Geologists map changes in carbon dioxide in Earth’s atmosphere over the past 66 million years

Geologists map changes in carbon dioxide in Earth’s atmosphere over the past 66 million years

Geologists map changes in carbon dioxide in Earth's atmosphere over the past 66 million years

This graph shows carbon dioxide concentrations in Earth’s atmosphere, expressed in parts per million (ppm), throughout the Cenozoic Era from preindustrial times to 65 million years ago. These are estimates based on proxies encoded in the geological record. Color-coded bars represent global temperature reconstructed from independent proxy data. The dashed line represents where carbon dioxide concentrations today reach 420 ppm. — Credit: Gabe Bowen, University of Utah.

Today, carbon dioxide in the atmosphere is at its highest levels in at least several million years, thanks to humans’ widespread burning of fossil fuels over the past two centuries.

But where does 419 parts per million (ppm) – the current concentration of greenhouse gases in the atmosphere – fall in Earth’s history?

This is a question that an international community of scientists, which includes key contributions by geologists at the University of Utah, is sorting out by examining a large number of markers in the geological record that provide clues about the contents of the ancient atmosphere. Their preliminary study, published this week in the journal Science, reconstructed carbon dioxide concentrations dating back to the Cenozoic, the era that began with the extinction of the dinosaurs and the appearance of mammals 66 million years ago.

The glaciers contain air bubbles, providing scientists with direct evidence that carbon dioxide levels go back 800,000 years, according to UCLA geology professor Gabe Bowen, one of the study’s authors. But this record does not extend deep into the geological past.

“Once you lose ice samples, you lose direct evidence. You no longer have samples of atmospheric gas that you can analyze,” Bowen said. “So you have to rely on indirect evidence, what we call proxies. “It is difficult to work with these agents because they are indirect.”

“Proxies” in the geological record

These clues include isotopes found in minerals, the formation of fossilized leaves, and other geological evidence that reflects the chemistry of the atmosphere. One proxy stems from fundamental discoveries by UT geoscientist Thor Serling, himself a co-author of the new study, and whose previous research has determined that carbon isotopes in ancient soils indicate past carbon dioxide levels.

But the strength of these proxies varies, and most cover narrow segments of the past. The research team, called the Cenozoic Carbon Dioxide Proxy Integration Project, or CenCO2PIP, and organized by Columbia University climate scientist Bärbel Hunesch, set out to evaluate, classify and integrate available proxies to create a high-resolution record of atmospheric carbon dioxide.

“This represents some of the most comprehensive and statistically rigorous approaches to interpreting carbon dioxide over the past 66 million years,” said co-author Dustin Harper, a postdoctoral researcher in the Bowen Lab. “Some of the new takeaways are that we’re able to combine multiple proxies from different archives of sediment, whether it’s in the ocean or on land, and that’s really not been done at this scale.”

The new research is a community effort involving about 90 scientists from 16 countries. Funded by dozens of grants from multiple agencies, the group hopes to eventually reconstruct a carbon dioxide record dating back 540 million years, to the dawn of complex life.

At the beginning of the Industrial Revolution — when humans began burning coal, then oil and gas to fuel their economies — carbon dioxide in the atmosphere was about 280 parts per million. A heat-trapping gas is released into the air when these fossil fuels burn.

Looking ahead, concentrations are expected to rise to between 600 and 1,000 ppm by 2100, depending on the rate of future emissions. It is not clear exactly how these future levels will affect the climate.

But having a reliable map of past carbon dioxide levels could help scientists more accurately predict what future climates might look like, according to UCLA biology professor William Anderegg, director of the UCLA Wilkes Center for Climate and Policy.

“This is a very important synthesis that has implications for future climate change as well, especially the key processes and components of the Earth system that we need to understand to anticipate the speed and magnitude of climate change,” Anderegg said.

Today, 419 ppm is the highest level of carbon dioxide in 14 million years

In earlier times, when Earth was a much warmer place, carbon dioxide levels were much higher than they are now. However, the 419 ppm recorded today represents a sharp, potentially dangerous, and unprecedented rise in modern geological history.

“Eight million years before the present, there might have been a 5% chance that carbon dioxide levels would have been higher than they are today, but in reality we have to go back 14 million years before we see the levels we think they were,” Bowen said. today”.

In other words, human activity has changed the atmosphere dramatically within a few generations. As a result, climate systems around the world are showing worrying signs of disruption, such as powerful storms, long-term droughts, deadly heat waves, and ocean acidification.

A solid understanding of atmospheric carbon dioxide change over geologic time is also essential for deciphering and learning from different features of Earth’s history. It is likely that changes in atmospheric carbon dioxide and climate contributed to mass extinctions, as well as evolutionary innovations.

During the Cenozoic, for example, long-term declines in carbon dioxide and associated climate cooling may have led to changes in plant physiology, species competition and dominance, which in turn influenced the evolution of mammals.

“A more precise understanding of past CO2 trends is key to understanding how modern species and ecosystems emerged and what might happen in the future,” the study notes.


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