Boron Isotope Based CO2 Reconstructions and Insights into the Sensitivity of the Climate System to CO2-forcing

Understanding the role of atmospheric CO₂ in driving global climate is one of the great challenges facing Earth Scientists today. Ultimately we are driven by the pressing need to predict how hot the Earth will get in the near future. Increasingly the palaeo-community has been willing to contribute to the debate, since the geological record is littered with examples of warm climate states, in part associated with elevated levels of atmospheric CO₂. Whilst these provide real world examples of how the Earth System responds to CO₂ forcing, no geological period is a perfect analogue for our warm future. Furthermore, climate models suggest that a significant level of state dependency exists for climate sensitivity. Therefore examining the response of the climate system to CO₂ forcing in the Eocene, for example, does not have direct relevance due to the different continental arrangements, ice sheet configurations and vegetation. When these factors are accounted for, it has however been recently shown that the available palaeo-data largely confirms the sensitivity of the Earth system to CO₂forcing determined by climate models alone [1].

Despite this overall conformity, there are several warmer time periods in the geological record (e.g. the Pliocene, the PETM) that apparently exhibit significantly elevated climate sensitivites (e.g. [1]). Importantly, these are above the range typically, but not always (e.g. [2]), achievable in climate models – clearly suggesting that the Earth can exist in states that appear to be particularly sensitive to CO₂ forcing. Such a state dependency to sensitivity has worrying implications for predictions of our warm future climate. In this contribution I will summarise the results of new boron isotope data from planktic foraminifera from a number of key periods. I will also focus on the challenges of generating accuract CO₂ data form boron isotope measurements and discuss methodologies to use such data to further probe the response of the Earth system to CO₂ change. This study provides new insights into the possibility of “hidden” feedbacks and the likelihood of a state dependency to climate sensitivity.

[1] Rohling, E.J., et al., 2012. Nature, 491: 683-691. [2] Stainforth, D.A. et al., 2005. Nature, 433(7024): 403-406.