Roy Warren Spencer

Global Carbon Project (GCP) data shows that natural processes have been sequestering atmospheric CO2 on a yearly basis in proportion to how much the atmospheric CO2 concentration has risen above pre-Industrial levels, the so-called CO2 “sink rate”. Here it is argued that the future trajectory of the sink rate has not been adequately addressed, which has led to overestimation of future atmospheric CO2 concentrations, and thus of global warming. Additionally, use of the CO2 “airborne fraction” concept has led to some misunderstanding regarding how natural processes remove CO2 from the atmosphere, including unrealistic projections of future sink rates. The 20 land models and 10 ocean models used to estimate rates of CO2 removal from the atmosphere produce a wide variety of results. The GCP averages all of these model results together to obtain a best estimate of the yearly CO2 fluxes. Based upon this average, assuming a linearly declining sink rate into the future derived from GCP data, emissions reductions of only 1% per year totaling 39% below 2023 emissions are required over the next 50 years to stabilize atmospheric CO2 near 457 ppm. Assuming the IPCC best estimate of climate sensitivity of 3 deg. C to a hypothetical doubling of atmospheric CO2, this would meet the 2015 Paris Agreement target of less than 2 deg. C of eventual global-average surface warming. But if observation-based estimates of climate sensitivity around 2 deg. C are assumed, then the 1.5 deg. C Paris goal is easily met. These results, though, are very dependent upon the assumed linear decrease of the future sink rates.