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 carbon dioxide emissions (With2) 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 ocean surface temperatures to atmospheric levels With2 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” are similar enough 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 the ocean surface at the time the shellfish lived. Using a sophisticated statistical model, they reconstructed sea surface temperatures and atmospheric With2 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 show that with the atmospheric concentration of With2 rose, as did global temperatures.
“Our planet and our atmosphere are affected in many ways by CO2 input, but in every case, regardless of the CO2 source, we see 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 reacted to these changes. With2. 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 With2 change when we look at these very long-term shifts. But overall we see a common range of climate sensitivity.”
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 with ice and ocean temperatures were around 35 degrees Celsius.
To determine the oceanic With2 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.
Small amounts of boron accumulate in the shells of foraminifera. According to Harper, the isotopes of this substance are an indicator of the CO2 concentration in the ocean at the time the shells were formed.
“We have measured the boron chemistry of the shells and can extrapolate these values to earlier seawater conditions using modern observations. We can use seawater With2 and translate that into atmospheric With2”, said Harper. “The goal of the study interval was to identify some new With2 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 Ridge in the subtropical North Pacific east of Japan, an ideal location for recovering seafloor deposits that reflect conditions in the distant past.
Carbonate shells dissolve as they sink into 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 With2 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.
