Abstract

Atmospheric CO2 is an important greenhouse gas, with elevated atmospheric CO2 levels contributing to anthropogenic climate change in the present day. Changing atmospheric CO2 levels have contributed to natural variations in past climate, such as during the Pleistocene glacial-interglacial cycles. Many geological and biogeochemical processes affect atmospheric CO2 levels over a range of timescales [e.g.1,2], from volcanic outgassing and silicate weathering on 106 year timescales to the seasonal balance between photosynthesis and respiration on sub-annual timescales. Numerical models of the Earth system are used to simulate atmospheric CO2 levels. However, the model output is often too complex to gain an understanding of why the model simulation achieves a particular atmospheric CO2 level. In particular, the non-linearity introduced when CO2 dissolves in seawater is problematic. Here, atmospheric CO2 is analytically related to biogeochemical processes on 100 to 100,000 year timescales by approximating aspects of ocean carbonate chemistry³. These approximate analytical solutions are in good agreement with numerical models, helping to both predict and understand their behaviour.

[1] Ridgwell and Zeebe (2005). Earth Plan. Sci. Letters, Vol 234 p299-315 [2] Archer et al (2009). Annu. Rev. Earth Planet. Sci. 2009. 37:117–34 [3] Goodwin et al (2008). Glob. Biogeochem. Cycles. doi:10.1029/2008GB003184