Higher levels of atmospheric carbon dioxide (CO2) are linked to rising ocean temperatures -- but the seas were not so sensitive to the gas around 13 million years ago, says a new study.
About five to 13 million years ago (late Miocene period), oceans were warmer than they are today -- even though atmospheric CO2 concentrations were considerably lower.
The unusual mismatch between sea temperatures and CO2 levels during this period hints that the relationship between climate and CO2 hasn't always been the same as it is today, said Petra Dekens, study co-author, the journal Nature reports.
"There was a transition, from the Earth's climate system being not as sensitive to changes in atmospheric carbon dioxide to becoming more sensitive to these changes," said Dekens, assistant professor of geosciences at the San Francisco State University, US.
"What's interesting is that we can see this transition happening within the last 13 million years," Dekens added.
The link between modern-day ocean warming and increased levels of atmospheric CO2 produced by human activities has been confirmed in numerous studies, many of them compiled in the recent report of the Intergovernmental Panel on Climate Change, according to a San Francisco statement.
Recent reconstructions of CO2 levels for the late Miocene period suggest that CO2 concentrations for the period were only 200-350 parts per million. Modern CO2 concentrations, by contrast, are around 390 parts per million.
Jonathan P. LaRiviere from the University of California (Santa Cruz), US, who led the study and colleagues including Dekens, sought information on late Miocene ocean temperatures to analyse alongside the Miocene CO2 reconstructions.
They used an organic compound called unsaturated alkenone as their "fossil thermometers." Ratios of the compound preserve a record of the water temperature in which the plankton lived.
These data provided the first evidence, Dekens said, that late Miocene sea surface temperatures were significantly warmer than today across a large swath of the north Pacific.