One of the enigmas of past climate is the event known as the Miocene Climatic Optimum (MCO), a period of global warming from about 17 million to 15 million years ago. The MCO was the most significant interruption of a long-term planetary cooling trend that has stretched from around 40 million years ago up to modern (preindustrial) times. Yet researchers have found scant evidence of high carbon dioxide (CO2) levels during the Miocene, leading to speculation as to what may have driven the temperature shift.
Research being presented at AGU’s Fall Meeting 2020 opens a new perspective on the MCO and the cooling period that followed. Timothy Herbert, a professor of oceanography and Earth sciences at Brown University, and colleagues said the MCO was indeed hot—significantly hotter than previous estimates. And they contend that the warming was the result of very high levels of atmospheric CO2.
The study also does more, using the MCO as a window into a neglected process that may be responsible for driving changes in climate over very long periods of time. The key to understanding the MCO and broader temperature changes over the past 20 million years, these researchers said, lies in plate tectonics and the release of deep-Earth carbon as new oceanic crusts are formed.
Hot and Hotter
Even before the MCO, Earth’s climate had shifted to warming during much of the early and mid-Miocene. “Most of the Miocene, especially from about 22–10 million years ago, was strikingly warm,” Herbert said. “The MCO was an anomaly even in that warm climate, a period of about 2 million years of extreme warmth.”
Previous MCO temperature estimates were based largely on microfossils of deepwater marine organisms. Although registering some unusual warming, Herbert believes these data underestimated it. His team instead studied remains of marine algae living at or near the ocean surface, focusing on the presence of biomolecules called alkenones. “The proportion of different alkenones that [the algae] synthesize while they’re alive reflects the temperature at the time they lived,” Herbert explained.
The researchers collected data from the Northern and Southern Hemispheres, at higher latitudes where the alkenone-temperature relationship is thought to be most accurate. “The picture is very different if you look at the surface ocean,” Herbert said. “Our estimates of [MCO] warming are 2–3 times higher than what [are] in the literature.” The researchers estimated that surface air temperatures were 11℃–12℃ warmer than those of the present day.
Such a hot climate is hard to reconcile with levels of CO2 suggested by other studies on the basis of analysis of plant leaf fossils and other data. Yair Rosenthal, a marine chemist at Rutgers University who was not part of the new study, said the “perplexing decoupling between temperature and CO2 raises two possibilities.” One is that Earth’s climate was somehow more sensitive to relatively small changes in CO2 during the Miocene. “The other is that the [previous] reconstructions are wrong, and changes in atmospheric CO2 were much larger than has been suggested,” Rosenthal said.
Herbert’s team believes the latter explanation is correct. In line with current climate models, they said the warm temperatures indicated by their data required atmospheric CO2 levels of 900–1,200 parts per million, over twice what they are today. “We had run into temperatures far warmer than we had expected to find,” Herbert said. “And so we had to figure out how to explain [them].”
A Tectonic Mechanism
That explanation, they said, is tectonic degassing, which occurs when deep reservoirs of CO2 are released to the atmosphere as sections of Earth’s crust collide or spread apart. It has long been widely believed that this tectonic mechanism operates at a constant rate, making it an unlikely explanation for changes in climate.
“Tim’s research questions this paradigm,” Rosenthal said. “At least for the Miocene, they are suggesting that changes in seafloor spreading were more important than previously assumed, and the consequent degassing raised atmospheric CO2, causing the warming of the MCO.”
This perspective is not without precedent. The idea that rates of tectonic activity and degassing change, and are an important driver of long-term shifts in climate, was first put forward in the 1980s, Herbert said. But this theory had been largely ignored because of the lack of supporting evidence.
Herbert said that evidence is now at hand. His team’s new model assumes seafloor spreading to be a reliable indicator of overall tectonic degassing. The researchers measured and dated seafloor spreading by studying anomalies on the ocean floor that represent shifts in Earth’s magnetic pole from north to south. New techniques of dating these polarity flips allowed them to reconstruct rates of plate creation with a high degree of precision.
“We now have good estimates of rates of plate creation over the last 20 million years,” Herbert said. “And what we show is that rates [at the time of the MCO] were 35% higher than today.”
The model explains not just the temperature spike of the MCO but also the longer period of early and mid-Miocene warming and the long cooling period that followed. “The most surprising conclusion to us is how rapidly rates of plate creation changed in the relatively recent geological past,” Herbert said.
Rosenthal added, “This new research challenges both the current reconstructions of atmospheric CO2 and…our understanding of the role of plate tectonics in controlling climate.”
—Scott Norris (@norris_sd), Science Writer