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Atmospheric CO2 hits 23-million-year peak, plant fossils reveal


It’s well known that the current levels of carbon dioxide in the atmosphere are the highest they’ve been in a long time – but is that a few hundred years? Thousands? Millions? According to a new study of fossil plant matter, CO2 concentrations haven’t been this high in at least 23 million years, and have never shot up this fast.

Lately we’ve been breaking a lot of records in terms of atmospheric carbon dioxide levels. In 2016 the South Pole became the last region on Earth to exceed a concentration of 400 parts per million (ppm). In May 2019, the Mauna Loa Observatory in Hawaii picked up a record new high of 415.26 ppm. Currently, levels are the highest they’ve been in all of human history.

And these are records we really don’t want to be breaking. Higher carbon dioxide levels are linked to climate change, and all the devastation that can bring. Long term studies have shown a sharp spike from the early 19th century onwards – “coincidentally” right around the time of the Industrial Revolution.

To see just how bad things may be, we need to look to the past. Our direct records go back a few hundred years, but it gets murky before that. Drilled ice cores give us a glimpse as far back as 2.7 million years – and unfortunately, revealed that levels were less than 300 ppm back then.

For the new study, researchers at the University of Louisiana set out to look even further back in time, stretching 23 million years back. The team managed this by examining fossil remains of ancient plants.

When plants grow, they take in carbon dioxide from the atmosphere, and their tissues retain certain stable isotopes of carbon – specifically, carbon-12 and carbon-13. When these plants fossilize, scientists can study the amounts of these isotopes and determine the concentration of CO2 that the plants grew in.

Using this method, the researchers found that throughout this 23-million-year period CO2 levels mostly fluctuated between around 230 ppm and 350 ppm. That’s far less than modern levels. The team also found no increase in that time as sharp as the climb we’re currently experiencing.

Worse still, the most dramatic warming episodes in the last 23 million years were associated with fairly small increases in CO2. That includes the middle Miocene, which occurred between 15 and 17 million years ago, and the middle Pliocene of three to five million years ago.

The new study gives us further evidence of the severity of the challenge we’re now facing (on top of all the other ones, of course).


This article was originally published on

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