CH4 is one of the most important anthropogenic greenhouse gases but it has significant biogenic sources ~ 50% of the total. Its present day mixing ratio in the northern hemisphere (NH) is ~ 1800 ppb while in the southern hemisphere (SH) it is 1600 ppb. We will use this observed gradient to determine the difference between sources in the NH and the SH. We take as the timescale for mixing air between the two hemispheres to be 1 year and the CH4 lifetime by oxidation in the atmosphere is ~ 9 years.
1) Using a two-box model for the NH and SH, write the mass balance equations for CH4 in these two boxes.
2)Assume that the gradient between the hemispheres is constant, i.e. d(mN - mS)/dt = 0, where mN is the amount of methane in the NH and mS is the amount in the SH. Calculate ?E, the difference between NH and SH methane emission rates in units of moles of methane per year.
3)If the total global emission rate of CH4 ETot = EN + ES ~ 5x1013 moles/yr, what do you conclude about the distribution of methane sources between the two hemispheres? Does your calculation support a large terrestrial (land-based) component to the methane source? Why or why not?