New estimate of effect of methane on climate

pattylou

I need to read through this a few more times before I really "get it" but it will be of general interest. After one read-through, it looks like the effects of methane on climate have been underestimated in the past because this gas was measured in the atmosphere after it has mixed with other GHG. It looks like if you separate out the components at the source of emission you end up with a different contributory effect of methane (twice that thought previously).

This may ultimately affect policy on climate change.

http://www.nasa.gov/centers/goddard/...5/methane.html

Garlic Bread?

Without looking it up, I think you'll find that natural additions of methane are twice as much as anthropogenic ones. Why would this effect our policy on CC? So what? If our methane effects it twice as much then natural emissions does to!

Garlic Bread?

Yeah. Pre-industrial baseline was 848 ppb. Natural additions have been 577 and man-made just 320.

pattylou

Of course. I am sure you understand that our society is working hard to understand the inputs into climate regardless of whether they are man made or not?

If we can cut methane emissions by reducing man-made methane, it help lower GHG in the atmosphere.

Also, any present policy that shifts us from reducing CO2 --- by increasing methane - may need to be revisited in light of this report.

Finally, natural sources of methane could at least theoretically be addressed as well, if necessary - although that is a risky proposition. For example, some microorganisms utilise methane. Swamps and bogs are natural sources of methane, but it may be possible to increase the methane-users there, I am not sure the end result of such a proposition. Alternatively, policy could entail capturing naturl methane. I am sure there are plenty of useful applications for it, although I don't know the "cost" of any of those applications.

Andre

Am I missing something? Isn’t chemistry about changing compounds. If methane is changed chemically, it’s not methane anymore. isn’t it?

This is a very misleading statement and it’s not true. CO2 is more than triple as strong as CH4 as greenhouse gas. But the logarithmic relation makes the first few ppbv and ppmv the most effective. If the CO2 was in the same very low concentrations as methane then the effect would have been about triple. This can be easily seen when playing a bit with the Modtran3 model Just enter
CO2 = 0, ch4 = 0 output:The basic value.

Then CO2=1, ch4=0,
Hence a difference of some 3.4 W/m2 with the basic value

Then CO2=0, CH4=1
Hence only some 0.9 W / m2 difference.

Since the decay time of CH4 in the atmosphere is about 12 years, I would not worry too much about methane.

pattylou

You got it! But the report is indicating that some of the chemistry (changing methane to non-methane) is creating trophospheric ozone. This is why the report recommends measuring methane emissions, not methane in the atmosphere.



You lost me. Can you restate that? Methane is frequently cited as 25 times as potent as CO2. Are you saying that the mass comes into play? Or that the reactivity does?

I am given to understand that a molecule of methane in the atmosphere traps 20 - 25 as much heat as a molecule of CO2. I am also given to understand that methane does not remain in the atmosphere for long.

Thanks for any clarification you can give.

Did you mean CO2? Or, is 12 years considered short? Is "decay time" the same as "half life?

Thanks Andre.

CobblyWorlds

The 12 year atmospheric residence time for CH4 is the mean time a molecule spends in the atmosphere. For water it's about 10 days, for CO2 it's of the order of a century.

I generally agree with Andre, that (I'd qualify by saying "at present") it's not as great a factor as CO2. Which given the increase in fossil fuel use by nations such as India and China is a far greater additive factor to radiative balance.

Garlic Bread also points out the amount of natural addition. However with issues such as the melting of permafrost, this ostensibly 'natural' addition, which is however driven by anthropogenic warming, could become far more significant later in the 21st century.

I agree it's a factor to watch, but nowhere near as serious as CO2. Although I'm prepared to be proven wrong on this.

As to the ModTran3 model I've not had the chance to check this out, so can't comment on the context of the figures it produces.

CobblyWorlds

Oh and PS, Andre thanks for the Modtran3 link, could be quite a useful tool.

Andre

Okay Let's play with Modtran3 a bit more. The model calculates the absorbtion of reradiated energy and gives as output the radiated energy to space after the greenhouse absorbtion part. The temp gadget appears not to be working yet.

In my previous example I showed that 1 ppm of CO2 absorbs over 3 times as much radiation as 1 ppm CH4, falsifying the statement:

But the relationship is approximately logaritmical, so you cannot compare a 1 ppm change on 375 ppm CO2 against 1 ppm change against 1,7 ppm CH4.

Now, I ran modtran3 with the default parameters (scroll down and hit "submit the calculation") and we see an output of "I out, W / m2 = 227.87".

Now, if we double the CO2 (750 ppm) the reradiated energy reduces to 225.546 W/m2. Not a lot really, because of the saturation effect that can be seen clearly in the graph in the model. The big CO2 chunk only grows minimally at the edges.

Now, put CO2 back (375ppm) and poke a double CH4 value (3.4 ppm). This gives an output of 227.367 W/m2, only 0.5 less than the original value. Note that we have to increase the CH4 to 19ppm, 11 times the current value to obtain the same effect as doubling CO2.

CH4 is oxidized in the atmosphere like this http://www.atmosp.physics.utoronto.c...d_93/m93-b.htm
however their life time estimate is only 7 years. I hear 12 years more frequently.

CobblyWorlds

Andre,

Having checked out an article here http://www.nasa.gov/centers/goddard/...5/methane.html. Notwithstanding their problems with 'Challenger' I find it hard to see how NASA, not to mention many other sources, could be wrong with statements such as "Molecule for molecule, Methane is 20 times more potent than carbon dioxide as a greenhouse gas" (from the above article).

So I suspect a problem with either your reasoning or the programme you link to. In the case of the programme, it'd be interesting to see it's code. But it seems likely it's a far more simple model than using a GCM to examine the same variation of parameters, I don't imagine it would account for issues such as cloud and convection.

However I'm just to busy to attend to this issue right now. However I think that, as you seem to agree, it's reasonable to view CH4 as a far lesser issue than CO2 at present(due to the current atmos concentrations and likely future trend in fossil fuel use).

I'll see if I can get the time to look at this over the weekend, but the way I'm going sometime next year looks more likely. ;)

Andre

No the statement is part of a slippery slope of a slight exagaration and older models. If the figures get less attractive there is a reluctance to update them.

There is nobody doubting modtran3. Global warmers use it. It's world famous. There is nothing wrong with the physics behind it.

So what happens when you increase the CO2 with one ppm to 376? The basic reradiation 227,87 w/m2 decreases with 0.032 W/m2, whilst increasing CH4 with one 1 ppm to 2,7 ppm, gives a decrease of 0.314 W/m2. So the most dramatic statement, that you could make accurately, is:

With the current concentrations, CH4 is tenfold more potent than CO2 with equal increases in concentrations

CobblyWorlds

Hi Andre,

I'm quite prepared to believe it is one of those oft repeated 'rumours' whose origins are lost/not updated with time (like the 'water is 98% of the greenhouse effect' one), but I like to check things out for myself. That said I'm happy to accept your reasoning as I can't see an immediate problem. But it may be a while before I come back on it. Just another issue to be attended to. Work eating sleeping etc all gets in the way.

pattylou

I think that is generally agreed upon. The article indicates simply that methane emissions may be twice as important (as a contributor to warming) than previously thought.

I don't know the various arguments on this, but it appears naively to me that the old estimate was around 15% (of warming was attributable to methane) and that the new estimate puts it closer to 30% (of warming is attributable to methane emissions, as opposed to methane in the atmosphere, as some of the methane is converted before becoming mixed in the atmosphere.)

At 30%, it seems to be something to take more notice of.

pattylou

That's at odds with this report from 2002, which indicates the reverse:

http://www.sciencedaily.com/releases...1010065923.htm

But this is a minor point.

CobblyWorlds

Hi Patty,

I've not really looked into it. In terms of satisfying myself that the observed global temp increase was due to CO2, CH4 was not a point I investigated. I'll have to check all of these figures at some point. PS the Guy on the BBC board was wrong. The NASA GISS shows a clear warming trend in the US area as stated, its all here; http://data.giss.nasa.gov/gistemp/graphs/
"GISS Graph Annual Mean Temperature Change in the United States"

pattylou

That has been my argument in the past. The most recent thing I had looked at indicated that US warming was less than global warming. Look at this figure:



Which is from this article (from the goddard institute.):

http://www.giss.nasa.gov/research/briefs/hansen_07/


...and I have emailed the author with a few questions, I hope he has time to answer them.


So I am happy to agree that the US is warming, but not as much as other parts of the world. THe point is irrelevant, as we agree, because local effects are expected, due to .... local effects!

Would you *disagree* that the US may have enjoyed less warming than most of the rest of the planet as a result of the NAO? As I see it, my present position is consistent with your own, but perhaps you understand there to have been a larger scale effect in the US than I do.

Andre

Another reason why the US may not have warmed is that worldwide most rural weather stations closed in the 1980-1990 timeframe, wereas the USA maintained just about all of them.