Decarbonization rate and the timing and magnitude of the CO2 concentration peak

Carbon-dioxide (CO2) is the main contributor to anthropogenic global warming, and the timing of its peak concentration in the atmosphere is likely to be the major factor in the timing of maximum radiative forcing. Other forcers such as aerosols and non-CO2 greenhouse gases may also influence the timing of maximum radiative forcing. This paper approximates solutions to a linear model of atmospheric CO2 dynamics with four time-constants to identify factors governing the timing of its concentration peak. The most important emissions-related factor is the ratio between average rates at which emissions increase and decrease, which in turn is related to the rate at which the emissions intensity of CO2 is reduced. Rapid decarbonization of CO2 can not only limit global warming but also achieve an early CO2 concentration peak. The most important carbon cycle parameters are the long multi-century time-constant of atmospheric CO2, and the ratio of contributions to the impulse response function of atmospheric CO2 from the infinitely long lived and the multi-century contributions respectively. Reducing uncertainties in these parameters can reduce uncertainty in forecasts of the radiative forcing peak. A simple approximation for peak CO2 concentration, valid especially if decarbonization is slow, is developed. Peak concentration is approximated as a function of cumulative emissions and emissions at the time of the concentration peak. Furthermore peak concentration is directly proportional to cumulative CO2 emissions for a wide range of emissions scenarios. Therefore, limiting the peak CO2 concentration is equivalent to limiting cumulative emissions. These relationships need to be verified using more complex models of Earth system's carbon cycle. •Factors controlling the timing and magnitude of peak CO2 are examined.•Ratio between the rates of increase and decrease of emissions is important.•This ratio depends mainly on the timescale with which decarbonization occurs.•Simple approximation for peak CO2 concentration is developed.•Important carbon cycle parameters are identified.