Moderate levels of Eocene pCO2 indicated by Southern Hemisphere fossil plant stomata

Margret Steinthorsdottir,Mike Pole,Vivi Vajda,Guy Holdgate

doi:10.1130/g46274.1

Abstract

Abstract Reducing the uncertainty in predictions of future climate change is one of today’s greatest scientific challenges, with many significant problems unsolved, including the relationship between pCO2 and global temperature. To better constrain these forecasts, it is meaningful to study past time intervals of global warmth, such as the Eocene (56.0–33.9 Ma), serving as climatic analogues for the future. Here we reconstructed pCO2 using the stomatal densities of a large fossil Lauraceae (laurel) leaf database from ten sites across the Eocene of Australia and New Zealand. We show that mostly moderate pCO2 levels of ∼450–600 ppm prevailed throughout the Eocene, levels that are considerably lower than the pCO2 forcing currently needed to recreate Eocene temperatures in climate models. Our data record significantly lower pCO2 than inferred from marine isotopes, but concur with previously published Northern Hemisphere Eocene stomatal proxy pCO2. We argue that the now globally consistent stomatal proxy pCO2 record for the Eocene is robust and that climate sensitivity was elevated and/or that additional climate forcings operated more powerfully than previously assumed.

Highlights

The anthropogenic rise in CO2 concentrations is predicted to result in a global average temperature increase of up to 4 °C by the year 2100 (IPCC, 2014), with severe socioeconomic and ecosystem impacts predicted
Scientists look to past time intervals of global warming, which can serve as “climatic analogues”, for clues about our future
Three methods of stomatal proxy paleo-pCO2 reconstructions are currently in use: 1. The semiquantitative empirical stomatal ratio method, which utilizes the ratio between the stomatal indices (SI) of fossil plants and the SI of extant nearest living relatives or equivalents (NLRs or nearest living equivalents (NLEs)), grown in known pCO2, to estimate paleo-pCO2 (McElwain, 1998); 2

Summary

Introduction

The anthropogenic rise in CO2 concentrations (pCO2) is predicted to result in a global average temperature increase of up to 4 °C by the year 2100 (IPCC, 2014), with severe socioeconomic and ecosystem impacts predicted. We contribute to this quest by presenting a new terrestrial record of Eocene pCO2 using a stomatal proxy method of paleo-pCO2 reconstruction.

Results

Conclusion