Our climate system reacts more sensitively to fluctuations in atmospheric carbon dioxide concentrations than previously thought. This is shown by a large study that reconstructed the history of carbon dioxide over the past 66 million years through a wealth of geological knowledge. Accordingly, the concentration of carbon dioxide in the atmosphere was similar to what it is today 14 million years ago. Since the study concerns time scales extending from thousands of years to millions of years, it is not suitable for predicting climate changes over the next few decades. However, it shows that we humans have changed the atmosphere in unprecedented ways, and that further increases in carbon dioxide concentrations are more likely to lead to longer-term warming than previously thought.
To draw conclusions about past climate, researchers use a variety of different clues. Air bubbles trapped in glaciers preserve atmospheric conditions at that time and allow direct measurements of carbon dioxide concentrations up to 800,000 years ago. In order to look further into the past, science relies on so-called indirect agents. For example, the structure of fossilized leaves can be used to determine how high the carbon dioxide content of their environment was when they were growing. Different isotopes in minerals taken 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 vary widely,” a consortium of more than 80 researchers explains in a recent publication in the journal Science. Led by climate scientist Barbel Honisch of Columbia University in New York, the team in the so-called Cenozoic Carbon Dioxide Proxy Integration Project (CenCO2PIP) has compiled, compared and re-evaluated previously available knowledge about past carbon dioxide concentrations.
The researchers looked at the entire Pliocene, the Pliocene, which began 66 million years ago when dinosaurs became extinct and the first mammals began to spread. “This is one of the most comprehensive and statistically sophisticated approaches to explaining carbon dioxide levels over the past 66 million years,” says co-author Dustin Harper of the University of Utah in Salt Lake City. “We have shown that it is possible to combine multiple proxies from different sediment archives, whether in the ocean or on land. This has never been done on this scale before.”
Importance for developing ecosystems
The result of the research was mapping atmospheric carbon dioxide concentrations over the past 66 million years. The team was also able to clarify some existing scientific uncertainties. Some previous studies seem to suggest that carbon dioxide concentrations were relatively low at the beginning of the Pleistocene, 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 carbon dioxide concentrations may have been around 600 to 700 parts per million 66 to 56 million years ago, which is more consistent with climate conditions at that time.
About 50 million years ago, carbon dioxide levels in the atmosphere rose to as much as 1,600 parts per million and average temperatures were up to 12 degrees Celsius higher than today – the warmest period in the modern era. Over the next millions of years, the concentration of carbon dioxide decreased again and the Antarctic ice sheet today began to form about 34 million years ago. With some fluctuations, the carbon dioxide content in the atmosphere has continued to decline. The ancestors of today’s animals and plants evolved during this phase of low carbon dioxide. “A more precise understanding of past CO2 trends is key to understanding how modern species and ecosystems emerged and how they may evolve in the future,” the research team wrote.
Man-made increase
About 16 million years ago, the value of carbon dioxide was consistently higher than it is today at about 480 parts per million; About 14 million years ago, it dropped slightly to 420 parts per million – a level we are at again today due to human greenhouse gas emissions. 2.5 million years ago, carbon dioxide concentrations dropped to about 270 to 280 parts per million, leading to several ice ages. This was on a similar level when modern humans appeared about 400,000 years ago. It was only when we began emitting greenhouse gases into the atmosphere on a large scale about 250 years ago that concentrations of greenhouse gases began to rise again.
Because the study looks at climate developments on time scales of hundreds of thousands of years, it is not designed to provide short-term predictions about temperatures in the coming decades. “However, it has important implications for current climate policy,” said co-author Dana Royer from Wesleyan University in Connecticut. “It confirms what we thought we already knew and shows that there are ripple effects that will last for thousands of years.” The results indicate that the increase in carbon dioxide is warming the atmosphere more than previously expected.
“No matter how many degrees of temperature change there are, it’s clear that we’ve already pushed the planet to a place our species has never been before,” says Harper’s colleague Gabriel Bowen. “This should make us stop and ask ourselves what is the right way forward.”
Source: Cenozoic CO Proxy Integration Project Consortium, Science, doi: 10.1126/science.adi5177
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