This geological outcrop in Argentina is a particularly rich location for siderite crystals. These minerals make it possible to reconstruct the climate 55 million years ago. Photo: Joep van Dijk / ETH Zürich
This news release was adapted from a story produced by ETH Zürich, the Swiss Federal Institute of Technology in Zurich
About 55 million years ago, the Earth’s atmosphere was flooded by carbon dioxide, with concentrations of the greenhouse gas reaching levels high enough that surface temperatures may have resembled those of a sauna. It was hot and humid, and the ice on the polar caps had completely disappeared.
Using tiny bits of iron compounds in soil samples taken from former swamps, a group of researchers led by ETH Zürich and including researchers from Pennsylvania State University, CASP (formerly known as Cambridge Arctic Shelf Programme) and Colorado State University reconstructed the climate that prevailed as the geological epoch known as the Paleocene gave way to another era, the Eocene.
Through this work, the team created a new dataset which not only provides soil temperatures in the late Paleocene and early Eocene but also the oxygen isotope composition of precipitation, which is a tracer for the hydrological processes that both respond to and drive extreme polar warmth.
Jeremy Rugenstein, assistant professor in the Department of Geosciences at CSU, said these data will allow scientists to better assess how our climate might look in the future. The findings suggest that today’s global warming goes hand-in-hand with increased transport of moisture, and by extension heat, in the atmosphere.
“With this new dataset, climate modelers can simulate future climates and compare the projected temperatures with our reconstructed temperatures from a period of extreme warmth,” he said. “It will also allow researchers to compare simulations of the hydrological cycle with our oxygen isotope data, to see if the models can accurately simulate the mechanisms that drive extreme warmth.”
The study, “Spatial pattern of super-greenhouse warmth controlled by elevated specific humidity,” is published Oct. 26 in the journal Nature Geoscience.
Iron crystals store details on temperature, moisture
The iron carbonate known as siderite is composed of one iron atom, one carbon and three oxygen atoms. During the growth of these crystals, different carbon and oxygen isotopes are incorporated into the crystal lattice, depending on soil temperature. The growing crystals also store information about atmospheric moisture and the climate of the epoch in which they are growing.
Researchers can “read” this information by determining the isotopic composition of the crystals in the laboratory. They can also draw conclusions about the moisture content of the atmosphere and the air temperature as the crystals formed.
To reconstruct the climatic conditions from the equator to the polar regions, the researchers studied siderites from 13 different sites in the Northern Hemisphere, covering all geographical latitudes from the tropics to the Arctic.
The climate in that era provides researchers with an indication as to how today’s climate might develop. While preindustrial levels of atmospheric CO2 stood at 280 parts per million, today’s measure 412 ppm. Climate scientists believe that CO2 emissions generated by human activity could drive this figure up to 1,000 ppm by the end of the century, similar to levels experienced 55 million years ago.
Finding the siderites was not easy. The minerals are tiny, and they occur solely in fossil swamps, which today are often found only several kilometres, or thousands of feet, below the Earth’s surface. This made it difficult or even impossible for the researchers to dig up siderites themselves.
Fortunately, one of the study’s co-authors – Tim White, a research professor from Pennsylvania State University – owns the world’s largest collection of siderite.
Prevailing high humidity
Lead author Joep van Dijk, who completed his doctoral degree at the ETH Zürich, said that the reconstruction of the climate based on the siderite samples shows that a hot atmosphere also comes with high levels of moisture.
By measuring the oxygen isotope composition of the siderites, the researchers also demonstrated that the global moisture content in the atmosphere, or the specific humidity, was much higher in the Paleocene and Eocene eras than it is today. In addition, water vapor remained in the air longer because specific humidity increased at a greater rate than evaporation and precipitation. However, the increase in specific humidity was not the same everywhere.
Since the research team had access to siderite from all latitudes, the scientists were also able to study the spatial pattern of the humidity. They found that the tropics and higher latitudes would have had very high humidity levels.
The researchers attribute this phenomenon to water vapor that was transported to these zones from the subtropics, where specific humidity rose the least. While evaporation increased, precipitation decreased. This resulted in a higher level of atmospheric water vapor, which ultimately reached the poles and the equator, carrying heat along with it.
Climate scientists still observe the flow of water vapor – and accompanying latent heat – from the subtropics to the tropics today.
“Latent heat transport was likely to have been even greater during the Eocene,” van Dijk said. “And the increase in the transport of heat to high latitudes may well have been conducive to the intensification of warming in the polar regions.”