According to a new study by the scientists Jonathan LaRiviere and Christina Ravelo of the University of California, there was a time in the Earth’s history, about 12 million to 5 million years ago, when the climate warmed without a corresponding increase in atmospheric CO2, perhaps because of vastly different circulation patterns in the world’s oceans.
The scientists reconstructed an open-ocean Pacific temperature record during the late Miocene epoch, finding that temperatures across a broad swath of the North Pacific were 9-14 degrees Fahrenheit warmer than today, while atmospheric carbon dioxide concentrations remained low and near values prior to the Industrial Revolution.
It was a time of nearly ice-free conditions in the Northern Hemisphere and warmer than modern conditions across the continents.
The research relies on evidence of ancient climate preserved in microfossils, that long-ago sank to the sea-floor and ultimately were buried beneath it in sediments. The microfossils contain clues to a time when the Earth’s climate system functioned much differently than it does today.
“In the late Miocene, there must have been some other way for the world to be warm. 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,” LaRiviere said.
The researchers speculated that the super-heated oceans may have resulted in a distribution of atmospheric water vapor and clouds that kept the globe warm with relatively low levels of CO2.
“This study highlights the importance of ocean circulation in determining climate conditions,” says Ravelo. “It tells us that the Earth’s climate system has evolved, and that climate sensitivity is possibly at an all-time high.”
According to the recent World Meteorological Organization (WMO)’s Annual Statement, 2011 was the 11th warmest since records began in 1850 and global temperature has increased since 1971 at an average estimated rate of 0.166°C per decade compared to the average rate of 0.06 °C per decade computed over the full period 1881-2010.