CO2 levels linked to cascading effects that last for millennia 

In a comprehensive new study, scientists have assembled a detailed 66 million-year history of Earth’s atmospheric carbon dioxide (CO2) levels and corresponding temperatures. The research offers critical insights into the current and future state of our planet’s climate. 

The study was conducted over the course of seven years by a consortium of over 80 researchers from 16 countries.

Sensitivity of Earth’s climate 

The experts determined that the last time Earth’s atmosphere contained CO2 levels comparable to today was 14 million years ago, which is much earlier than previously estimated. 

This revelation underscores the sensitivity of Earth’s climate to greenhouse gases and human-driven concentrations of CO2, with potential long-term effects spanning millennia.

“We have long known that adding CO2 to our atmosphere raises the temperature,” said Bärbel Hönisch, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory who coordinated the consortium. “This study gives us a much more robust idea of how sensitive the climate is over long time scales.”

CO2 levels and global temperature rise 

The research suggests that for every doubling of atmospheric CO2, average global temperatures could increase by 1.5 to 4.5 degrees Celsius (2.7 to 8.1 Fahrenheit) in the near future, with some studies proposing even higher increases.

The study highlights a concerning trend: from around 280 parts per million (ppm) of CO2 in the late 1700s, we have reached approximately 420 ppm today, a 50% increase. 

This trajectory could lead to levels of 600 ppm or more by the century’s end, pushing the planet along a path of uncertain but significant warming, with an increase of about 1.2 degrees C (2.2 degrees F) already observed since the late 19th century.

Studying historical CO2 levels

The researchers used a range of scientific methods, including analysis of air bubbles in ice cores, ancient soil and ocean sediment chemistry, and fossil plant leaf anatomy, to track historical temperature trends relative to CO2 levels. 

The team meticulously reviewed and recalibrated existing data, excluding less reliable studies in light of new findings, to create a more accurate and comprehensive CO2 versus temperature curve over 66 million years.

Dramatic prediction 

This new curve suggests a dramatic prediction: a doubling of CO2 could increase global temperatures by 5 to 8 degrees C. However, this “Earth system sensitivity” metric reflects climate changes over hundreds of thousands of years, not the shorter timescales directly relevant to human experience.

The study authors explained that over long periods, increases in temperature may emerge from intertwined Earth processes that go beyond the immediate greenhouse effect created by CO2 in the air.

These include melting of polar ice sheets, which would reduce the Earth’s ability to reflect solar energy; changes in terrestrial plant cover; and changes in clouds and atmospheric aerosols that could either heighten or lower temperatures.

Cascading effects of CO2 levels

“If you want us to tell you what the temperature will be in the year 2100, this does not tell you that. But it does have a bearing on present climate policy,” said study co-author Dana Royer, a paleoclimatologist at Wesleyan University. “It strengthens what we already thought we knew. It also tells us that there are sluggish, cascading effects that will last for thousands of years.”

Hönisch said the study will be useful for climate modelers trying to predict what will happen in coming decades, because they will be able to feed the newly robust observations into their studies, and disentangle processes that operate on short versus long time scales. She noted that all the project’s data are available in an open database, and will be updated on a rolling basis.

Critical insights 

Covering the Cenozoic era, the study offers a refined understanding of the CO2-temperature relationship, particularly for certain time periods. It confirms that the hottest period was around 50 million years ago, with CO2 levels reaching up to 1,600 ppm.

The researchers conclude that the last time CO2 was consistently higher than now was about 16 million years ago at about 480 ppm. By 14 million years ago, atmospheric CO2 had decreased to today’s human-induced level of 420 ppm. 

Messing with the atmosphere

The decline continued, and by 2.5 million years ago, atmospheric CO2 reached about 270 or 280 ppm, kicking off a series of ice ages. It was at or below that when modern humans came into being about 400,000 years ago, and persisted there until we started messing with the atmosphere on a grand scale 250 years ago, said the experts.

“Regardless of exactly how many degrees the temperature changes, it’s clear we have already brought the planet into a range of conditions never seen by our species,” said study co-author Gabriel Bowen, a professor at the University of Utah. “It should make us stop and question what is the right path forward.”

The study is published in the journal Science.

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