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Author Topic: Comparison: forcings from CO2, CH4, N2O  (Read 4962 times)

oren

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #200 on: October 08, 2018, 12:03:04 AM »
Quote
In recent decades, the methane forcing has been extremely small. 
Indeed. But not because nothing happened with methane. Rather methane emissions were almost cancelled out with atmospheric methane decay. But had there been no methane emissions, we would have had a significant negative methane forcing. Ignoring this dynamic while posting comments that methane is not important due to its stable concentration (and relegating emissions to policy), is I think the reason why this thread is eliciting emotional responses.
Humans are increasing CO2 concentrations (visibly affecting RF) and maintaining methane concentrations from dropping (invisibly affecting RF). I think both of these are important.
As the focus of this thread is narrowly defined as the RF with a 1 year moving baseline, would you be willing to calculate and present RF resulting from a reduction of 10% in methane concentration over 1 year while all other GHG concentrations remain stable at current levels? (This would occur naturally should humans decide to shift policy and stop emitting GHGs). This RF information could help readers who desire to discuss the relative merits of various policies in the yet-unopened policy thread.

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #201 on: October 08, 2018, 01:53:04 AM »
Thanks for the comment, Oren.  You've brought up this idea before and I'm not sure what to think about it, mainly because I'm not sure what the relative costs and obstacles are for different approaches to reducing the total forcing from greenhouse gases.  But you made a very straightforward request, so let's tackle it. 

We can start with the 2017 NOAA AGGI data on atmospheric concentrations for CO2, CH4, and N2O.  Assume that CH4 is cut by 10% over some period of time (it actually doesn't matter how long) while CO2 and N2O stay the same. 

Using the formulations from Etminan (2016), that 10% drop in CH4 would provide a (negative) radiative forcing of -0.08 W/m2.  How much cooling that would produce depends on what value you choose for climate sensitivity.  Likewise, it would take some time to have its full effect (just as we now speak of "warming in the pipeline", in a hypothetical case of negative forcings we would need to speak of "cooling in the pipeline"....)

Is that a large amount or a small amount?  Depends on how you choose to look at it, I suppose.  For comparison, the normal annual increase in CO2 in recent years has created an annual forcing around +0.03 W/m2.  So dropping the concentration of methane in the atmosphere by 10% would be equivalent to just under three years' worth of the annual increment in CO2 concentration.

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #202 on: October 08, 2018, 02:09:28 AM »
As a check on my calculations ... a 10% cut in atmospheric methane concentration would put us back at 1984-era values.  Per NOAA AGGI, the methane forcing in 2017 was 0.509, while in 1984 it was 0.432, giving a negative forcing of -0.077 W/m2, basically the same as my -0.08.

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #203 on: October 10, 2018, 02:09:32 PM »
Here's some more context: a comparison of the WMGHGs to all the other major categories of anthropogenic forcing:


The darkest bars are the forcing over the past 30 years (1988-2018).  Lighter bars are the two previous 30-year periods (1958 to 1988, and 1928 to 1958) for comparison.

What's kind of stunning is that during the past 30 years, every other bar [positive or negative] is less than 10% of the magnitude of the CO2 forcing.

Other comments

* Methane, N2O, halocarbons, fluorocarbons, and tropospheric ozone were the largest non-CO2 forcings [in that order], but all were pretty small.

* Over this period, there has been no large negative anthropogenic forcing.  Aerosols barely produced any forcing at all over the past 30 years, and if you combine the direct and indirect (cloud albedo) effects, they nearly cancel out entirely.

* Non-anthro forcings (solar and volcanic) probably each produced a small negative forcing over the past 30 years (not shown).

* Oren asked about the impact of a 10% decrease in atmospheric methane.  As shown in the previous post, that would create a negative forcing of -0.08 W/m2, basically dropping the methane bar in this graph to near 0.

* a 10% decrease in aerosols would produce a positive forcing of around +0.11 W/m2 (direct+indirect).

Key:

CO2_RF: CO2 Forcing
CH4_RF: Methane Forcing
N2O_RF: Nitrous Oxide Forcing
FGASSUM_RF: Total forcing from all fluorinated gases controlled under the Kyoto Protocol (HFCs, PFCs, SF6)
MHALOSUM_RF: Total forcing from all gases controlled under the Montreal Protocol
TOTAER_DIR_RF: Total direct aerosol forcing
CLOUD_TOT_RF: Cloud albedo effect
STRATOZ_RF: Stratospheric ozone forcing
TROPOZ_RF: Tropospheric ozone forcing
CH4OXSTRATH2O_RF: Stratospheric water-vapour from methane oxidization
LANDUSE_RF: Land-use albedo
BCSNOW_RF: Black carbon on snow

Source:

http://www.pik-potsdam.de/~mmalte/rcps/
http://www.pik-potsdam.de/~mmalte/rcps/data/RCP85_MIDYEAR_RADFORCING.xls

TerryM

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #204 on: October 10, 2018, 05:54:24 PM »
Ned
Per your chart the Montreal protocol seems to have been a huge success.
Is there anything on the table that could yield rapid results of like magnitude?
Thanks
Terry

Ned W

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #205 on: October 10, 2018, 06:43:31 PM »
It sure reversed what had been a terrifying trend.  You normally see that shown in terms of ozone depletion, but it's interesting to see it also represented in the direct climate forcing from halocarbon gases.

AbruptSLR

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #206 on: October 10, 2018, 07:19:28 PM »
With some countries cheating on the Montreal Protocol agreement, who knows how future hydrofluorocarbon and chlorofluorocarbons will have on atmospheric hydroxyl ion and ozone concentrations (with associated impacts of radiative forcing):

Montzka, S.A., G.S. Dutton, P. Yu, E. Ray, R. Portmann, J.S. Daniel, L. Kuijpers, B.D. Hall, D. Mondeel, C. Siso, D. Nance, M. Rigby, A. Manning, L. Hu, F. Moore, B.R. Miller, and J.W. Elkins, An unexpected and persistent increase in global emissions of ozone-depleting CFC-11, Nature, doi:10.1038/s41586-018-0106-2, 2018.

http://www.nature.com/articles/s41586-018-0106-2

Abstract:
The Montreal Protocol was designed to protect the stratospheric ozone layer by enabling reductions in the abundance of ozone-depleting substances such as chlorofluorocarbons (CFCs) in the atmosphere1,2,3. The reduction in the atmospheric concentration of trichlorofluoromethane (CFC-11) has made the second-largest contribution to the decline in the total atmospheric concentration of ozone-depleting chlorine since the 1990s1. However, CFC-11 still contributes one-quarter of all chlorine reaching the stratosphere, and a timely recovery of the stratospheric ozone layer depends on a sustained decline in CFC-11 concentrations1. Here we show that the rate of decline of atmospheric CFC-11 concentrations observed at remote measurement sites was constant from 2002 to 2012, and then slowed by about 50 per cent after 2012. The observed slowdown in the decline of CFC-11 concentration was concurrent with a 50 per cent increase in the mean concentration difference observed between the Northern and Southern Hemispheres, and also with the emergence of strong correlations at the Mauna Loa Observatory between concentrations of CFC-11 and other chemicals associated with anthropogenic emissions. A simple model analysis of our findings suggests an increase in CFC-11 emissions of 13 ± 5 gigagrams per year (25 ± 13 per cent) since 2012, despite reported production being close to zero4 since 2006. Our three-dimensional model simulations confirm the increase in CFC-11 emissions, but indicate that this increase may have been as much as 50 per cent smaller as a result of changes in stratospheric processes or dynamics. The increase in emission of CFC-11 appears unrelated to past production; this suggests unreported new production, which is inconsistent with the Montreal Protocol agreement to phase out global CFC production by 2010.

Also see:

Title: "Observational evidence for interhemispheric hydroxyl parity"

https://m.phys.org/news/2014-09-evidence-interhemispheric-hydroxyl-parity.html

Extract: "Although state-of-the-art chemistry-transport models predict that the OH concentration in the NH is ~28% (13–42%) higher than in the SH, the present detailed study derives a NH/SH OH ratio of 0.97±0.12 (Figure 3). Uncertainties about the relative abundance of OH in the two hemispheres have persisted since the early 1990s due to uncertainties about how much CH3CCl3 was actually released into the atmosphere and also due to imperfections in interhemispheric transport in models used to estimate OH concentrations."

&

P. K. Patra, M. C. Krol, S. A. Montzka, T. Arnold, E. L. Atlas, + et al. (2014), "Observational evidence for interhemispheric hydroxyl-radical parity", Nature 513, 219-223 DOI: 10.1038/nature13721

http://www.nature.com/articles/nature13721
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TerryM

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #207 on: October 10, 2018, 08:17:55 PM »
Not sure if it's still in place, but the US at one time insisted on exempting it's military from compliance with the Montreal Protocol.


Terry

oren

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Re: Comparison: forcings from CO2, CH4, N2O
« Reply #208 on: October 15, 2018, 03:51:54 AM »
We can start with the 2017 NOAA AGGI data on atmospheric concentrations for CO2, CH4, and N2O.  Assume that CH4 is cut by 10% over some period of time (it actually doesn't matter how long) while CO2 and N2O stay the same. 

Using the formulations from Etminan (2016), that 10% drop in CH4 would provide a (negative) radiative forcing of -0.08 W/m2.

Is that a large amount or a small amount?  Depends on how you choose to look at it, I suppose.  For comparison, the normal annual increase in CO2 in recent years has created an annual forcing around +0.03 W/m2.  So dropping the concentration of methane in the atmosphere by 10% would be equivalent to just under three years' worth of the annual increment in CO2 concentration.
Thank you Ned for this calculation, and apologies for my delay in responding.
My layman's choice of how to look at this result:
10% would be the annual reduction of methane concentration should methane emissions stop. Therefore, annual methane emissions are equal to a 0.08 W/m2 forcing: 0 W/m2 compared to "natural baseline" of -0.08 W/m2. (I am ignoring non-anthropogenic and feedback methane emissions, so this is an overstatement).
What would be the annual reduction in CO2 concentration should CO2 emissions stop? I read somewhere that 50% of emitted anthropogenic CO2 is absorbed each year. If true, CO2 concentration would drop annually by the magnitude it currently rises each year, and result in an annual forcing of negative 0.03 W/m2, instead of the current positive forcing of similar magnitude. (Or CO2 would drop by much less than it currently rises, leading to a negative forcing <<0.03 W/m2. I am not sure which is correct)
Therefore, annual CO2 emissions are equal to a 0.06 W/m2 forcing: +0.03 W/m2 compared to "natural baseline" of -0.03 W/m2.
So the bottom line of this very crude "analysis" is that methane emissions cause roughly the same annual forcing as CO2 emissions, and are certainly not negligible, although at face value methane concentration is unchanging and therefore contributes almost nothing to changes in forcing. Note this result is independent of costs and obstacles and all the policy issues of reducing anthropogenic emissions.
(Of course, after a few years the methane effect would diminish as its concentration dropped).
I could be quite wrong in my rough numbers (I am sure you can improve this quite a bit), but I think this is the proper way of looking at the RF data and using it to assess emissions data. From  this point onward it's policy discussions of how to tackle emissions and what is more cost-effective to do, but until this point IMHO it's pure science and belongs in this thread. When discussing concentration changes and the resultant RF as a basis for informing policy we cannot ignore the effects of emissions and natural sinks. We must consider a baseline of no anthropogenic emissions, calculating a natural baseline RF to which all should be compared.