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Is CO2 nothing more than a M2C2 companion GHG?


Is CO2 nothing more than a M2C2 companion GHG?26-10-2015 01:19
trafn
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In medicine, when AIDS was first being studied, there were many contenders for its cause, one of which was HIV. At first, no one was sure if HIV really caused AIDS, or if it was merely a companion virus: a virus that happened to travel with whatever caused AIDS and was, therefore, always present when AIDS occurred, but was not the actual cause of AIDS.

In M2C2 (man-made climate change) CO2 is one of the most common and easily measurable GHG's. But is it really the GHG of greatest concern when it comes to M2C2? In other words, are there GHG's that should be of greater concern to us when considering both the long and short-term impacts of M2C2?

As examples of GHG's which might be more important than CO2, what about methane or nitrous oxide?


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Edited on 26-10-2015 01:20
26-10-2015 02:45
Totototo
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There are many factors to consider if you want to point which gas has the most important role in M2C2. That's a good question I don't have the answer to but would love to get one.

In the meantime, you should chech the Global-Warming potential values. That won't give you all the answers since the GWP doesn't consider the amount of emissions of each GHG I think, not sure about that.
26-10-2015 10:20
trafn
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@Totototo - here's a link to a GHG GW potential page by the IPCC 4 (2007):

https://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-2.html

I'm still looking for the data for IPCC 5 (2013).

It's amazing to see that as far as GW potential goes, CO2 is not the big concern.

Nitrous oxide may also be called laughing gas, but when it comes to GW it's no laughing matter.


The 2015 M2C2 (Global 9/11) Denialist Troll Awards

1st Place - Jep Branner - Our Stupid Administrator!
2nd Place - IBdaMann - Science IS cherry picking!
3rd Place - Into the Night - Mr. Nonsense numbers!
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26-10-2015 11:58
climate scientist
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Hi trafn and Totototo

The GHG GW potentials indicate the potency of different greenhouse gases, but there are two other factors to consider:

1. As Totototo said, the emissions and atmospheric concentration of a particular gas is important.
2. As can be seen from the IPCC AR4 table, the lifetime of the gas in the atmosphere is also important.

Although table 2.14 from the IPCC AR4 shows that CO2 is one of the weakest gases, on both short and longer time scales, this information does not reflect the amount of CO2 in the atmosphere. For example, today, there is about 400 ppm of CO2 in the atmosphere, but only about 1800 ppb of CH4 (e.g. 1.8 ppm). This means that there is about 220 times more CO2 in the atmosphere than CH4, so even though on a 20 year time horizon, CH4 is 72 times as potent as CO2, because there is a lot more CO2, the total radiative forcing from CO2 dominates.

This information is shown in Figure SPM.5 here, from the IPCC AR5 Summary for Policy makers:

http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_SPM_FINAL.pdf

CH4 is the second largest contributor to anthropogenic radiative forcing. However, since it has such a short atmospheric lifetime, the if we stopped all anthropogenic CH4 emissions today, then the CH4 contribution to anthropogenic radiative forcing would decline very rapidly within a decade or so. If we stopped emitting CO2 today, however, atmospheric CO2 would probably continue to rise slightly for a few years, and then level off. It will take thousands of years for atmospheric CO2 levels to return to pre-industrial levels, owing to the extremely long lifetime of CO2 in the atmosphere (and potentially tens of thousands of years, depending on how much CO2 we continue to emit in the coming century).

Of course, historically, CO2 has not been the most dangerous gas. The rise of CFCs and subsequent destruction of ozone was probably the closest that humans have come so far to a global environmental catastrophe. It is therefore entirely possible that there are gases other than CO2 that are potentially much more dangerous, but I suspect that they will be dangerous for other reasons that are not directly related to M2C2.
28-10-2015 21:30
climate scientist
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Hi trafn

This is slightly off topic of this thread, but I couldn't find the old thread where it was mentioned. Really interesting article in Nature about turning atmospheric CO2 into useful products:

http://www.nature.com/news/how-to-make-the-most-of-carbon-dioxide-1.18653?WT.ec_id=NATURE-20151029&spMailingID=49884916&spUserID=MzY4MjIzMjg5NjcS1&spJobID=783976831&spReportId=NzgzOTc2ODMxS0
29-10-2015 00:28
trafn
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@climate scientist:


1. That is an intersting article on uses of captured atmospheric CO2. By the way, the thread this was referring to started here:

Can petrochemicals exist in a carbon-neutral world?


2. Radiative forcing is a good way to look at this. But what if we keep putting these GHG's into the atmosphere without reducing them? Then, assuming their ratios stay the same, perhaps sheer volume is not the most important factor. Consider this:



This is a 2013 EPA assessment of U.S. GHG emissions, only. In the diagram, the three major GHG's by volume/concentration are:

1. CO2 = 82%
2. Methane = 10%
3. Nitrous oxide = 5%

In the 2007 IPCC report, they quoted the following "GW potential" numbers for these three gases over a 20 year time frame:

1. CO2 = 1 (the baseline gas)
2. Methane = 79
3. Nitrous oxide = 289

Global warming potential is the radiative forcing effect of a GHG over a specified time period. So, if we assume there are no GHG reductions over the next 20 years and that the concentrations of these 3 gases stay relatively the same for their volume percentages, then when we adjust those volume percentages into GW potential percentages as follows, we get:

1. CO2: 82 x 1 = 82 --> 82 / (82 + 790 + 1445) = 3.5%

2. Methane: 10 x 79 = 790 --> 790 / (82 + 790 + 1445) = 34.1%

3. Nitrous Oxide: 5 x 289 = 1445 --> 1445 / (82 + 790 + 1445) = 62.4%

In other words, if we do nothing about M2C2 and things stay unchecked, then though CO2 represents 82% of atmospheric GHG's by volume, it only accounts for 3.5% of AGW due to it's low GW potential. On the other hand, nitrous oxide which is only 5% of the GHG's by volume accounts for 62.4% of the GW potential of these three gases.

This is why I wonder if maybe we're placing too much emphasis on CO2. I'm not saying that CO2 is unimportant, but that it's long-term effects under a "non-reduction" scenario may not be as significant or important as those from other GHG's which have lower percentage volumes (i.e. - methane and nitrous oxide).

Of course, the above is only based on U.S. emissions (not global emissions), it assumes unchecked M2C2 emissions which stay relatively constant to one another, and it's a limited 20 year projection, but I still find it interesting

Thoughts?


The 2015 M2C2 (Global 9/11) Denialist Troll Awards

1st Place - Jep Branner - Our Stupid Administrator!
2nd Place - IBdaMann - Science IS cherry picking!
3rd Place - Into the Night - Mr. Nonsense numbers!
4th Place - Tim the plumber - The Drivel Queen!
29-10-2015 11:37
climate scientist
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Yes, it is interesting that currently, US GHG CH4 and N2O emissions are contributing more radiative forcing. But the thing that matters for climate change, is the total GHGs in the atmosphere, not the emissions. And most of the CH4 that is emitted today will be converted into CO2 and H2O within a few decades. Also, CH4 has a large natural atmospheric sink, which plays a large role in determining atmospheric CH4 mole fraction, so a lot of the CH4 emissions do not contribute to the long-term trend in CH4.

So I think that in the long-term, CO2 will remain the dominant driver of anthropogenic climate change, mainly because CH4 has a relatively short lifetime in the atmosphere. Since the industrial revolution, the contribution of CH4 to the total anthropogenic radiative forcing is about half the contribution from CO2 (see IPCC AR5, WG1, Summary for policy makers, Figure SPM.5). And N2O is about 1/5th of CO2. I think that we will see a shift towards CH4 contributing a larger proportion, but I'm not sure if CH4 will end up taking over CO2, but this depends on how future emissions play out, which is very uncertain.
29-10-2015 16:34
trafn
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@ climate scientist - I get what your saying about GHG decay time, and that CO2 has, by far, the longest active life-span in the atmosphere. But, even though MH4 and NO2 have shorter half-lives (the time it takes for half of a given quantity to be removed from the atmosphere), they are also, like CO2, being constantly replenished, most likely at increasing rates. So, though a given molecule of MH4 will only be around for about a decade, it will rapidly be replaced by another molecule of MH4, thereby functionally extending the atmospheric activity.

I would analogize it like this:

1. Over 200 years, a single CO2 molecule will be remain present in our atmosphere.

2. Over 200 years, 20 generations of MH4 molecules will sequentially be present in our atmosphere.

3. Over 200 years, the 20 sequential generations of MH4 molecules will have the equivalent impact to a single theoretical MH4 molecule that lasted for 200 years.

By the way, here's a very nice graph which gives relative atmospheric life spans for 4 GHG's:

CO2 - about 200 years
N20 - about 114 years
MH4 - about 12 years


The 2015 M2C2 (Global 9/11) Denialist Troll Awards

1st Place - Jep Branner - Our Stupid Administrator!
2nd Place - IBdaMann - Science IS cherry picking!
3rd Place - Into the Night - Mr. Nonsense numbers!
4th Place - Tim the plumber - The Drivel Queen!




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