Methane and Global Warming

Early Warning Signs

Before devling into a discussion about the possible important role of methane in global warming, let's first take a look at some early indicators that this process is now happening.

Note: On Thursday we will be using the laptops to do an exercise in "weather event" detection to see how difficult it is to find a real event buried in noisy climate data.


All About Methane

 

Table 1

Characteristics of some major (climate changing) greenhouse gases

Greenhouse gas

Sources

Sinks

Importance for climate

Carbon Dioxide

(CO2)

1) Burning of fossil fuel

2) Land-use change (deforestation)

1) Ocean Uptake

2) Plants!& photosynthesis

Absorbs infrared radiation; affects stratospheric O3

Methane

(CH4)

1) Biomass burning

2) Enteric fermentation

3)Rice paddies

1) Reactions with OH

2) Microorganisms uptake by soils

Absorbs infrared radiation; affects tropospheric O3 and OH; affects stratospheric O3 and H2O; produces CO2

Nitrous Oxide

(N2O)

1) Biomass burning

2) Fossil-fuel combustion

3) Fertilizers

1) Removal by soils

2) Stratospheric photolysis and reaction with O

Absorbs infrared radiation; affects stratospheric O3

Ozone

(O3)

Photochemical reactions involving O2

Catalytic chemical reactions involving NOx, ClOx and HOx species.

Absorbs ultraviolet and infrared radiation

Carbon Monoxide

(CO)

1) Plant emissions

2) Man-made release (transport, industrial)

1) Soil uptake

2) Reactions with OH

Affects stratospheric O3 and OH cycles; produces CO2

Chlorofluorocarbons

(CFCs)

Industrial production

Insignificant in troposphere, dissociated in stratosphere (photolysis and reaction with O)

Absorbs infrared radiation; affects stratospheric O3

Sulphur Dioxide

(SO2)

1) Volcanoes

2) Coal and Biomass burning

1) Dry and wet deposition

2) Reactions with OH

Forms aerosols, which scatter solar radiation

The Relative Importance of Methane:

Current concentration of CO2 is 366 ppm

Current concentration of CH4 is 1.8 ppm and is responsible for 20% of the greenhouse gases in the atmosphere (tho that 20% depends on how you model the ratio of IR absorption between CO2 and CH4.)

The relative absorption power of methane to carbon dioxide is still uncertain. The "offical" value of its GWP (global warming potential) is 21.

The higher IR absorption cross section results because CH4 is a physically bigger molecule than CO2 so it preferentially absorbs more long wavelength radiation.


A more recent version of this table is below where the ranges in parantheses represent the uncertainy in the variable:

Distribution of wetlands emission and soil moisture potential:


Recent build up of methane in the atmosphere seems to closely follow the world's population strong argument that build up is human induced:

However, in recent years, there appears to be a leveling off of methane buildup. There is no viable explanation for this (yet) but we will speculate a bit.




Methane Sinks:

Methane Removal Mechanism (some estimates suggest the methane residence time is up to 12 years so there is short term equilibrium control if methane emissions from earth's surface were reduced):

Atmospheric Chemistry End Result:

CH4 + 2O2 CO2 + 2H20

Details:

A recent finding shows that methane is more effectively processed by soil bacteria in an aspen/birch forest than in either a coniferous forest or grassland.

Under conditions of high soil mositure, the processing rate was up to 65% higher (this is quite significant).

If changing precipitation and temperature pattners serve to enhance the likely northward migration of these kinds of forces, then this is a biological negative feedback channel to global warming.

In the US, anthropogenic sources are stabilizing

This is also seen in the UK

But these results probably are insufficient to explain the worldwide levelling off and there hasn't been sufficient wetlands loss in the last 10 years to explain this either.

The most probable explanation, therefore, is the fall of the Soviet Union:

A major concern, a positive feedback loop: (note: approximately 3000 times the current volume of methane in the atmosphere is currently stored in hydrate form).

Note, however, that "catastrophic" release of CH4 during period of Ice Ages may be the natural way that the Earth accelerates out of a glacial period. There is lots of evidence discovered in the last 2-3 years for such sudden warm ups in geologic history.

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

Good overview of the hydrate "problem"

Methane is more directly related to food production and population growth so it could also dominate in the near future

Frozen methane is also found in the Arctic Ice Caps and will be released due to global warming thus exacerbating the problem. This is far more serious that people realize. In the last 2 years, the scientific community, however, has done lots of research on this problem and have conclude its a potentially serious effect.