Glacial Cycles:
Methane Oxidation

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Background and Instructions

This tool computes cumulative CO2 from excess CH4 emissions, and vice versa. The idea is to illustrate that the famous ice core correlation of temperature and CO2 can be reproduced solely by incremental CH4 release at deglaciation. That is, the CO2 curve is an effect of temperature change, not a cause. The source of this excess CH4 is posited to be boreal/peat/tundra carbon buried by continuous snowfall at the onset of glaciation. This carbon is crushed, ground and anaerobically "fermented" by the increasing weight of ice, producing a methane-rich paste which is sequestered for thousands of years.

Correlation of methane and CO2 emissions from the EPICA ice cores;
modified version of this LBL original.  Citation
This model has these advantages. 1) The ice core data show a repetitive CO2 cycle to only 280-300ppmv rather than a wider swing. The limit is reached because the system absorbs in proportion to the quantity present in the atmosphere and eventually a new, higher equilibrium is reached. The methane source is also more or less a fixed quantity, related to the carrying capacity of northern forests. 2) The phase relationship naturally appearing in ice cores, that of temperature change preceding CO2 change follows naturally from the fact that as glaciers melt back (due to Milankovitch cycles) they expose and release the methane-saturated silt underneath. 3) As compared to the alternate hypothesis that warming causes release of CO2 from the ocean, while true in part, this model has the advantage of producing a carbon isotope signature associated with land-plants, as has been found in ice cores.

The main tool computes absolute CO2 levels from changing levels of methane, CH4. Oxidation of methane produces CO2 and water vapor. It assumes rates of CO2 addition (CH4 oxidation) and removal (weathering, ocean absorption, biomass accumulation) are directly proportional to their levels in the atmosphere. Similarly, it assumes the rates of stratospheric H2O addition and removal are proportional to levels of CH4 and H2O, respectively.

The ODEs used are:

At the bottom of this tool is the "constants" section. The values should be self-explanatory by reference to the above equations. CO2 absorption corresponds with k1. CH4 oxidation corresponds with k2. Note that all CO2 change is assumed to come from methane input - other sources and sinks are assumed in balance. k2 also determines the rate of CH4 depletion and stratospheric H2O generation. This simple tool models methane emissions as bimodal, thus there are two settings for k3: "Normal" and "Elevated". k4 is the rate of water vapor removal in the stratosphere. Note: the tool originally modeled CH4 oxidation as primarily occurring in the stratosphere. However, neither where the oxidation occurs nor how fast the water vapor is removed changes the end result as the generated CO2 becomes well-mixed into the troposphere.

The simulation is assumed to start at the depths of a glacial cycle when a rise in insolation causes a glacial retreat. The retreat allows methane clathrates formed underneath the glacier to decompose, and the resulting meltwater runoff carries the methane from under the glacier to the surface. This runoff further facilitates the formation of boglands and other wetlands, which lead to a significant uptick in methane production, modeled here as a stepwise increase, k3. (Note: if the North American icesheets were 25 million km3 at their peak, the per annum meltwater flow could not have been less than 4X the current maximum flow of the Mississippi every year for 9000 yrs.) Oxidation of the methane produces CO2 and H2O vapor.

Note that the default rate of CO2 removal, 0.00024/yr, corresponds to a half-life in the atmosphere of almost 3000 yrs, much longer than some current models predict for the present day. This does, however, correspond to the actual CO2 levels as seen in ice core data which relax slowly. One possible explanation may be that the eruption of polar oceanic methane clathrates saturates the polar oceans with CO2, altering the surface fugacity so that ocean absorption is much reduced.

H2O removal is determined by the rate of overturn of the stratosphere into the troposphere: 3-8 yrs, typically. The default value is a compromise to give observed levels of water in the stratosphere for "reasonable" values for other variables. Stratification is the ratio of stratospheric methane ppmv to that in the troposphere. It's tropospheric values that are captured in ice core data. However, the values used for CH4 in the equations are total methane relative to the whole atmosphere, and have to be converted to and from sea-level (SL) values.

The second toolset, ΔCH4 ↔ ΔCO2 simply illustrates that an increase in CH4 emissions, from whatever source, can drive CO2 from a glacial minimum of 180ppm to a interglacial maximum of 280ppm or more, if sustained for thousands of years. This is due to the relative longevity of CO2 in the atmosphere. Note that this ultrasimplified tool unrealistically assumes infinite CO2 longevity. The second half of this tool computes the inverse of the first calculation.

Excess CO2 mass: +{{compute_rev_co2_mass()|number:0}} Gtonnes
Final ΔCO2: {{compute_rev_co2_ppmv()|number:1}} ppmv

Excess CO2 mass: {{compute_co2_mass()|number:0}} Gtonnes
Source CH4 mass: {{compute_ch4_mass()|number:0}} Gtonnes
Excess CH4/yr: {{compute_ch4_mass_per_yr()|number:5}} Gtonnes/yr
Avg. ΔCH4 @ SL: +{{compute_ch4_ppbv()|number:0}} ppbv
H2O mass: {{compute_h2o_mass()|number:0}} Gtonnes

{{convert2mass("yearly_methane2_ppbv",1e-9,16)/1e12|number:4}} Gt/yr

{{convert2mass("yearly_methane1_ppbv",1e-9,16)/1e12|number:4}} Gt/yr

t1/2 = {{inp.compute(,0.01)|number:1}} yrs

t1/2 = {{inp._nxt.compute(,0.01)|number:1}} yrs