forrester, I used these techniques within my own PhD a long time ago – but not quite that long ago! I used them in a biological context, although the principles are the same as is needed to model flows and stocks of GHGs.

Recognition that short and long-lived GHGs need to be considered differently has escaped both policy makers and the general public. Our current Commissioner of the Environment stands out for his recognition of the issue, but he has largely been a voice in isolation.

Nobody likes hearing messages that run counter to prior beliefs, and so I can already hear the howls of protest from those who want to do away with ruminant agriculture in general and the dairy industry in particular. So, let me identify the key counter arguments in advance.

The first attack will be that the carbon equivalent system already takes into account the short-lived gas effect. The answer is that it does indeed do so in terms of comparing the gross emissions over a 100-year period, and this allows for the fact that the methane molecules are rapidly departing from the atmosphere over that time. But it does not take account of the net emissions (inflows versus outflows) and relative heating effects that are occurring at any point in time. 

To reinforce that point, what the current measurement system does not do is allow for the fact that methane does its damage quickly and then goes away. In contrast, whatever damage carbon dioxide does is long, drawn out, and irreversible. 

There is also a remarkable assumption buried within the current measuring system that we are only interested in the global warming potential for the next 100 years. This is called the GWP100. This means that we effectively capture all of the methane effects but miss most of the carbon dioxide effects because these relate to beyond 100 years.

If we were to measure the relative effects of methane and carbon dioxide over a 500-year time period, then we would be saying that whereas each methane molecule equates to about 28 molecules of carbon dioxide within the GWP100 (the current best estimate), then that number is reduced to around 8 carbon dioxide molecules using a GWP500.

The second argument is likely to be around the issue that, to quote Climate Change Minster James Shaw from a letter forwarded to me by its recipient: “There has been a 2.5-fold increase in global concentrations of methane in the atmosphere since the pre-industrial era”. Clearly, it would seem, we have to do something.

What was left unstated in that letter was that on this global scale most of the methane has nothing to do with ruminants. Reducing methane leakage from oil and gas fields would be a good place to start.  Another source we could work on would be from landfills and associated wastes, plus wastewater, which jointly contribute about 20 percent of the world total.  Rice paddies are also a major source, but there is no current or likely solution to that issue.

The third argument against what I am saying will be ‘ad hominem’. This is a standard approach of attack when someone says something that is controversial. The starting point is to criticise their credentials.

As a personal example, way back in 2006 I wrote an article published in Primary Industry Management about this particular issue of 100-year versus 500-year GWP indices. The logic was irrefutable, so instead I was attacked in the media as not being qualified to talk on such matters.

Unfortunately, there is no such things as Queensberry Rules when to comes to environmental politics. And that is precisely the reason that many people shy away from raising their head above the parapet. 

In a broader context, I have written about what I (and others) call ‘noble cause corruption’. These are situations where ‘advancing the cause’ is seen to justify one or all of data cherry picking, using data out of context, even fudging of data, and also throwing in some ad hominem attacks. Put simply, it is where desired and supposedly noble ends justify means. The bias can be either conscious or sub-conscious. 

The other part of ad hominem attacks is the straw man strategy. This goes along the lines of falsely arguing that the target person holds a particular stance, which is then destroyed as being built on straw. But in reality, the target person did not hold that stance at all; rather it was a twisted version thereof, with just enough truth to be credible to the uninitiated, so as to destroy the credibility of the target person.

So, let me make my own position clear. Within the current context of the Paris Agreement, agriculture emissions are indeed important and that includes methane. Also, agriculture produces another gas called nitrous oxide and this is a long-lived gas. So, in a world that is worried by climate change, agriculture along with others does have to step up to the plate. Fortunately, there are some technologies we can use to nibble away at agricultural GHGs without destroying pastoral agriculture.

In stepping forward to the plate, rural leaders must come to grips with the underlying science and associated GHG issues. These issues are indeed complex. We need to see more intellectual grunt and less emotion.


*Keith Woodford was Professor of Farm Management and Agribusiness at Lincoln University for 15 years through to 2015. He is now Principal Consultant at AgriFood Systems Ltd.  His articles are archived at http://keithwoodford.wordpress.com. You can contact him directly here.

We welcome your help to improve our coverage of this issue. Any examples or experiences to relate? Any links to other news, data or research to shed more light on this? Any insight or views on what might happen next or what should happen next? Any errors to correct?

We welcome your comments below. If you are not already registered, please register to comment.

Remember we welcome robust, respectful and insightful debate. We don't welcome abusive or defamatory comments and will de-register those repeatedly making such comments. Our current comment policy is here.

45 Comments

Comment Filter

Highlight new comments in the last hr(s).

Well done Keith, this is the first article I have read with a straightforward explanation of Agricultures effect on climate. I have read that as the planet warms Methane emissions form Tundra Perma Frost in Siberia will add to GHG as will the Methane locked in Ocean depths so surely our immediate interest is in adapting to the climate changes that are/will occur? Mitigating CO2 at the same time will also help and if we are able and successful in mitigating Ruminant Methane and more accurately calculate the CO2 that gets locked up in soil the future climate may well leave the planet a stable place to live on?

There is strong scientific evidence (outside of the political and conflicted interest domains) that is predicting global cooling is going to happen over the next 20-30yrs+. To not have this evidence debated in the mainstream media is bordering on outright fraud and deceit. Bread and circuses and the decline of Rome is a good analogy to the current state of the western world.
Even if the planet was warming, we'd have no chance of stopping the trend, and the so-called greenies would be better off spending their time on trying to reduce pollution, which is a real concern and one that can be addressed effectively.

I agree, many poor oil and gas companies from around the world have been actively trying to warn us that 'climate change' is a hoax perpetrated by 'liberal scientists' who are receiving trillions of dollars in research grants because ??? conspiracy. They've even been kind enough to pay people to prevent the spread of this dangerous 'climate change hoax'

I too have read some good internet blog posts from non-climate scientists about how it's all a big fraud. In fairness scientists have been in collusion since they first reported the greenhouse effect in the 1820's.

Even if human's are responsible for the warning through our emissions, reducing them would result in some of us earning less money which is untenable. Surely we'd be better stuffing the planets climate up rather risk our earnings.

Careful there Plutocracy your sarcasm will be completely missed by the likes of Boatman and Ludwig. Indeed Boatman has completely lost the last vestiges of my respect for his opinions. I often wonder if humanity deserves this planet.

Yeah, I don't know how they've concluded we're ending a cooling period when global mean temperature has been consistently tracking upwards. Some postulated that we would be entering a cooling period due to the Milankovitch cycle but the greenhouse emissions are currently overwhelming this if that is the case.

Heck, even Exxon's own scientists concluded climate change was real in the 1970's and 1980's (although these findings were suppressed by Exxon and instead they paid millions in an attempt to delay action being taken).

I view Boatman as being a 'baby boomer parody'. He's the quintessential stereotype of everything that most other people hate about that generation - denies housing crisis, fiercely anti-science, pro-neoliberal, hypocritical etc. I generally try to ignore him but sometimes his posts are just too ignorant to ignore.

Plutocracy get yourself informed or put your money where your mouth is.
https://www.youtube.com/watch?v=TVdKuNLmcCc

Oh look, you have linked to a video presented by someone who's not even a climate scientist and who has been found to have received money from the Oil and Gas industry.

Anyway, thanks, after watching that video I think we can now safety ignore the tens of thousands of peer-reviewed scientific publications which support anthropogenic climate change and continue emitting knowing there are no consequences.

@Ludwig , you are correct , there is existing research -based evidence of Global cooling , and thats why the lunatic fringe no longer use the term Global Warming ................ they now use the term "climate change "

Its a load of false nonsense, as false as a plastic rose by another name

11
up

The depths of ignorance you seem willing to dive to never cease to amuse me.

To clarify, we are seeing "Global warming" which causes "climate change". Its really a short, and simple sentence to understand the two terms relationship.

PS I would suggest whaleoil is more up your street than interest.co.nz.

Rubbish, there is no scientific evidence of cooling as going to happen.

What you are saying in effect is the non-scientific nutjobs you want to believe because they are of a similar world view are what you are going to listen to.

Since we are producing the CO2 of course we can stop it, we will simply not choose to as we are un-willing to make any sacrifices in our lifestyles.

Every breath you are exhaling 40,000 parts per million of CO2! So yes you can stop it......

Maybe you could try learning about the carbon cycle? Here's a good link to a helpful child's science website, hope it helps.

Lol classic - so you think air pollution and climate change occur independently of each other?

Imagine if indeed man-made climate change was all a hoax or over-reported and by reducing the burning of fossil fuels for cars and electricity we "accidentally" made the atmosphere cleaner and people healthier. I'm in, either for the cleaner air bit or the reducing climate change bit - where do I sign up?

Frankly I am sick to death of this cows and methane nonsense .

We have had methane on the planet since the dawn of time and in far larger quantities than a a couple of hundred thousand cows can emit .

And , whatsmore , to suggest that cows are causing global warming or climate change is farcical

While we all acknowledge that pollution ( air a water ) is a widespread problem and that we are consuming the planet at a rapid rate , those issues should be our focus .

Not some skewed and haphazard implementation model as is envisaged by the Paris Accord , where we have to comply immediately and China and India have 100 years to comply .

Being sick of hearing something does not make it less true.

If you want to bury your head in the sand and live out your twilight years on your yacht or wherever then go to it, but leave the good fight to others and stop whinging from the sidelines.

Excellent article, Keith, as is the linked presentation from Simon Upton. A glance at the Industry-Crown Research Consortium for the mitigation of agricultural greenhouse gases appears to indicate that most of the consortium's work is on CH4 and it appears the more thorny to solve. I wonder if NO2 can be more easily captured or managed than CH4, e.g. freestall or compost barns; or capturing, diluting, spreading effluent; or low N green feed?

While NZ needs to be linked to international fora like the IPCCC, NZ's approach should be unique and suited to us. As the Upton presentation notes "land, water, biodiversity and climate do not exist in separate silos. So understanding the synergies between them is really important in choosing tools to address the array of environmental pressures we face." We must avoid being fixated on one, e.g. at present it's all about water quality, while GHGs are forgotten about by the general public; but unarguably our biodiversity is also massive for NZ and the amazing bird life it needs to support. A pine monoculture is not the answer for NZ and this billion pine trees initiative is ridiculous and counter-productive.

I also like Upton's idea that "it might be worth thinking about a second “basket” linking methane and other short-lived gases with improved forest cover". If a farm's CH4 emissions could be offset by planting natives in gullies or riparian strips or poplars/willows in slip-prone areas, farmers might really get behind it. Obviously also benefits to mitigating P movement and sediment for water quality and supporting bird-life and biodiversity which farmers should also get a credit for -- and the aesthetic/amenity values.

Environmental offsets with native plantings - all for it.

Much, much better than the current Labour government approach which is softly, softly (or slowly, slowly) bring ag into our emissions trading scheme.

Given Simon Upton's current position as PCE, hopefully this alternate solution might get some traction.

Yep. Behaviour change is not always a function of money. Central and/or regional government could get proactive and establish native plant nurseries relatively cheaply (I think there was one in Otago until fairly recently), then provide the native plants (or poplar poles) as a public/mixed good contribution with a farmer responsible for planting and maintenance. If a farmer decides to place a QEII covenant on the land, the plants get planted for them, maybe as a community project. A shared project or two might help with this urban/rural divide.

A new climate feedback loop from wetlands. http://www.bbc.com/news/science-environment-43990403

It is not explained that methane, CH4, is oxidized to CO2 in the atmosphere, so a long term effect of methane emission is an increase in CO2, which outlives the original methane obviously by far.
So methane emission is a double whammy

Kiwichas is correct

Wikipedia Atmospheric Methane

"Stratosphere
If it is not destroyed in the troposphere, methane will last approximately 120 years before it is eventually destroyed in Earth’s next atmospheric layer: the stratosphere. Destruction in the stratosphere occurs the same way that it does in the troposphere: methane is oxidized to produce carbon dioxide and water vapor. Based on balloon-borne measurements since 1978, the abundance of stratospheric methane has increased by 13.4%±3.6% between 1978 and 2003.[69]"

Be good if Keith would reply to this point.

The equivalence numbers used for methane do take account of the fact that it ends up in C02. But in any case, the CO2 warming number is only '1', so it makes little difference to the overall calculation, which, depending on whose estimate one uses, is either 28 or 34.
Every time a methane molecule is formed by ruminant nutrition, it draws on a carbon molecule in the feed, which itself has been drawn from the atmosphere. And when the methane degrades back to CO2, that molecule re-enters the atmosphere.

The key effect is the warming that occurs while it is in the form of CH4 in the atmosphere. And it is not legitimate to ignore this warming. But it needs to be accounted for in a way that recognises that although the greenhouse gas effect of a methane molecule is powerful, it is short term, and that the ruminant CH4 'bathtub' of greenhouse gas is no longer increasing given that outflows now equal, and probably even exceed, the inflows.
KeithW

Keith, OK so the bathtab requires us to use the land in the equation as a CO2 sink. OOH I see the reason for this, comparing the land use of dairy, sheep, cropping etc there is a net CO2 gain / loss depending on use.

So if we could silo various industries using the bathtub we could assign their respective CO2 loading.

Of course if you remove the land sink the bathtub will fill up I guess.
However OOH In terms of total CO2 inventory , the effect of siloing various industries could simply result in some clever accounting but higher levels of CO2.
I can see the business case.

Bathtub analogies and settled science. "During the early 2000s, scientists studying methane noticed the global concentrations of atmospheric methane inexplicably leveled off."

"When atmospheric concentrations of methane increase, it may not be correct to chalk it up solely to an increase in methane emissions, says Caltech's Christian Frankenberg, co-corresponding author of a study on the decadal trends of methane concentrations that was published the week of April 17 in the early online edition of the Proceedings of the National Academy of Sciences.

..."Think of the atmosphere like a kitchen sink with the faucet running," Frankenberg explains. "When the water level inside the sink rises, that can mean that you've opened up the faucet more. Or it can mean that the drain is blocking up. You have to look at both."

In this analogy, hydroxyl represents part of the draining mechanism in the sink. Hydroxyl is the neutral form of the negatively charged hydroxide molecule (OH−). It is described as a "radical" because it is highly reactive and, as such, acts like a detergent in the atmosphere, breaking down methane into oxygen and water vapor.

Tracking decadal trends in both methane and hydroxyl, Frankenberg and his colleagues noted that fluctuations in hydroxyl concentrations correlated strongly with fluctuations in methane.

However, the authors do not yet have a mechanistic explanation for the last decade's global changes in hydroxyl concentrations. Future studies are needed to investigate this further, Frankenberg says. The researchers also would like to see the trends they detected verified with a more detailed study of both methane sources and sinks."

https://www.sciencedaily.com/releases/2017/04/170417182818.htm

CO2 is a trace gas measured in ppm or parts per million.

CH4 is measured in ppb or parts per billion - It is just very difficult to imagine it has warming effects orders of magnitude greater than CO2. Think less than 2 mm per kilometre on a comparative linear basis !

Whilst these warming theories have been around since the 1820's and late 1800's - we did not invent Quantum Mechanics until the early 1900's and without a knowledge of quantum mechanics it is very difficult to analyse heat flows in a gas.

These early theories compared CO2 to the warming effect of a blanket which of course is a totally incorrect analogy as blankets simply prevent the cold atmosphere from cooling via conduction. They do not reflect IR as is commonly postulated.

In addition a blanket is a physical shield - CO2 in the atmosphere is a gas and can in no way be compared to a blanket yet that's what's used.

Similarly the whole concept of a greenhouse " trapping " IR " is of course also incorrect. Again they simply allow the sun's rays in but keep the colder atmosphere out which is why they open the roof when it gets too hot.

We need to keep a level of sanity on these issues - not apparent at the moment.

Interesting to read the IPPC Scientific Report to the Policy Advisers - a very different document from what finally emerged that Paris targets are based upon.

You don't need to understand quantum mechanics to pump a tube full of different gas mixes, shine various wavelengths of light down it and measure how much is absorbed, reflected or passes straight through. That is relatively simple experimental physics.

Global Warming in the mainstream narrative IS FAKE NEWS ...... absolute rubbish .

There is no such thing as human -caused global warming , its a scam just like Bitcoin .

There may have been some climate change over the past 100 years and a rise in temps of 0,04 degrees since 1880 , but this is simply cyclical .

NASA has found evidence of GLOBAL COOLING as well as evidence of small amounts of cyclical warming , so those who think NASA is lying, probably also think that NASA landing a man on the moon is a hoax , or maybe they believe the earth is flat .

Oh puh-lease, we are sh*tting this planet up and it is affecting climate. The jury is IN! Your refusal to understand changes not one thing.

Reality is not permitted to intrude into the little right wing bubble Boatman inhabits, this isn't exactly news.

The scariest thing is there are other idiots out there espousing the same ignorance as Boatman. Humanity really is in dire straits.

You mean this NASA https://climate.nasa.gov?

"Seventeen of the warmest years on record have been recorded since 2001"

"Carbon Dioxide levels are at their highest in 650,000 years"

"Global Sea level have risen 7" over the past 100 years"

In your analogy it is you who is proclaiming the earth to be flat and landing a man on the moon is a hoax.

https://climate.nasa.gov/evidence/

https://www.esrl.noaa.gov/gmd/ccgg/trends_ch4/ is a graph showing methane levels are rising.
They will rise faster as the permafrost melts

Yeah, nah if you're worried about that sort of thing. "When permafrost thaws, wetlands are formed. Because typical wetland soils are water‐saturated and rich in organic carbon, they create a favorable environment for methanogens, the microorganisms that produce CH4. According to this conventional paradigm, the Arctic under global warming may act as a source of CH4 to the atmosphere. However, the carbon‐poor mineral soils studied by Lau et al. occupy five times the spatial area of carbon‐rich soils in the Arctic. Given that these mineral soils have now been shown to have the capacity to consume atmospheric CH4, large parts of the Arctic may actually act as CH4 sinks in the 21st century."
http://www.princeton.edu/geosciences/news/archive/?id=14884

BM, the last paragraphs in Keith article give a good explanation of our beliefs and how we resist change to them. Would be worth you reading it.

"ExxonMobil, one of the defendants of the municipalities’ lawsuits, filed a petition in a Texas District Court in January describing dramatic inconsistencies between the municipalities’ disclosure of climate risks in their bonds and their allegations of past and future damage from climate change in their lawsuits.

ExxonMobil suggested that the municipalities were either downplaying the risks of climate change in their bond disclosures or exaggerating the risks of climate change in their lawsuits. The Competitive Enterprise Institute and National Association of Manufacturers later petitioned the SEC to investigate the municipalities for possible securities fraud."

http://eidclimate.org/in-defending-calif-municipalities-bonds-former-sec...

...and it still won't warm faster today than 100 years ago. The southern hemisphere a full 0.1 degrees warmer than 30 years ago. Run for the hills!

"Global climate trend since Nov. 16, 1978: +0.13 C per decade". Indistinguishable from inter glacial warming and a lower rate than 150 and 100 years ago when we didn't have middle class guilt.

http://news.bbc.co.uk/2/hi/8511670.stm
https://www.nsstc.uah.edu/climate/

That commentary from the BBC is almost 10 years old, here's commentary from the same climate scientist in the same publication one year later -

http://www.bbc.com/news/science-environment-13719510

So what? It is tabulated historical data and that doesn’t change if written today or yesterday. The satellite data is from last month. Your posted article does nothing to dismiss the fact it was warming faster 100 years ago when hand wringers were too busy fighting wars to feel guilty about trace gase theories.

Note in your link Jones states “It just shows the difficulty of achieving significance with a short time series, and that's why longer series - 20 or 30 years - would be a much better way of estimating trends and getting significance on a consistent basis." We now have 30 years of satellite data showing no difference in pre WW2 warming and the satellite record. 0.13 degrees per decade is indisguishable from inter glacial warming or the pre WW2 warming of 0.16 degrees per decade.

What do you think the inter glacial warming rate should be?

You posted an article a decade old in defense of your position, I corrected that with a newer article from the same person, saying something significantly different, so yes that does matter. Because science.

https://www.independent.co.uk/news/science/climate-change-sceptics-satel...

http://www.theclimateconsensus.com/content/satellite-data-show-a-cooling...

Post away. Doesn’t change the fact 0.15 historic is faster than 0.13 in the UAH satellite record. Or do you disagree with that simple fact? Orbital decay has been know about for decades and is corrected for in the UAH dataset. A data set that is calibrated with weather balloon data and other methods. You can read about in peer reviewed lit if that turns your dial.

You didn’t tell me what you think the correct inter glacial warming rate should be.

And as for the Independent - the news paper that bought us the “children aren’t going to know what snow is” in 2000.

The equivalence numbers used for methane do take account of the fact that it ends up in C02. But in any case, the CO2 warming number is only '1', so it makes little difference to the overall calculation, which, depending on whose estimate one uses, is either 28 or 34.
Every time a methane molecule is formed by ruminant nutrition, it draws on a carbon molecule in the feed, which itself has been drawn from the atmosphere. And when the methane degrades back to CO2, that molecule re-enters the atmosphere.

The key effect is the warming that occurs while it is in the form of CH4 in the atmosphere. And it is not legitimate to ignore this warming. But it needs to be accounted for in a way that recognises that although the greenhouse gas effect of a methane molecule is powerful, it is short term, and that the ruminant CH4 'bathtub' of greenhouse gas is no longer increasing given that outflows now equal, and probably even exceed, the inflows.
KeithW

"If we were to measure the relative effects of methane and carbon dioxide over a 500-year time period, then we would be saying that whereas each methane molecule equates to about 28 molecules of carbon dioxide within the GWP100 (the current best estimate), then that number is reduced to around 8 carbon dioxide molecules using a GWP500."

What's the break even point Keith? i.e. how many years have to pass before the GWP of methane is 1.

Doris,
There is no break even point. Whatever time-frame is used, then the effect of a molecule of CH4 is much greater than a molecule of CO2. But there are of course a lot less molecules of CH4 in the atmosphere than CO2.

The 500 year equivalence is not a bad place to start in terms of the total effect, because this is capturing most but not all of the CO2 effects. But there are also value judgements involved here. Do we only care about the next 100 years or do we consider that we have a moral duty to think beyond there?

But whatever time-frame is used, there is still the issue that trying to compare short and long term effects is highly problematic. Effects from short-lived gases are quickly observable and largely reversible; effects of long-lived gases are insidious and irreversible. These effects build up over time and once they are seen it is too late.
KeithW