2010-12-14 04:37:00CH4 and CO2
Bob Guercio
Robert Guercio
robertguercio@optonline...
24.187.94.227

Hi All,

I started studying climate change about six months ago but it is a very confusing subject.  I feel like I am constantly reinventing the wheel.  

It is my understanding that a molecule of methane absorbs much less infrared radiation than one of carbon dioxide.  There is also much less methane in the environment than CO2. So why is CO2 such a problem?

It is such a problem because there is so much less of it.  It takes much less CH4 to double the concentration than that of CO2.

Is my understanding of this correct?

Thank you,

Bob 

2010-12-14 05:40:36
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.233.144

Bob, that's not right. Methane is a far more powerful greenhouse gas than carbon dioxide (approx 25 times). It atmospheric lifetime is around 10-12 years before it breaks down to CO2. You are right however, about the fact that because the levels of methane in the atmosphere are low it takes much less of any extra methane to double it's concentration. 

Then of course there is all that methane locked up in the permafrost and as clathrates at the bottom of the oceans. 

2010-12-14 11:42:10
Bob Guercio
Robert Guercio
robertguercio@optonline...
24.187.94.227
Hi Rob, You're probably correct because that is what was stated on the "Science of Doom" website. The reason I thought differently was because of the following graph which I must be misinterpreting. I noticed the absorption spectrums of both carbon dioxide and methane. As I see it, there is more area under the curve of the absorption spectrum for CO2 than for CH4. How should I be interpreting this figure. Thank you, Bob
2010-12-14 14:57:35
Bob Guercio
Robert Guercio
robertguercio@optonline...
24.187.94.227

Rob,

I think that I am correct.

Please go here:  

 

Methane/Carbon Dioxide

 

I went to David Archer's site and confirmed that methane is actually weaker molecule for molecule than carbon dioxide.

 

Bob 

2010-12-14 18:23:11
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.202.99

Bob, the Global Warming Potential (GWP) of Carbon Dioxide is 1 (it's the baseline which other GHG's are compared to). The GWP of methane is 72 at 20 years, and drops to 25 at 100 years.

http://www.nature.com/nature/journal/v344/n6266/abs/344529a0.html

http://www.epa.gov/climatechange/emissions/downloads/ghg_gwp.pdf

This stuff was covered back in the 80's and 90's so that the Kyoto Protocol could be implemented, although refinements have been made and discussion of better yardsticks, such as Global Temperature Potential featured in the last IPCC report.

As for all the gory details, I thought Science of Doom would have covered this?.  

 

 

 

 

 

 

2010-12-14 22:31:43
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.202.99

Bob, the intimate details of physics don't generally interest me, however you piqued my curiousity and I wanted to know why methane was a more potent GHG, This originates from David Archer:

http://geoflop.uchicago.edu/forecast/docs/archer.ch4.greenhouse_gases.pdf

 "The band saturation for CO2 makes CO2 a less potent greenhouse gas than it would be if we had no CO2 in the air to start with. Let’s revisit our comparison of the CO2 and methane as greenhouse gases. Methane had a disadvantage because its absorption band sort of fell in the suburbs of the earth-light spectrum whereas CO2 fell right downtown. Now we see the advantage shifting the other way. Methane has a much lower concentration in the atmosphere. You can see from the jagged edges of the methane peak in Figure 4-3 that the methane absorption band is not saturated. For this reason, in spite of the suburban location of the methane band, a molecule of methane added to the atmosphere is 20 times more powerful than is a molecule of CO2"

"Band saturation: A greenhouse gas at relatively high concentration like CO2 will be less effective, molecule per molecule, than a dilute gas like methane" .

Maybe Chris Colose is addressing some urban myth, I don't know. The point is, with our current atmosphere methane is a more potent GHG. The Molecule vs molecule comparison in a hypothetical atmosphere is a somewhat pointless, since it's an atmosphere we do not have.

 

2010-12-15 02:38:38
Bob Guercio
Robert Guercio
robertguercio@optonline...
24.187.94.227

He is addressing an urban myth.

Imagine taking two vessels of the same volume with methane in one and carbon dioxide in the other in equal concentrations.  Now send IR radiation through both and measure which one has absorbed more of the IR energy.

The myth is that the vessel with the methane will absorb more of the IR energy where the reality is that the CO2 actually absorbs more.

However, for other reasons as explained above, Methane is a more powerful greenhouse gas.

I do think that the details are important because the contrarians take the details out of context to say that we are wrong.  We are being forced to be very precise even though, as you said and I agree, it really doesn't matter regarding the real atmosphere. 

Bob 

 

 


2010-12-15 21:36:07
Riccardo

riccardoreitano@tiscali...
192.84.150.209
When it's said that methane is more potent than CO2, it refers to the effect of the increase in concentration in current atmosphere. For example, what's the effect if we double CH4 or CO2 from current values? Here CH4 wins due to the logarithmic dependence of the forcing on concentration. Note that in this case we're are talking about a doubling of very different initial concentrations.
A completely different question is what is the contribution to the total greenhouse effect. Here CO2 wins due to his higher concentration and the position of its absorption band near the maximum of outgoing thermal radiation. As Chris Colose shows, the effect of CO2 is larger even at equal concentration.