Saturday, 14 February 2015: 8:30 AM-11:30 AM Physical Science of the Global Carbon Cycle as It Relates to Carbon Dioxide Removal Ken Caldeira  Carnegie Institution for Science, Stanford, CA 94305 USA kcaldeira@carnegiescience.edu

Carbon Dioxide Removal and the Natural Carbon Cycle Many carbon dioxide removal and storage proposals involve increasing rates of processes that already occur in the natural carbon cycle. Removing CO2 from the atmosphere causes outgassing of CO2 from the land biosphere and ocean. For storage times of many thousands of years or more, there must be a physical or chemical barrier preventing the CO2 from contacting the atmosphere.

CO2 stays in the atmosphere for a long time. Archer et al., 2009 CO2 stays in the atmosphere for a long time.

Carbon Cycle Time Scales Years to centuries CO2 taken up by biosphere CO2 + H2O  CH2O + O2 CO2 taken up oceans CO2 + H2O  H+ + HCO3- Thousands of years dissolution of carbonate minerals  oceans CO2 + CaCO3 + H2O  Ca2+ + 2 HCO3- Hundreds of thousands of years weathering of silicate rocks  carbonate rocks and sediments CO2 + CaSiO3  CaCO3 + SiO2

Carbon Cycle Time Scales Years to centuries CO2 taken up by biosphere CO2 + H2O  CH2O + O2 CO2 taken up oceans CO2 + H2O  H+ + HCO3- Thousands of years dissolution of carbonate minerals  oceans CO2 + CaCO3 + H2O  Ca2+ + 2 HCO3- Hundreds of thousands of years weathering of silicate rocks  carbonate rocks and sediments CO2 + CaSiO3  CaCO3 + SiO2

Carbon Cycle Time Scales Years to centuries CO2 taken up by biosphere CO2 + H2O  CH2O + O2 CO2 taken up oceans CO2 + H2O  H+ + HCO3- Thousands of years dissolution of carbonate minerals  oceans CO2 + CaCO3 + H2O  Ca2+ + 2 HCO3- Hundreds of thousands of years weathering of silicate rocks  carbonate rocks and sediments CO2 + CaSiO3  CaCO3 + SiO2

Carbon Cycle Time Scales Years to centuries CO2 taken up by biosphere CO2 + H2O  CH2O + O2 CO2 taken up oceans CO2 + H2O  H+ + HCO3- Thousands of years dissolution of carbonate minerals  oceans CO2 + CaCO3 + H2O  Ca2+ + 2 HCO3- Hundreds of thousands of years weathering of silicate rocks  carbonate rocks and sediments CO2 + CaSiO3  CaCO3 + SiO2

CO2 CaCO3 Ca2+ HCO3- Ridgwell, Kennedy and Caldeira (2003)

Carbon Cycle Time Scales Years to centuries CO2 taken up by biosphere CO2 + H2O  CH2O + O2 CO2 taken up oceans CO2 + H2O  H+ + HCO3- Thousands of years dissolution of carbonate minerals  oceans CO2 + CaCO3 + H2O  Ca2+ + 2 HCO3- Hundreds of thousands of years weathering of silicate rocks  carbonate rocks and sediments CO2 + CaSiO3  CaCO3 + SiO2

Carbon Cycle Time Scales Years to centuries CO2 taken up by biosphere CO2 + H2O  CH2O + O2 CO2 taken up oceans CO2 + H2O  H+ + HCO3- Thousands of years dissolution of carbonate minerals  oceans CO2 + CaCO3 + H2O  Ca2+ + 2 HCO3- Hundreds of thousands of years weathering of silicate rocks  carbonate rocks and sediments CO2 + CaSiO3  CaCO3 + SiO2

Geological carbon cycle Weathering of rocks Sediment burial Williams and Follows (2011)

Carbon Cycle Time Scales Years to centuries CO2 taken up by biosphere CO2 + H2O  CH2O + O2 CO2 taken up oceans CO2 + H2O  H+ + HCO3- Thousands of years dissolution of carbonate minerals  oceans CO2 + CaCO3 + H2O  Ca2+ + 2 HCO3- Hundreds of thousands of years weathering of silicate rocks  carbonate rocks and sediments CO2 + CaSiO3  CaCO3 + SiO2

CO2 stays in the atmosphere for a long time. Archer et al., 2009 CO2 stays in the atmosphere for a long time.

If we removed all excess CO2 from the atmosphere today, that would offset only about half the warming Cao et al 2011

Carbon dioxide removal proposals Caldeira et al., 2013

Carbon Dioxide Removal and the Natural Carbon Cycle Many carbon dioxide removal and storage proposals involve increasing rates of processes that already occur in the natural carbon cycle. Removing CO2 from the atmosphere causes outgassing of CO2 from the land biosphere and ocean. For storage times of many thousands of years or more, there must be physical or chemical barrier preventing the CO2 from contacting the atmosphere.