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Warm and Wet or Hot and Dry Future Earth? The Role of CO2 in Climate Change Explored with Fossil Plants

Photo: Margret Steinthorsdottir

An Early Miocene (ca. 20 million years old) leaf, Victoria, Australia.


CO2 is a principal greenhouse gas that is increasing in the atmosphere at an unprecedented rate due to human activity, causing global climate change. It is predicted to continue to rise further and result in a perilous temperature increase of up to 4°C by the year 2100, but the relationship between concentrations of CO2 and temperature – the so-called climate sensitivity – is still not well understood.

Existing long-term CO2 records and climate models still lack the resolution to precisely constrain the role of CO2 in climate change, severely impeding our ability to project the consequences of future CO2 rise.This research project aims to reduce these uncertainties, by studying selected intervals of the Cenozoic era (the last 66 million years), which will serve as ‘climate change analogues’ for the future.

Stomata are pores on plant leaf surfaces, through which gas exchange takes place, i.e. CO2 is acquired for photosynthesis and water vapour and oxygen are lost by diffusion. An inverse relationship exists between the density of stomata and CO2, due to plants aiming to preserve water by reducing the density of stomata when CO2 is abundantly available. The inverse relationship can consequently be employed to reconstruct CO2 concentrations in the past, using fossil leaves – this is called the stomatal proxy. In addition to tracking rising CO2 levels, decreases in stomatal densites suppress the plant transpiration stream, potentially with major implications for Earth’s hydrological cycle. Therefore, CO2 has a unique dual role in global climate change.

The inverse relationship between stomatal density and CO2 in the atmosphere allows the reconstruction of palaeo-CO2, using fossil plant leaves. When stomatal density (and/or stomatal size) decreases, plants take up less water into the transpiration stream, leading in some cases to excess surface water, or runoff (M. Steinthorsdottir, unpublished. Illustration by Steinthor Thoroddsson).

The main focus of this project is on transitions of rapid climate change and/or extreme warmth periods, such as the Mid Miocene Climatic Optimum (MMCO, ca. 16 million years ago), the Eocene (ca. 56-34 million years ago), the Eocene-Oligocene climate transition (EOT, ca. 34 million years ago) and the Paleocene-Eocene Thermal Maximum (PETM, ca. 56 million years ago). New high-resolution stomatal proxy CO2 records are under way, using the stomatal proxy method with fossil plant leaves, collected from the USA, Europe, Australia and New Zealand.

Fieldwork – collecting leaves from the Clarkia lagerstätte deposit, Idaho, USA, in summer 2017. The Clarkia flora was deposited during the Mid Miocene Climatic Optimum, ca. 16 million years ago.

Gaining deeper understanding of the dual role of CO2 in climate change is paramount to allow us to assess the likely repercussions on present and future ecosystems and biodiversity, and suggest strategies to mitigate these. Will our future Earth be warm and humid or hot and dry? Will it be both simultaneously in different regions? Or mostly warm and humid initially, then hot and dry ultimately? What thresholds crossed enable global climate to move from one state to another? Studying fossil plant stomata may help provide some of the answers.

This research is funded by the Swedish Research Council (VR): Starting grant NT7-2016 04905

External Project Participants

University of Idaho, USA

Selected Publications

Steinthorsdottir, M., Vajda, V. & Pole, M., 2018. Significant transient pCO2 perturbation at the New Zealand Oligocene-Miocene transition recorded by fossil plant stomata. Palaeogeography, Palaeoclimatology, Palaeoecology 98: 49-69.

Steinthorsdottir, M., Vajda, V., Pole, M., 2016. Global trends of pCO2 across the Cretaceous-Paleogene boundary supported by the first Southern Hemisphere stomatal proxy-based pCO2 reconstruction. Palaeogeography, Palaeoclimatology, Palaeoecology 464:143-152.

Steinthorsdottir, M., Porter, A., Holohan, A., Kunzmann, L., Collinson, M., and McElwain, J.C., 2016. Fossil plant stomata indicate decreasing atmospheric CO2 prior to the Eocene-Oligocene boundary. Climate of the Past, 12, 439-454.

Steinthorsdottir, M., Woodward, F.I., Surlyk, F. and McElwain, J.C., 2012. Deep-time evidence of a link between elevated CO2 concentrations and perturbations in the hydrological cycle via drop in plant transpiration. Geology 40, 815-818.

McElwain, J. C. & Steinthorsdottir, M., 2017. Palaeoecology, ploidy, palaeoatmospheric composition and developmental biology: A review of the multiple uses of fossil stomata. Plant Physiology linkuexternal linkllexternal link.pdf+htmlexternal link

Mays, C., Steinthorsdottir, M., and Stillwell, J.D., 2015. Climatic implications of Ginkgoites waarrensis Douglas emend. from the south polar Tupangi fora, Late Cretaceous (Cenomanian), Chatham Islands. Palaeogeography, Palaeoclimatology, Palaeoecology 439: 308-326.

A laurel leaf from the Clarkia deposit, displaying the original autumnal red colour.

Clarkia Betula (birch) leaf cuticle surface under the microscope – stomata are marked with dots.