How Long Does CO2 Stay in The Air? A Simple Analysis
Atmospheric CO2 concentration only makes up for about 44–50% of total CO2 emitted since the industrial era. The concept of an “airborne fraction” has spurred the writing of numerous papers. Due to its consistency over time, it has become known as the “airborne fraction” and sparked much debate.
Some when as far as programing entire coupled climate models. Others attempted to calculate the size of Earth’s carbon sinks. But most kept it simple.
Below is a typical experiment used by IPCC describing the amount of CO2 remaining in the atmosphere following a single large CO2 injection, or pulse. The blue line titled 100 PgC is equivalent to 100 Gt — ten times today’s annual emissions.
Oddly, each stage of uptake acts discreetly when everything should work simultaneously. Like an assembly line, a creates a product for b, which makes some tweaks and gives it to c for final assembly. This is hardly natural.
The model posits a fast uptake in land and ocean, moderate uptake during ocean invasion and a slow phase during reaction with CaCO3. In the end, emitted carbon remains in the atmosphere essentially forever in the human timeline.
I present a simpler model with a single rate of decline.
Cumulative CO2 emissions is the serial addition of each year’s emissions. The problem is growth in atmospheric CO2 has not kept up with human emissions. Here I propose emitted CO2 has a 35 year atmospheric half-life, resembling exponential decay.
Atmospheric CO2 growth will naturally be slower than cumulative emissions. Here I extrapolate a 35 year exponential decay of each year’s emissions, conducted iteratively to project atmospheric CO2 concentrations, or the airborne fraction.
In the chart below: cumulative CO2 emissions (blue), modeled atmospheric CO2 (yellow), and observed atmospheric CO2(black). 1880 serves as the baseline.
It’s a pretty damn good fit between modeled and observed data. This analysis suggests CO2 emissions may behave with a 35 year half life, or residence time.
With coverage of over 140 years, this model suggests there has been no reduction in the capacity of the carbon sinks. It also implies CO2 levels are sensitive to changes in emissions. A slowdown and flattening of emissions alone will lead to steadying of atmospheric CO2 concentrations. Modest reductions will result in CO2 decline.
