The Potential Roles of Methane in Global Warming

As of May 2013 the concentration of CO2 is now 400 ppm and the current concentration of CH4 is 1.9 ppm

The efficiency of absorption of infrared light by a molecule depends largely on its physical shape. A CO2 molecule is a dumbbell shape:

However, the physical size of Methane is much larger and therefore this molecule presents considerably more cross section with respect to outgoing long wavelength IR emission.

The ratio of absorption cross sections between methane and carbon dioxide is temperature and wavelength dependent on temperature and wavelength, but an aggregate value is around 21.

Hence CH4 has 21 times the global warming potential as CO2

or 21x1.9 = 40 ppm CO2 equivalent or about 10% of CO2

So current CO2e = 400 + 40 = 440 ppm

That methane has increased due to human industrialization is very clear.

And it tracks the world population growth pretty well.

However, over the last 20 years, there has been some odd behavior in the build up of methane in the atmosphere. This is shown below:

The period from about 1999 through 2007 is when the methane concentration stayed relatively flat(which would be very good news). At the time this was a serious mystery. One of the strong things that controls terrestrial methane emission is the amount and vitality of wetlands: Wetlands emission are key here!

The resolution to the flat period as well as the recent, unfortunate rise in emissions seems to be a combination of the following two items:

After declining in the 1990s due to the industrial collapse of the Soviet Union, and the combination of more efficient use of natural gas and efforts to reduce landfill methane emissions in the West, emissions have risen since 1999 due possibly, in part, to the booming Chinese economy.

Had it not been for this reduction in methane emissions from wetlands, atmospheric levels of methane would most likely have continued rising," said Dr Paul Steele, from a scientist from Australia's CSIRO Marine and Atmospheric Research and one of the paper's authors. "This suggests that, if the drying trend is reversed and emissions from wetlands return to normal, atmospheric methane levels may increase again, worsening the problem of climate change."

But it now appears that methane emissions are on the rise again at a rate of about 6 ppb(parts per billion).

If the current rates in both CO2 and CH4 continue then by the year 2050 we will have this situation:

  • CO2 = 400 + 37*2 = 474 ppm
  • CH4 = 1.9 + .006*37 = 2.15 ppm
  • CO2e = 474 +2.15*21 = 530 ppm (you should be concerned about this!)

But we have a potentially larger problem which is the possibility of substantial methane releases from arctic permafrost: and elsewhere Methane release

Methane releases from ocean floor are well known:

Standard Methane Sources:

In the US the rate of many sources is either slowly shrinking or stabilizing. This may indicate that developed nations can effectively control their methane emission through better landfill management and possible co-generation or capture and burn. Note that burning methane releases CO2 but that is 21 times less than letting CH4 reach the atmosphere.

Long term sources of global methane emissions. Note the post WWII inflection point.

Yet more Controversy: Is the Amazon rain forest a net sink or net source of greenhouse gas emissions?

THIS JUST IN New satellite measurements indicated the Amazon Rain Forest produces more methane than previously thought. Other forests may also be strong methane contributors.

Methane Removal Mechanism (some estimates suggest the methane residence time is up to 12 years so there is no short term equilibrium control):

Atmospheric Chemistry (which can be modified by volcanic eruptions like Pinatubo):

CH4 + 2O2 CO2 + 2H20

  • OH + CH4 + 02 H2O + CH3O2
  • CH3O2 + NO +02 H2CO + HO2 + NO2
  • CH3O2 + HO2 CH3OOH + O2
  • H2CO + uv H2 + CO
  • CH3OOH +OH H2O + CH3O2a

  • However, these atmospheric chemistry attributes are likely not the explanation for the various observed yearly changes in the methane growth rate:

    In sum, it is very difficult to predict the future growth of methane retention in the atmosphere but its likely to remain on the rise, especially with more aggressive mining of remaining natural gas reserves and the continued consumption of rice and cows.

    There is, however, potentially, a much worse problem. A major concern, a positive feedback loop:

    Note, however, that "catastrophic" release of CH4 during period of Ice Ages may be the natural way that the Earth accelerates from a glacial period:

    Known Hydrate deposits. Found typically in continental shelf where marine organisms (microbes) actively feed on carbon and carbon related products to produce methane in the sediments.

    Location of Major Clathrate Deposits

    Good overview of the hydrate "problem"

    And here is a likely scenario which has happened in the geological past

    And there is Evidence of this happening within the last 10,000 years