This article was originally published in "Chemistry Review", a journal written for A-level students with articles that will broaden their understanding of chemistry.
Burning of fossil fuels has increased the concentration of carbon dioxide and other greenhouse gases in the earth’s atmosphere over the last 200 years. If we continue to burn fossil fuels at current rates, by the end of the century, the average temperature of the earth will have increased by 3 to 5°C (Figure 1). This is a worldwide challenge which will have potentially devastating consequences. For example, changes in temperature will impact the production of crops and the availability of freshwater while low-lying regions, such as the Netherlands, are more likely to be affected by rising sea level1. To understand how the earth will respond to increased carbon dioxide (CO2) concentrations, we can go back in time to a period when CO2 was much greater than today. To investigate this we use biomarkers or ‘molecular fossils’. These are organic compounds which have a definitive biological origin and can be preserved in the sedimentary record over millions of years2. Using these compounds, we can then use the past to better predict the future.
Figure 1: Global average surface temperature change from 2006 to 2100 as determined by multi-model simulations. All changes are relative to 1986–2005. Time series of projections and a measure of uncertainty (shading) are shown for scenarios RCP2.6 (blue) and RCP8.5 (red). Source: IPCC AR5
The Eocene epoch
The Eocene epoch was 34-56 million years ago (Ma). For the majority of the Eocene, Antarctica was attached to South America and Australia3,4 (Figure 2). Sea level was much higher, around 70 to 120 m above today’s level5. There were no large ice sheets on Antarctica or Greenland6,7. Carbon dioxide estimates, derived from the shells of tiny marine micro-organisms, are greater than 800 parts per million (ppm)8. As a result, the Earth was a much warmer planet. Part of the evidence for this extreme warmth is biological. Fossilised spores of Macadamia, palm and baobab trees have been found in the Antarctic and can only live in warm and wet subtropical climates7. Fossils of the Hippo-like Coryphodon and pig-like Tapir have been found in the Arctic, along with 13-metre long snakes and giant crocodilians6. These fossils provide a snapshot of the climate; however, long-term records of temperature are required to unravel ancient climate change.
Figure 2: Orography and bathymetry for the early Eocene. Credit: Lunt et al., 2016 (GMD)
How do you measure temperature millions of years ago? We can use biomarkers or ‘molecular fossils’. Biomarkers are organic molecules which are derived from living organisms. Such compounds are very resistant to degradation over millions of years and can be retrieved from sedimentary rocks using traditional chemical techniques.
One set of biomarkers are glycerol dialkyl glycerol tetraethers (GDGTs: Figure 3) which are present in most marine settings. GDGTs are produced by single-celled marine organisms (Thaumarchaeota) and change their structure depending upon the temperature of the ocean9. Experimental10 and field-based11 studies indicate that molecules with fewer rings are produced in cooler waters whereas those with more rings are produced in warmer waters. A simple index, based upon the number of rings (the TEX86), can be used to estimate the temperature of the ocean in the past.
Isoprenoidal GDGTs used to calculate TEX86 sea surface temperature estimates.
When results from Eocene sedimentary rocks are analysed, two periods of temperature change are observed. The early Eocene (56 to 48 Ma) was very warm and relatively stable (Figure 4). During this interval, polar regions, such as East Antarctica, exceeded 20°C during summer months and prevented the growth of ice sheets. Higher temperatures were likely caused by higher concentrations of carbon dioxide (>1000 ppm) as indicated by other geochemical records12. During the middle and late Eocene (48-34 Ma) carbon dioxide concentrations began to decrease and this apparently caused a decrease in sea surface temperatures13-15. (Figure 4). During this interval, a stronger temperature gradient develops between the tropics and the polar regions. By the end of the Eocene (~34 Ma), CO2 concentrations became low enough to allow ice sheets to grow on Antarctica16.
Figure 4: Relative changes in high southern latitude sea surface temperature during the Eocene derived from molecular fossils (TEX86). Credit: Bijl et al., 2009; Hollis et al., 2009; Bijl et al., 2013
Studying the Eocene is particularly useful because it allows us to better constrain climate sensitivity (i.e. the degree of warming associated with a doubling in carbon dioxide concentrations). Recent studies suggest that during the Eocene, a doubling of CO2 is associated with ~2.1–4.6 °C warming17. Although this is not a straightforward measurement18, it does agree with predictions of future climate change1.
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