Sediments drilled from the sea floor under what are normally ice-covered, wind-battered parts of the Bering Sea are revealing the surprising nature of the last, most recent era in which that arm of the Pacific Ocean experienced a serious case of global warming.
The Pliocene warm period lasted from 3.5 to 4.5 million years ago, just before the Earth went into a repeating cycle of ice ages. The Pliocene is of special interest to scientists because it bears the closest resemblance -- at least in terms of heat and carbon dioxide -- to the world that is now resulting from global warming.
"It's the first time we've got sediments from that far back," said Christina Ravelo, a researcher at the University of California at Santa Cruz.
Ravelo was part of the International Ocean Drilling Program's Expedition 323, which managed to gather cores of Early Pliocene sediments from the bottom of the Bering Sea in two locations.
These cores underwent detailed analysis to figure out how productive the waters were at that time, as well as a host of other details about that long lost time. The results were reported at the recent meeting of the American Geophysical Union.
"The Bering Sea is at the edge," said Ravelo. It's extremely sensitive to climate changes, she said, and has revealed some surprises about what it was like during the last big warming event.
For one thing, the productivity of the Bering Sea was very high, Ravelo told Discovery News. That means there was a lot of animal life. For another, the sea was not neatly layered -- stratified -- with the coldest waters on the bottom and the warmest on the top, as is expected of oceans in warmer times.
"I think that's generally true in lower latitudes," she said. "But in high latitudes it's very sensitive to salinity."
In lower latitudes it's thought that the increased rainfall from global warming would freshen the surface waters of the oceans, and since fresher waters are less dense and float atop more saline waters, the overturning of ocean waters would be less. That, in turn, should lower ocean productivity, because it's upwelling water from the depths that is the source of nutrients for the world's greatest fisheries.
Yet in the Early Pliocene's Bering Sea, there was loads of productivity, probably due to strong winds stirring things up on an ice-free sea.
"I guess her results would support a more limited region where stratification occurred," commented Mark Pagani, a geology professor and researcher at Yale University.
As for how much this helps us predict the future, that's less clear. The carbon dioxide levels during the height of the Pliocene warm period were about where today's levels are, Ravelo said. But ours are still rising, and it's all happening much faster now than in the past.
"I think it's pretty hard to draw a direct analogy," Ravelo said.
Other things to keep in mind are that today's Earth has more ice than it did when the warming happened in the Pliocene, said Pagani. That suggests it might require more carbon dioxide to melt that ice (which helps cool the planet by reflecting sunlight back into space) and get as much warming today.
That said, there are some things in the future that are likely to look a lot like that particular time in the past, said Pagani.
"The early Pliocene is now considered a model for future warming...dramatically higher sea levels, temperatures 3 to 4 degrees C warmer than today, with CO2 levels comparable to modern values," he said.
"The lesson we take from this is that the Earth has a relatively high climate sensitivity to CO2 and we can expect future warming over the next few centuries even if CO2 levels are held at current levels."