Guest Post by Willis Eschenbach

I have a category that I call “scientific urban legends”. These include things like the idea that rising seas will drown atolls, when Darwin showed 150 years ago that rising seascreate atolls. Another scientific urban legend is the claim that we’re in the middle of the “Sixth Wave of Extinctions”, when there is no evidence to support that claim.  Despite flying in the face of scientific observations, these urban legends show amazing persistence. From my observations in fighting them, each legend will require the equivalent of an oak stake through its heart at a lonely midnight crossroads in order to eventually kill it.

I got to thinking about methane today. It’s supposed to be the doomsday gas of all the greenhouse gases, many times more powerful than CO2. People discuss things like the “methane time bomb”, which is supposed to be ticking somewhere or other, and ready to blow us all to Thermageddon, or at least to the Climatory … the proposed location of said explosive device has changed over time …

So I googled “methane times more powerful co2”, and I got the following top six results, from number one on down:

EPA: 20 times more powerful

EDF: 84 times more powerful

thinkprogress: 34 times more powerful

onegreenplanet: 100 times more powerful

psehealthyenergy: 20 times more powerful

global-warming-forecasts: 72  times more powerful

In those numbers you see an initial confirmation that the methane alarmism actually is a scientific urban legend … one of the red flags for such legends is, nobody knows what the exact number is, but by gosh, everyone is very sure that it is really, really big and really, really bad for us.

So I wondered … the IPCC says that the change in atmospheric absorption from a doubling of CO2 is a 3.7 watt per square metre increase. How much change would there be from a doubling of the methane levels?

To answer this question, I went to the wondrous MODTRAN site. Using todays values for CO2 (~ 400 ppmv) and methane (~1.81 ppmv) gives me upwelling radiation of 287.5 watts per square metre (W/m2).

Then I doubled the methane to 3.62 ppmv, re-ran the calculations, and got 286.7 W/m2 emitted from the TOA …

…

… which means that if by some chance the methane levels were to double in the next hundred years, the total effect would be an increase in the atmospheric absorption of 0.8 W/m2. Less than a quarter of the effect of a doubling of CO2 … say what? This is supposed to be the dread methane, eleventy times more powerful than CO2? Less than one watt per doubling?

So of course, I wanted to check my figures. To do that, I used the formulas from the IPCC for calculating the change in forcing resulting from a given change in methane. They are available here, see Table 6.2. I won’t bore you with the calculations, but they say if the atmospheric methane level doubles from the current level of 1.81 ppmv to 3.62 ppmv, the forcing will increase change by 0.54 W/m2. Somewhat smaller than the 0.8 W/m2 from MODTRAN but the same order of magnitude, well under one watt per square metre …

Let me slow that down for you to make sure you understand what I’m saying. IF methane concentrations double over the next century we would expect and increase in forcing of

One half

Of one watt per square metre

Per century.

So … how likely is it that the methane levels will double within a hundred years? To answer that, we can look at the recent changes in the methane levels. Here is the recent observational data:

Figure 1. Source: NOAA/ESRL 

To double from today (1810 ppbv or 1.81 ppmv) would be another 1810 parts per billion. As you can see, the methane levels rose more rapidly until about 1992, and rose roughly linearly at a slower rate after that. The period of record is about a third of a century (36 years). Over that time, it rose by about 250 ppbv. This means that over the next century, with a “business-as-usual” scenario we’d expect something on the order of three times that, or 750 parts per billion. This is a long ways from a doubling, which would be 1,810 parts per billion

And the increased forcing from that 750 ppbv? Well … it’s a measly quarter of one watt per square metre. Again, let me slow that down. With a “business-as-usual” scenario, we would expect an increase in forcing from methane of

One quarter

Of one watt per square metre

Per century

How about if the rate goes wild, and the methane starts rising at say three times the current rate? That would be an additional 2,250 ppbv per century, which in turn will result in an additional forcing of, wait for it … two-thirds of one poor lonely watt per square metre. MODTRAN puts it slightly higher, but still under one W/m2. Pathetic.

And what are the odds of the rate being that high, 2,250 ppbv per century, three times the recent rate of 750 ppbv per century? Very slim. We can see that by looking at the last thousand years of methane levels. Note that these are not global values as in Figure 1. Since there is a methane gradient from the north to the south pole, the Antarctic values are somewhat less than in Figure 1. However, we’re interested in the trend, which will be about the same globally:

Figure 2: Source: NASA GISS

From 1900 to 2000, which was the fastest-rising century in the last millennium regarding atmospheric methane, the concentration went up by about 800 ppbv, a bit larger than the recent increase shown above in Figure 1 of 750 ppbv per century. So there is no acceleration in the rate of methane level increase. To the contrary, there is deceleration, since the recent two decades of the record show an increase of only around 400 ppbv. And indeed, my “business-as-usual” estimate is about as fast as the record rise over the last thousand years.

As a result, I’d say there is very little chance that the rate of methane increase will be doubled, much less tripled, over the coming hundred years … and even in the very unlikely chance that it did triple, the increase in forcing would still be under one watt per square metre per century. Not per decade. Per century.

I gotta say, that’s not some fearsome gas. That’s a downright wimpy example of a Chicken Little gas, a laughing gas if you will. Anyone who is worried about methane, good news. You can stop worrying. Even an extreme methane increase sustained for a hundred years will only make a trivial difference in downwelling forcing. The idea that methane is a major player in the temperature game is a scientific urban legend.

w.

AS USUAL, I request that if you disagree with someone, please quote the exact words that you disagree with. That way, we can all understand just what you object to.

PS—Yes, I know that people claim that methane has some strong feedbacks. And yes, I took a look at them. One is that increasing temperature causes increasing methane, because methane is a byproduct of life, and life likes warmth. More warmth = more life = more decay = more methane.You can see the relationship here.

The problem with that feedback is that whatever increased methane emissions the recent global temperature increase might have caused are already included in both graphs above, Figures 1 and 2. So that feedback is already accounted for in the 750 ppmv/century predicted increase.

The second feedback is due to the fact that methane only lasts about ten years in the atmosphere, at which time it breaks down as follows (simplified):

CH4 ==> CO2 + 2 H20

So when the CH4 is gone, you still have two different greenhouse gases remaining, carbon dioxide and water vapor. Oooh, frightening!

But the problem with that feedback is that the methane numbers are so tiny. The atmospheric levels of the three gases are approximately as follows:

Methane: 1.8 ppmv

CO2: 400 ppmv

Water Vapor: 6,400 ppmv

Now, turnover time in the atmosphere for methane is on the order of ten years. This means that every decade a tenth of the methane turns over, or .18 ppmv/decade, which is .02 ppmv per year.

This means that the amount of methane that decays into CO2 and H20 each year increases the CO2 levels by about 0.02 ppmv per year (or 2 ppmv per century), and the water vapor levels go up by twice that or about 0.04 ppmv … meaninglessly small.