New research indicates climate instability at end of last ice age
Released on 01/04/2007, at 1:00 PM
Office of University Communications
University of Nebraska–Lincoln
As carbon dioxide levels in Earth's atmosphere continue to rise, new geological evidence suggests the planet could begin to experience climatic instability unlike anything seen in the last 34 million to 40 million years.
Writing in the Jan. 5 issue of the international journal Science, a team of scientists that includes four current and former University of Nebraska-Lincoln researchers, reported finding evidence of repeated major climate changes during the period when Earth last came out of a major ice age more than 250 million years ago.
In a multipronged study of the period from 305 million to 265 million years ago -- near the end of the Paleozoic era -- the scientists found evidence of several major climate shifts, alternating between extreme cold, or glacial, periods and periods of relative warmth. In each instance, an increase in atmospheric carbon dioxide (CO2) accompanied a shift to a warmer climate and a decrease in atmospheric CO2 accompanied a shift to a glacial climate.
"What's considerably different from previous perceptions of this ice age is that we see evidence that there were periods of really austere glacial climate that alternated with periods of relative warmth. This is news because previous work has seen it as more or less a single, protracted period, with maybe some ups and downs," said UNL geologist Christopher Fielding. Fielding is professor of geosciences at UNL and one of the Science paper's UNL co-authors, along with his wife, Tracy Frank, assistant professor of geosciences.
Frank, Fielding and their colleagues found atmospheric CO2 at 280 parts per million in the early portion of their study period and as high as 3,500 parts per million near its end, with many shifts in between. The level of atmospheric CO2 these days is approximately 380 parts per million, the highest it has been in at least 650,000 years. That's nearly half-again as high as it was before the start of the Industrial Revolution in the early 19th century. It's also rising at a faster rate than anything found in the geologic record.
"The rate of change that we may undergo here in the next 100 or 200 years might be a lot faster than we've seen in the past," Fielding said. "We really don't know how the Earth is going to respond to such rapid change . . . That's a level of CO2 (3,500 parts per million) that we've never experienced as human beings, and yet it's been fairly common at times in the past. We're at 380 (parts per million) and people think it's getting warm."
Frank said the study of the late Paleozoic era is significant because it has some important similarities to today in that Earth is again coming out of a major ice age, one that began between 34 million and 40 million years ago.
"This period of time represents the latter half of the last major ice age that the Earth has gone through, and for that reason, it's a good time period to compare to what's happening today," she said. "This was the last time the Earth went through a major ice age, and it was vegetated much like it is today. It looks like we're coming out of the major ice age, with CO2 building in the atmosphere and acting as a greenhouse gas."
The UNL geologists, Fielding, Frank, doctoral candidate Lauren Birgenheier and postdoctoral researcher Michael Rygel (now at the State University of New York-Potsdam), examined the sedimentary record of glacial activity in eastern Australia, which was at polar latitudes in Paleozoic times.
The rest of the nine-person team, including lead author Isabel Montanez of the University of California, Davis, and UNL alumnus Neil Tabor of Southern Methodist University, looked at changes in late Paleozoic tropical areas. They analyzed the isotopic compositions of soil-formed minerals and fossil plant matter to determine CO2 levels and used fossil shallow-water brachiopods as a proxy, or indicator, for sea-surface temperatures and glacial ice volume. Brachiopods, commonly known as lamp shells, are two-shelled marine animals. The oxygen isotope composition of their shells responds to changes in water temperature and the amount of water stored in ice caps.
Comparison of the data sets collected by the different groups indicated CO2 played a major role in forcing climate change during that period.
"The various different records all show coincident trends," Fielding said. "If the evidence for CO2 goes up, so does the apparent sea surface temperature proxy. When that happens, we see our nonglacial record in Australia, and vice versa. When the CO2 levels and the surface temperature go down, we see glacial deposits during those times. All these sets of data were collected independently of each other. That's really the great thing about this paper."
The research was supported by the National Science Foundation.