Friday, December 23, 2011

Making sense of methane

I'm traveling today, but here are a few review articles about methane which are free online:

"Atmospheric Methane: Trends and Impacts"
"As discussed earlier, increasing water vapor from methane could be leading to an increased amount of polar stratospheric clouds. Ramanathan (1988) notes that both water and ice clouds, when formed at cold lower stratospheric temperatures, are extremely efficient in enhancing the atmospheric greenhouse effect. He also notes that there is a distinct possibility that large increases in future methane may lead to a surface warming that increases nonlinearly with the methane concentration."
"Archer: Destabilization of Methane Hydrates: A Risk Analysis"
"Methane is less concentrated than CO2, and its absorption bands less saturated, so a single molecule of additional methane has a larger impact on the radiation balance than a molecule of CO2, by about a factor of 24 [Wuebbles and Hayhoe, 2002]. The radiative impact of CH4 follows the concentration to roughly the 1/3 power, while the CO2 impact follows the log of the concentration. To get an idea of the scale, we note that a doubling of methane from present-day concentration would be equivalent to 60 ppm increase in CO2 from present-day, and 10 times present methane would be equivalent to about a doubling of CO2." 

"Strong atmospheric chemistry feedback to climate warming from Arctic methane emissions"
"The indirect contribution to RF of additional methane emission is particularly important. It is shown that if global methane emissions were to increase by factors of 2.5 and 5.2 above current emissions, the indirect contributions to RF would be about 250% and 400%, respectively, of the RF that can be attributed to directly emitted methane alone. Assuming several hypothetical scenarios of CH4 release associated with permafrost thaw, shallow marine hydrate degassing, and submarine landslides, we find a strong positive feedback on RF through atmospheric chemistry. In particular, the impact of CH4 is enhanced through increase of its lifetime, and of atmospheric abundances of ozone, stratospheric water vapor, and CO2 as a result of atmospheric chemical processes."

. . . so make sense of it your own damn self! Kidding. Here are a some things I gleaned:

* The East Siberian Arctic Shelf is uniquely vulnerable, and this vulnerable formation has its own vulnerable sub-sections. So a leak, while serious, would not necessarily imply a planetary disaster.

* Doubling methane would increase forcing by about 0.4 - 0.6 W/m^2 (that is a harder number to find then you might think.) The calculation is complicated, because the effect of methane on water vapor, ozone, and reactive O2 species effects both the warming caused by the methane and the lifespan of the methane in the atmosphere.

* The impact of an event similar to the Storegga landslide I found helpfully described as "similar in magnitude and duration but opposite in sign to a large volcanic eruption." The largest known "mud volcanoes" have similar potential.


Overall, this is a complex but not unapproachable subject. Worriers like me will find plenty to worry about, but there are also good reasons why oceanic methane release is not the thing keeping methane scientists up at night. And the science and research is really cool.

2 comments:

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