Source: Communication with The Right Climate Stuff Team and others
Fundamental point about human carbon dioxide in the atmosphere. Don Bogard April 23, 2018 1) Atmospheric CO2 continually exchanges with other, even larger carbon reservoirs -oceans, plants, and soils. The yearly rate at which CO2 exchanges with these other reservoirs is considerably greater than the rate humans add CO2 to the atmosphere. 2) The time for new CO2 added to the atmosphere to equilibrate with these other reservoirs is on the order of days (green plants) to perhaps months and years for soil and surface ocean (deeper ocean has mixing times of centuries). 3) When a new amount of human CO2 is added to the atmosphere, that new CO2 gets mixed (through exchange) with the natural CO2 in these other reservoirs. This decreases the fraction of human CO2 present in the atmosphere, AND drives the fraction of human CO2 in all reservoirs toward a common value. This is an efficient mixing process. 4) Human CO2 is added to the atmosphere continually and has been for over a century -- times longer than the rate of C exchange in most C reservoirs. Consequently, to the first order, human CO2 is generally well mixed with natural CO2 among ALL reservoirs. Change in the rate by which human CO2 is added, or moderate change in the C exchange rate among all reservoirs, has no first-order effect on the fraction of human CO2 in the atmosphere or in these other C reservoirs. 5) The concentration of human CO2 in the atmosphere, to the first order, thus depends on the size of the other C reservoirs, and not the exchange rate for C among reservoirs. The larger these other C reservoirs, the smaller will be the fraction of human C in each and all. Moderately changing the mixing rates will have little effect. 6) There is a second-order effect (minor) by which human C is not well mixed into another C reservoir and thus the C fraction differs among reservoirs. This is the case for the deep oceans, where mixing times are longer than the time human C has been added. Thus deep oceans will absorb more human C than they return to the upper ocean, causing the upper oceans to have slightly less fractional C than the atmosphere. This second-order effect will have a dependence on the rate by which C mixing occurs (along with reservoir size), but this rate dependence applies to long times (centuries). 1