Images of foraminifera taken with a scanning electron microscope. Photo credit: Dustin Harper
At the end of the Paleocene and the beginning of the Eocene, between 59 and 51 million years ago, the Earth experienced dramatic periods of warming, both gradual periods that lasted millions of years and sudden warming events called hyperthermia.
The trigger for this global warming was massive emissions of carbon dioxide (CO2) and other greenhouse gases, but other factors such as tectonic activities may also have played a role.
New research led by geoscientists at the University of Utah links sea surface temperatures to atmospheric CO2 levels2 during this period, showing that the two are closely linked. The results also provide case studies for testing feedback mechanisms and sensitivities of the carbon cycle, which are critical for predicting human-induced climate change as we continue to emit greenhouse gases into the atmosphere at rates unprecedented in the planet’s history.
“The main reason we’re interested in these global carbon release events is that they can provide analogues for future changes,” said lead author Dustin Harper, a postdoctoral fellow in the Department of Geology and Geophysics. “We really don’t have a perfect analogue event with exactly the same background conditions and the same carbon release rate.”
But the study, published Monday in the Proceedings of the National Academy of Sciences (PNAS), suggests that emissions during two historical “thermal maxima” bear enough similarity to today’s human-caused climate change to help scientists predict its consequences.
The research team analyzed microscopic fossils found in cores from an underwater plateau in the Pacific to characterize the chemistry of ocean surfaces at the time the shellfish lived. Using a sophisticated statistical model, they reconstructed ocean surface temperatures and atmospheric CO2 Temperatures rose over a 6 million year period that included two hyperthermal phases: the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago and the Eocene Thermal Maximum 2 (ETM-2) 54 million years ago.
The results suggest that atmospheric CO2 rose, as did global temperatures.
“We have several ways in which our planet, our atmosphere, is affected by CO.2 additives, but in any case, regardless of the source of CO2we are seeing similar effects on the climate system,” said co-author Gabriel Bowen, professor of geology and geophysics at the University of Illinois.
“We are interested in how sensitive the climate system is to these changes in CO2. And what we see in this study is that there is some variation, perhaps a slightly lower sensitivity, a lower warming associated with a certain amount of CO2 change when we look at these very long-term shifts. But overall we see a common range of climate sensitivity.”
Dustin Harper (right), geoscientist at the University of Utah, discusses core drilling with his colleague Weimu Xu from University College Dublin on board a research vessel of the International Ocean Discovery Program. Photo credit: Sandra Herrmann
Today, human activities related to fossil fuels are releasing carbon four to ten times faster than during these ancient hyperthermal events. However, the total amount of carbon released during these events is similar to the amount predicted for human emissions, potentially giving researchers insight into what might be in store for us and future generations.
First, scientists need to figure out what happened to the climate and oceans during these periods of global warming over 50 million years ago.
“These events could be a kind of case study that represents a medium to worst case scenario,” Harper said. “We can study them to answer the question of what environmental changes occur as a result of this carbon release.”
During the PETM, the Earth was very warm. The poles were not covered in ice and water temperatures were around 35 degrees Celsius.
To determine oceanic CO2 At these levels, the researchers turned to the fossil remains of foraminifera, a single-celled organism with shells that resembles plankton. The research team based its study on cores previously collected by the International Ocean Discovery Program at two sites in the Pacific.
The foram shells accumulate small amounts of boron, whose isotopes are an indicator of CO.2 Concentrations in the ocean at the time the shells formed, says Harper.
“We have measured the boron chemistry of the shells and can extrapolate these values to earlier seawater conditions using modern observations. We can detect CO2 and converts this into atmospheric CO2,” said Harper. “The goal of the target study interval was to establish some new CO2 and temperature records for PETM and ETM-2, which provide two of the best analogues for modern changes and also provide a longer-term background assessment of the climate system to better contextualize these events.”
The cores Harper studied were taken from the Shatsky Rise in the subtropical North Pacific, an ideal location for recovering seafloor deposits that reflect prehistoric conditions.
Carbonate shells dissolve when they settle in the deep sea, so scientists must look to underwater plateaus like the Shatsky Rise, where water depths are relatively shallow. While their inhabitants lived millions of years ago, the foraminiferal shells record conditions at the ocean surface.
“Then they die and sink to the sea floor, where they are deposited in about two kilometers of water,” Harper said. “We can recover the complete sequence of dead fossils. In these places in the middle of the ocean, there’s not really much sedimentary material from the continents, so it’s mainly these fossils and that’s all. It’s a really good archive for what we want to do.”
The study “Long- and short-term coupling of sea surface temperature and atmospheric CO2 during the late Paleocene and early Eocene”, was published August 26 in the Proceedings of the National Academy of Sciences (PNAS). Funding was provided by the National Science Foundation. The research was conducted in collaboration with colleagues at Columbia University, the University of California Santa Cruz, Vassar College, Utah State University, and the University of Hawaii.
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