Comparison: forcings from CO2, CH4, N2O

[Ned W]

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]

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]

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]

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]

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

[TerryM]

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]

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.

[Ned W]

Thanks for sharing your thoughts, Oren.  I think I understand where you're coming from and what you're trying to show.  But I think that if you look closely at it, you'll see that your approach doesn't actually work.  Bear with me here:

You are trying to calculate an annual radiative forcing that would represent the impact of a single year's anthropogenic emissions.  In essence, you're trying to back-calculate something like an emissions-based forcing, or GWP, or GTP.  But you're only considering the impact of this year's emissions on climate for one year.  In other words, it's like GSY's insistence on calculating a 1-year GWP for methane.  Yes, it can be done, but it's not a physically meaningful representation of the impact of methane vs CO2 emissions on the climate. 

What you're actually showing is that a given mass of methane has a much stronger radiative forcing than CO2 does over the very short term (1 year).  But we already all know that.  If you instantaneously stopped all anthropogenic emissions of both methane and CO2, the cooling from the loss of methane would be very high in year 1, but would rapidly taper off.  In contrast, the cooling from the loss of CO2 would keep going for centuries, and it would soon eclipse the effect of methane.

[Ned W]

FWIW, in working on the previous post I repeatedly wrote, deleted, and re-wrote some additional paragraphs arguing that what you're deriving is not in fact a radiative forcing (W/m2) at all, but should really be considered a "radiant exposure" (J/m2). 

But the explanation was probably too subtle to be worth dealing with here.  The more important issue is just that your comparison of the effects of CO2 and CH4 is ignoring the great difference in their residence time, and thus leading to an unrealistic result.

[AbruptSLR]

Stratospheric ozone is under assault from anthropogenic emissions (primarily from East Asia) of several chlorine-containing very short-lived substances (Cl-VSLSs).  This could extend the life of methane in the atmosphere:

Oram, D. E., Ashfold, M. J., Laube, J. C., Gooch, L. J., Humphrey, S., Sturges, W. T., Leedham-Elvidge, E., Forster, G. L., Harris, N. R. P., Mead, M. I., Samah, A. A., Phang, S. M., Ou-Yang, C.-F., Lin, N.-H., Wang, J.-L., Baker, A. K., Brenninkmeijer, C. A. M., and Sherry, D.: A growing threat to the ozone layer from short-lived anthropogenic chlorocarbons, Atmos. Chem. Phys., 17, 11929-11941, https://doi.org/10.5194/acp-17-11929-2017, 2017.

https://www.atmos-chem-phys.net/17/11929/2017/?utm_content=bufferb397a&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

Abstract. Large and effective reductions in emissions of long-lived ozone-depleting substance (ODS) are being achieved through the Montreal Protocol, the effectiveness of which can be seen in the declining atmospheric abundances of many ODSs. An important remaining uncertainty concerns the role of very short-lived substances (VSLSs) which, owing to their relatively short atmospheric lifetimes (less than 6 months), are not regulated under the Montreal Protocol. Recent studies have found an unexplained increase in the global tropospheric abundance of one VSLS, dichloromethane (CH2Cl2), which has increased by around 60 % over the past decade. Here we report dramatic enhancements of several chlorine-containing VSLSs (Cl-VSLSs), including CH2Cl2 and CH2ClCH2Cl (1,2-dichloroethane), observed in surface and upper-tropospheric air in East and South East Asia. Surface observations were, on occasion, an order of magnitude higher than previously reported in the marine boundary layer, whilst upper-tropospheric data were up to 3 times higher than expected. In addition, we provide further evidence of an atmospheric transport mechanism whereby substantial amounts of industrial pollution from East Asia, including these chlorinated VSLSs, can rapidly, and regularly, be transported to tropical regions of the western Pacific and subsequently uplifted to the tropical upper troposphere. This latter region is a major provider of air entering the stratosphere, and so this mechanism, in conjunction with increasing emissions of Cl-VSLSs from East Asia, could potentially slow the expected recovery of stratospheric ozone.

[AbruptSLR]

The linked reference shows the significant impact of methane on climate change, when appropriately considering both the direct and indirect forcing contributions (see the attached image):

Ocko, I. B., Naik, V., and Paynter, D.: Rapid and reliable assessment of methane impacts on climate, Atmos. Chem. Phys., 18, 15555-15568, https://doi.org/10.5194/acp-18-15555-2018, 2018.

https://www.atmos-chem-phys.net/18/15555/2018/

Abstract. It is clear that the most effective way to limit global temperature rise and associated impacts is to reduce human emissions of greenhouse gases, including methane. However, quantification of the climate benefits of mitigation options are complicated by the contrast in the timescales at which short-lived climate pollutants, such as methane, persist in the atmosphere compared to carbon dioxide. Whereas simple metrics fail to capture the differential impacts across all timescales, sophisticated climate models that can address these temporal dynamics are often inaccessible, time-intensive, require special infrastructure, and include high unforced interannual variability that makes it difficult to analyse small changes in forcings. On the other hand, reduced-complexity climate models that use basic knowledge from observations and complex Earth system models offer an ideal compromise in that they provide quick, reliable insights into climate responses, with only limited computational infrastructure needed. They are particularly useful for simulating the response to forcings of small changes in different climate pollutants, due to the absence of internal variability. In this paper, we build on previous evaluations of the freely available and easy-to-run reduced-complexity climate model MAGICC by comparing temperature responses to historical methane emissions to those from a more complex coupled global chemistry–climate model, GFDL-CM3. While we find that the overall forcings and temperature responses are comparable between the two models, the prominent role of unforced variability in CM3 demonstrates how sophisticated models are potentially inappropriate tools for small forcing scenarios. On the other hand, we find that MAGICC can easily and rapidly provide robust data on climate responses to changes in methane emissions with clear signals unfettered by variability. We are therefore able to build confidence in using reduced-complexity climate models such as MAGICC for purposes of understanding the climate implications of methane mitigation.

[AbruptSLR]

The linked Columbia website (reflective James Hansen's thinking) on effective radiative forcing, and it calculates the effective radiative forcing of methane to include: "… simulated CH4-induced changes of O3 and stratospheric H2O …".

Title: "Radiative Forcings"

http://www.columbia.edu/~mhs119/Forcings/

Extract: "Fe provides a good prediction of the response to different forcing amounts.

The time dependent effective forcings relative to 1850 for the agents used in our computations are shown individually and as the total in the graph below."

See also:

Hansen et al (2017), "Young people’s burden: requirement of negative CO₂ emissions", Earth Syst. Dynam., 8, 577–616, https://doi.org/10.5194/esd-8-577-2017

https://www.earth-syst-dynam.net/8/577/2017/esd-8-577-2017.pdf

Extract: "The CH4 forcing includes its indirect effects, as increasing atmospheric CH4 causes tropospheric ozone (O3) and stratospheric water vapor to increase (Myhre et al., 2013)."

[Tom_Mazanec]

Rising methane levels imperils Paris agreement:

https://eos.org/research-spotlights/rising-methane-emissions-could-derail-the-paris-agreement

EDIT: Here are Feb 2019 stats on CH4:
https://www.esrl.noaa.gov/gmd/ccgg/trends_ch4/

EDIT 2: Industrial emissions of CH4 100 times greater than previously thought:
http://news.cornell.edu/stories/2019/06/industrial-methane-emissions-are-100-times-higher-reported-researchers-say

EDIT 3: Permafrost releasing twelve times as much nitrous oxide as thought:
https://news.harvard.edu/gazette/story/2019/06/harvard-chemist-permafrost-n2o-levels-12-times-higher-than-expected/

[Tom_Mazanec]

What Is Nitrous Oxide and Why Is It a Climate Threat?
https://insideclimatenews.org/news/11092019/nitrous-oxide-climate-pollutant-explainer-greenhouse-gas-agriculture-livestock

For years, experts have warned about the risks from one pollutant in particular—nitrous oxide—and yet there's been little global action on it.

The reason: "It is intimately connected to food," said Ravi Ravishankara, an atmospheric chemist at Colorado State University who co-chaired a United Nations panel on stratospheric ozone from 2007 to 2015.