About 56 million years ago, something mysterious caused the concentrations of atmospheric carbon dioxide to spike to levels much higher than they are right now. After this event, the planet warmed, leading to the acidification of oceans and major die-offs of numerous marine organisms. These events caused the Palaeocene-Eocene Thermal Maximum (PETM) and although experts still debate on its cause, a new study reveals that the rate at which we are putting carbon into the atmosphere is higher than the rate seen during this time period.
"If you look over the entire Cenozoic, the last 66 million years, the only event that we know of at the moment, that has a massive carbon release, and happens over a relatively short period of time, is the PETM," said Richard Zeebe, who led the study. "We actually have to go back to relatively old periods, because in the more recent past, we don't see anything comparable to what humans are currently doing."
In order to better understand the PETM, Zeebe and his team examined a deep ocean core of sediment from off the coast of New Jersey and analyzed the ratios between different isotopes of carbon and oxygen. Using this information, the team was able to determine how atmospheric carbon dioxide levels influenced temperatures by looking at the ratio of carbon 12 to carbon 13 and the oxygen isotopes in the ocean, respectively.
"In terms of these two systems, the first shows us when the carbon went into the system, and the second tells us when the climate responded," Zeebe said.
The team found a connection between the lag in time between the massive carbon releases and subsequent warming. This lag stems from the large thermal inertia of the oceans, meaning that the bigger the lag, the greater the carbon release. Conversely, no lag at all points to slow carbon release.
Although the new core did not show a lag, the authors believe that this means the gigantic carbon release that entered the atmosphere during the PETM - between 2,000 to 45,000 tons - took place over the course of around 4,000 years. However, humans are now releasing approximately 10 billion tons of carbon each year, leading to a much more rapid pace of climate change.
"The two main conclusions is that ocean acidification will be more severe, ecosystems may be hit harder because of the rate [of carbon release]," Zeebe said.
Although the results help us examine the effects of the current climate warming, there are many other changes that will likely arise without our knowledge due to the high rate of current climate change having no ancient parallel.
"Given that the current rate of carbon release is unprecedented throughout the Cenozoic, we have effectively entered an era of a no-analogue state, which represents a fundamental challenge to constraining future climate projections," the researchers concluded.
The findings were published in the March 21 issue of Nature Geoscience.
