14 DAY TRIAL // As hard as it may seem to believe today, there was a time in Earth's history – about 12 to 15 million years ago – when the climate was not coupled to atmospheric carbon dioxide concentrations. Nowadays, the climate is extremely sensitive to CO2 amount variations in the air.
Climate experts were convinced that periods of CO2 increase were correlated with global warming, while reduced amounts of the gas in the air were tied to lower global temperatures. The new study shows that this may not always be the case.
The team behind the new study says that these conclusions should not be construed as implying that CO2 is not responsible for the current warming we are experiencing. They are merely pointing out that exceptions can occur.
The new investigation was led by Jonathan LaRiviere and Christina Ravelo, who are both based University of California at Santa Cruz (UCSC). Their research is detailed in this week's issue of the top scientific journal Nature.
They base their conclusions on an analysis of deep-sea sediment cores collected from the North Pacific Ocean, which were determined to date back to the late Miocene period. Temperatures at that time were 9-14 degrees Fahrenheit higher than they are today, even though CO2 concentrations were relatively low.
The reason carbon dioxide can now influence the climate is that the global ocean circulation suffered a series of changes starting around 5 million years ago. This is what ultimately made the climate so sensitive to the greenhouse gas, and it could be that the process is still going on.
“This work represents an important advance in understanding how Earth's past climate may be used to predict future climate trends,” explains US National Science Foundation (NSF) Division of Ocean Sciences program director, Jamie Allan. The DOS funded this investigation.
Researchers used the Integrated Ocean Drilling Program's JOIDES Resolution scientific drilling ship to collect the samples. The microfossils they discovered in the sediments allowed them to assess temperatures and CO2 concentrations that existed on Earth 12 million years ago.
“It's a surprising finding, given our understanding that climate and carbon dioxide are strongly coupled to each other. In the late Miocene, there must have been some other way for the world to be warm,” LaRiviere explains.
“One possibility is that large-scale patterns in ocean circulation, determined by the very different shape of the ocean basins at the time, allowed warm temperatures to persist despite low levels of carbon dioxide,” the expert concludes.