The analysis of sediments from the Pacific Ocean near California, conducted by researchers at NIOZ, the Universities of Utrecht and Bristol, indicates that a doubling of CO2 in the atmosphere may lead to an increase in the average Earth’s temperature by as much as 14 degrees.
“The temperature rise we found is much larger than the 2.3 to 4.5 degrees that the UN climate panel, IPCC, has been estimating so far”, said the first author, Caitlyn Witkowski.
The researchers utilized a 45-year-old drill core retrieved from the depths of the Pacific Ocean for this study. “I realized that this core is very attractive for researchers because the ocean floor at that spot has had oxygen-free conditions for many millions of years,” said Professor Jaap Sinninghe Damsté, senior scientist at NIOZ and professor of organic geochemistry at Utrecht University.
“As a result, organic matter is not broken down as quickly by microbes, and more carbon is preserved,” Damsté said. He was also the supervisor of Witkowski, whose doctorate thesis included this research.
“CO2 over the past 15 million years has never before been examined from a single location,” Witkowski said.
The drill core’s upper thousand meters represent the last 18 million years. With this data, the researchers could discern past seawater temperature and ancient atmospheric CO2 levels using a new approach.
The temperature was determined using the TEX86 method, which was developed 20 years ago at NIOZ by the researchers. “That method uses specific substances that are present in the membrane of archaea, a distinct class of microorganisms,” Damsté explains.
“Those archaea optimize the chemical composition of their membrane depending on the temperature of the water in the upper 200 meters of the ocean. Substances from that membrane can be found as molecular fossils in the ocean sediments and analyzed to this day.”
The scientists devised a novel method to estimate atmospheric CO2 levels by analyzing the chemical makeup of two specific compounds commonly present in algae: chlorophyll and cholesterol. This marks the first instance of using cholesterol to measure CO2 levels and the first instance of employing chlorophyll for this specific time frame. Algae need to absorb CO2 from the water and convert it through photosynthesis to produce these compounds.
“A very small fraction of the carbon on Earth occurs in a ‘heavy form,’ 13C instead of the usual 12C. Algae have a clear preference for 12C. However, the lower the CO2 concentration in the water, the more algae will also use the rare 13C. Thus, the 13C content of these two substances is a measure of the CO2 content of the ocean water. And that, in turn, according to solubility laws, correlates with the CO2 content of the atmosphere,” Damsté says.
With this innovative approach, it seems that the concentration of CO2 decreased from approximately 650 parts per million 15 million years ago to 280 just before the industrial revolution.
When scientists graph the recorded temperature and atmospheric CO2 levels from the last 15 million years in relation to each other, they observe a significant correlation. The average temperature 15 million years ago exceeded 18 degrees, which is 4 degrees higher than the current temperature and aligns with the UN climate panel, IPCC’s forecast for the year 2100 under the most extreme scenario.
“So, this research gives us a glimpse of what the future could hold if we take too few measures to reduce CO2 emissions and also implement few technological innovations to offset emissions,” Damsté said. “The clear warning from this research is: CO2 concentration is likely to have a stronger impact on temperature than we are currently taking into account!”
Journal reference:
Caitlyn R. Witkowski, Anna S. von der Heydt, Paul J. Valdes, Marcel T. J. van der Meer, Stefan Schouten & Jaap S. Sinninghe Damsté. Continuous sterane and phytane δ13C record reveal a substantial pCO2 decline since the mid-Miocene. Nature Communications, 2024; DOI: 10.1038/s41467-024-47676-9
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