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

Variability in the Global Ocean Carbon Sink From 1959 to 2020 by Correcting Models With Observations

“…(2020) estimates the global‐mean air‐sea CO 2 flux that occurs in response to the observed growth of atmospheric pCO 2 . It also has the option to include upper ocean heat content anomalies driven by the three most climatically impactful volcanic eruptions of the last 60 years: Agung in 1963, El Chichon in 1982, and Mt Pinatubo in 1991 (Crisp et al., 2022). Comparing air‐sea CO 2 fluxes estimated by the box model for 1960–2019 allows consideration of flux variability with and without large volcanic influences and puts LDEO‐HPD into context with previous comparisons of the box model to observation‐based products (McKinley et al., 2020).…”

Explicit Physical Knowledge in Machine Learning for Ocean Carbon Flux Reconstruction: The pCO2‐Residual Method

“…This is the net CO 2 flux ( F net = F natural + F ant ) (Crisp et al., 2022). The natural outgassing of riverine carbon is understood to be the dominant component of F natural (Aumont et al., 2001; Crisp et al., 2022; Hauck et al., 2020). Additional non‐riverine components such as outgassing due to ocean circulation change have been proposed for 1994–2007 (Gruber et al., 2019), with large uncertainty in magnitude and no evidence of long‐term impact (Crisp et al., 2022); these are assumed to be zero here.…”

“…Observation‐based products that incorporate observations of surface ocean pCO 2 include both natural and anthropogenic carbon in the resulting pCO 2 and CO 2 flux product. This is the net CO 2 flux ( F net = F natural + F ant ) (Crisp et al., 2022). The natural outgassing of riverine carbon is understood to be the dominant component of F natural (Aumont et al., 2001; Crisp et al., 2022; Hauck et al., 2020).…”

“…Since the beginning of the Industrial Revolution, the ocean has absorbed about a third of the total anthropogenic emissions (Khatiwala et al., 2013; Sabine et al., 2004). The processes governing the large‐scale distribution of ocean pCO 2 and the drivers of seasonality are well understood (Crisp et al., 2022; Takahashi et al., 2002, 2009). Yet, the quantification of year‐to‐year variability and long‐term changes in this carbon sink remains a challenge (Crisp et al., 2022; Hauck et al., 2020; McKinley et al., 2016, 2017).…”

“…The processes governing the large‐scale distribution of ocean pCO 2 and the drivers of seasonality are well understood (Crisp et al., 2022; Takahashi et al., 2002, 2009). Yet, the quantification of year‐to‐year variability and long‐term changes in this carbon sink remains a challenge (Crisp et al., 2022; Hauck et al., 2020; McKinley et al., 2016, 2017). This quantification is necessary for climate policies worldwide in order to separate the impact of any mitigation policies from interannual variability in the ocean carbon sink (Peters et al., 2017).…”