E.7 Carbon and Other Biogeochemical Cycles (future change)

Climate change will affect carbon cycle processes in a way that will exacerbate the increase of CO2  in the atmosphere (high confidence). Further uptake of carbon by the ocean will increase ocean acidification. {6.4}

 

  • Ocean uptake of anthropogenic CO2  will continue under all four RCPs through to 2100, with higher uptake for higher concentration pathways (very high confidence). The future evolution of the land carbon uptake is less certain. A majority of models project a continued land carbon uptake under all RCPs, but some models simulate a land carbon loss due to the combined effect of climate change and land use change. {6.4}

 

  • Based on Earth System Models, there is high confidence that the feedback between climate and the carbon cycle is positive in the 21st century; that is, climate change will partially offset increases in land and ocean carbon sinks caused by rising atmospheric CO2. As a result more of the emitted anthropogenic CO2 will remain in the atmosphere. A positive feedback between climate and the carbon cycle on century to millennial time scales is supported by paleoclimate observations and modelling. {6.2, 6.4}

 

  • Earth System Models project a global increase in ocean acidification for all RCP scenarios. The corresponding decrease in surface ocean pH by the end of 21st century is in the range13 of 0.06 to 0.07 for RCP2.6, 0.14 to 0.15 for RCP4.5, 0.20 to 0.21 for RCP6.0 and 0.30 to 0.32 for RCP8.5 (see Figures SPM.7 and SPM.8). {6.4}

 

  • Cumulative CO2 emissions20 for the 2012–2100 period compatible with the RCP atmospheric CO2 concentrations, as derived from 15 Earth System Models, range from 140 to 410 GtC for RCP2.6, 595 to 1005 GtC for RCP4.5, 840 to 1250 GtC for RCP6.0, and 1415 to 1910 GtC for RCP8.5 (see Table SPM.3). {6.4}

 

Table SPM.3: Cumulative CO2 emissions for the 2012–2100 period compatible with the RCP atmospheric concentrations simulated by the CMIP5 Earth System Models. {6.4, Table 6.12}

 

  • By 2050, annual CO2  emissions derived from Earth System Models following RCP2.6 are smaller than 1990 emissions (by 14% to 96%) (see Figure TS.19). By the end of the 21st century, about half of the models infer emissions slightly above zero, while the other half infer a net removal of CO2 from the atmosphere. {6.4}

 

  • The release of CO2 or CH4 to the atmosphere from thawing permafrost carbon stocks over the 21st century is assessed to be in the range of 50 to 250 GtC for RCP8.5 (low confidence). {6.4}
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