During the last 540 million years, Earth’s climate has oscillated between three basic states: Icehouse, Greenhouse (subdivided into Cool and Warm states), and Hothouse (Kidder & Worsley, 2010). The “Hothouse” condition is relatively short-lived and is consequence from the release of anomalously large inputs of CO2 into the atmosphere during the formation of Large Igneous Provinces (LIPs), when atmospheric CO2 concentrations may rise above 16 times (4,800 ppmv), while the “Icehouse” is characterized by polar ice, with alternating glacial–interglacial episodes in response to orbital forcing. The ‘Cool Greenhouse” displays some polar ice and alpine glaciers, with global average temperatures between 21° and 24°C. Finally, the ‘Warm Greenhouse’ lacked of any polar ice, and global average temperatures might have ranged from 24° to 30°C.
Reconstructions of Earth’s history have considerably improved our knowledge of episodes of rapid emissions of greenhouse gases and abrupt warming. Consequently, the development of different proxy measures of paleoenvironmental parameters has received growing attention in recent years.
The early Eocene was characterized by a series of short-lived episode of global warming, superimposed on a long-term early Cenozoic warming trend. Atmospheric CO2 was the major driver of the overall warmth of the Eocene. For the Paleocene-Eocene Thermal Maximum (PETM; 55.8 million years ago), and the Early Eocene Climate Optimum (EECO; 51 to 53 million years ago) the transient rise of global temperatures has been estimated to be 4 to 8° (Hoffman et al., 2012).
Reconstructions using multiple climate proxy records, identified the EECO as the warmest interval of the past 65 million years. One such proxy measure is the stomatal frequency of land plants, which has been shown in some species to vary inversely with atmospheric pCO2 and has been used to estimate paleo-pCO2 for multiple geological time periods. Stomata are the controlled pores through which plants exchange gases with their environments, and play a key role in regulating the balance between photosynthetic productivity and water loss through transpiration. (Smith et al., 2010).
Pollen and other palynomorphs proved to be an extraordinary tool to palaeoenvironmental reconstruction. Terrestrial microflora from the EECO indicates a time period with warm and humid climatic conditions and displays a higher degree of tropicality than the microflora of the PETM.
A new high-fidelity record of CO2 can be obtained by using the boron isotope of well preserved planktonic foraminifera. The boron isotopic composition of seawater is also recquiered to estimate the pH. The global mean surface temperature change for the EECO is thought to be ~14 ± 3 °C warmer than the pre-industrial period, and ~5 °C warmer than the late Eocene.
Since the start of the Industrial Revolution the anthropogenic release of CO2 into the Earth’s atmosphere has increased a 40%. Glaciers from the Greenland and Antarctic Ice Sheets are fading away, dumping 260 billion metric tons of water into the ocean every year. The ocean acidification is occurring at a rate faster than at any time in the last 300 million years, and the patterns of rainfall and drought are changing and undermining food security which have major implications for human health, welfare and social infrastructure. These atmospheric changes follow an upward trend in anthropogenically induced CO2 and CH4. If fossil-fuel emissions continue unstoppable, in less than 300 years pCO2 will reach a level not present on Earth for roughly 50 million years.
Eleni Anagnostou, Eleanor H. John, Kirsty M. Edgar, Gavin L. Foster, Andy Ridgwell, Gordon N. Inglis, Richard D. Pancost, Daniel J. Lunt, Paul N. Pearson. Changing atmospheric CO2 concentration was the primary driver of early Cenozoic climate. Nature, 2016; DOI: 10.1038/nature17423
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Robin Y. Smith, David R. Greenwood, James F. Basinger; Estimating paleoatmospheric pCO2 during the Early Eocene Climatic Optimum from stomatal frequency of Ginkgo, Okanagan Highlands, British Columbia, Canada; Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 293, Issues 1–2, 1 (2010).