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Previous Projects: Palaeoatmospheric CO2

Carbon dioxide (CO2) is a critically important greenhouse gas that has risen by over 35% in the last hundred years, due to fossil fuel burning and land use change. It is anticipated under a ‘business as usual’ scenario that atmospheric CO2 concentration will reach 500 ppmV by 2050 and 700mmV by 2100. Climate models predict that global mean temperatures will increase by ~4°C by the year 2100 as a direct consequence of raised atmospheric CO2 levels. In order to project the potential biological and climatic consequences of future increases in atmospheric carbon dioxide, time intervals in the past of high global warmth can serve as ‘climatic analogues’ for our future. The Eocene period in Earth history (~55 to 34 million years ago (m.y.a.)) was one such time interval, with average global temperatures 4 to 12°C higher than present. However, available estimates for atmospheric CO2 concentration for the Eocene and entire Tertiary period (last 65 million years) are highly contradictory, with reports of both highly elevated (three times present levels) and similar to modern ambient CO2 levels. Similarly high variability in estimated CO2 levels exists for the Oligocene (34 to 24 mya), Miocene (24 to 5 m.y.a.) and Pliocene (5 to 2 m.y.a.). These varying estimates for Tertiary CO2 concentrations have led to arguments for both coupling and uncoupling of CO2 concentration and global climate on geological time scales.

This uncertainty in long-term CO2 records, particularly for the past 65 million years has severely hampered our ability to project the biotic and abiotic consequences of future CO2 rise using ‘past climatic analogues’ such as the Eocene. Put in more simple terms, there is uncertainty on the role of past CO2 fluctuations in the modulation of global temperature trends over the past 65 million years. This Science Foundation Ireland funded PhD research project addressed this uncertainty by developing two parallel records of Tertiary CO2 fluctuations from one extinct gymnosperm genus (Quasisequoia) and one extant angiosperm genus using the stomatal-CO2 proxy method. This method used a well documented inverse relationship between atmospheric CO2 concentration and the frequency of stomata of the leaf surface (measured using stomatal density: number of stomata per mm2 leaf area, and stomatal index (SI): ratio of stomata to total number of cells on leaf surface) to estimate past atmospheric CO2 levels from fossil plant stomatal frequency. The project was co-supervised by Jennifer McElwain, Margaret Collinson, Royal Holloway University of London and Lutz Kunzmann (des Staatlichen Museums für Mineralogie and Geologie, Dresden ).



UCD Plant Palaeoecology and Palaeobiology Group Updated: July 2013
Professor J.C. McElwain