finding 1.4 : key-message-1-4

Net CO2 uptake by land and ocean removes about half of annually emitted CO2 from the atmosphere, helping to keep concentrations much lower than would be expected if all emitted CO2 remained in the atmosphere. The most recent estimates of net removal by the land, which accounts for inland water emissions of about 1 petagram of carbon (Pg C) per year, indicate that an average of 3.0 ± 0.8 Pg C per year were removed from the atmosphere between 2007 and 2016. Removal by the ocean for the same period was 2.4 ± 0.5 Pg C per year. Unlike CO2, CH4 has an atmospheric chemical sink that nearly balances total global emissions and gives it an atmospheric lifetime of about 9 to 10 years. The magnitude of future land and ocean carbon sinks is uncertain because the responses of the carbon cycle to future changes in climate are uncertain. The sinks may be increased by mitigation activities such as afforestation or improved cropping practices, or they may be decreased by natural and anthropogenic disturbances (high confidence).



This finding is from chapter 1 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

Description of evidence base: Net CO2 uptake by land and ocean removes about half of annually emitted CO2 from the atmosphere, helping to keep concentrations much lower than would be expected if all emitted CO2 remained in the atmosphere. The most recent estimates of net removal by the land, which accounts for inland water emissions of about 1 petagram of carbon (Pg C) per year, indicate that an average of 3.0 ± 0.8 Pg C per year were removed from the atmosphere between 2007 and 2016. Removal by the ocean for the same period was 2.4 ± 0.5 Pg C per year. Unlike CO2, CH4 has an atmospheric chemical sink that nearly balances total global emissions and gives it an atmospheric lifetime of about 9 to 10 years. The magnitude of future land and ocean carbon sinks is uncertain because the responses of the carbon cycle to future changes in climate are uncertain. The sinks may be increased by mitigation activities such as afforestation or improved cropping practices, or they may be decreased by natural and anthropogenic disturbances (high confidence).

New information and remaining uncertainties: The partitioning of carbon fluxes between land and ocean has significant uncertainty resulting from sparse observational coverage of atmospheric concentration and fluxes. Models of ocean-land carbon exchange must be evaluated against observations of carbon fluxes and storage in ecosystems, but in general there is not enough global coverage. Similarly, large regions that are important for understanding the global carbon budget, such as the tropics and Siberia, are not covered by atmospheric observations. This lack of observational coverage makes accurate estimates of the partition of carbon uptake between global land and ocean difficult to achieve using inverse modeling. Uncertainties in atmospheric transport models add to the problem of sparse observational coverage. Increased observational coverage offered by space-based instruments may improve the situation in the future, assuming technical limitations can be understood and overcome. The future evolution of the carbon cycle, including climate–carbon cycle feedbacks, is highly uncertain (e.g., Friedlingstein et al., 2014), and the use of inverse techniques to understand the carbon budget over recent decades could help to improve simulations of the future carbon budget. Future carbon cycle–climate feedbacks are expected to be positive (Ciais et al., 2013).

Assessment of confidence based on evidence: Observations and models clearly demonstrate that about half of annually emitted CO2 is absorbed by the terrestrial biosphere and by oceans. However, the exact partitioning between the land and ocean sinks is somewhat uncertain, while projections of the future of this uptake are highly uncertain.

Provenance
This finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation

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