New study suggests that Earths climate has evolved to become more sensitive to greenhouse gases
By Summit Voice
SUMMIT COUNTY — The modern era’s link between atmospheric carbon dioxide levels and temperatures is nearly undisputed as both have climbed in tandem during recent decades.
But new studies of fossilized plankton suggests 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, led by Jonathan LaRiviere and Christina Ravelo of the University of California at Santa Cruz, 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–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 microscopic plankton skeletons–called 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.
The study suggests that the modern climate responds more readily to changing carbon dioxide levels than it has during the past 12 million years, possibly through changes in ocean circulation.
“It’s a surprising finding, given our understanding that climate and carbon dioxide are strongly coupled to each other,” LaRiviere said.
“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.”
The Pacific Ocean in the late Miocene was very warm, and the thermocline, a boundary that separates warmer surface waters from cooler underlying waters, was much deeper than now.
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.
“The results explain the seeming paradox of the warm — but low greenhouse gas — world of the Miocene,” said Major, program director in NSF’s Division of Ocean Sciences.
“This work represents an important advance in understanding how Earth’s past climate may be used to predict future climate trends,” says Jamie Allan, program director in the National Science Foundation’s (NSF) Division of Ocean Sciences, which funded the research.
Several major differences in the world’s waterways could have contributed to the deep thermocline and the warm temperatures of the late Miocene.
For example, the Central American Seaway remained open, the Indonesian Seaway was much wider than it is now, and the Bering Strait was closed.
These differences in the boundaries of the world’s largest ocean, the Pacific, would have resulted in very different circulation patterns than those observed today.
By the onset of the Pliocene epoch, about five million years ago, the waterways and continents of the world had shifted into roughly the positions they occupy now.
That also coincides with a drop in average global temperatures, a shoaling of the thermocline, and the appearance of large ice sheets in the Northern Hemisphere–in short, the climate humans have known throughout recorded history.
“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.”
The study was published in the most recent issue of the journal Nature. Other co-authors of the paper are Allison Crimmins of UCSC and the U.S. Environmental Protection Agency; Petra Dekens of UCSC and San Francisco State University; Heather Ford of UCSC; Mitch Lyle of Texas A&M University; and Michael Wara of UCSC and Stanford University.