A massive new 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 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—much longer ago than some existing assessments indicate. It asserts that long-term climate is highly sensitive to greenhouse gas, with cascading effects that may evolve over many millennia.
The study was assembled over seven years by a consortium of more than 80 researchers from 16 nations. It appears today in the journal Science.
The consortium’s 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 They excluded some that that they found outdated or incomplete in the light of new findings, and recalibrated others to account for the latest analytical techniques. Then they 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 degrees C.
The giant caveat: Earth system sensitivity describes climate changes over hundreds of thousands of years, not the decades and centuries that are immediately relevant to humans. The authors say 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.
The consortium has now evolved into a larger project that aims to chart how CO2 and climate have evolved over the entire Phanerozoic eon, from 540 million years ago to present.