Zeke, this is a great article and comparison between CO2 and CH4 and their time horizons. I think it highlights that time horizons matter, not just for greenhouse gases, but for all aspects of environmental sustainability issues. Thank you!
One thing I can’t quite figure out: if methane decomposes into CO2, doesn’t it (at least partly) also stay in the atmosphere forever? Does that get taken into account here?
Yes, you are correct. However, methane's addition to CO2 is a pretty small contribution — it's main contribution is the warming you get when it's in the form of methane.
Oh, and pretty much all* methane is biogenic, either from anaerobes alive in the distant past (forming fossil gas reservoirs) or those currently fed by the availability of thawed permafrost (and its old store of previously inaccessible carbon).
We have increasing number of cattle worldwide, a large percent of which are being fed by cultivated crops rather than wild vegetation. I would think that accelerated the "natural carbon cycle" turnover.
This post covers some key science but it fails to cover the relevance of that science to meeting the Paris Agreement Article 2 commitments made by all nations: (1) efforts to achieve 1.5; (2) via equitable implementation. In this crucial context for climate action, methane mitigation (and CDR) become critically important at global and wealthy nation levels due to already existing or imminent overshoot of a 1.5ºC or 1.5ºC fair share goal.
Of course, defining a national air-share of 1.5ºC effort is not value neutral (see Dooley et al. 2021, doi 10.1038/s41558-021-01015-8), but the Paris Agreement is "bottom-up", so it is up to nations to state their values in claiming remaining 1.5ºC temperature budget. Not saying is a value choice too. Given imminent global 1.5ºC overshoot, for "developed" nations such effort means that early, deep and sustained methane mitigation (and future CDR) is now essential IN ADDITION TO CO2 MITIGATION by cutting fossil fuel and land use emissions.
As you correctly say, SLCP mitigation is not a substitute for CO2 mitigation, but it becomes critically important if stringent temperature targets are to be met. This was a key IPCC AR6 conclusion. Correct GWP* use supports this reality. It is GWP* misuse that is being wrongly leveraged to suggest that "no additional warming" is a reasonable net zero target for wealthy nations – it is not.
Cutting methane emissions from milk and meat production in developed nations is a major lever to limit 1.5ºC fair share overshoot and to enable an early return toward the goal. Even if global 1.5ºC overshoot is inevitable, the Paris Article 2 commitment remains the internationally agreed gauge of how bad that failure might be. Every fraction of a degree counts and cutting annual methane emissions from fossil, livestock, land and rice sources are a major temperature mitigation lever, as GWP* warming analysis shows.
In our journal article, McMullin et al. 2024, we point to the case of Ireland, where, in 2021 and since, the national advisory expert body used GWP* to assess a (questionably fair) national share of remaining warming from 2020 and carried out a "Paris Test" of emissions scenarios to 2050. This GWP*-based test evidenced the critical importance of early, deep and sustained reductions in agricultural (ruminant) emissions in meeting a stringent temperature goal. For more see: https://iopscience.iop.org/article/10.1088/1748-9326/ad3660
Unfortunately, the originators of GWP* have directed their briefings at the livestock industry and their "net zero" science contributions generally fail to point to Article 2 commitments to 1.5ºC and equity (on the basis of Common But Differentiated Responsibilities). Addressing this science-based political agreement reality is not about removing all cattle and sheep (a favourite strawman response), but the analysis shows that reducing ruminant production and focusing more on non-animal derived food farm production instead is far more aligned with a resilient future for society and farming.
By ignoring these commitments, too many journal articles and livestock industry outputs make it appear that simply stabilising methane emissions is enough as a "net zero" goal. But globally and for developing nations, attaining future net zero CO2fe/yr in GWP* terms merely allows reaching some indeterminate level of temperature stabilisation in permanent overshoot of a 1.5ºC fair share goal. Attaining net zero CO2e/yr in GWP100 terms means that residual N2O and CH4 also have to be continually offset in future by CDR in CO2e terms – a questionable assumption in scientific and equity terms.
But BOTH of these net zero annual emission futures – GWP* and GWP100 – are potentially very societally costly ways of excusing near-term action to cut livestock production methane (and N2O) emissions for due 1.5ºC fair share effort. So cows are NOT like closed power staions: we can still choose to cut livestock methane, the closed coal power station is already closed.
An additional point is that GWP100 for the different gases, being directly related to GHG emissions, remains a reasonable gauge of emission mitigation action, whereas GWP* annual values do not as they relate to 20-year change in emissions. GWP* provides different information, on mitigation impact rather than mitigation emissions.
As per UNFCCC SBSTA 2022 advice, GWP* can supply useful supplementary information to GWP100 reporting, but GWP* is not a GWP100 replacement. GWP* is a useful micro climate model approximation of the forcing impact (temperature commitment) from a given date on the basis of cumulative CO2fe, which is a proxy for temperature commitment contribution. A common reference year for differentiated action (and prior historical responsibility) must be asserted – another choice that is not value neutral.
SLCP's are analogous to heat loads then? i.e. thermall gen waste heat and albedo chnages from PV.
It seems that we should use GWPX where X is the time until peak GHGe concentrations in the atmosphere. Thats where the SLCPs will have their biggest impact, in raising the peak. At any other time they are just time shifting the path to that peak...
Just to be sure to have well understood : on your graph showing the delta global mean temperature linked to the emission of 1Mt of CO2 or 30kt of CH4, the delta T is not the same after 100yrs. It is due to the fact that the GWP is not based on warming but rather on energy put in the system ?
In fact, with this name I thought that GWP was based on the warming. Could it be a solution to the problem to define another GWP on temperature rather than energy put in the system ? I.e to have a parameter based on a given period, for which the warming (=Delta T) is the same after this period ? (for CH4, it would lead to very high overshoot, though, as is shown on your graph).
GWP is based on comparing average radiative forcing (which is similar to but not the same as warming) over a chosen time horizon. So its essentially saying that 1 Mt CO2 has the same average forcing as 30 kt CH4 over 100 years, but that masks a very different time evolution in that forcing!
I appreciated the article, including the graphs, but note that it is a bit unrealistic for CO2 since it doesn't correctly address drawdown by the natural sinks.
Note that the natural carbon sinks would continue to draw down CO2 at a rate that is directly proportional to the partial pressure of atmospheric CO2. The current drawdown rate is a bit greater than 20 GT CO2 per year, which coincidentally works out to be a little more than half the annual CO2 emission rate, but from the physics (eg., Henry's Law, for ocean uptake) we know that it's driven by the partial pressure (in the ocean case, more precisely, by the difference in partial pressure of CO2 between the air and dissolved CO2 within the surface water).
In more realistic (but still ambitious) cases, say reductions in anthropogenic CO2 emission rates that reach 1-2% per year, both atmospheric levels and drawdown rates would initially continue to rise but at increasingly slower rates, then peak, and then begin to fall.
The proof of this physics/math is annually demonstrated by the convenient fact that natural drawdown rates are seasonally-varying. During the spring and summer each year, for example, atmospheric CO2 levels at Mauna Loa currently decline by a remarkable amount of up to about 5 ppm (though in lesser amounts during El Niño years), despite the fact that anthropogenic emissions continue during these same months. This tells us that, during these months, the drawdown rate is considerably greater than the rate of anthropogenic emissions! Imagine how much they would fall if, at the start of spring, we really could halt anthropogenic CO2 emissions!
The overall point is that the actual physical system shows that CO2 doesn't simply stay in the atmosphere. Aggressive (but plausible) emission reductions can lead to substantial cumulative reductions in atmospheric levels during this century, such as was explicitly targeted in the Paris Agreement (halting the annual increase in anthropogenic emissions, as soon as possible, and then reaching a balance between sources and sinks during the second half of the 21st Century). We can thus plan to gain control of atmospheric CO2 levels and should start talking about where we want to be in, say, around 50-75 years. 400 ppm perhaps, or perhaps 350? Overall, this is, IMO, very good news and, if widely understood, could help people be optimistic about the future. And it is also important information for policy-makers, to help them make good decisions, so that we can achieve that better future.
More importantly, the warming from CO2 stays well after atmospheric concentrations fall due to the counterbalancing continued warming of the oceans: https://www.pnas.org/doi/10.1073/pnas.0812721106
The climate model used to produce the figures in this article includes both the behavior of the carbon cycle and the relevant timeframes of methane atmospheric chemistry.
Thanks for that response. I've read David Archer's technical publications on the topic and thoroughly enjoyed his book, "The Long Draw", and have also reviewed the AR6 material (as well as previous AR reports) on their treatment of drawdown from the natural sinks.
I'm also aware that cumulative, warming-based, heat uptake by the oceans is about an order of magnitude greater than heat uptake by the atmosphere, but given that the thermal mass of the ocean (mass x specific heat) is about 1000 times that of the atmosphere, the oceans and the atmosphere are not close to being in thermal equilibrium. We really should not expect that substantial amounts of heat will be returned to the atmosphere rapidly after we begin succeeding in reducing atmospheric CO2 levels.
I'm interested in the model that you used. Can you comment on how it addresses dynamic energy and mass transfer within the ocean (or provide a link to a paper on the model). IMO, the chief uncertainty, in projecting continued ocean drawdown over a long period of time, comes from uncertainties in our understanding of the ocean's biological carbon pumps which remove carbon from the upper ocean more rapidly than lateral transport by currents. But this is less critical for projections over the next few decades and should not substantially affect projections for the earliest years in which we reduce emissions.
I'm using the calibrated constrained ensemble in FaIR, which is in turn sampling from a parameter space of carbon cycle responses (and other uncertainties) taken from the CMIP6 ensemble and the AR6 assessed ranges: https://gmd.copernicus.org/articles/17/8569/2024/
Thanks! I'll review the linked articles and some of its references. As I said, I'm interested in how the emulator (or its sources) addresses heat and mass transport in the ocean, especially including the biological carbon pumps which amplify carbon transport rates from surface water into the deep ocean. I believe this is a critical area for projections of the ocean's "carbon sink function" as we continue to flatten emissions and then begin to reduce atmospheric CO2 levels. It should be clear from direct observations (e.g., the Keeling curve) that carbon dioxide doesn't simply stay in the atmosphere for hundreds of years, as is often asserted.
"Note that the natural carbon sinks would continue to draw down CO2 at a rate that is directly proportional to the partial pressure of atmospheric CO2."
----
Is that really true at the time scales we're dealing with?
I can see that, say, masses of exposed calcium rock might keep up, but oceans have temperature increases that reduce their ability to draw down CO2 for a given partial pressure. Likewise, we're managing to *remove* sizeable carbon sinks (either chopping down forests or losing them to massive high-temp crown fires).
Furthermore, warming creates more greenhouse gas emissions from natural sources:
(1) warming wetland
(2) thawing permafrost feeding more microbes
(3) thawing permafrost mechanically weakening caps over fossil gas reservoirs (resulting in those gas blowout craters in the arctic, like the Yamal Peninsula)
Air pollutants including metallic chemical stuff in industrial exhaust has a negative effect on CCN and with the introduction of EM(electro magnetism) in our world, predictions and effects of more or less rain are is being goverened by something new that little study of how EM affects rain and water droplets, size abnd quantity
Clean air works wonders for rain and natural pollutants from the environment spores, etc are much more effective at producing rain. Something to think about. Of course it seems that you have all the answers already
Um, electromagnetism is one of the four fundamental forces at the basic physical level: weak force, strong force, EM force and gravity. It's been here long before us.
Do you mean our devices create more of the products that used to be tied to just lightning and static (like generating ozone at ground level)?
And are you talking about an effect in addition to what the increase of heat in the atmosphere does (i.e., warmer air can hold more water)?
How should the increase in "natural pollutants" from mass combustion events (e.g., kilometer-long smoke plumes from increased crown fires in boreal forests) affect rainfall rates downwind?
Zeke, this is a great article and comparison between CO2 and CH4 and their time horizons. I think it highlights that time horizons matter, not just for greenhouse gases, but for all aspects of environmental sustainability issues. Thank you!
Thank you for this useful comparison of CO2 and CH4. It's appreciated.
One thing I can’t quite figure out: if methane decomposes into CO2, doesn’t it (at least partly) also stay in the atmosphere forever? Does that get taken into account here?
Yes, you are correct. However, methane's addition to CO2 is a pretty small contribution — it's main contribution is the warming you get when it's in the form of methane.
If the methane is biogenic (e.g. from cows) the CO2 is not additional – it would have otherwise been emitted by the vegetation the cows ate decaying.
But yes, it matters for fossil CO2 (but is only ~2% of the instantaneous radiative forcing of the CH4!).
Oh, and pretty much all* methane is biogenic, either from anaerobes alive in the distant past (forming fossil gas reservoirs) or those currently fed by the availability of thawed permafrost (and its old store of previously inaccessible carbon).
_______________
*Hail Pedantia!
We have increasing number of cattle worldwide, a large percent of which are being fed by cultivated crops rather than wild vegetation. I would think that accelerated the "natural carbon cycle" turnover.
The third graph says 1 Mton CO2 and 29.4 kton CH4. The graph caption says 29.4 Gton of CH4. Shouldn't these be the same unit of mass?
Good catch! I've fixed it to say kilotons.
This post covers some key science but it fails to cover the relevance of that science to meeting the Paris Agreement Article 2 commitments made by all nations: (1) efforts to achieve 1.5; (2) via equitable implementation. In this crucial context for climate action, methane mitigation (and CDR) become critically important at global and wealthy nation levels due to already existing or imminent overshoot of a 1.5ºC or 1.5ºC fair share goal.
Of course, defining a national air-share of 1.5ºC effort is not value neutral (see Dooley et al. 2021, doi 10.1038/s41558-021-01015-8), but the Paris Agreement is "bottom-up", so it is up to nations to state their values in claiming remaining 1.5ºC temperature budget. Not saying is a value choice too. Given imminent global 1.5ºC overshoot, for "developed" nations such effort means that early, deep and sustained methane mitigation (and future CDR) is now essential IN ADDITION TO CO2 MITIGATION by cutting fossil fuel and land use emissions.
As you correctly say, SLCP mitigation is not a substitute for CO2 mitigation, but it becomes critically important if stringent temperature targets are to be met. This was a key IPCC AR6 conclusion. Correct GWP* use supports this reality. It is GWP* misuse that is being wrongly leveraged to suggest that "no additional warming" is a reasonable net zero target for wealthy nations – it is not.
Cutting methane emissions from milk and meat production in developed nations is a major lever to limit 1.5ºC fair share overshoot and to enable an early return toward the goal. Even if global 1.5ºC overshoot is inevitable, the Paris Article 2 commitment remains the internationally agreed gauge of how bad that failure might be. Every fraction of a degree counts and cutting annual methane emissions from fossil, livestock, land and rice sources are a major temperature mitigation lever, as GWP* warming analysis shows.
In our journal article, McMullin et al. 2024, we point to the case of Ireland, where, in 2021 and since, the national advisory expert body used GWP* to assess a (questionably fair) national share of remaining warming from 2020 and carried out a "Paris Test" of emissions scenarios to 2050. This GWP*-based test evidenced the critical importance of early, deep and sustained reductions in agricultural (ruminant) emissions in meeting a stringent temperature goal. For more see: https://iopscience.iop.org/article/10.1088/1748-9326/ad3660
Unfortunately, the originators of GWP* have directed their briefings at the livestock industry and their "net zero" science contributions generally fail to point to Article 2 commitments to 1.5ºC and equity (on the basis of Common But Differentiated Responsibilities). Addressing this science-based political agreement reality is not about removing all cattle and sheep (a favourite strawman response), but the analysis shows that reducing ruminant production and focusing more on non-animal derived food farm production instead is far more aligned with a resilient future for society and farming.
By ignoring these commitments, too many journal articles and livestock industry outputs make it appear that simply stabilising methane emissions is enough as a "net zero" goal. But globally and for developing nations, attaining future net zero CO2fe/yr in GWP* terms merely allows reaching some indeterminate level of temperature stabilisation in permanent overshoot of a 1.5ºC fair share goal. Attaining net zero CO2e/yr in GWP100 terms means that residual N2O and CH4 also have to be continually offset in future by CDR in CO2e terms – a questionable assumption in scientific and equity terms.
But BOTH of these net zero annual emission futures – GWP* and GWP100 – are potentially very societally costly ways of excusing near-term action to cut livestock production methane (and N2O) emissions for due 1.5ºC fair share effort. So cows are NOT like closed power staions: we can still choose to cut livestock methane, the closed coal power station is already closed.
An additional point is that GWP100 for the different gases, being directly related to GHG emissions, remains a reasonable gauge of emission mitigation action, whereas GWP* annual values do not as they relate to 20-year change in emissions. GWP* provides different information, on mitigation impact rather than mitigation emissions.
As per UNFCCC SBSTA 2022 advice, GWP* can supply useful supplementary information to GWP100 reporting, but GWP* is not a GWP100 replacement. GWP* is a useful micro climate model approximation of the forcing impact (temperature commitment) from a given date on the basis of cumulative CO2fe, which is a proxy for temperature commitment contribution. A common reference year for differentiated action (and prior historical responsibility) must be asserted – another choice that is not value neutral.
Thanks for explaining what CO2e really signifies. I’d seen figures before and wondered if they had real meaning.
SLCP's are analogous to heat loads then? i.e. thermall gen waste heat and albedo chnages from PV.
It seems that we should use GWPX where X is the time until peak GHGe concentrations in the atmosphere. Thats where the SLCPs will have their biggest impact, in raising the peak. At any other time they are just time shifting the path to that peak...
Great most and makes good sense. Feels like the thrust is to not view them as zero-sum. We can walk and chew gum at the same time (ideally).
Thank you for this article !
Just to be sure to have well understood : on your graph showing the delta global mean temperature linked to the emission of 1Mt of CO2 or 30kt of CH4, the delta T is not the same after 100yrs. It is due to the fact that the GWP is not based on warming but rather on energy put in the system ?
In fact, with this name I thought that GWP was based on the warming. Could it be a solution to the problem to define another GWP on temperature rather than energy put in the system ? I.e to have a parameter based on a given period, for which the warming (=Delta T) is the same after this period ? (for CH4, it would lead to very high overshoot, though, as is shown on your graph).
GWP is based on comparing average radiative forcing (which is similar to but not the same as warming) over a chosen time horizon. So its essentially saying that 1 Mt CO2 has the same average forcing as 30 kt CH4 over 100 years, but that masks a very different time evolution in that forcing!
I appreciated the article, including the graphs, but note that it is a bit unrealistic for CO2 since it doesn't correctly address drawdown by the natural sinks.
Note that the natural carbon sinks would continue to draw down CO2 at a rate that is directly proportional to the partial pressure of atmospheric CO2. The current drawdown rate is a bit greater than 20 GT CO2 per year, which coincidentally works out to be a little more than half the annual CO2 emission rate, but from the physics (eg., Henry's Law, for ocean uptake) we know that it's driven by the partial pressure (in the ocean case, more precisely, by the difference in partial pressure of CO2 between the air and dissolved CO2 within the surface water).
In more realistic (but still ambitious) cases, say reductions in anthropogenic CO2 emission rates that reach 1-2% per year, both atmospheric levels and drawdown rates would initially continue to rise but at increasingly slower rates, then peak, and then begin to fall.
The proof of this physics/math is annually demonstrated by the convenient fact that natural drawdown rates are seasonally-varying. During the spring and summer each year, for example, atmospheric CO2 levels at Mauna Loa currently decline by a remarkable amount of up to about 5 ppm (though in lesser amounts during El Niño years), despite the fact that anthropogenic emissions continue during these same months. This tells us that, during these months, the drawdown rate is considerably greater than the rate of anthropogenic emissions! Imagine how much they would fall if, at the start of spring, we really could halt anthropogenic CO2 emissions!
The overall point is that the actual physical system shows that CO2 doesn't simply stay in the atmosphere. Aggressive (but plausible) emission reductions can lead to substantial cumulative reductions in atmospheric levels during this century, such as was explicitly targeted in the Paris Agreement (halting the annual increase in anthropogenic emissions, as soon as possible, and then reaching a balance between sources and sinks during the second half of the 21st Century). We can thus plan to gain control of atmospheric CO2 levels and should start talking about where we want to be in, say, around 50-75 years. 400 ppm perhaps, or perhaps 350? Overall, this is, IMO, very good news and, if widely understood, could help people be optimistic about the future. And it is also important information for policy-makers, to help them make good decisions, so that we can achieve that better future.
CO2 does stay in the atmosphere for quite a bit of time: ~40% of it for >100 years, and ~20% of it for >10,000 years: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2004JC002625
More importantly, the warming from CO2 stays well after atmospheric concentrations fall due to the counterbalancing continued warming of the oceans: https://www.pnas.org/doi/10.1073/pnas.0812721106
The climate model used to produce the figures in this article includes both the behavior of the carbon cycle and the relevant timeframes of methane atmospheric chemistry.
Thanks for that response. I've read David Archer's technical publications on the topic and thoroughly enjoyed his book, "The Long Draw", and have also reviewed the AR6 material (as well as previous AR reports) on their treatment of drawdown from the natural sinks.
I'm also aware that cumulative, warming-based, heat uptake by the oceans is about an order of magnitude greater than heat uptake by the atmosphere, but given that the thermal mass of the ocean (mass x specific heat) is about 1000 times that of the atmosphere, the oceans and the atmosphere are not close to being in thermal equilibrium. We really should not expect that substantial amounts of heat will be returned to the atmosphere rapidly after we begin succeeding in reducing atmospheric CO2 levels.
I'm interested in the model that you used. Can you comment on how it addresses dynamic energy and mass transfer within the ocean (or provide a link to a paper on the model). IMO, the chief uncertainty, in projecting continued ocean drawdown over a long period of time, comes from uncertainties in our understanding of the ocean's biological carbon pumps which remove carbon from the upper ocean more rapidly than lateral transport by currents. But this is less critical for projections over the next few decades and should not substantially affect projections for the earliest years in which we reduce emissions.
Again, thanks for responding to my comment!
I'm using the calibrated constrained ensemble in FaIR, which is in turn sampling from a parameter space of carbon cycle responses (and other uncertainties) taken from the CMIP6 ensemble and the AR6 assessed ranges: https://gmd.copernicus.org/articles/17/8569/2024/
Thanks! I'll review the linked articles and some of its references. As I said, I'm interested in how the emulator (or its sources) addresses heat and mass transport in the ocean, especially including the biological carbon pumps which amplify carbon transport rates from surface water into the deep ocean. I believe this is a critical area for projections of the ocean's "carbon sink function" as we continue to flatten emissions and then begin to reduce atmospheric CO2 levels. It should be clear from direct observations (e.g., the Keeling curve) that carbon dioxide doesn't simply stay in the atmosphere for hundreds of years, as is often asserted.
"Note that the natural carbon sinks would continue to draw down CO2 at a rate that is directly proportional to the partial pressure of atmospheric CO2."
----
Is that really true at the time scales we're dealing with?
I can see that, say, masses of exposed calcium rock might keep up, but oceans have temperature increases that reduce their ability to draw down CO2 for a given partial pressure. Likewise, we're managing to *remove* sizeable carbon sinks (either chopping down forests or losing them to massive high-temp crown fires).
Furthermore, warming creates more greenhouse gas emissions from natural sources:
(1) warming wetland
(2) thawing permafrost feeding more microbes
(3) thawing permafrost mechanically weakening caps over fossil gas reservoirs (resulting in those gas blowout craters in the arctic, like the Yamal Peninsula)
Air pollutants including metallic chemical stuff in industrial exhaust has a negative effect on CCN and with the introduction of EM(electro magnetism) in our world, predictions and effects of more or less rain are is being goverened by something new that little study of how EM affects rain and water droplets, size abnd quantity
Clean air works wonders for rain and natural pollutants from the environment spores, etc are much more effective at producing rain. Something to think about. Of course it seems that you have all the answers already
Um, electromagnetism is one of the four fundamental forces at the basic physical level: weak force, strong force, EM force and gravity. It's been here long before us.
Do you mean our devices create more of the products that used to be tied to just lightning and static (like generating ozone at ground level)?
And are you talking about an effect in addition to what the increase of heat in the atmosphere does (i.e., warmer air can hold more water)?
How should the increase in "natural pollutants" from mass combustion events (e.g., kilometer-long smoke plumes from increased crown fires in boreal forests) affect rainfall rates downwind?
Ils veulent nous faire consommer plus, alors qu'il faut consommer moins.