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Author Topic: Comparison: forcings from CO2, CH4, N2O  (Read 12616 times)

Ned W

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Comparison: forcings from CO2, CH4, N2O
« on: August 20, 2018, 02:57:23 PM »
From time to time on ASIF (e.g. here) there seems to be some mistaken ideas about the relative importance of CO2 vs methane.

Over the past three-plus decades, the annual change in forcing from CO2 has consistently been much larger than from methane (or N2O, for that matter):



In fact, the gap between them is growing, not shrinking.  The trend in this gap (dF_CO2 minus dF_CH4) is statistically significant.  From 1985 to 2017 it increased by approximately 77%. 

At this point in time, the increase in the methane forcing is simply not very important compared to the increase in the CO2 forcing.

The fine print:  Data from here, here, and here.  All forcings calculated using the updated models from Etminan et al. (2016)

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #1 on: August 20, 2018, 03:26:13 PM »
Looking back in time, this is a continuation of a longer-term trend.  In 1950, the annual increases in forcings from CO2 and methane were basically equal.  But the CO2 forcing increased rapidly, while the methane forcing increased more slowly.

This graph uses the annual concentrations from CMIP5 here up to 2005, and NOAA compilations for 2006-present (see previous post for sources).   All have been converted to annual increases in forcing using the updated models from Etminan et al. (2016).  :



For some reason there's a disproportionate emphasis on methane here on ASIF.  It was very important in the past but over the next few decades it will be basically trivial in comparison to the warming from CO2.

------------------
Edited to add:  I verified my calculations of forcings by comparison with Table S1 from the supplementary information of Etminan et al.  The calculations match nicely for all three gases.
« Last Edit: August 20, 2018, 03:36:44 PM by Ned W »

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #2 on: August 20, 2018, 04:30:25 PM »
One more post. 

Papers like this are cited as reason to be alarmed about methane releases from the Arctic:

Quote
Schneider von Deimling, T., Grosse, G., Strauss, J., Schirrmeister, L., Morgenstern, A., Schaphoff, S., Meinshausen, M., and Boike, J.: Observation-based modelling of permafrost carbon fluxes with accounting for deep carbon deposits and thermokarst activity, Biogeosciences, 12, 3469-3488, doi:10.5194/bg-12-3469-2015, 2015.

http://www.biogeosciences.net/12/3469/2015/bg-12-3469-2015.html

But at the global scale, the impact of that is relatively small.  Per Table 3, the total additional warming from newly thawed permafrost is 0.03 C in 2050, and 0.06 to 0.09 C in 2100 (depending on RCP pathway).  And a majority of that is from CO2, not methane. 

So according to that paper, the global warming attributable to *methane* from newly thawing permafrost in 2050 and 2100 will be around 1% of the total predicted warming across each RCP scenario.

Sure, it's worth studying.  But keep in mind that what you're talking about is approximately 1% of the overall problem.

wili

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #3 on: August 20, 2018, 04:50:12 PM »
Ned, first, please state specifically who said exactly that this article was "a reason to be alarmed about methane releases from the Arctic"

Second, I am having trouble accessing the tables from this article. Could you link them or copy them here? Your claims about what they say seem to not jibe with how Science Daily covered the article:

Quote
Existing models currently attribute about 20 percent of the permafrost carbon feedback this century to methane, with the rest due to carbon dioxide from terrestrial soils. By including thermokarst lakes, methane becomes the dominant driver, responsible for 70 to 80 percent of permafrost carbon-caused warming this century.

Adding thermokarst methane to the models makes the feedback’s effect similar to that of land-use change, which is the second-largest source of human-made warming.

But maybe I'm missing something?

And finally, thank you for your admission that: "Sure, it's worth studying"

Perhaps a bit of an understatement, though?

I'm sure we can agree that slashing CO2 emissions as quickly and as deeply as possible is the thing that is of central importance. But looking at the plethora of feedbacks waiting in the wings or already kicking in certainly seems to be a responsible and relevant thing to do, right? Especially as every GW exacerbating ('positive') feedback also feeds back on all the others in a dizzyingly complex way...
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Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #4 on: August 20, 2018, 05:17:48 PM »
It's a general pattern across several threads, several posters, and various similar papers. 

Here's Table 2:



Note the additional warming values listed in 2050 (0.03 C) and in 2100 (0.06 to 0.09 C).  Using the CMIP5 multimodel means (downloaded from KNMI Climate Explorer), the additional warming represents around 3% of the CMIP5-projected warming in 2050 and 2100 across the four RCP pathways.

Meanwhile, section 3.4 of the paper states that:

Quote
Despite CH4 release contributing only a few percent to total permafrost carbon release, our analyses suggest that it can cause up to about 40 % (upper 68 % range) of permafrost-affected warming.

So the total additional warming from this source (CO2 plus methane) is about 3% of all projected warming, but the methane fraction is 40% or smaller ... so around 1% of the global mean temperature change, as I said.

I do think it's worth studying.  But methane from thawing permafrost is just not all that big a factor for the globe as a whole. 

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #5 on: August 20, 2018, 05:31:52 PM »

I'm sure we can agree that slashing CO2 emissions as quickly and as deeply as possible is the thing that is of central importance. But looking at the plethora of feedbacks waiting in the wings or already kicking in certainly seems to be a responsible and relevant thing to do, right? Especially as every GW exacerbating ('positive') feedback also feeds back on all the others in a dizzyingly complex way...

Full article at https://www.nature.com/articles/s41467-018-05738-9 (open access)

and this is what the paper concludes...
Quote
The moderate climate mitigation strategy (RCP4.5) requires a > 50% reduction in anthropogenic CO2 emissions (i.e., −20 Gt CO2 yr−1) by 2100 compared to the current level61. Our projected permafrost emissions are comparatively small (1.5–4.2 Gt CO2e yr−1 by 2100 for RCP4.5 and 8.5, respectively). However, they are of similar magnitude to the second most important anthropogenic source after fossil fuels [Land Use Change emissions 3.5 ± 1.8 Gt CO2 yr−1], which has been relatively constant during the last 60 years62, implying that our projected permafrost emissions will provide a headwind in the goal to aggressively mitigate CO2 emissions.

Yet another +ve feedback. There is a grave shortage of studies showing -ve feedbacks.
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Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #6 on: August 20, 2018, 05:52:11 PM »
Your claims about what they say seem to not jibe with how Science Daily covered the article

That Science Daily quote was about a different paper, not the one I cited (though some of the same authors were on both papers). 

From that other paper (Katey Walter Anthony et al.) the additional radiative forcing (relative to 1950) from "abrupt permafrost thaw beneath thermokarst lakes" is as follows:

2050:  +0.030 or 0.032 W/m2 (depending on method) for RCP 8.5
2100:  +0.066 or 0.137

Meanwhile, the CMIP5 radiative forcing under RCP 8.5 (relative to 1950) is:

2050: +3.77 W/m2
2100: +7.50

So ... the additional RF from thermokarst lakes, according to the Katey Walter Anthony et al. paper, is about 1.3% of the overall (global) forcing.  The KWA paper also provides the fraction that is due to methane:

2050: 70% (under RCP 8.5)
2100: 64%

So the methane fraction of RF from "abrupt permafrost thaw beneath thermokarst lakes" would be around 0.9% of the global total RF.

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #7 on: August 20, 2018, 06:17:04 PM »
Yet another +ve feedback. There is a grave shortage of studies showing -ve feedbacks.

There are some; for example:

Climate mitigation from vegetation biophysical feedbacks during the past three decades

How important is carbon storage by southern polar benthos as a negative feedback on climate change?

Interactive ozone induces a negative feedback in CO2‐driven climate change simulations

Large‐scale ocean circulation‐cloud interactions reduce the pace of transient climate change

Mechanisms of the Negative Shortwave Cloud Feedback in Middle to High Latitudes

etc. 

That said, my qualitative impression is that negative feedbacks on climate are less important (aside from the obvious and already well-understood Planck feedback).  But they probably also tend to be discussed less here, because we (collectively) do like to focus on the idea that The World Is Going To Hell In A Handbasket.

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #8 on: August 20, 2018, 06:27:59 PM »
I feel like I'm taking crazy pills. How does it make any sense calculating ANNUAL change in forcing, using a number derived from a 100 year time scale? Maybe I'm missing something, but why is the GWP1 never discussed? Would that not be a more appropriate number to calculate annual forcing that GWP100?
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Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #9 on: August 20, 2018, 06:58:16 PM »
I feel like I'm taking crazy pills. How does it make any sense calculating ANNUAL change in forcing, using a number derived from a 100 year time scale? Maybe I'm missing something, but why is the GWP1 never discussed? Would that not be a more appropriate number to calculate annual forcing that GWP100?

RF isn't derived from GWP. 

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #10 on: August 20, 2018, 07:15:49 PM »
I feel like I'm taking crazy pills. How does it make any sense calculating ANNUAL change in forcing, using a number derived from a 100 year time scale? Maybe I'm missing something, but why is the GWP1 never discussed? Would that not be a more appropriate number to calculate annual forcing that GWP100?

RF isn't derived from GWP.

Ya, it's an input for GWP. So whats CH4's GWP1? It's the question I've asked a dozen times in various threads, and no one is interested...why?
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kassy

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #11 on: August 20, 2018, 07:32:51 PM »
See

https://www.epa.gov/ghgemissions/understanding-global-warming-potentials

It varies on the time scale you look at.
And then there is a range of estimates.

You could easily summarize it as methane is bad especially as a feedback loop.

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #12 on: August 20, 2018, 07:47:32 PM »
So whats CH4's GWP1? It's the question I've asked a dozen times in various threads, and no one is interested...why?

Probably because it ignores all the warming that happens after that first year is over.  The entire point of GWP is to account for the integrated warming produced by a molecule of gas X over a long period of time.

You could easily summarize it as methane is bad especially as a feedback loop.

Well, again, see the first two posts in this thread.  On a global scale, methane is currently pretty minor compared to CO2, and the gap is actually widening -- radiative forcing from CO2 is increasing faster than radiative forcing from methane. 
« Last Edit: August 20, 2018, 07:54:29 PM by Ned W »

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #13 on: August 20, 2018, 08:12:37 PM »
So whats CH4's GWP1? It's the question I've asked a dozen times in various threads, and no one is interested...why?

Probably because it ignores all the warming that happens after that first year is over.  The entire point of GWP is to account for the integrated warming produced by a molecule of gas X over a long period of time.

You could easily summarize it as methane is bad especially as a feedback loop.

Well, again, see the first two posts in this thread.  On a global scale, methane is currently pretty minor compared to CO2, and the gap is actually widening -- radiative forcing from CO2 is increasing faster than radiative forcing from methane.

I'm not the dullest tack on the board, but I can't mentally integrate this information to my understanding of the climate system. If you are calculating how much warming happens in a year, shouldn't that inherently ignore all the warming that will happen after that, and solely concentrate on that year?

If the GWP1 of CH4 is 100X CO2, and since pre industrial CH4 has gone up 1200 ppb (or 1.2 ppm) isn't that the equivalent of a 120 ppm increase of CO2? Where is the fault in this logic?
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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #14 on: August 20, 2018, 08:20:48 PM »
Is that so?

@ZLabe
Monthly atmospheric global methane (CH₄; potent greenhouse gas) through 2017

https://www.esrl.noaa.gov/gmd/ccgg/trends_ch4/#global
« Last Edit: August 20, 2018, 08:31:33 PM by A-Team »

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #15 on: August 20, 2018, 08:30:04 PM »
If you are calculating how much warming happens in a year, shouldn't that inherently ignore all the warming that will happen after that, and solely concentrate on that year?

Each CO2 molecule sticks around longer and continues adding to the warming.

As a simple example, consider two gases, X and Y.  Gas X has a lifetime of 1 year, gas Y has a two-year lifetime.  Emissions for each start at 1 unit and increase by 1 unit per year:

Emissions:
Year 1: X = 1, Y = 1
Year 2: X = 2, Y = 2
Year 3: X = 3, Y = 3
Year 4: X = 4, Y = 4
Year 5: X = 5, Y = 5

Atmospheric concentration for gas X is always the same as emissions -- because each molecule disappears after one year.  But for gas Y, concentration is the sum of the two most recent years' emissions, because it sticks around for two years:

Atmospheric concentration:
Year 1: X = 1, Y = 1
Year 2: X = 2, Y = 3
Year 3: X = 3, Y = 5
Year 4: X = 4, Y = 7
Year 5: X = 5, Y = 9

So, to answer your question: Let's say you are interested in the warming that occurs in year 3.  You are correct that you don't need to know about warming that occurs a year later (year 4).  But you do need to know about warming caused by molecules emitted a year earlier (year 2) because in the case of gas Y, molecules emitted during year 2 are still producing warming during year 3.

wili

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #16 on: August 20, 2018, 08:30:16 PM »
"Science Daily quote was about a different paper, not the one I cited"

Thanks for the clarification, Ned. I should have checked!  :-[
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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #17 on: August 20, 2018, 08:51:18 PM »
Yet another +ve feedback. There is a grave shortage of studies showing -ve feedbacks.

There are some; for example:

Climate mitigation from vegetation biophysical feedbacks during the past three decades
How important is carbon storage by southern polar benthos as a negative feedback on climate change?
Interactive ozone induces a negative feedback in CO2‐driven climate change simulations
Large‐scale ocean circulation‐cloud interactions reduce the pace of transient climate change
Mechanisms of the Negative Shortwave Cloud Feedback in Middle to High Latitudes

That said, my qualitative impression is that negative feedbacks on climate are less important (aside from the obvious and already well-understood Planck feedback).  But they probably also tend to be discussed less here, because we (collectively) do like to focus on the idea that The World Is Going To Hell In A Handbasket.
Thanks Ned W,

Followed your links and ended up here..
https://www.gfdl.noaa.gov/bibliography/related_files/bjs0601.pdf
An Assessment of Climate Feedbacks in Coupled Ocean–Atmosphere Models

and have to say that excluding the Planck, it does not look so good.

But maybe it will be the forcing effects from human activities that will do for us. E.g.s mitigation from the greening of earth can't happen if we chop everything down, co2 sequestration from animals living in new habitats in the polar regions won't happen if we have eaten the food chain.

But going off-topic. halt!
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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #18 on: August 20, 2018, 09:52:26 PM »
Is this thread concern trolling, Ned?

Of course CH4 is tiny today compared to CO2 forcing. But CH4 levels are accelerating, we know of many CH4 feedbacks waiting to kick in, and we are already seeing significant reductions in atmospheric OH (Hydroxyl Radicals) necessary to break down CH4.

CH4 will be a very big factor in decades to come. This is now almost fact, Ned.

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #19 on: August 20, 2018, 10:09:48 PM »
Is that so?

@ZLabe
Monthly atmospheric global methane (CH₄; potent greenhouse gas) through 2017

https://www.esrl.noaa.gov/gmd/ccgg/trends_ch4/#global

It seems like this is in response to my post. It doesn't totally make sense, but I can't figure out what other post it would be regarding. What are you getting at? Do you believe that industry began in 1983?
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GoSouthYoungins

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #20 on: August 20, 2018, 10:14:28 PM »
Is this thread concern trolling, Ned?

Of course CH4 is tiny today compared to CO2 forcing. But CH4 levels are accelerating, we know of many CH4 feedbacks waiting to kick in, and we are already seeing significant reductions in atmospheric OH (Hydroxyl Radicals) necessary to break down CH4.

CH4 will be a very big factor in decades to come. This is now almost fact, Ned.

CH4 forcing today tiny compared to CO2? REALLY? 1.9 ppm vs 410 ppm. 100 times more potent (conservatively) means it is almost half as strong a forcing as C02. I guess you could think as that as tiny, but not to me.
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TeaPotty

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #21 on: August 20, 2018, 10:21:08 PM »
CH4 forcing today tiny compared to CO2? REALLY? 1.9 ppm vs 410 ppm. 100 times more potent (conservatively) means it is almost half as strong a forcing as C02. I guess you could think as that as tiny, but not to me.

I guess tiny is the wrong word. It would be be interesting to see a graph comparing raw forcing.

In any case, CH4 will undeniably be a major climate change factor in the future.

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #22 on: August 20, 2018, 10:39:41 PM »
1.9 ppm vs 410 ppm. 100 times more potent (conservatively) means it is almost half as strong a forcing as C02.

That ... Is not how this works.  Sorry.

It would be be interesting to see a graph comparing raw forcing.

See the first two posts of this thread, which compare forcings from CO2, Methane, and N2O. 

CH4 isn't necessarily "tiny", but even the total global CH4 forcing -- which includes non-Arctic sources, e.g. the tropics -- is pretty small compared to CO2.

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #23 on: August 20, 2018, 10:51:10 PM »
1.9 ppm vs 410 ppm. 100 times more potent (conservatively) means it is almost half as strong a forcing as C02.

That ... Is not how this works.  Sorry.

It would be be interesting to see a graph comparing raw forcing.

See the first two posts of this thread, which compare forcings from CO2, Methane, and N2O. 

CH4 isn't necessarily "tiny", but even the total global CH4 forcing -- which includes non-Arctic sources, e.g. the tropics -- is pretty small compared to CO2.

isn't the main risk coming from CH4 the huge "dormant" part that depending on temperature developments at depth and in permafrost regions can kind of blow out in mass within very short future time periods. at least this has been my impression when reading about CH4 over the last 10-15 years.
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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #24 on: August 20, 2018, 11:08:24 PM »
The total radiative forcings, RFs, from the linked ORNL website article by Blasing, T.J. (that updates such RF values reported in April 2016) are used in the linked Wikipedia article to calculate a CO2e value of 526.6ppm:

https://en.wikipedia.org/wiki/Carbon_dioxide_equivalent


Extract: "To calculate the CO2e of the additional radiative forcing calculated from April 2016's updated data: ∑ RF(GHGs) = 3.3793, thus CO2e = 280 e3.3793/5.35 ppmv = 526.6 ppmv."

http://cdiac.ornl.gov/pns/current_ghg.html

Per the following linked NOAA website the change in CO2e in 2017 was 4 ppm, which would give a total CO2e at the end of 2017 of about 530.6 ppm

https://www.esrl.noaa.gov/gmd/aggi/aggi.html

This value of 530.6 ppm for CO2e for 2017 is well above the 425ppm value assumed by RCP 8.5 (used to force climate models for AR5, see the attached image).
« Last Edit: August 21, 2018, 05:37:03 PM by AbruptSLR »
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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #25 on: August 20, 2018, 11:37:52 PM »
1.9 ppm vs 410 ppm. 100 times more potent (conservatively) means it is almost half as strong a forcing as C02.

That ... Is not how this works.  Sorry.

It would be be interesting to see a graph comparing raw forcing.

See the first two posts of this thread, which compare forcings from CO2, Methane, and N2O. 

CH4 isn't necessarily "tiny", but even the total global CH4 forcing -- which includes non-Arctic sources, e.g. the tropics -- is pretty small compared to CO2.

Alright, then explain to me how it works, in plain english. Or at least tell me what is wrong with the way I am describing it.

CH4 atmospheric life = 12.4 years;  GWP100 equal 28. 100/12.4 = 8;  28*8 = 224. Why is that wrong? This is by no means my area of expertise, but if you actually understand it, you should be able to explain it. And if you can explain it, I will be able to understand it.
« Last Edit: August 21, 2018, 02:46:06 AM by GoSouthYoungins »
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GoSouthYoungins

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #26 on: August 21, 2018, 12:01:20 AM »
The total radiative forcings, RFs, from the linked ORNL website article by Blasing, T.J. (that updates such RF values reported in April 2016) are used in the linked Wikipedia article to calculate a CO2e value of 526.6ppm:

https://en.wikipedia.org/wiki/Carbon_dioxide_equivalent


Extract: "To calculate the CO2e of the additional radiative forcing calculated from April 2016's updated data: ∑ RF(GHGs) = 3.3793, thus CO2e = 280 e3.3793/5.35 ppmv = 526.6 ppmv."

http://cdiac.ornl.gov/pns/current_ghg.html

Per the following linked NOAA website the change in CO2e in 2017 was 1.6 ppm, which would give a total CO2e at the end of 2017 of about 528.2 ppm

https://www.esrl.noaa.gov/gmd/aggi/aggi.html

This value of 528.2ppm for CO2e for 2017 is well above the 425ppm value assumed by RCP 8.5 (used to force climate models for AR5, see the attached image).

Do the math yourself. They are clearly using a multiplier of 28 for CH4. This understates the current effect methane is have 8 ****ING FOLD.

CO2e is closer to 700, LLOL. (My best friend from college had an australian father who used to say, "Laugh and the world laughs with you, cry and you cry alone.)
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Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #27 on: August 21, 2018, 04:15:50 AM »
No, no, no.  We went over this exact point last year, and now ASLR is repeating the same incorrect claims. 

The big problem is that he's comparing apples to oranges.  ASLR cites Wikipedia as evidence that something called "CO2e" ("CO2 equivalent") was 528 ppmv (in 2017) and uses this to suggest that the IPCC's CMIP5 RCP 8.5 forcings are too low because their value for "CO2e" was projected to be only 425 ppmv in 2017.

That would be dramatic indeed -- the IPCC's fastest-warming scenario is already 100 ppmv too low after only a few years?!  But this is a fundamentally dishonest comparison.  ASLR is comparing numbers from two very different definitions of "CO2e":

* ASLR's Wikipedia number (528 ppmv) is based only on forcing from greenhouse gases. 

* ASLR's RCP 8.5 number (425 ppmv) is based on all anthropogenic forcings -- greenhouse gases, aerosols, black carbon, land-use albedo, everything.

What ASLR is doing is quite literally equivalent to suggesting that the IPCC has erroneously calculated the area of New York [city] because its number is much smaller than Wikipedia's value for the area of New York [state].   Yeah, they're both called "New York" but the numbers are based on two different definitions of "New York". 

That's the main problem with ASLR's comment.  The other curious thing is his reliance on an anonymous Wikipedia user for a greenhouse-gas-based value of CO2e (528 ppmv).  I could say more about that, but this comment is already too long and we went over all this a year ago already.

The total radiative forcings, RFs, from the linked ORNL website article by Blasing, T.J. (that updates such RF values reported in April 2016) are used in the linked Wikipedia article to calculate a CO2e value of 526.6ppm:

https://en.wikipedia.org/wiki/Carbon_dioxide_equivalent


Extract: "To calculate the CO2e of the additional radiative forcing calculated from April 2016's updated data: ∑ RF(GHGs) = 3.3793, thus CO2e = 280 e3.3793/5.35 ppmv = 526.6 ppmv."

http://cdiac.ornl.gov/pns/current_ghg.html

Per the following linked NOAA website the change in CO2e in 2017 was 1.6 ppm, which would give a total CO2e at the end of 2017 of about 528.2 ppm

https://www.esrl.noaa.gov/gmd/aggi/aggi.html

This value of 528.2ppm for CO2e for 2017 is well above the 425ppm value assumed by RCP 8.5 (used to force climate models for AR5, see the attached image).

GoSouthYoungins

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #28 on: August 21, 2018, 04:48:51 AM »

That would be dramatic indeed -- the IPCC's fastest-warming scenario is already 100 ppmv too low after only a few years?!  But this is a fundamentally dishonest comparison.  ASLR is comparing numbers from two very different definitions of "CO2e":

* ASLR's Wikipedia number (528 ppmv) is based only on forcing from greenhouse gases. 

* ASLR's RCP 8.5 number (425 ppmv) is based on all anthropogenic forcings -- greenhouse gases, aerosols, black carbon, land-use albedo, everything.

What ASLR is doing is quite literally equivalent to suggesting that the IPCC has erroneously calculated the area of New York [city] because its number is much smaller than Wikipedia's value for the area of New York [state].   Yeah, they're both called "New York" but the numbers are based on two different definitions of "New York". 


1) Thanks for not answering my simple question. I guess you don't understand it, at least not well enough to explain it.

2)Much more important to slander ASLR. Notice how he doesn't actually make an hard apples to apples, but instead simply states a correct fact... "This value of 528.2ppm for CO2e for 2017 is well above the 425ppm value assumed by RCP 8.5" True it is a bit misleading, but intellectually dishonest? What's intellectually dishonest is your willingness to say that my math is wrong, but not be willing to show what the correct math is. My guess is that you can't, but you read the papers and the papers conclusion differs from mine, so you just go with that.

big time oops

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #29 on: August 21, 2018, 06:25:52 AM »
Um ... what is the point of this thread and why is the DELTA in annual forcing a relevant metric?

CH4 lasts in the atmosphere for 10-12 years after which it converts to CO2 and H2O. CO2 lasts for 200-ish years. The delta in annual forcing for CH4 will obviously stay close to 0, if the same amount of CH4 is released today, as was released 10-12 yrs ago.

Delta in annual forcing is a somewhat pointless metric when you compare compounds with such massively varying lifespans. You provided as evidence, a 40 year graph comparing the RF of a compound that decays in 12 yrs and 200 yrs.


Methane release from the Arctic is an insanely important metric because there is more buried methane hydrate than the world's oil, gas and coal resources combined. A sudden release of even a fraction of this, will cause the 'ridiculous' change in forcing metric to skyrocket.

Additionally, nuanced topics cannot be reduced to 1 simple graph. For example, the IPCC report clearly states that every 1% increase in methane released, causes a 0.32% decrease in OH which is necessary to convert methane to CO2 and H2O. This is also a feedback loop, more methane released, the longer it stays in the atmosphere.

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #30 on: August 21, 2018, 06:53:29 AM »
Mr Ned W is correct.

land use and aerosols (and some others) subtract in the IPCC number. wiki does not consider those.

sidd

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #31 on: August 21, 2018, 07:39:43 AM »
Ned, why not post a chart of the current total radiative forcing of all GHGs, instead of just the annual delta?
Then we can judge for ourselves how negligible each gas is.
GSY, I think global warming potential, GWP is the sum, over the atmospheric lifetime of a molecule, of its radiative forcing RF.

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #32 on: August 21, 2018, 11:57:03 AM »
I'm not convinced that a real understanding of the mathematical meaning of the term "radiative forcing" is as widespread on ASIF as it ought to be.

For a greenhouse gas, a "radiative forcing" (RF) is simply the change in the Earth's radiation balance when the concentration of that gas changes from some initial value c0 to a different value c1.

Time, per se, is utterly irrelevant to this definition [1].  The times at which c0 and c1 are measured don't matter.  They could be millions of years apart.  Or c0 and c1 could be purely hypothetical, for thought experiments about how much warming would occur under different concentrations of gases.  All that matters are the actual concentrations present in the atmosphere at times c0 and c1.

Mathematically, the forcing from 1765 to present is literally identical to the sum of the forcing from 1765-1766 plus the forcing from 1766-1767 plus the forcing from 1767-1768 plus ... up to the forcing from 2017-2018.  Literally, you can calculate the individual one-year forcings and add them up over some longer period of time and you'll get the same result as if you calculated the forcing for that longer period of time directly using its starting and ending concentrations.

Most of us are used to reading about RF in the context of a change from pre-industrial (say, 1765) to now.  In that case, c0 is the concentration in 1765 and c1 is the concentration today.  But there's nothing special about using 1765's concentration as the baseline. 

And in fact if you do use 1765 as the baseline (c0) value, it loses the ability to differentiate between the effects of changing concentrations within that 250-year time window.  If we want to understand what is happening in the atmosphere in recent years, using 1765's concentration as the baseline will obscure that, because the resulting forcing includes changes that happened a century ago.

So, it's incorrect to think of 1765-present as some kind of "true" radiative forcing and 2016-2017 as some kind of sketchy "delta" that isn't a "true" radiative forcing. Anyone inclined to doubt me on this should check out Table S1 from Etimnan et al. (2016), in which the authors use a roughly 2011-ish set of CO2, CH4, and N2O concentrations as the baseline and then calculate the radiative forcings for all kinds of crazy hypothetical combinations of the three gases.

[1] The standard version of RF that I am using here includes the stratospheric temperature adjustment resulting from the change in concentrations.  If the time elapsed between when concentrations c0 and c1 are measured is extremely short, a literalist interpretation of the meaning of this RF would be incorrect.  But that's really only of academic interest.
« Last Edit: August 21, 2018, 01:02:42 PM by Ned W »

gerontocrat

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #33 on: August 21, 2018, 12:48:58 PM »
I can't say I am much clearer about RF after all that. So I stick with the NOAA emissions data and their calculation from that of their AGGI measure.

The only cautionary note I have is the measure of CH$ emissions given the absence of sampling stations in the tundra and over the shallow waters of Arctic shelves.
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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #34 on: August 21, 2018, 01:01:26 PM »
I can't say I am much clearer about RF after all that.

Sorry.  It's not the easiest thing to explain, and is perhaps not all that intuitive.  I'll keep thinking about how to clarify it.

Quote
So I stick with the NOAA emissions data and their calculation from that of their AGGI measure.

The only cautionary note I have is the measure of CH$ emissions given the absence of sampling stations in the tundra and over the shallow waters of Arctic shelves.

Honestly, I'm not trying to be argumentative, but just like radiative forcing, AGGI is calculated from atmospheric concentration measurements, not from emissions.  Because methane is a well-mixed greenhouse gas (unlike, say, tropospheric ozone), whatever methane is generated from the tundra and Arctic shelves and makes its way into the atmosphere will contribute to the globally-averaged atmospheric concentration measurements and thus into AGGI.

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #35 on: August 21, 2018, 01:09:16 PM »
I'm not convinced that a real understanding of the mathematical meaning of the term "radiative forcing" is as widespread on ASIF as it ought to be.

For a greenhouse gas, a "radiative forcing" (RF) is simply the change in the Earth's radiation balance when the concentration of that gas changes from some initial value c0 to a different value c1.

Time, per se, is utterly irrelevant to this definition [1].  The times at which c0 and c1 are measured don't matter.  They could be millions of years apart.  Or c0 and c1 could be purely hypothetical, for thought experiments about how much warming would occur under different concentrations of gases.  All that matters are the actual concentrations present in the atmosphere at times c0 and c1.

Mathematically, the forcing from 1765 to present is literally identical to the sum of the forcing from 1765-1766 plus the forcing from 1766-1767 plus the forcing from 1767-1768 plus ... up to the forcing from 2017-2018. 

Ned, thanks for addressing a confusing question.  But what you've described here doesn't match the definitions of radiative forcing to be found at:
https://en.wikipedia.org/wiki/Radiative_forcing
No at:
http://news.mit.edu/2010/explained-radforce-0309

By these definitions, you don't need two points in time, nor do you need any change in any concentration of any GHG. 
Total radiative forcing at any point in time is the imbalance between incoming radiation and  outgoing radiation.  That total imbalance can presumably be broken down by attributable effects of individual gases and particulates.  We have a positive total radiative balance right now, which is ultimately the physical cause of warming across the planet. 

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #36 on: August 21, 2018, 01:22:41 PM »
[I see that SteveMDFP has posted another comment while I was writing this ... I'll address that later.]

Discussions about radiative forcing here are often made needlessly complicated when GWP (global warming potential) is kind of mashed into the discussion.  RF and GWP are different quantities and are typically used to answer different kinds of questions.  It's rarely if ever helpful to try to combine them.

RF is a measure of the change in the Earth's radiation balance at time X (with some concentrations of greenhouse gases) relative to some baseline at time Zero (with some other concentrations).  The RF at time X is purely dependent on how many molecules of the gas(es) in question are present in the atmosphere at time X, compared to how many were present at the baseline.

GWP is a measure of how much warming a given quantity of emitted greenhouse gas (e.g., one tonne of methane) will produce, relative to how much warming the same quantity of CO2 would produce, over a specified (long) time period.  It is used primarily by economists, policymakers, etc. to assign costs to anthropogenic emissions of different mixtures of gases, for accounting purposes.

GWP takes into account how long a pulse of a given gas remains in the atmosphere.  RF can also take that into account by calculating the evolution of RF over time, as the emitted pulse of greenhouse gases accumulates and then decays.

For example, consider two hypothetical Earths, both of which have an RF of 3.1 in 2018, relative to a baseline of 1765:

* On Earth 1, that RF comes solely from methane -- concentrations of CO2, N2O, CFCs etc. have not changed at all from 1765 to 2018, only methane has increased, and it's increased exactly enough to produce an RF of 3.1 in 2018.

* One Earth 2, that RF comes solely from CO2 -- concentrations of methane, N2O, etc. are unchanged and only CO2 has increased.  And like on Earth 1, it's created an RF of 3.1 in 2018.

Now, we know that CO2 lasts much longer than methane in the atmosphere.  So while both Earth 1 and Earth 2 have the same RF in 2018, in the following years Earth 1's RF will taper off faster, while Earth 2's RF will take a longer time to decrease.  Eventually (if no more excess emissions are produced) both of them will return to their 1765 concentrations, and have an RF of 0 again.  But that will take a lot longer for Earth 2 than Earth 1.

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #37 on: August 21, 2018, 01:40:36 PM »
Ned, thanks for addressing a confusing question.  But what you've described here doesn't match the definitions of radiative forcing to be found at:
https://en.wikipedia.org/wiki/Radiative_forcing
No at:
http://news.mit.edu/2010/explained-radforce-0309

By these definitions, you don't need two points in time, nor do you need any change in any concentration of any GHG. 
Total radiative forcing at any point in time is the imbalance between incoming radiation and  outgoing radiation.  That total imbalance can presumably be broken down by attributable effects of individual gases and particulates.  We have a positive total radiative balance right now, which is ultimately the physical cause of warming across the planet.

Steve, thanks so much for your comment.  You're right that the language used doesn't seem to match, e.g., the opening paragraph of the Wikipedia definition. 

But if you read further, it does match exactly.  For example, on the Wikipedia page, scroll down to "Sample Calculations" and within that, look at "Forcing due to atmospheric gas".  The equation given for RF from CO2 is:

dF = 5.35 * LN(c/c0)

dF is the radiative forcing from CO2.  Look at the remainder of the sentence in your Wikipedia link:

Quote
where C is the CO2 concentration in parts per million by volume and C0 is the reference concentration.

I called them "c1" and "c0" rather than "c" and "c0" but that's just a trivial difference in notation.  The calculation for RF on your Wikipedia page is based on a change in CO2 (or whatever) relative to some "reference" (or "baseline" as I called it) concentration.

See also the quote from IPCC AR4 higher up on the Wikipedia page, which ties together the two seemingly different (but equivalent) definitions of radiative forcing.

Likewise, your link to the MIT page also shows the same thing.  Scroll down on that page to this part:

Quote
For convenience, most researchers choose a “baseline” year before the beginning of world industrialization — usually either 1750 or 1850 — as the zero point, and compute radiative forcing in relation to that base. The IPCC uses 1750 as its base year and it is the changes in the various radiative forcing agents since then that are counted.

So, again, the language used to introduce the concept of RF on that page "feels" different, but when you get down to the nuts and bolts, it turns out to be the same as the definition used elsewhere:  RF is the effect on the Earth's radiation balance of a change in concentration of a greenhouse gas relative to some "baseline" or "reference" concentration.

Hope that helps.

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #38 on: August 21, 2018, 02:25:34 PM »
Ned, you’re doing a whole lot of talking, and not making any sense.

Is this how you try to win the argument you’ve started, while slandering posters like AbruptSLR?

Stop rambling and try to give a logical explanation for your method.

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #39 on: August 21, 2018, 02:55:18 PM »
I've been trying to respond as clearly as possible to specific questions that are raised.  See for example SteveMDFP's post directly above this and my reply. 

If there are specific things that make no sense, point them out and I or someone else can try to address them.

As for ASLR's post and my response, recall that he wrote this:

Quote
This value of 528.2ppm for CO2e for 2017 is well above the 425ppm value assumed by RCP 8.5 (used to force climate models for AR5, see the attached image).

That comparison (528 vs 425 ppmv) is deeply misleading.  It implies that the IPCC's fastest-warming scenario is already underestimating climate forcings by over 100 ppm CO2-equivalent after only a very few years.

Either ASLR understands why those two numbers should not be compared, or he doesn't understand it.  Neither option is very appealing. 

Plus, he and I went over this one year ago (August 2017) and it's not exactly an obscure problem.  See my analogy above:

What ASLR is doing is quite literally equivalent to suggesting that the IPCC has erroneously calculated the area of New York [city] because its number is much smaller than Wikipedia's value for the area of New York [state].   Yeah, they're both called "New York" but the numbers are based on two different definitions of "New York". 

This isn't a minor thing.  People reading ASLR's post who are not familiar with the meaning of "CO2e" will come away from it believing -- incorrectly! -- that the IPCC's scenarios are so wildly wrong that even the fastest-warming one is way, way too low. 

I am used to seeing that kind of thing (in the opposite direction) at WattsUpWithThat.  Let's not put up with it here, please. 

GoSouthYoungins

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #40 on: August 21, 2018, 03:25:43 PM »

I am used to seeing that kind of thing (in the opposite direction) at WattsUpWithThat.  Let's not put up with it here, please.

Babble babble babble babble, babble babble babble babble. Ya, he was comparing different things. But I think you are concentrating on that to hide that you don't understand what you post so prolifically about in the thread you started.  If GWP100 and GWP20 are able to be calculated, why not GWP1?  Why not explain with some math so we all understand?!?
big time oops

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #41 on: August 21, 2018, 03:36:48 PM »
If GWP100 and GWP20 are able to be calculated, why not GWP1?  Why not explain with some math so we all understand?!?

You asked that question in this post:

Quote from: GoSouthYoungins
So whats CH4's GWP1? It's the question I've asked a dozen times in various threads, and no one is interested...why?

I replied in this post:

Quote from: Ned W
Probably because it ignores all the warming that happens after that first year is over.  The entire point of GWP is to account for the integrated warming produced by a molecule of gas X over a long period of time.

You then replied in this post:

Quote from: GoSouthYoungins
If you are calculating how much warming happens in a year, shouldn't that inherently ignore all the warming that will happen after that, and solely concentrate on that year?

And I replied with this explanation, complete with numbers and tables:

Quote from: Ned W
As a simple example [... simple example omitted ...]

So, to answer your question: Let's say you are interested in the warming that occurs in year 3.  You are correct that you don't need to know about warming that occurs a year later (year 4).  But you do need to know about warming caused by molecules emitted a year earlier (year 2) because in the case of gas Y, molecules emitted during year 2 are still producing warming during year 3.

As far as I can see, you never acknowledged or responded in any way to that.  Now you're insistently demanding that I answer some other questions.  Meanwhile I'm getting flack from people for posting too much already. 

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #42 on: August 21, 2018, 04:22:09 PM »
To (hopefully) halt the increasing acrimony and get back to where this thread started, here's a graph similar to the one in the second post, but for rolling 30-year windows:


So each point along one of those lines is the radiative forcing at a given year (the X axis) relative to a baseline 30 years earlier.  In other words, it's the magnitude of the radiative forcing from each gas over the previous 30 years.

During the 30-year periods ending in the 1950s and 1960s (i.e., encompassing data from the 1920s/30s to the 1950s/60s) the forcing from methane was only slightly lower than that from CO2.  But that changed in later years, and the most recent 30-year period (far right end of chart) has the biggest gap between CO2 forcing and CH4 forcing.

That said, methane concentrations did resume rising a decade ago, so the orange line will start to turn back up at some point. 

-----------------

The data are from CMIP5 (prior to 2005) and from NOAA (post 2005).  The CMIP5 estimates for CH4 concentration during the period of overlap (1984-2005) are offset relative to the NOAA series, so I shifted them by ~20 ppb for consistency.

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #43 on: August 21, 2018, 06:03:46 PM »
In the linked thread entitle: "Radiative forcing and CO2eq", I explain in Reply #38 that what people really care about is effective radiative forcing, ERF, rather than radiative forcing, RF; while in Replies #45 & #46  I explain that AR5 had a number of shortcomings about near-term climate forcers, NTCFs (including methane and aerosols), that AerChemMIP is working to address:

https://forum.arctic-sea-ice.net/index.php/topic,2158.0.html#lastPost

From Reply #38:

"What people really care about is how big will the climate impacts be and how fast will they occur.  Thus people are really interested in effective radiative forcing, ERF, rather than RF (see the following linked lecture visuals & attached image) due to the combination of forcings and feedbacks.

Title: "Forcing and feedbacks", from Climate Dynamics (Summer Semester 2017) by J. Mulmenstadt

https://home.uni-leipzig.de/~jmuelmen/lehre/cd/cd-2017/lec11.pdf

The link makes it clear that considering the fast response from CO₂ is relatively straight forward, but the impacts from slow response feedbacks, particularly from the ocean (note that OHC is higher than AR5 assumed, as the oceans have been absorbing extra heat since 1750) and ENSO can rapidly increase effective values of ECS up to the range of 4 to 5C.  If so then the Zhang et al (2016) values (which include feedbacks) are not 'outliers' but rather are an indication that the effective ECS this century is higher than other researchers are assuming.  Furthermore, the attached image illustrates how ERF can be determined but due to uncertainties accurate estimates of ERF will take decades, and by that time we may have crossed several tipping points if AR5 is erring on the side of least drama."

From Reply #45:

Obviously, the issues raised in this thread do not represent 'settled science' as indicated by the linked reference about CMIP6's efforts to better quantify these effects:

Collins et al (2017), "AerChemMIP: quantifying the effects of chemistry and aerosols in CMIP6", Geosci. Model Dev., 10, 585–607, doi:10.5194/gmd-10-585-2017

https://www.geosci-model-dev.net/10/585/2017/gmd-10-585-2017.pdf

Abstract: "Abstract. The Aerosol Chemistry Model Intercomparison Project (AerChemMIP) is endorsed by the Coupled-Model Intercomparison Project 6 (CMIP6) and is designed to quantify the climate and air quality impacts of aerosols and chemically reactive gases. These are specifically near-term climate forcers (NTCFs: methane, tropospheric ozone and aerosols, and their precursors), nitrous oxide and ozone depleting halocarbons. The aim of AerChemMIP is to answer four scientific questions.

1. How have anthropogenic emissions contributed to global radiative forcing and affected regional climate over the historical period?

2. How might future policies (on climate, air quality and land use) affect the abundances of NTCFs and their climate impacts?

3. How do uncertainties in historical NTCF emissions affect radiative forcing estimates?

4. How important are climate feedbacks to natural NTCF emissions, atmospheric composition, and radiative effects?

These questions will be addressed through targeted simulations with CMIP6 climate models that include an interactive representation of tropospheric aerosols and atmospheric chemistry. These simulations build on the CMIP6 Diagnostic, Evaluation and Characterization of Klima (DECK) experiments, the CMIP6 historical simulations, and future projections performed elsewhere in CMIP6, allowing the contributions from aerosols and/or chemistry to be quantified.  Specific diagnostics are requested as part of the CMIP6 data request to highlight the chemical composition of the atmosphere, to evaluate the performance of the models, and to understand differences in behaviour between them."

From Reply #46:

To reiterate the point of my last post, the linked reference concludes that there is a lot of uncertainty associated with model projections of the impacts of aerosols and chemically reactive gases:

Heyn, I., K. Block, J. Mülmenstädt, E. Gryspeerdt, P. Kühne, M. Salzmann, and J. Quaas (2017), Assessment of simulated aerosol effective radiative forcings in the terrestrial spectrum, Geophys. Res. Lett., 44, doi:10.1002/2016GL071975.

https://spiral.imperial.ac.uk/bitstream/10044/1/43885/7/Heyn_et_al-2017-Geophysical_Research_Letters.pdf

Extract: "The analysis in this paper relies on global climate models. Beyond climate model results, very little is known quantitatively about the global forcing due to the reaction of ice-phase, mixed-phase, and deep convective clouds to aerosol perturbations. A truly realistic estimate, or even just reliable uncertainty interval, thus requires substantial further research, especially for effects in the terrestrial spectrum. Observational estimates are urgently needed. The current state of the art, however, suggests that the effective forcing in the terrestrial spectrum is either small, or, for models where it is large, is accompanied by a large negative effective forcing in the solar spectrum."
« Last Edit: August 21, 2018, 06:11:43 PM by AbruptSLR »
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GoSouthYoungins

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #44 on: August 21, 2018, 07:13:30 PM »

As far as I can see, you never acknowledged or responded in any way to that.  Now you're insistently demanding that I answer some other questions.  Meanwhile I'm getting flack from people for posting too much already.

Sorry if I was too harsh before. My question has always been the same: What is the CH4 multiplier (or CO2e) for a time less than or equal to its atmospheric lifespan?  GWP1 or GWP10 would work fine. Any time horizon longer than its lifespan will downplay its short term effect.

GPW100  =  30
GWP20    =  85
GWP10    =  100-300???

big time oops

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #45 on: August 21, 2018, 07:29:03 PM »

Likewise, your link to the MIT page also shows the same thing.  Scroll down on that page to this part:

Quote
For convenience, most researchers choose a “baseline” year before the beginning of world industrialization — usually either 1750 or 1850 — as the zero point, and compute radiative forcing in relation to that base. The IPCC uses 1750 as its base year and it is the changes in the various radiative forcing agents since then that are counted.

So, again, the language used to introduce the concept of RF on that page "feels" different, but when you get down to the nuts and bolts, it turns out to be the same as the definition used elsewhere:  RF is the effect on the Earth's radiation balance of a change in concentration of a greenhouse gas relative to some "baseline" or "reference" concentration.

Hope that helps.

Thanks Ned.  I see that you're correct.  If one assumes that the climate was in thermodynamic equilibrium in 1750 or 1850 (quite reasonable as a first approximation, I think), then the radiative forcing at those moments would be zero.  In this approximation, the two wordings of the definition would be synonymous.  The positive radiative forcing today is (approximately) the true heat imbalance resulting in continued warming of the globe.

Now, that pesky methane question.  Appropriate answers depend on precise questions and precise definitions.  I think ASLR is correct that, for example, real people care about projected effects under reasonable scenarios, and different answers are true from some of the precise, technical answers.

If we project zero rise or fall in methane concentrations over the next century (a very conservative assumption, I think) then for any meaningful answer to what methane will do to the climate, the lifetime of a methane molecule becomes irrelevant.  Each oxidized molecule is replaced.

The GWP50 for methane then becomes severely underestimated for assessing what that specific methane concentration will do to the climate.

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #46 on: August 21, 2018, 07:29:15 PM »
The attached image presented by the chairman of Chapter 8 of AR5, shows the time evolution of the ERF by agent, and which shows the relative importance of CO₂ compared to other well mixed GHGs:

Title: "Radiative Forcing in the AR5" presented by Drew Shindell

http://climate.envsci.rutgers.edu/climdyn2013/IPCC/IPCC_WGI12-RadiativeForcing.pdf

« Last Edit: August 21, 2018, 09:01:17 PM by AbruptSLR »
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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #47 on: August 21, 2018, 07:41:58 PM »
This article may be relevant to the current discussion (though it is somewhat old now--relevant updates on the subject would be most welcome, especially with links!):

Quote
It is shown that if global methane emissions were to increase by factors of 2.5 and 5.2 above current emissions, the indirect contributions to RF would be about 250% and 400%, respectively, of the RF that can be attributed to directly emitted methane alone.

Assuming several hypothetical scenarios of CH4 release associated with permafrost thaw, shallow marine hydrate degassing, and submarine landslides, we find a strong positive feedback on RF through atmospheric chemistry. In particular, the impact of CH4 is enhanced through increase of its lifetime, and of atmospheric abundances of ozone, stratospheric water vapor, and CO2
as a result of atmospheric chemical processes

https://darchive.mblwhoilibrary.org/bitstream/handle/1912/4553/2010GB003845.pdf?sequence=1

See also (with similar caveats):

Shindell, D.T., G. Faluvegi, D.M. Koch, G.A. Schmidt, N. Unger, and S.E. Bauer, 2009: Improved attribution of climate forcing to emissions. Science, 326, 716-718, doi:10.1126/science.1174760.

(from the note under figure 2):

“Our calculations for the shorter 20-year GWP, including aerosol responses, yield values of 79 and 105 for methane”

(the latter number is for full accounting of both direct and indirect aerosol effect. Bar chart shows 33 for 100 year value.)

http://www.see.ed.ac.uk/~shs/Climate%20change/Data%20sources/Shindell%20methane.pdf
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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #48 on: August 21, 2018, 08:11:02 PM »
Quote from: AbruptSLR
what people really care about is effective radiative forcing, ERF, rather than radiative forcing, RF

For the subjects of this thread (CO2, CH4, and N2O) the difference between ERF and stratospherically adjusted RF is extremely small (e.g., for CO2, ERF is only 2% smaller than RF).  See page 667 of AR5, starting with "In many cases, however, ERF and RF are nearly equal...." 

Thus, since the difference is fairly trivial, and since it's very easy to calculate stratospherically adjusted RF, and very hard to calculate ERF, in this thread and elsewhere I encourage people to basically ignore ERFs for the well-mixed greenhouse gases and just use RFs. 

Anyone can calculate the RF for CO2, CH4, or N2O and do various thought experiments, e.g. see for themselves what effect adding 100 ppb of methane would have done in 1750 vs today, or whatever. 

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #49 on: August 21, 2018, 09:43:51 PM »
AR5 Chapter 8 does a reasonably good job of calculating current ERF (see linked pdf) it is not hard for most people to see per Figure 8.15, the solid bars are for ERF and the solid uncertainty lines are also for ERF.  Thus if one take the high end of uncertainty for Other WMGHGs its ERF in 2011 may be close to equaling the low uncertainty end of the ERF for carbon dioxide.

Also, per Figure 8.32 that on a ten year horizon that for the current anthropogenic GHG emissions that the global warming potential of methane exceeds that of that for carbon dioxide.  So if the combine natural and anthropogenic methane emissions keep pace with the combined natural and anthropogenic carbon dioxide emissions then we could all be in trouble (see Isaksen et al. 2011).  Also, I note that what is difficult to calculate is the true value of ECS by say 2100 (the ACME model projects this value to be about 5.2C), as ECS increases with continued global warming:

Title: "Chapter 8:  Anthropogenic and Natural Radiative Forcing"

https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter08_FINAL.pdf

&

Isaksen, I. S. A., Gauss M., Myhre, G., Walter Anthony, K. M.  and Ruppel, C.,  (2011), "Strong atmospheric chemistry feedback to climate warming from Arctic methane emissions", Global Biogeochem. Cycles, 25, GB2002, doi:10.1029/2010GB003845.

http://onlinelibrary.wiley.com/doi/10.1029/2010GB003845/abstract

“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson