The results of a sizable new study released on Thursday provide a dire picture of where Earth’s climate is headed: 14 million years ago was the last time carbon dioxide in the atmosphere continuously matched today’s human-driven levels.
The work, which was published in the journal Science, examines biological and geochemical clues from the distant past to reconstruct the historical CO2 record more precisely than ever before. It spans the 66 million years ago to the present.
According to lead author Baerbel Hoenisch of the Lamont-Doherty Earth Observatory at the Columbia Climate School, “it really brings it home to us that what we are doing is very, very unusual in Earth’s history,” as reported to AFP.
The new analysis reveals, among other things, that the last time carbon dioxide levels in the air reached 420 parts per million (ppm) was between 14 and 16 million years ago, during a period when Greenland was devoid of ice and human ancestors were just beginning to move from forests to grasslands.
That is a significant amount of time past the 3-5 million years that previous analyses suggested.
The amount of atmospheric carbon dioxide that humans have contributed to has increased by approximately 50% since the late 1700s. This greenhouse gas traps heat in the atmosphere and has caused the planet to warm by 1.2 degrees Celsius since before industrialization.
“What’s important is that Homo, our species, has only evolved 3 million years ago,” Hoenisch stated. “And so our civilization is tuned to sea level as it is today, to having warm tropics and cool poles and temperate regions that have a lot of rainfall.”
By the year 2100, if global CO2 emissions keep rising, we might be between 600 and 800 parts per million.
The last time those levels were observed was 30–40 million years ago, during the Eocene, before Antarctica was covered in ice and before the world’s flora and fauna underwent significant changes. For instance, enormous insects were still commonplace.
The new study, which took seven years to complete and involved 80 researchers from 16 different countries, is currently regarded as the most recent consensus within the scientific community.
Instead of gathering fresh data, the team combined the best-rated published works into a new timeline by synthesizing, reevaluating, and validating them in light of new scientific findings. They did this by classifying the works based on confidence levels.
Drilling into ice sheets or glaciers to retrieve ice cores, which are known to contain air bubbles that reveal the composition of the past atmosphere, is a concept that is widely known. However, these cores are typically only recovered from a few thousand years ago.
Paleoclimatologists use “proxies” to delve deeper into the past. Through analyzing the chemical makeup of minerals, ancient leaves, and plankton, they can infer atmospheric carbon at a specific time indirectly.
According to the research, the hottest time in the previous 66 million years occurred 50 million years ago, when temperatures were 12 degrees hotter and CO2 levels reached as high as 1,600 ppm. After that, a protracted decline began.
A series of ice ages began when carbon dioxide levels reached 270–280 parts per million by 2.5 million years ago.
That was the same level 400,000 years ago when modern humans first arrived, and it remained that way until our species started using fossil fuels extensively.
According to the team, a doubling of CO2 is expected to warm the planet by 5-8 degrees Celsius. However, these temperatures will have an impact on Earth’s systems over a very long time, perhaps hundreds of thousands of years.
For instance, if the polar ice caps melted, the planet’s capacity to reflect solar radiation would be diminished, creating a feedback loop that would reinforce itself.
However, Hoenisch emphasized that policy makers can still directly benefit from the new research.
According to the carbon record, Earth experienced a similar sudden release of carbon dioxide 56 million years ago. This caused significant changes to ecosystems and took about 150,000 years to dissipate.
“We are in this for a very long time, unless we sequester carbon dioxide, take it out of the atmosphere, and we stop our emissions sometime soon,” she stated.