An extensive new analysis of historical atmospheric carbon dioxide concentrations and associated temperatures paints a frightening picture of the possible future direction of Earth’s climate. The study places current concentrations in the context of deep time by examining geologic data spanning the previous 66 million years.
It suggests, among other things, that atmospheric carbon dioxide last steadily reached the levels it does today approximately 14 million years ago—a significant amount of time longer than some current estimates suggest. It claims that greenhouse gas emissions have a significant impact on the long-term climate, with cascade effects that could last for millennia.
A group of more than 80 academics from 16 countries worked together for seven years to put the study together. It can be found in the Science journal.
Coordinating the group was Bärbel Hönisch, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory. “We have long known that adding CO2 to our atmosphere raises the temperature,” she said. “This study gives us a much more robust idea of how sensitive the climate is over long time scales.”
According to mainstream predictions, the average global temperature will rise by 1.5 to 4.5° Celsius (2.7 to 8.1° Fahrenheit) for every doubling of atmospheric CO2 over timescales ranging from decades to centuries. The current consensus, according to at least one widely read study published recently, underestimates planetary sensitivity, placing it between 3.6 and 6°C warmer per doubling.
In any case, all estimates place the world dangerously close to or above the 2° warming that many scientists say we must avoid if at all possible, given current trends.
There were roughly 280 parts per million (ppm) of CO2 in the air in the late 1700s. We could reach 600 ppm or more by the end of the century; we are currently up to 420 ppm, a rise of around 50%. As a result, with an increase of roughly 1.2°C (2.2°F) since the late 19th century, we are already halfway down the uncertain warming curve.
The majority of projections of future warming use data from studies that show how temperatures have historically linked with CO2 levels, regardless of how high temperatures end up getting. In order to do this, scientists examine a variety of materials, such as air bubbles preserved in ice cores, the composition of prehistoric soils and ocean sediments, and the structure of leaves from extinct plants.
Instead of gathering fresh data, the consortium members pooled their resources to review prior research and evaluate their dependability in light of new information. They recalculated some to take into account the most recent analytical methods and eliminated others that they felt were out-of-date or lacking in light of fresh information. Then, using all available data, scientists created a new 66 million-year graph of CO2 vs temperature and reached a consensus on what they refer to as “Earth system sensitivity.” According to this estimate, the world is expected to warm by a staggering 5 to 8°C upon a doubling of CO2.
The enormous disclaimer is that climate changes spanning hundreds of thousands of years—rather than the decades and centuries that directly affect humans—are what is meant to be described by Earth system sensitivity. According to the scientists, long-term temperature rises could result from interconnected Earth processes that go beyond the direct greenhouse effect brought on by atmospheric CO2. These include changes in terrestrial plant cover, clouds, and atmospheric aerosols that might raise or lower temperatures, as well as melting of the polar ice sheets, which would lessen the Earth’s capacity to reflect solar energy.
This doesn’t tell you what the temperature will be like in 2100 if that’s what you want to know. However, it does have an impact on current climate policy, according to Wesleyan University paleoclimatologist and co-author Dana Royer. What we already believed to be true is strengthened. It also indicates that there will be slow, cascading impacts that persist for millennia.”
According to Hönisch, the work will help climate modelers make predictions about the future decades by enabling them to separate systems that operate on short vs long time scales and incorporate the newly robust observations into their research. She said that every piece of information related to the project is accessible in an open database and will be updated periodically.
The recently published study, which focuses on the so-called Cenozoic era, improves our understanding of some time periods and sharpens our measurements of others, but it does not fundamentally alter the generally known relationship between CO2 and temperature.
The most remote era, between around 66 and 56 million years ago, has always been a mystery since, although the Earth was mostly free of ice, some research had shown that CO2 concentrations were comparatively low. This raised some questions about the connection between temperature and CO2. The consortium discovered that CO2 was actually rather high—between 600 and 700 parts per million, similar to what could be achieved by the end of this century—after excluding estimations that they thought to be the least reliable.
The findings supported the long-held theory that the hottest time on record occurred roughly 50 million years ago, during which time CO2 levels reached as high as 1,600 ppm and temperatures rose by up to 12°C. However, CO2 levels had sufficiently decreased by around 34 million years ago for the current Antarctic ice sheet to start forming.
This was followed by another long-term CO2 fall, with occasional ups and downs, during which the progenitors of many current plants and animals evolved. According to the paper’s authors, this shows that changes in CO2 have an impact on ecosystems as well as climate.
According to the updated assessment, CO2 was last continuously higher than it is today, at roughly 480 parts per million, some 16 million years ago; by 14 million years ago, it had dropped to the 420 parts per million that it is today due to human activity. As the drop persisted, CO2 levels eventually hit 270–280 ppm approximately 2.5 million years ago, which caused a sequence of ice ages to begin. When modern humans first appeared about 400,000 years ago, it was at or below that, and it remained there until approximately 250 years ago, when we began to significantly alter the atmosphere.
“It’s obvious we have already brought the planet into a range of conditions never seen by our species, regardless of exactly how many degrees the temperature changes,” said University of Utah professor and research co-author Gabriel Bowen. “It should make us stop and question what is the right path forward.”
With the goal of mapping the evolution of CO2 and climate during the Phanerozoic eon—540 million years ago to the present—the group has now grown into a broader effort.