Around 18 to 14 million years ago Earth experienced a sharp rise in temperature, which was probably initiated by rising CO2 levels from increased volcanic activity. The heating happened during the Miocene epoch and is therefore called the Miocene Climatic Optimum. The phenomenon is often used as a sort of climate template to predict what might happen if we allow the present CO2 level to rise.
The concentration of CO2 in the atmosphere back then is thought to have reached around 500 parts per million (ppm) – at present that number is 414. Then, the temperature rose up to six degrees, which is three times more than we hope to stay below at the end of this century according to the Paris Treaty.
This is the reason why it is very important to understand the mechanisms that caused the rise in temperature during Miocene, and a team of researchers from the Geological Survey of Denmark and Greenland (GEUS) has just taken a huge leap doing just that. With a new technique they have been able to measure how the temperature changed before, during and after the Miocene Climatic Optimum. A study recently published in the journal Paleoceanography and Paleoclimatology.
”We have managed to place a very important bit in a very hard jigsaw puzzle. It is the first time we have been able to take measurements for such a long, continuous part of the Miocene epoch in the North Atlantic region, and this part has really been missing in climate research for many years. We just haven’t had the methods to take the temperature measurements in this region until now,” says one of the authors of the study, Kasia Sliwinska from GEUS’ Department of Stratigraphy.
Until now, the researchers’ knowledge about the rise in temperature back then has been based on core samples from the southern hemisphere and around the equator or a few shorter time spans in the north.
Microalga with an indestructible molecule
A few years ago, researchers announced the happy news that now they were able to measure prehistoric temperature via molecules called alkenones. They are produced by a microalga that exists in oceans everywhere and has done for millions of years. The great thing is that the alkenones are made slightly differently depending on the temperature of the ocean. And the temperature of the ocean follows that of the atmosphere.
The alkenones turned out to be indestructible, and therefore the researchers were able to measure temperature changes through time by measuring what the different types of alkenones in the seabed contained. They sank here when the alga died, and they were slowly buried under new layers that were buried under new layers etc.
”We had an old core sample whose sediments represent an unbroken timeline through the part of the seabed that was deposited during the Miocene epoch. It is the only core sample we have in Denmark covering that large a time interval in such great detail and that is why it was a really good source of knowledge about the temperature rise during the Miocene epoch, using the new method,” says Kasia Sliwinska.
When the team of researchers measured the distribution of alkenones all the way down through the core sample, the results showed the same sharp rise in temperature around 18 to 14 million years ago that was earlier measured in core samples from southern regions.
”That means that the rise in temperature in the middle of the Miocene epoch was a global phenomenon. We haven’t been able to say that with certainty previously,” she says.
The new measurements also show what happened before and after the rise in temperature.
”The samples showed that the temperature actually fell to a lower level after the optimum than before the temperature rose. It did not just return to the starting point and that is pretty interesting,” says the researcher.
"Overall, our study shows that the global climate is sensitive to CO2 and that the rise in levels have previously had large consequences.”