CO2 puts heavier stamp on temperature than thought

A doubling of the amount of CO₂ in the atmosphere could cause an increase in the average temperature on earth from 7 to even a maximum of 14 degrees. That is shown in the analysis of sediments from the Pacific Ocean off the coast of California, by researchers at NIOZ and the Universities of Utrecht and Bristol. The results were published in this week’s Nature Communications. “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.

45-year-old drill core

The researchers used a 45-year-old drill core extracted from the bottom of the Pacific Ocean. “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.

Unique time series

“CO₂ over the past 15 million years has never before been examined from a single location,” Witkowski said. The upper thousand meters of the drill core correspond to the past 18 million years. From this record, the researchers were able to extract an indication of the past seawater temperature and an indication of ancient atmospheric CO₂ levels, using a new approach.

Derived temperature

The researchers derived the temperature using a method developed 20 years ago at NIOZ, called the TEX₈₆ method. “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.”

CO₂ from chlorophyll and cholesterol

The researchers developed a new approach to derive past atmospheric CO₂ content by using the chemical composition of two specific substances commonly found in algae: chlorophyll and cholesterol. This is the first study to use cholesterol for quantitative CO2 and the first study to use chlorophyll for this time period. To create these substances, algae must absorb CO₂ from the water and fix it via photosynthesis.

Damsté: “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.”

Using this new method, it appears that the CO₂ concentration dropped from about 650 parts per million, 15 million years back, to 280 just before the industrial revolution.

Stronger relationship

When the researchers plot the derived temperature and atmospheric CO₂ levels of the past 15 million years against each other, they find a strong relationship. The average temperature 15 million years back was over 18 degrees: 4 degrees warmer than today and about the level that the UN climate panel, IPCC, predicts for the year 2100 in 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!”