Humanity's CO2 emissions blow past last high 56 million years ago

The climate change we're experiencing now is often compared with another period of extreme warming some 56 million years ago. But today's climatic event blows the prehistoric one out of the water.

Carbon is being released into the atmosphere today ten times faster than it was during the Paleocene-Eocene Thermal Maximum (PETM), a period that has been considered the closest recent analogue to today's human-caused global warming, according to new research.

It turns out that while a record of about 37 billion metric tons (40.8 billion US tons) of carbon dioxide were released into the atmosphere in 2014, probably no more than 4 billion metric tons (4.4 billion US tons) of carbon dioxide was released in a peak year during the PETM, scientists report in a paper published Monday in the journal Nature Geoscience. 

"As far as we know, the PETM has the largest carbon release during the past 66 million years," study lead author Richard Zeebe of the Mānoa School of Ocean and Earth Science and Technology at the University of Hawai'i said in a press release. 

"Because our carbon release rate is unprecedented over such a long time period in Earth's history, it also means that we have effectively entered a 'no-analogue' state. This represents a big challenge for projecting future climate changes because we have no good comparison from the past," Dr. Zeebe said.

But that doesn't mean researchers can't gain any insights into our current climate crisis from the PETM. 

"We don't have an analogue but we do have a comparison that tells us that we can create extreme changes on the planet," Bärbel Hönisch, paleogeochemistry researcher at Columbia University's Lamont-Doherty Earth Observatory who was not part of the study, tells The Christian Science Monitor.

But "the changes were extreme, what we're doing today is much more extreme than what happened in Earth's history," she says. "The implication of that is, first of all, we don't know where this is going to take us, we can't make really good projections, but they're probably going to be larger than what we've seen at the PETM."

So what do we know about the PETM?

At this transition from the Paleocene to the Eocene epoch temperatures are thought to have risen at least 5 degrees Celsius (about 9 degrees Fahrenheit). That heat lasted some 200,000 years before the extra carbon dioxide in the atmosphere balanced out.

In addition to the heat, this transition period was punctuated by droughts and floods in different regions. A few species went extinct, and some had to migrate in order to stay in environments conducive to their lives. 

But it took at least 4,000 years for the extra carbon released into the atmosphere to kick off these hot times. Human activity has been producing elevated levels of carbon emissions for just around 150 years.

"It's not a mystery, we know that increases in carbon in the atmosphere causes warming," Marina Suarez, a paleogeologist at the University of Texas at San Antonio who was not part of the study, tells the Monitor. "But what becomes a little bit more controversial is how fast and how quickly the Earth's system will respond."

"And that's why we look at past instances," she says. 

But at this point all scientists can say is the dramatic difference between the PETM and current climate change does not bode well. 

"It tells us is that we have to expect very extreme changes in the future," Dr. Hönisch says. And "that means that if we don't act now, it will make it so much worse in the future."

Climate change could also be ushering a new geological time period. The PETM marked the birth of the Eocene epoch, so could today's dramatic changes in the Earth system be doing the same?

Possibly, says Dr. Suarez. 

"There's a big debate going on as to whether or not we have entered a new geologic time period," she says. That suggested epoch has been dubbed the Anthropocene. 

As the proposal goes, the Anthropocene was kicked off by human activity significantly impacting the Earth's natural systems. 

But here's the catch: geological time periods are usually marked by significant changes in the rocks. The centuries that humans have been altering the Earth might not be enough to leave a mark. "In sedimentary rocks, that's barely a layer," Suarez says. "It's such a small and insignificant time compared to all of the rest of, say, an epoch."

So, she says, a lot of the debate hinges on the question "would we even see evidence of what we're doing in the rock record?"

The new paper was written by Richard E. Zeebe, Andy Ridgwell and James C. Zachos, who were not available for comment at the time of this publication.