How Well Do We Understand the Land-Ocean-Atmosphere Carbon Cycle?

Abstract

Since the beginning of the industrial age, human activities have increased the atmospheric concentrations of carbon dioxide (CO 2 ) and other greenhouse gases (GHGs) to levels never before seen in human history. These large increases are driving climate change because CO 2 is an efficient greenhouse gas with atmospheric residence times spanning years to millennia (see Box 6.1 of Ciais et al. [2013]). Bottom-up statistical inventories indicate that fossil fuel combustion, industry, agriculture, forestry, and other human activities are now adding more than 11.5 Pg of carbon (Pg C) to the atmosphere each year (Friedlingstein et al., 2019(Friedlingstein et al., , 2020(Friedlingstein et al., , 2021. Direct measurements of CO 2 in the atmosphere and in air bubbles in ice cores (Etheridge et al., 1996) indicate that human activities have increased the globally averaged atmospheric CO 2 dry air mole fraction from less than 277 parts