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
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.