The first direct evidence supporting the idea that a recently-discovered period of global warming, one of the hottest in Earth’s history, was caused by CO2 has been published this week. Before the Middle Eocene Climatic Optimum (MECO), which occurred 40 million years ago, temperatures were much higher than today, but steadily falling. However, the MECO was a 400,000 year warming reversal of this trend. Researchers from the Netherlands and UK have now shown that it was accompanied by at least a doubling in atmospheric CO2 levels. Utrecht Univesity’s Peter Bijl and his colleagues also provide cautious estimates for just how much warming can be expected from adding CO2 in top journal Science this week.
Scientists already suspected that the approximately 4°C MECO temperature rise was caused by CO2. “There are only three ways to cause a large and lasting increase in Earth’s average surface temperature,” pointed out Paul Pearson from Cardiff University, UK, who was not involved in the study. “Turn up the heat from the Sun, reflect less sunlight back into space, or trap more heat in the atmosphere.” The heat from the Sun is relatively stable, Pearson comments in a separate article in this week’s Science giving his perspective on the research. Changes in the Earth’s reflectivity due to melting ice tend to happen after climate change has already begun, making greenhouse gases that trap heat around the Earth a likely culprit.
To test this suspicion, Bijl and coworkers studied a column of rock drilled from the sea bed near Antarctica. They measured atmospheric CO2 by looking at the ratio between a radioactive isotope of carbon and its normal isotope in chemicals called alkenones, produced by a type of marine algae. “This proxy is useful because the carbon isotope ratio is related to the dissolved CO2 concentration in seawater, which in turn is in equilibrium with the atmosphere in most places,” Pearson explains. “The method works well in more recent geological periods and in the modern ocean, so it seems reasonable to apply it to the MECO, if enough alkenones can be found.”
Bijl’s team found clear evidence of MECO warming, and relatively high alkenone levels showed similar temperature and CO2 profiles, with a matching peak in each. They found that the baseline CO2 levels in the broader Eocene period were around 1000 to 2000 parts per million (ppm). During the temperature peaks atmospheric CO2 levels reached 4000 ppm or higher, backing the theory of the greenhouse gas cause. By comparison, current atmospheric CO2 concentrations have grown from around 280 parts per million (ppm) before the industrial revolution to almost 390 ppm today.
These findings allowed the scientists to estimate the Earth’s CO2 “climate sensitivity” – the global temperature change caused by a doubling of CO2 – during this period. While this calculation is consistent with recent models that predict sea surface temperature increases of 2-5ºC for a doubling of carbon dioxide, Bijl and his colleagues concede that their temperature rise values are “tentative”. That’s because so far the evidence is from just a single site and the CO2 proxy data are limited, the global temperature change and the CO2 estimates are based on a number of assumptions, plus the data are not completely consistent with more detailed temperature records. “Data from more sites, and other CO2 proxies, are urgently required to flesh out the details,” Pearson commented.