Middle Miocene called and wants its CO2 level back. Photo: © Al.geba / Shutterstock.com

66 million years of CO2 data reveals future climate risks

A new study suggests that a doubling of CO2 could warm the planet by up to 8ºC. It also indicates that the last time CO2 levels matched today's levels was 14 million years ago. 

A massive review of ancient atmospheric carbon-dioxide levels and corresponding temperatures lays out a daunting picture of where the Earth’s climate may be headed. The study published in Science was assembled over seven years by a consortium of more than 80 researchers from 16 nations.

The study covers geologic records spanning the past 66 million years, putting present-day concentrations into context with deep time. Among other things, it indicates that the last time atmospheric carbon dioxide consistently reached today’s human-driven levels was 14 million years ago.
Mainstream estimates indicate that on scales of decades to centuries, every doubling of atmospheric CO2 will drive average global temperatures 1.5 to 4.5 degrees Celsius higher. The study calculated a new 66-million-year curve of CO2 versus temperatures based on all the evidence so far, coming to a consensus on what they call “earth system sensitivity.” By this measure, they say, a doubling of CO2 is predicted to warm the planet a whopping 5 to 8°C.
The new assessment says that about 16 million years ago was the last time CO2 was consistently higher than now, at about 480 ppm; and by 14 million years ago it had sunk to today’s human-induced level of 420 ppm. The decline continued, and by about 2.5 million years ago, 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 about 250 years ago.
In the late 1700s, the air contained about 280 parts per million (ppm) of CO2. We are now up to 420 ppm, an increase of about 50%. By the end of the century, we could reach 600 ppm or more. As a result, we are already somewhere along the uncertain warming curve, with a rise of about 1.2°C since the late 19th century.

The Columbia Climate School press release 7 december 2023: A New 66 Million-Year History of Carbon Dioxide Offers Little Comfort for Today



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