Global cooling in the past 15 million years was caused by a decrease in the levels of carbon dioxide. A team of Spanish scientists led by the University of Oviedo has just provided evidence to prove the relationship between CO2 and climate. The results have been published today in the journal Nature Communications.

The principal researcher, Heather M. Stoll, professor of the Department of Geology of the University of Oviedo highlights that up to now, the temperature decrease in the earth´s surface, between 7 and 9 degrees, in the last 15 million years had never been identified with changes in the levels of CO2. In fact, the scientific community was divided between those who believed that CO2 did not regulate climate, and those who stated that temperature was sensitive to carbon dioxide. "We can prove–says Heather Stoll—that, 15 million years ago, increased CO2 implied warmer temperatures, so the same can be expected with the current increased CO2 levels" .

The work published in Nature Communications is entitled Decrease in coccolithophore calification and CO2 since the middle Miocene (Disminución en la calcificación de los cocolitofóridos y CO2 desde el Mioceno medio). Spanish scientists, led by geologists of the University of Oviedo, have come to those conclusions after analyzing coccolithopores, single-celled algae that live on the surface of the ocean. "These scales arevery important–points out Heather Stoll—because they allow experts to study how these organisms, basis of the trophic chain, have reacted to climate and oceanic changes in the past".

Beyond the specific results for this type of organisms, the principal researcher emphasizes that the study provides new evidence of the relationship between climate and the so called greenhouse gas emissions. Heather Stoll adds that for more than a decade, they have been collecting information that shows how the planet has been cooling down in the last 15 million years. However, it is difficult to figure out the reason for this temperature decrease, due to the lack of information about the evolution of CO2 in such a long period.

The work published in ‘Nature Communications' highlights the capacity of some living organisms to adapt to increased amounts of CO2.

New evidence provided by this research shows a decrease in carbon dioxide levels, which explains lower temperatures in that period. Heather Stoll reminds that 15 million years ago the earth´s temperature was a lot warmer than nowadays, between 7 and 9 degrees in mid-latitude conditions. Global temperature has been gradually cooling down up to the present days, with slight variations. This tendency has been broken down in recent years due to the so called anthropogenic CO2, that is, the one originated by human activity.

The study also suggests that increased levels of CO2 may not always be harmful for some living organisms like the coccolithophores. Heather Stoll points out that it is obvious that some species have adapted in a better way to changes in the levels of carbon dioxide, maybe because they have had more time to get used to them. The work proves that the thickness of the scales of these single-celled algae was reduced by half in the last 10 million years. Amazingly, this process was developed at the same time the levels of CO2 decreased. This geologist highlights that on the time scale of hundreds of years, increased levels of CO2 could help cells produce thicker scales, which could prevent them from suffering calcification problems.

Heather Stoll aclara, pese a todo, que el hecho de encontrar conchas de cocolitofóridos más gruesas en periodos de CO2 elevados no excluye los riesgos para los organismos calcificantes. La investigadora explica que los cocolitofóridos poseen una particularidad: son plantas, y necesitan el carbono tanto para la fotosíntesis como para la calificación. Es previsible, por lo tanto, que organismos animales que no realizan fotosíntesis, como corales o almejas, responderán de otra forma a los cambios de CO2.

Researchers took the microscopic scales from samples in the Atlantic and Indian Ocean and measured the amount of light that passed through them with the help of a special microscope to measure thickness. Combining the dimensions of thousands of scales, researchers were able to prove that their thickness started to decrease 9 million years ago, and as those changes took place in two distant places, they came to the conclusion that they were caused by a global variation in the climate conditions.

In order to find out what caused those thickness variations, they performed geochemical measurement of the scales and the drops stuck to the sediments for millions of years. The results confirmed changes in the atmospheric concentration of CO2. It is worth mentioning that the scales became thinner when the CO2 levels decreased, because carbon was bounced back so as to be used in the photosynthesis process. These results are in line with a previous study developed by the same group, published in Nature in 2013, which showed how these algae react at low levels of CO2, reducing carbon concentration to form scales.

Heather Stolle explains that, finding thicker coccolithopores in periods of increased levels of CO2 does not preclude risks for calcifying organisms. The researcher explains that coccolithopores have a peculiar feature: they are plants, and they need carbon for both, photosynthesis and calcification. Therefore, it may be expected that animal organisms that do not carry out photosynthesis, like corals or clams, will react in a different way to the changes of CO2.

She also states that, while our politicians discuss about how to mitigate irreversible global climate changes, the chemical properties and circulation of oceans are quickly changing. "It is foreseeable that calcifying organisms are at risk when facing the chemical changes developed as the ocean absorbs large amounts of CO2 resulting from human activities.", she points out.

The research is provided with funding from the European Union, which has allocated 1,77 million euros to develop a 6-year lasting project. In addition to Heather Stoll, Clara Bolton, as author of the work, has been part of the research group of the University of Oviedo for 4 years, and she is currently working in the CNRS in France. Other scientists from the University of Salamanca have also participated, developing a special microscopy technique to measure the scales thickness; other participating entities are the Woods Hole Oceanographic Institute and the University of New Hampshire in the United States, which supplied the samples of the exploration in the Indian Ocean.