Near‐Future <i>pCO<sub>2</sub></i> During the Hot Miocene Climatic Optimum
Margret Steinthorsdottir, Phillip E. Jardine, William C. Rember
Abstract
Abstract To improve future predictions of anthropogenic climate change, a better understanding of the relationship between global temperature and atmospheric concentrations of CO 2 ( p CO 2 ), or climate sensitivity, is urgently required. Analyzing proxy data from climate change episodes in the past is necessary to achieve this goal, with certain geologic periods, such as the Miocene climatic optimum (MCO), a transient period of global warming with global temperatures up to ~7°C higher than today, increasingly viewed as good analogues to future climate under present emission scenarios. However, a problem remains that climate models cannot reproduce MCO temperatures with less than ~800 ppm p CO 2 , while most previously published proxies record p CO 2 < 450 ppm. Here, we reconstructed MCO p CO 2 with a multitaxon fossil leaf database from the well‐dated MCO Lagerstätte deposits of Clarkia, Idaho, USA, using four current methods of p CO 2 reconstructions. The methods are principally based on either stomatal densities, carbon isotopes, or a combination of both—thus offering independent results. The total of six reconstructions mostly record p CO 2 of ~450–550 ppm. Although slightly higher than previously reconstructed p CO 2 , the discrepancy with the ~800 ppm required by climate models remains. We conclude that climate sensitivity was heightened during MCO, indicating that highly elevated temperatures can occur at relatively moderate p CO 2 . Ever higher climate sensitivity with rising temperatures should be very seriously considered in future predictions of climate change.