Global Analysis of Ginkgo biloba Stomatal Frequencies and other Key Traits shows no Influence by Climate, indicating that the Stomatal Paleo-pCO2 Proxy is Robust.

The aim of the research project 'Global Ginkgo' was to improve the stomatal proxy for atmospheric concentrations of CO2 (pCO2) using the "living fossil" Ginkgo biloba - arguably the most important plant species on Earth for paleoclimate research focusing on the relationship between climate and pCO2. In order to improve future predictions of climate change, Earth's climate sensitivity (the exact relationship between pCO2 and temperature) needs to be better understood. The expected warming for a doubling of pCO2 is under-constrained by current models at between 2 °C and 5 °C, with limited success in reducing this range despite decades of research. Therefore, scientists turn to past observations, integrating climate modelling with records of past greenhouse global warming using proxy reconstructions from geological archives. Studying Cenozoic intervals of global warming is an important tool for this, but often climate models cannot reproduce temperatures recorded using the moderate pCO2 reconstructed by proxies. This suggests that either climate sensitivity was elevated, that current climate models are missing important forcings, and/or that proxies systematically underestimate pCO2 during these warm periods. We tested a large global dataset of G. biloba as a tool to explore this mismatch. Importantly, we tested whether trees growing at higher temperatures recorded a lower pCO2 than those growing in colder climates, since this could help explain the proxy's apparent underestimates during Cenozoic warm episodes. pCO2 was reconstructed using the three stomatal proxy methods currently in use, as well as the more recently proposed C3 plant proxy. All the stomatal proxy methods reproduced ambient pCO2 reasonably well and consistently with each other, whereas the C3 proxy underestimated pCO2. Further, our data analysis found very high natural variability in key traits, but that neither high temperatures nor other climate parameters resulted in underpredictions of pCO2. We conclude that the lack of climatic control on stomatal parameters strengthens the reliability of the stomatal proxy for pCO2 reconstructions and from this infer that most likely climate sensitivity was elevated during warm episodes in the Cenozoic.