Our climate system reacts even more sensitively to fluctuations in atmospheric CO2 concentrations than previously thought. This is shown by a large study that reconstructed the history of carbon dioxide over the last 66 million years from a wealth of geological knowledge. Accordingly, the CO2 concentration in the atmosphere was last similar to today 14 million years ago. Since the study relates to time scales of millennia to millions of years, it is not suitable for predicting climate changes within the next few decades. However, it shows that we humans have changed the atmosphere in unprecedented ways and that further increases in CO2 concentrations are likely to lead to even greater warming in the long term than previously thought.
To draw conclusions about the climate of the past, researchers use a variety of different clues. Air bubbles trapped in glaciers preserve the atmospheric conditions of that time and enable direct measurements of CO2 concentrations from up to 800,000 years ago. In order to look even further into the past, science relies on so-called indirect proxies. For example, the structure of fossilized leaves can be used to determine how high the CO2 content was in their environment when they were growing. Different isotopes in minerals from soil samples or microfossils can also provide information.
Geological data collected
“Although each of these proxies has been extensively validated, reconstructions based on different proxies often differ significantly,” explains a consortium of more than 80 researchers in a recent publication in the journal Science. Led by climate scientist Bärbel Hönisch from Columbia University in New York, the team in the so-called Cenozoic CO2 Proxy Integration Project (CenCO2PIP) compiled, compared and re-evaluated the previously available knowledge on past CO2 concentrations.
The researchers looked at the entire Cenozoic Era, i.e. the modern era, which began 66 million years ago when the dinosaurs died out and the first mammals began to spread. “This is one of the most comprehensive and statistically sophisticated approaches to interpreting CO2 levels over the last 66 million years,” says co-author Dustin Harper from the University of Utah in Salt Lake City. “We have shown that it is possible to combine multiple proxies from different sediment archives, be it in the ocean or on land. This has never been done on this scale before.”
Importance for the development of ecosystems
The result of the research is a mapping of atmospheric CO2 concentrations over the last 66 million years. The team was also able to clarify some existing scientific uncertainties. Some previous studies seemed to indicate that CO2 concentrations were relatively low at the beginning of the Cenozoic, even though temperatures were high and the poles were not covered by ice caps. However, as the CenCO2PIP team explains, some of these studies had methodological weaknesses. Assessments using more recent methods have shown that CO2 concentrations were probably around 600 to 700 ppm 66 to 56 million years ago, which is more consistent with the climate conditions of the time.
About 50 million years ago, CO2 levels in the atmosphere rose to up to 1,600 ppm and average temperatures were up to 12 degrees Celsius higher than today - the warmest period of the modern era. Over the following millions of years, the CO2 concentration decreased again and today's Antarctic ice sheet began to form around 34 million years ago. With some fluctuations, the CO2 content of the atmosphere continued to fall. The ancestors of today's animals and plants developed during this phase of CO2 decline. “A more precise understanding of past CO2 trends is therefore central to understanding how modern species and ecosystems emerged and how they might develop in the future,” writes the research team.
Man-made increase
Around 16 million years ago, the CO2 value was last consistently higher than today at around 480 ppm; About 14 million years ago it fell slightly to 420 ppm - the level we have reached again today due to human greenhouse gas emissions. By 2.5 million years ago, CO2 concentrations had fallen to around 270 to 280 ppm, leading to several ice ages. It was at a similar level when modern humans emerged around 400,000 years ago. Only when we began emitting greenhouse gases into the atmosphere on a large scale around 250 years ago did the concentration begin to rise again.
Because the study looks at climatic developments on time scales of hundreds of thousands of years, it is not designed to make short-term predictions about temperatures in the coming decades. “Nevertheless, it has important implications for current climate policy,” said co-author Dana Royer of Wesleyan University in Connecticut. “It confirms what we already thought we knew and shows that there are cascading effects that will last for thousands of years.” The results suggest that the increase in CO2 is warming the atmosphere even more than previously predicted.
“Regardless of exactly how many degrees the temperature changes, it is clear that we have already pushed the planet into a place where our species has never been before,” says Harper's colleague Gabriel Bowen. “This should cause us to pause and ask ourselves what the right path forward is.”
Source: Cenozoic CO2 Proxy Integration Project Consortium, Science, doi: 10.1126/science.adi5177