66 million-year history of carbon dioxide shows climate is highly sensitive to greenhouse gases

The last time atmospheric carbon dioxide consistently reached today’s human-driven levels was 14 million years ago, researchers have found.

Their study, assembled by an international consortium of over 80 researchers, including experts at Cardiff University, covers geologic records spanning the past 66 million years, putting present-day concentrations into context with deep time.

Their findings, published in Science, show the long-term climate is highly sensitive to CO₂, with cascading effects that can evolve over many millennia.

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 team used existing publications to calculate a new 66-million-year curve of CO₂ against global average surface temperatures   based on existing evidence, to describe a process they call “Earth System Sensitivity.”

By this measure, they say, a doubling of CO₂ is predicted to warm the planet by 5 to 8 degrees celsius  . Earth system sensitivity describes climate changes over hundreds of thousands of years, not the decades and centuries that are immediately relevant to humans.

Professor Caroline Lear, one of the report’s co-authors from Cardiff University’s School of Earth and Environmental Sciences, said:   “Atmospheric CO₂ concentrations have varied naturally in the past. Although these changes were far slower than the modern changes caused by burning fossil fuels, we can use them to understand how our planet responds to changes in CO₂.

The new assessment shows the last time CO₂ was consistently higher than now was about 16 million years ago, at about 480 parts per million (ppm) – the unit used to report CO₂ concentrations.

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 280ppm kicking off a series of ice ages. It was at or below this figure when modern humans came into being about 400,000 years ago, and persisted there until the effects of widespread combustion of fossil fuels by humans took effect on a grand scale about 250 years ago.

Dr Sindia   Sosdian, another of the report’s co-authors and a Senior Lecturer at Cardiff University’s School of Earth and Environmental Sciences, added: “‘This new review will be of use to climate modellers looking to make predictions about future climate change and understand how the Earth System will respond.”

The study, covering the so-called Cenozoic era, does not radically revise the generally accepted relationship between CO₂ and temperature, but does strengthen the understanding of certain time periods, and refines measurements of others.

Project data are available in a regularly updated open database.

The consortium now aims to chart how CO₂ and the climate have evolved over the entire Phanerozoic eon, from 540 million years ago to present  , to better understand the links between climate and the evolution of life on Earth.