Abstract

The physical and biogeochemical controls of greenhouse gases (GHGs) is a  central motivation for this chapter, which identifies biogeochemical feedbacks that have led or could lead to a  future acceleration, slowdown or abrupt transitions in the rate of GHG accumulation in the atmosphere, and therefore of climate change. A  characterization of the trends and feedbacks lead to improved quantification for the remaining carbon budgets for climate stabilization, and the responses of the carbon cycle to atmospheric carbon dioxide removal (CDR), which is embedded in many of the mitigation scenarios, to achieve the goals of the Paris Agreement. Changes in the abundance of well-mixed GHGs –  carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) – in the atmosphere play a large role in determining the Earth’s radiative properties and its climate in the past, the present and the future (Chapters  2, 4, 6 and 7). Since 1950, the increase in atmospheric GHGs has been the dominant cause of the human-induced climate change (Section 3.3). While the main driver of changes in atmospheric GHGs over the past 200 years relates to the direct emissions from human activities, the net accumulation of GHGs in the atmosphere is controlled by biogeochemical source-sink dynamics of carbon that exchange between multiple reservoirs on land, oceans and atmosphere. The combustion of fossil fuels and land-use change for the period 1750–2019 released an estimated 700 ± 75 PgC (1 PgC = 1015 g of carbon) into the atmosphere, of which less than half remains in the atmosphere today (Sections  5.2.1.2; 5.2.1.5) (Friedlingstein et al., 2020). This emphasizes the central role of terrestrial and ocean CO2 sinks in regulating its atmospheric concentration (Ballantyne et al., 2012; W. Li et al., 2016; Le Quéré et al., 2018a; Ciais et al., 2019; Gruber et al., 2019b; Friedlingstein et al., 2020).