A new 66-million-year-old carbon dioxide story offers little comfort for today

A massive new examination of ancient atmospheric carbon dioxide levels and corresponding temperatures paints a daunting picture of where Earth’s climate could be heading. The study covers geological records spanning the last 66 million years, putting current concentrations in context with deep time.

Among other things, this indicates that the last time atmospheric carbon dioxide reached current human-caused levels was 14 million years ago, much longer than some existing assessments indicate. . He says the long-term climate is very sensitive to greenhouse gases, with cascading effects that can evolve over several millennia.

The study was carried out over seven years by a consortium of more than 80 researchers from 16 countries. It appears in the newspaper Science.

“We have known for a long time that adding CO₂ “This study gives us a much more robust idea of ​​long-term climate sensitivity.”

Prevailing estimates indicate that, on scales of decades to centuries, each doubling of atmospheric CO₂ will cause average global temperatures to rise by 1.5 to 4.5° Celsius (2.7 to 8.1° Fahrenheit). However, at least one widely read recent study claims that the current consensus underestimates global sensitivity, placing it between 3.6 and 6°C of warming per doubling.

Regardless, given current trends, all estimates place the planet dangerously close to or beyond the 2° warming that could be achieved this century, and which many scientists agree we must avoid as much as possible.

In the late 1700s, the air contained about 280 parts per million (ppm) of CO₂. We are now at 420 ppm, an increase of approximately 50%; by the end of the century, we could reach 600 ppm or more. As a result, we already find ourselves somewhere on an uncertain warming curve, with an increase of about 1.2°C (2.2°F) since the late 19th century.

Regardless of what temperatures eventually manifest, most estimates of future warming derive information from studies of how temperatures are tracked by CO.₂ levels in the past. To do this, scientists analyze materials such as air bubbles trapped in ice cores, the chemistry of ancient soils and ocean sediments, and the anatomy of fossil plant leaves.

Consortium members did not collect new data; rather, they came together to sort through published studies to assess their reliability, based on evolving knowledge. They excluded some that they deemed obsolete or incomplete in light of new findings, and recalibrated others to account for the latest analytical techniques. Then, they calculated a new CO curve over 66 million years.₂ in relation to temperatures, based on all the evidence available so far, to reach a consensus on what they call “Earth system sensitivity”. By this measure, they say, a doubling of CO₂ global warming of 5 to 8°C is predicted.

The big caveat: Earth system sensitivity describes climate change over hundreds of thousands of years, not the decades and centuries that immediately concern humans. The authors argue that over long periods of time, temperature increases can result from intertwined earth processes that go beyond the immediate greenhouse effect created by CO.₂ in the air. These include melting polar ice caps, which would reduce the Earth’s ability to reflect solar energy; changes in terrestrial vegetation cover; and changes in clouds and atmospheric aerosols that could increase or decrease temperatures.

“If you want us to tell you what the temperature will be in 2100, this doesn’t tell you. But it does impact current climate policy,” said co-author Dana Royer, a paleoclimatologist at Wesleyan University. “It reinforces what we thought we already knew. It also tells us that there are slow, cascading effects that will last for thousands of years.”

Hönisch said the study will be useful to climate modelers trying to predict what will happen in the coming decades, because they will be able to integrate the robust new observations into their studies and unravel the processes that operate on short and long time scales. She noted that all project data is available in an open database and will be updated on an ongoing basis.

The new study, which covers the so-called Cenozoic era, does not radically revise the generally accepted relationship between CO₂ and temperature, but it strengthens the understanding of some periods and refines the measurements of others.

The earliest period, about 66 to 56 million years ago, has been an enigma because Earth was largely ice-free, but some studies suggested that CO₂ concentrations were relatively low. This casts doubt on the relationship between CO₂ and temperature. However, once the consortium excluded the estimates it deemed least reliable, it determined that CO₂ was actually quite high – around 600 to 700 parts per million, comparable to what could be achieved by the end of this century.

Researchers have confirmed the long-held belief that the warmest period occurred about 50 million years ago, when CO₂ reached up to 1,600 ppm and temperatures were up to 12°C higher than today. But about 34 million years ago, CO₂ had declined enough for the current Antarctic ice sheet to begin to grow.

With some ups and downs, this situation was followed by a further decline in CO emissions in the long term.₂ decline, during which the ancestors of many modern plants and animals evolved. This suggests, say the authors of the article, that variations in CO₂ affect not only the climate, but also ecosystems.

The new assessment indicates that about 16 million years ago was the last time CO₂ was consistently higher than today, at around 480 ppm; and by 14 million years ago it had fallen to the current human-induced level of 420 ppm. The decline continued and, around 2.5 million years ago, CO₂ reached around 270 or 280 ppm, triggering a series of ice ages. It was at or below this level when modern humans appeared about 400,000 years ago and persisted there until we began disrupting the atmosphere on a large scale about 250 years ago.

“Regardless of the exact degree of temperature change, it is clear that we have already brought the planet into a range of conditions never before seen by our species,” said Gabriel Bowen, co-author of the study and professor at the University from Utah. “This should make us stop and ask ourselves what is the right path forward.”

The consortium has now evolved into a larger project to determine how CO₂ and climate have evolved throughout the Phanerozoic era, from 540 million years ago to today.