Sunday, June 7, 2009

Methane and Greenhouse Gases

I came across some interesting data on the sources and sinks of methane in the atmosphere, and it led me into the Greenhouse Gas issue. As most of us know, methane is the number two greenhouse gas, as shown on this figure. It shows the effect of each gas on radiation flux, which is called "Radiative Forcing" in the jargon of climate change. I graphed this from data by International Panel on Climate Change via Wikipedia. Their graph leaves off water, which is more important than all -- which is a little sleazy. 

Different gases have different effects on solar radiation because of their different IR (infra-red) spectra, which is something obvious that I did not know. The UV and visible light shines to the earth's surface, is converted to heat, and then the IR absorptive qualities of the gases prevent them from leaving. This chart shows the absorbances of these gases. The chart is done in frequency rather than wavelength in the backwards format of organic chemists. The data have been normalized to show the locations of the absorbances and not the relative intensities. 

This second chart shows the intensities better. Blue is water which is the most important. Red is carbon dioxide. Black is methane. (This is in nanometers -- a measure of wavelength, so it is backwards from the first chart.)

So what are the sources of methane? Remember when right wing politicians tried to debunk global warming by funding studies of cow gas? Well it turns out digestive gases from ruminants is a significant source of methane, but not the biggest. It is third - called "enteric fermentation" in the jargon. The data is for the United States in 2003 totals 545 teragrams which is 10^9 kg or 10^12 g.

There are natural sources too, and for the whole world the Intergovernmental Panel on Climate Change estimated 190 Tg or 1.9E11 kg. This is about one third of the US human based emissions. I graphed all of them. I took the US Data and tripled it, which is a quick-and-dirty way of turning domestic data into world data. I find tripling works pretty well with marketing data at work. So the following chart is a global.

The Ranking of Methane Sources
1.  First is the anaerobic decay of garbage in landfills is the primary source. It is interesting to consider if burning the garbage would be better or worse.

2.  The second sources is natural gas systems, and this means that natural gas has some methane in it, and it escapes to the atmosphere. 

3. Enteric fermentation is the methane generated from ruminant animals like cows and goats -- animals with multiple stomachs. One might argue this a natural source, but I think that domesticated animals are a human source. 

4. Coal mining is methane that escapes from the mines -- one might try to define this as natural too.

5. Anaerobic fermentation in wetlands is the first natural source.

Well this is really just scratching the surface, and I have already killed a lot of time on this. I will probably put together a post on how methane leaves the atmosphere, which is also pretty interesting. This stuff is pretty sobering in terms of the magnitude of the greenhouse gas problem, and the very limited means of action available. That is it seems hopeless.